AfiUNICWAL WASTE DI‘SPGSAL: TEMNOLOGY MED WHEY Thesis for the Degree of M. U. P. MICHQAN SYA‘FE UNEVERSFFY ROGER W BARDSLEY 1972 ABSTRACT MUNICIPAL WASTE DISPOSAL: TECHNOLOGY AND POLICY BY Roger W. Bardsley The problems of liquid and solid waste disposal and municipal responsibility concerning them have changed dramatically in the past 30 to 40 years. Americans are producing much more solid waste and using much more water which must be municipally treated than their grandparents did. To deal with these problems there is a great array of waste disposal techniques, some old and some very new. In the area of solid waste disposal these include sanitary landfill, refuse milling, compression, composting, inciner— ation, garbage grinding, and experimental systems such as pyrolisis and oil conversion. In the area of liquid waste disposal such techniques as activated sludge, trickling filters, lagoons, spray irrigation, and complex chemical proceSses are available to treat wastewater. This technology is sufficient to dispose of liquid and solid waste efficiently and without causing pollution. It is also sufficient to begin recycling some of the Roger W. Bardsley valuable minerals and organic nutrients which are lost in most present waste disposal processes. The problem is to persuade municipalities which are using environmentally damaging, inefficient, and resource-destroying disposal methods to reasses their situations and adopt better techniques. This reassessment processes requires a set of community-wide goals endorsing pollution abatement and resource reuse. It also requires a full working knowledge of available disposal techniques and their applicability to various situations. Lastly it requires that waste disposal be integrated into the general planning process to take full advantage of the community's knowledge about its physical, economic, and social make-up. This information will enable municipal decision makers to develop waste disposal techniques which are economically feasible and which further the goals stated before of pollution abatement and resource reuse. As more and more munici- palities adopt these goals and go through the reassessment process needed to achieve them, a set of solutions to waste disposal problems will appear which will make the national achievement of these goals possible. MUNICIPAL WASTE DISPOSAL: TECHNOLOGY AND POLICY BY Roger W. Bardsley A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF URBAN PLANNING School of Urban Planning and Landscape Architecture 1972 W ,1 fi‘ 1 TABLE OF CONTENTS Chapter Page I. INTRODUCTION . . . . . . . . . . . l PhYSical. O O O O O O O O O O 0 5 SOCial O O O O O O O O O O O O 6 Cultural. . . . . . . . . . . . 6 Environmental . . . . . . . . . . 8 II. WASTE CLASSIFICATION . . . . . . . . 10 Solid Wastes . . . . . . . . . . 10 Liquid Wastes . . . . . . . . . . l3 Gaseous Wastes. . . . . . . . . . 14 Heat Wastes. . . . . . . . . . . 14 III. AVAILABLE DISPOSAL SYSTEMS . . . . . . 16 Solid Wastes . . . . . . . . . . 16 Sanitary Landfill . . . . . . . . 16 Refuse Milling . . . . . . . . . 25 Refuse Compression. . . . . . . . 27 Composting . . . . . . . . . . 29 Incineration. . . . . . . . . . 36 Uri-Site. o o o o o o o o o o 36 Central Incineration . . . . . . 37 Refractory-Lined Furnaces. . . . . 40 Water-Wall Furnaces. . . . . . . 43 Rotating Kiln. . . . . . . . . 43 Waste Heat Boilers . . . . . . . 44 Baffled Spray Chamber . . . . . . 44 Wet Scrubber . . . . . . . . . 4S Cyclone. . . . . . . . . . . 4S Electro-static Precipitator . . . . 4S Swine Feeding . . . . . . . . . 46 Garbage Grinding . . . . . . . . 48 Salvage . . . . . . . . . . . 50 ii w“"4|lllllllllf|lll llllllill'lllllrlllll Chapter Production Site Separation . . . . Disposal Site Separation. . . . . Experimental Systems. . . . . . . Recycling and Heat Recovery Systems . Pyrolisis Systems . . . . . . . Conversion Systems. . . . . . . Rural Disposal. . . . . . . . . Convenience Centers . . . . . . Sanitary Landfills. . . . . . . Liquid Wastes. . . . . . . . . . Primary Treatment. . . . . . . . Secondary Treatment . . . . . . . Chemical Treatment. . . . . . Activated Sludge . . . . . . Contact Bio-Stabilizers . . . . . Trickling Filters . . . . . . Intermittent Sand Filters . . . . Lagoons . . . . . . . . . . Spray Irrigation . . . . . . . Soil Mantle Filtration . . . . . Tertiary Treatment . . . . . . . Sludge Disposal . . . . . . . . Rural Disposal. . . . . . . . . IV. COMPARATIVE ADVANTAGES OF AVAILABLE DISPOSAL SYSTEMS O O O O O O O O O O O 0 Solid Waste Matrix . . . . . . . . Pollution Danger . . . . . . . . Land Requirements. . . . . . . . Cost Per Ton . . . . . . . . . Design Capacity Limitations . . . Limitations on Types and Percentages of Wastes Handled . . . . . . . Heat, Metal, Glass, and Paper Recovery. Soil Conditioner or Landfill Use. . . Liquid Waste Matrix. . . . . . . . iii Page 51 52 54 54 55 56 57 58 59 80 80 80 81 82 82 83 83 84 84 Chapter Page Pollution Danger . . . . . . . . . 84 Land Requirements. . . . . . . . . 86 Cost . . . . . . . . . . . . . 87 Design Capacity Limitations . . . . . 87 Solids Removal. . . . . . . . . . 87 BOD Removal. . . . . . . . . . . 87 COD Removal. . . . . . . . . . . 87 Inorganics Removal . . . . . . . . 88 Sludge Production. . . . . . . . 88 Methods of Sludge Disposal. . . . . . 90 Methods of Water Disposal . . . . . . 90 V. CHOOSING THE SYSTEM . . . . . . . . . 91 Goals . . . . . . . . . . . . . 91 Existing and Future Situation . . . . . 95 Resources . . . . . . . . . . . . 99 VI. CONCLUSION . . . . . . . . . . . . 106 BIBLIOGRAPHY O O C O O C O O O O O O O O 110 iv CHAPTER I INTRODUCTION Disposal of liquid and solid wastes has an important part to play in the planning process. Often in the past it has been left to engineers who did not have the re- sources to relate disposal to other urban needs and services, or it was ignored until a crisis point was reached. Waste disposal should first be part of general land use and environmental planning. In the same way that a city needs residential, commercial, and industrial land, it needs waste disposal land. Land suitable for the disposal of wastes has certain physical and site re- quirements which can be delineated. There is a trend in land use planning and zoning best expressed by Ian McHarg. This trend is towards looking at the natural suitability of land for a variety of purposes and managing land use accordingly. Since waste disposal processes have delimitable land requirements, certain sites can be designated as being particuarly suitable for waste disposal. This can then be related to surrounding land uses and certain sites can be picked for acquisition or easement requests. Similarly, waste disposal processes have environmental impacts and these can be used as inputs into any ecological planning framework that is being developed for a particular area. Waste disposal should be a part of transportation planning because of the requirements of the heavy trucks which collect and haul solid wastes to disposal sites. Liquid waste disposal sites are less dependent upon location adjacent to major roads but are dependent upon adequate sewer easements. Waste disposal should be a part of social and economic planning for a number of reasons. Solid waste problems are aggravated or alleviated by various patterns of residential, industrial, and commercial development. Solid waste disposal is also greatly affected by collection schedules, burning ordinances, and dumping ordinances, all matters of the police power of local government. Liquid waste is more strongly affected by the rate of development of an area, the pattern of development, and the extent of development than by any other factors. For example, permitting large-scale subdivision of land away from sewer connections can result in water pollution from septic tank overflow. This can only be remedied at a later date by constructing expensive sewer lines out to the development. Rapid development of land can overload sewage treatment facilities more quickly than they can be expanded to meet the demand. Overdevelopment in an area can tax the receiving stream's capacity to accept sewage effluent from the treatment facilities. Disposal of wastes, then, is a planning function intimately related to all other facets of planning for urban areas. Cities and counties all over the country are presently having to deal with monumental waste disposal problems, generally because waste disposal was not an integral part of their planning process. These problems range from water and air pollution due to poor disposal practices, to a lack of sufficient land to handle ever larger quantities of waste. In the following decade many of these cities and counties will be forced to completely reassess their waste disposal situations and find new answers to their problems. This paper provides a framework for that reassessment. In the following pages wastes and waste disposal are discussed from a technological point of view in an attempt to answer the question "Are techniques available for disposing of wastes without damaging the environment," and "Can we begin to reuse some of the natural resources now being lost through the waste disposal process." Once the technological picture is clear, a method is laid out by which each governmental unit charged with disposing of wastes can assess its situation and decide which of the technological options available will best suit its needs. The waste disposal problem in this country today is one of both quantity and quality. An estimated one billion pounds of solid waste and billions of gallons of waste water are produced each day. The nature of these wastes has changed in past decades. The wet garbage and ashes of our parents day has been replaced by paper and plastic. Exotic chemicals, detergents, and food wastes which were not present thirty years ago now enter the sewage stream. Population growth has accounted for some of the increase in quantity while technology and pre- packageing have accounted for the remainder of the increase plus the changing composition of the wastes. There has been a strong long-term trend towards greater per capita waste production. In 1920 the average person produced less than three pounds of solid waste and used less than twenty gallons of water per day. In 1972 the average person is producing more than five pounds of solid waste and uses between thirty and fifty gallons of water each day. These trends have recipitated a twin challenge. First we must be able to dispose of this waste safely, without causing pollution of the air, water, and soil. Second, we must find ways to reuse the natural resources which are being irretrievably lost in the waste disposal process. One way to solve both problems might be to return to some of the practices of the 1920's, and indeed this is what the demand for returnable bottles and the movement towards home composting of garbage is all about. In the end, this answer may be the best one, and dwindling natural resources may force us to conserve at home as well as at the community level. In the forseeable future, however, the problems of waste disposal and waste reuse will be dealt with and solved on a large scale. Therefore, this paper does not treat home disposal of solid waste, voluntary recycling movements, or any of the other recently proposed community-effort solutions to waste disposal. Viewing waste disposal as an isolated problem is a grave mistake. Most urban problems are interlinked and can best be viewed in an ecological context. Waste disposal affects and fits into the physical, social, cultural, and environmental subsystems of the urban whole, just as it is an integral part of planning for these subsystems. Physical Waste disposal provides for the physical removal of waste, liquid, and solid. Waste disposal uses urban and rural land both for the transportation of waste and for its ultimate treatment and disposal. Waste disposal costs money and it provides jobs both for the construction of facilities designed to process waste and for the pickup of scattered wastes and their delivery to central locations. Social Waste disposal is a social function. People classify certain items as waste and they dispose of them in socially prescribed ways. In some neighborhoods garbage day is on Tuesday, in others garbage is picked up Monday and Thursday. A few years ago plastic garbage bags were unheard of; today, they are rapidly becoming the most common method of household disposal. Waste disposal is a social function which must fit into the urban social system as a whole. Related to this is the way in which waste disposal is linked to production economics. Some segments of our economy require a high rate of turnover in production, and thus a high generation of waste, in order to function. Cultural Waste and waste disposal are integrally linked to cultural values. We classify items as either being waste or non-waste (i.e., having some value) in cultural terms. Some containers we view as having reuse value. Others are used only once and thrown away. Some food wastes are considered reusable and others are not. Cultural values tell us when an item such as a piece of clothing or furniture or even a newspaper no longer has value. The item is then discarded. These values are not completely uniform, which is what makes scavenging in junk heaps interesting and profitable; however, cultural standards are consistent enough that every disposal area in this country will contain large quantities of glass bottles, metal cans, and paper. In other countries in the world. bottles may be saved for reuse, cans may furnish a source of metal for tool-making, and paper may be reused or burned. Thus these items will not appear in large quantities in waste heaps. The other aspect of waste disposal which is most strongly tied to culture is aesthetics. In general, waste is considered ugly and is avoided by most people. However, the types of waste considered most obnoxious and the level of accumulated waste needed to impinge upon peoples' aesthetic sense is culturally determined. Two types of waste are very noticeable on the American landscape, litter, which is composed mainly of paper, plastic, and metal and glass containers, and abandoned automobiles. In Europe, neither type of waste is evident in the same quantities that it is in this country because the level at which these wastes become obnoxious is lower there. In Europe, however, cattle manure and dried sewage sludge are much more commonly seen than they are in this country because in Europe these are not seen as wastes, but rather are valued as fertilizer. While recognizing the fertilizer value of these substances, most Americans classify them as obnoxious wastes and will not tolerate their presence near where they live. In both cases, the types of wastes in evidence are culturally determined. Environmental Wastes and waste disposal play an extremely important part in the environmental subsystem of the urban system. First, waste can be unsanitary and its improper disposal can be detrimental to community health. Prior to the introduction of organized solid waste collection in the mid-nineteenth century, rats, flies, and periodic epidemics were part of the normal urban scene. In sections of some cities and in some rural areas today, rats, flies, and the threat of epidemic outbreaks are still a reality. Safe disposal of sewage is even more recent. Although the first sewers were laid in this country during the eighteenth century, complete sewerage and adequate sewage treatment is still not available in many areas. Untreated sewage flowing into a water supply can contaminate it with a variety of dangerous microorganisms. Second, waste disposal plays a strong role in the ecological system of which man is a part. In order for man to exist he needs air, water, and the products of the soil. Improper waste disposal can affect all three of these. Solid waste leachate and sewage threaten water supplies with contamination. Burning waste can pollute the air, and careless land disposal of both liquid and solid waste can damage the soil. Thus, understanding the problems surrounding waste disposal requires an under- standing of the part waste plays in the urban system. The waste, non-waste dichotomy is an integral part of our cultural, and socio-economic systems, and its disposal affects our health and welfare. There are many types of wastes produced in this country and many different agencies whose responsibility it is to dispose of them. Probably only a third of the total wastes produced are municipal wastes. However it is these wastes which are causing the majority of all disposal problems, and it is also these wastes which are of most concern to planners. The following chapter is a classification of wastes by type. Only some types of wastes are urban municipal wastes and it is these products of the city which concern local governments and form the subject matter for this paper. Chapter III is a long and detailed report of the available technology in the field of liquid and solid waste disposal. The purpose of this chapter is to lay the technical groundwork for Chapter IV, a com- parison among these disposal systems as to cost, pollution danger, effectiveness, suitability for recycling programs, and a series of other factors. This information is summa- rized in two matrices which match particular disposal systems against their ratings in these various performance areas. Chapter V takes technology a step further and describes a method whereby any city can choose the waste disposal systems which will best suit its needs. CHAPTER II WASTE CLASSIFICATION In order to deal with waste disposal it is useful to classify wastes into types. The categories most often used are liquid, gaseous, solid, and heat wastes. These often correspond in the real world to different methods of waste disposal. Solid Wastes Solid wastes are usually divided into: Garbage, the portion of solid waste that results from food preparation and consumption. "Garbage" also connotes that the material is putrescible and has a high moisture content. Rubbish, non-putrescible solid wastes, both combustible and non-combustible, made up largely of paper, metal, wood, glass, and garden discards. "Rubbish" is sometimes also taken to mean demo- lition wastes which would include brick, stone, 10 11 and cement. Discussions of rubbish will include these wastes. Ashes, that portion of wastes which are the residue from burning. In the past, ashes were an important part of the collected waste. They were produced in large quantities in the home, in industry, and in institutions. Since the advent of oil and gas- fired furnaces, however, the domestic and insti- tutional production of ashes has dropped almost to nothing. Industrial production has remained high, particularly in steam generating plants. However, these plants are usually large enough to have their own disposal facilities. The secondary production of ashes has risen in past decades. Incinerators which burn previously collected solid waste are becoming more popular in large cities. Thus the collection of ashes has ceased to be a significant part of the waste disposal picture, although ultimate disposal of ashes remains important. Special Wastes, that portion of waste which requires special handling due to size or limited occurance. Auto hulks, construction debris of large size, appliances, machinery, dead animals, and the solids accumulated in sewage treatment all are considered special wastes. 12 Agricultural Wastes, that portion of solid waste which is produced during farming, dairying, and poultry and cattle raising. Except in limited circumstances, agricultural wastes are not a municipal problem. They are either recycled back to the land or are dealt with specially by the operator. With the exception of a few cases in which agricultural wastes are handled by munici- palities, they will not be considered as part of the solid waste problem. Industrial wastes, that portion of waste which is produced in industrial processes. Industrial wastes are usually dealt with specially by the operator of the plant. In general, industrial wastes with economic value such as plastic and metal scraps are recycled directly back to the producer. Exotic chemical and radioactive wastes are handled by special contractors and do not affect the municipal situation. However when industrial wastes are disposed of along with domestic and institutional solid they must be considered as part of the municipal waste problem. Mining wastes, that portion of solid waste which is produced during mining operations. This is by far the largest category of solid waste in this country. Each person produces about 5.5 pounds of 13 domestic solid waste each day. If mining wastes were averaged into the amount, the total would be over 100 pounds per day. However, except for very infrequent circumstances, mining wastes cannot be handled by municipal services and are thus a separate problem. Liquid Wastes Liquid wastes are those waste products generated which have the capacity to flow. The largest form of liquid waste is waste water. It is generated domestically as water is used for drinking, cooking, cleaning, and as a medium to carry away garbage and human wastes. It is also generated in large quantities by institutional and industrial sources. These sources add metal ions, organic and inorganic chemicals, heat, and other wastes to the water. Agriculture and land runoff also contribute to waste water, since animal wastes, fertilizers and oils are often carried off by rainfall and irrigation water. Other liquid wastes are those petroleum products and chemicals which are the unusable by-product of industrial processes that are not dumped into the water. In general, only those liquid wastes entering the sewage and storm sewer systems can be considered as part of the urban disposal problem. 14 Gaseous Wastes This is a less satisfactory category than the two because it may include such wastes as dust and fly ash which if collected would be considered solid waste. However, the nature of gaseous waste is such that it requires a separate category. This form of waste has a number of sources. Fuel consumption is a major source, contributing hydrocarbon, oxides of sulfur, carbon, and nitrogen, alkali salts, and a number of trace elements to the atmosphere. Some industrial processes release ozone, leads, oils, ammonia, and other chemicals. In addition there is the dust and fly ash mentioned before which results from incineration and some manufacturing processes. Gaseous wastes are almost entirely out of the realm of municipal waste disposal except where they are generated by the disposal process itself. Landfill can create significant amounts of dust and incineration of rubbish creates fly ash and these are distinct municipal concerns . Heat Wastes This is a recent addition to the waste disposal problem. Waste heat occurs because in industrialized countries the burning of fossil and nuclear fuels has contributed significant amounts of heat over normal solar energy to particular parts of the environment. The effects of this heat are only beginning to be charted but among the 15 known results are disruptions of fish populations and local weather changes. Waste heat is generally not a municipal concern except as it is generated by either power gener- ation or waste incineration. Waste heat from power generation can be solved on a plant-by-plant basis with existing technology and the disposal of waste heat from incineration is thoroughly discussed in a following chapter. The concern, then, is municipal wastes; liquid, solid, gaseous, and heat; methods of treating them, and means of ultimately disposing of them; and comparative advantages and disadvantages among the various waste disposal techniques. A flow chart which will aid any municipality in choosing the best disposal techniques for its situation given the inputs of local and national goals, available funds, soil and site analysis, population size served, existing facilities, and future needs has been included. This chart should enable any governmental agency concerned with waste disposal to take a rational systems approach to its disposal problems and determine the best solution to those problems. CHAPTER III AVAILABLE DISPOSAL SYSTEMS There are a great number of techniques for dis- posing of liquid and solid wastes. Some are centuries old and some have only recently been developed. The following is a technological overview of these systems and provides the basis for a comparison among these systems in Chapter IV. Solid Wastes Sanita£y_Landfill Dumping on land was probably the first refuse disposal method used by man, perhaps because it was the most convenient.1 As cities grew and people gathered together in greater and greater numbers, the practice of dumping in the streets and at the backs of houses became increasingly unsatisfactory. Refuse disposal was organized to the point that wastes were hauled to the nearest available open space outside the city and dumped. 1American Public Works Association, Municipal Refuse Disposal, 1961, p. 89. 16 17 By 1900 the health dangers of open dumps were clearly recognized but little was done about the open dump situation except to haul the wastes further from town. In some cities in the early 1900's refuse was used to reclaim marginal land along lake and river fronts. Champaign, Illinois, Columbus, Ohio, and Davenport, Iowa all suc- cessfully used garbage as fill before 1910. While the dump was being operated it was in no way sanitary. Refuse was simply disposed of and allowed to settle until the desired level was reached. At that point, the refuse was covered with earth and reclaimed. During the 1930's heavy machinery became commonly available and several cities including New York experimented successfully with compacting the refuse as it was dumped. This saved landfill space and hastened the settling process which enabled land to be reclaimed more quickly after dumping had ceased. During this period also the term "sanitary landfill" was coined in Fresno, California where the cut and cover trench method was first used. The Army Corps of Engineers perfected the sanitary landfill technique during World War II and it came into wide- spread municipal practice after the War. By 1960, over 1,400 cities had adopted sanitary landfill. Sanitary landfill is essentially a sophisticated refuse burial system. Refuse is dumped in a trench, gully, marshy diked-in area, or on open ground and thoroughly compacted by bulldozers. A three-to-one compaction ratio 18 over loose refuse is often obtained. At the end of the day's dumping, the refuse surface is covered with earth. Since the refuse is compacted and covered each day, a completed landfill consists of a series of refuse cells, each surrounded by earth. Compaction continues after the refuse is buried due to decomposition but tapers off after two to five years. During this time the landfill is unstable and gas produced by anaerobic bacteria is seeping to the surface. Sanitary landfill is the easiest, cheapest, and one of the safest methods of refuse disposal available. Actual disposal costs (not including land cost) average about $1.50 per ton. Any skilled equipment operator can run a landfill operation. Rats, flies, and odors are no problem due to daily covering of the refuse by dirt. Dust is sometimes a problem but can be controlled by occasionally spraying the working area with water. Blowing paper can be a serious problem if precautions are not taken. Storm fences and snow fences 6-10 feet high placed downwind of the site will trap most blowing paper. Noise is a definite problem if the site is located in a populated area. Noise protection measures include erecting earth berms around the site, planting dense vegetative screens, and locating fill sites away from populated areas. The greatest dangers from landfills occur when surface water runs through the site and on into a receiving l9 stream or when surface water percolates through a site and into an acquifer. When these situations exist, the following types of contamination can occur:2 Virus contamination—~refuse, particularly when the refuse includes sewage sludge, contains a large variety of viruses. These viruses can cause polio, hepatitis, gastro-intestinal diseases, and respiratory ailments. Although viruses do not travel far when leachate (the water that has traveled through the fill site) percolates through sand or clay, they will travel long distances through factured rock and in surface water. Micro-organism contamination--many types of micro- organisms are found in refuse. They may include bacteria, molds, yeasts, algae, rotifers, insect larvae, and worm eggs. These contaminants also will travel only a few feet when percolated through sand or clay but will travel great distances through fractured underground rock and in surface waters. Inorganic chemical contamination--inorganic contaminants include many metals and salts which become toxic when dissolved. Among the most toxic are lead, mercury, copper, silver, cadmium, zinc, aluminum, arsenic, nickle, antimony, and tin. Other chemicals such as iron, calcium, magnesium, potassium, chloride, iodide, and 2Student Water Publications Club, "Livingston County Solid Waste Study," I (1970), A-2. 20 bromide are not toxic but harden water and can impart a bad taste. Inorganic chemical pollutants are not as easily removed by filtration through a soil medium as are biological contaminants. Organic chemical contamination--organic chemicals which contaminate aquifers frequently give a bad taste and odor to the water. Principally they are carbohydrates, fats, proteins, greases, and oils. These contaminants are effectively removed by filtration through sand or clay, but once again, they can travel long distances through fractured rock or in surface water. The only way to insure protection from leachate contamination is through careful site selection. Once a landfill has been in operation for any length of time, it is nearly impossible to stop subsurface water contamination and difficult to halt surface drainage contamination. However, test wells should be drilled around the site and surface waters nearby should be checked periodically so that dumping can be halted if pollution is detected. Subsurface water contamination can be avoided by locating the landfill on a site having a base of uncon- solidated material 50 feet deep (30 feet is sufficient if no trenching is to be done).3 This unconsolidated material can be composed of sand, clay, silt, or a 31bid., p. A-l4. 21 combination of these. There should be no direct way for water percolating through the refuse to reach aquifers below without first filtering through this unconsolidated medium. Below the sand or clay but above the shallowest aquifer should be an impermeable layer to prevent direct contamination of the water-bearing strata. In addition the base of the landfill should be at least two feet above the highest ground water table. This prevents the refuse from being saturated and standing in water. To prevent surface water contamination, landfill sites should be located away from springs and drainage channels. Any surface drainage which develops as the site is being operated should be routed around the fill area. The heads of gullys make ideal landfill sites as no surface water can pass through the refuse and into a larger stream, thereby contaminating it. Marshy areas should not be filled, due to possible pollution of nearby lakes or rivers. However, since refuse will probably continue to be used as a cheap means of reclaiming land, guidelines can be set which will reduce the risk of contamination. The site area should be diked with an earthen berm and then filled. This will prevent direct contact by the refuse of nearby waters and the berm will act as a filter for sideways percolating water. Tests in Michigan indicate that a five foot earth bank effectively screens out most pollutants. 22 Ideal sanitary landfill terrain is uneven with gullys that can be filled. The landfill should be located at the head of the gully to prevent surface water from draining through it. The cut and cover trench method is suitable for level ground. With this technique, cover material is excavated from one trench while the previously dug trench is being filled with refuse. The newly excavated trench in turn is filled with refuse and covered with material dug from still another trench. In areas where strip mining or quarrying has taken place, the old excavations can be used provided that they meet the previously mentioned standards with regards to pollution control. Strip coal mines are particularly suitable as ample cover material is available next to the trench. The problems of site location have probably kept sanitary landfill from becoming the exclusive method of solid waste disposal in this country. In addition to the problems of noise, dust, blowing papers, and water pollution, citizen opposition to location of close-in disposal sites has often prevented sanitary landfill from being economical. Collection costs, no matter what the ultimate disposal method used is, usually average from two-thirds to three-fourths of the total disposal cost. A rule of thumb used by waste disposal experts4 is that 4Fargo Engineering Company, Regional Plan for Solid Waste Disposal, Jackson County, MiChigan, 1971, p. I7. 23 fifteen miles is the absolute maximum one-way haul distance that collector trucks should make. Beyond that distance, economic factors dictate some form of intermediate bulk reduction process. Because of these problems, and because of the limited distance that collector trucks can eco- nomically travel, adequate close-in disposal sites are at a premium in many urban areas. Oftentimes it is not possible to dispose of solid wastes directly by burial simply because in an unreduced state the wastes take up too much space. Every municipality must have a sanitary landfill somewhere. Whether it uses this landfill to dispose of refuse directly, or whether it uses it to dispose of the ashes after the refuse has been incinerated is dictated chiefly by the availability of good landfill sites. In addition, municipalities need landfills in order to dispose of bulky wastes such as tree stumps and appliances which are difficult to reduce by incineration, and of dangerous special wastes such as chemical sludges, dried sewage sludge, and industrial wastes. If close-in sites are limited or the only landfill sites are distant, a munici- pality cannot afford to bury refuse directly but must first reduce its bulk. A rule of thumb used by waste disposal experts is that a city should have one acre per 10,000 people per year for sanitary landfill. At current solid waste gener- ation rates, the acre would be covered with compacted 24 refuse eight feet deep. If significant amounts of refuse are recycled, salvaged, incinerated, or dumped elsewhere, the acreage needed can be reduced. Oftentimes munici- palities will dump construction debris in areas such as marsh or tidelands which are unsuitable for mixed refuse. Debris and rubble are normally non-putrescible and do not contain toxic wastes. Cities often also encourage salvage operations at landfill sites. Salvagers remove many bulky metal objects which saves on landfill space. Reuse of the landfill site should be included in any discussion of sanitary landfills. Sanitary landfill has the potential to reclaim marginal land and to add terrain features to an otherwise flat and dull landscape. In several instances, including a case in Chicago, landfill has been used to create large hills with recreational value. Landfill sites are generally used for parks, schools, golf courses, and occasionally for subdivisions. If the intended use is planned for while the landfill is in operation, the ground can be sculpted by the bulldozers working the site into the desired forms. This can save a great deal of time and money later when the area is put to another use. Immediately after completion, a landfill site should be seeded with grass to check erosion. After several years when gas seepage has subsided, trees may be planted. If trees are planted too early, they may be killed by the methane. 25 Refuse Milling Refuse milling or grinding has been used suc- cessfully in Weisbaden, Germany, in Montreal, and recently in Madison, Wisconsin. Experience in demonstration projects has shown the following: (1) Results have suggested that daily cover is not necessary. In fact it is felt an operation can be satisfactorily run by covering only when final grades are reached. (2) Milled refuse compacts more readily and uniformly than unmilled material often reducing bulk by one half. In addition the lack of required intermediate cover further reduces fill volumes thereby extending the life of the site. Another advantage of increased compaction is that leaching into ground water will be reduced [not proven]. (3) The problem of blowing paper as well as dust is almost entirely eliminated. Milled refuse is easier to handle on a continuous basis; and be trans- ferred on conveyors. (5) Cold and wet weather problems with daily covering operations are eliminated and costly equipment can be put to other uses. (6) Milled refuse can be used as a road bed on the landfill site during inclement weather. Trucks can drive on the material even when wet, with no injury to tires. (7) Milled refuse does not burn as readily as raw refuse and the milling process itself prevents hot ashes from reaching the fill area. (9) Milled refuse does not attract insects and rodents because the organic matter is ground extremely fine [note true] and distributed evenly. . . . (10) Finally, milled refuse can be easily contoured to create an attractive countryside.5 Refuse milling, then, is simply a method of making solid waste easier to handle and more compact in a landfill. The claims made for ground refuse being less of a nuisance than unmilled waste has not been proven. Flies and rodents are definitely discouraged by the process, but will cause problems if the refuse is left untended and uncovered. 5Student Water Publications Club, "Livingston County Solid Waste Study," I (1970), D-S. 26 Odor is not eliminated by milling, but it is controlled by daily covering of the refuse with earth. Refuse is not actually ground extremely fine, at least this is not possible if only one grinding is performed. Commercial hammermills can reduce municipal refuse to pieces that average one inch by one inch after two grindings. A single grinding yields pieces that average three inches by three inches.6 The average cost per ton processed at the Gainsville, Florida composting plant for primary grinding was $.90. The average cost per ton for secondary grinding was $1.12. A large portion of these costs were labor for maintenance and parts and labor for repairs. It is probable that a carefully run operation could reduce both these costs significantly. However, hammer wear is inevitable when the mill is used to process mixed municipal refuse and hammer surfaces must be rebuilt frequently. If refuse is pre-sorted to remove rubble, large branches and stumps, and large metal objects, this wear can be reduced. However, the pre-sorting represents an added cost. A variety of companies make shredders large enough to handle almost all municipal wastes, although not without the significant wear mentioned previously. The star shredder used in the Gainsville plant could process twenty tons per hour. The Edial shredder mentioned in the Livingston 6Gainesville Municipal Waste Conversion Authority, Inc., Gainesville Compost Plant, 1969, Appendix F. 27 County, Michigan study could process eighty tons per hour, while the Hazemag shredder could process fifty tons per hour.7 In general, then, refuse grinding is only economical when landfill space is at a premium but for reasons of cost or otherwise incineration cannot be considered. Refuse milling may also be practical if extensive landscaping with the solid waste is anticipated. Shredded refuse is more consistent in texture, density, and weight than un- shredded refuse and is therefore easier to work. If a project such as "Mt. Trashmore" in James Park, Chicago8 is comtemplated, shredded refuse may be practical. Refuse Compression Refuse compression is not a disposal method but since it canextend the life of a landfill and increase economic hauling distance it should be included in this discussion on variations of the sanitary landfill process. Refuse compressors are available which can squeeze refuse from its loose weight of about 300 pounds per cubic yard to 1,000 pounds per cubic yard, the weight of unconsolidated 9 soil. The compressed refuse can either be baled or hauled 7Op. cit., p. D—4. 81bid., III, p. w-13. 9Richard B. Engdahl, Solid Waste Processing: A State-of-the-Art Report on Unit Operations and Processes, I969, p. 4. 28 as is to the landfill site. Sufficiently compacted refuse blocks can be stacked in a landfill without covering because the refuse is not attractive to pests and decom- position takes place slowly. When the landfill is complete, however, it must be covered and should be treated in the same manner as a landfill using on-site compaction. Packer trucks, introduced about 1950, have made refuse collection more economical by allowing the truck to collect about three times as much refuse as an open truck before proceeding to the disposal site. However, this has not significantly increased the economic one-way haul distance to the landfill. To increase this distance, transfer stations which further compact the refuse to the 1,000 pounds/cu. yard density mentioned above and load it on trains or semi-trailer trucks must be used. Several large cities, including San Francisco, California have contemplated rail-hauling compressed refuse up to 200 miles to a disposal site. Refuse compaction and transfer stations become economically attractive at about the 1,000 tons per day size. Disposal costs for units of this size might be as low as $4 per ton,10 comparable with inciner- ators of the same capacity. Because of this, refuse compaction and transfer may become an alternative to incineration for larger municipalities in the future. 10Student Water Publications Club, "Mecosta County, Michigan Waste Disposal Study," 1970, p. 27. 29 Composting Composting as a technique for solid waste disposal and reuse has shown so much potential that dozens of demonstration projects have been run in this country in the past twenty years. Unfortunately, the problems have outweighed the benefits so far and as a result there are no permanent composting operations in the United States today. Composting is a biological aerobic stabilization process. The material to be composted is first adjusted to about 60 per cent moisture. It is then continuously or intermittently aerated for a period of from five days to a month until it is biologically stable. The phrase "biologically stable" means that if the material is again adjusted to 60 per cent moisture and aerated its temper- ature will not rise greatly. The temperature of unstable compost rises quickly when dampened and aerated. The biological processes which take place during composting are complex. During the first stage which lasts from two days to one week, various bacteria act upon the material and raise its temperature to between 120 and 140 degrees Fahrenheit. At this point the first popu- lations of bacteria die off and are replaced by thermo- philic bacteria which rapidly decompose the material. In two days to a week, the thermophilic stage tapers off and is replaced by the activities of fungi. These attack the 30 remaining cellulose, which is the most difficult portion of refuse to decompose. The last stage continues until all the organic matter or humus has been broken into its constituent elements. This stage, however, can take place in the soil and compost is ready to use about a week after the thermophilic stage has ended. During the thermophilic stage, all known pathogens are killed by the heat and the actions of other micro-organisms. Compost has thus been labeled "sanitary" and is safe for use anywhere. Weed seeds are also killed by the heat and compost will conse- quently not harm agricultural operations. To compost effectively the following conditions must be met: 1. Raw materials should have a carbon to nitrogen ratio of 50 to l or less. They should have no serious deficiency of essential food elements and should be within a normal pH range of from 5.5 to 8. 2. Material should be mixed and ground (fine for mechanical composting, medium to coarse grind for windrow or area composting). 3. Moisture should be controlled to 50 to 60 per cent throughout the process. 4. Air should be thoroughly dispersed throughout the composting material with an excess of oxygen remaining. 5. Seed compost should be recycled in the amount of about 1 to 10 per cent by weight. If high-rate composting is used the following additional conditions have been found desirable: l. Constant slow stirring or intermittent stirring every 5 to 10 minutes or a combination of forced air and less frequent stirring. 2. Temperatures should be controlled throughout the process. 3. The pH should be controlled to prevent nitrogen loss. 31 4. Digestion should be a continuous flow process in three or four stages, including recycling of seed and thorough mixing for each stage. The last stage may combine slower digestion with natural drying from the heat.11 The end product of the composting process is a relatively dry, odor-free substance which has good soil conditioning properties and the approximate fertilizer value of steer manure. This material does not have to be sold at a profit for a composting plant to be economically sound. However the entire end product must be disposed of and the sale of the compost must at least bring the total costs of the process in line with other disposal methods. In Europe where compost is valued as a soil conditioner there has been little problem making the process pay. In the United States where there is now a surplus of steer manure the sale of compost is more difficult. The best composting mixture is a combination of rubbish (dry solid waste), garbage (wet solid waste, mostly food scraps), and sewage sludge. Sewage sludge is about 97 per cent water and it is used to raise the water content of the refuse from an average of about 30 per cent to the required 50 to 60 per cent. In addition the sewage solids provide a good nitrogen source which helps ensure an acceptable carbon/nitrogen ratio. 11American Public Works Association, Municipal Refuse Disposal, 1961, p. 227. 32 The refuse is first sorted to remove as much metal, glass, and stone as possible. In areas where the refuse contains a higher percentage of paper as many municipal wastes do in the United States, some paper should be removed to gain an acceptable carbon/nitrogen ratio. If the amount of plastic in the refuse is higher than the usual 1-2 per cent, at least some of the plastic scraps should be removed. Plastic is non-biodegradable and a high percentage of plastic in compost reduces its value.) This separation has in the past been done chiefly by hand picking although ballistic separators, magnetic separators, and vibrating screens are available which will mechanically sort refuse. After sorting the refuse is fed into a grinder or series of grinders which pulverize the compost- able material. Hammermills and rasps have proven effective in this process. The resultant mass is passed beneath an electronic sensor which determines its moisture content and adds sufficient water or sewage sludge to bring the material up to the required 60 per cent moisture. At this point the refuse is ready to be composted. If a mechanical composter is used the material is fed into the first stage of the container. Mechanical composters are essentially gigantic drums divided into from three to seven sections. The drum is rotated slowly while air is forced through it. Each day the compost passes from one section to the next until it is removed from the last stage 33 in near-completed form. Windrow and area composting calls for the material to be heaped in piles and turned occasionally during a composting period which lasts several weeks. The resulting material can be sold directly for agricultural use or reground, bagged and retailed to home gardeners. The problems involved with composting have been detailed in many studies. Composting is a successful process in Europe and its application in this country has been attempted many times since Sir Albert Howard first developed the technique in India in the 1920's. The Dano biostabilizer is a popular European package plant and can be ordered in various sizes to handle a number of differ- ent situations. Most United States failures have been blamed on the lack of a ready market for compost. Indeed, this problem is difficult to overcome in a nation of giant farms and few small gardeners. Large farming enterprises have not been interested in soil conditioners, and the small gardener of Europe who cultivates his back yard and nearby vacant lots to raise vegetables is not found in the United States. The more important reason for the failure of the composting technique in this country is the unsuitable nature of American solid waste to the composting process. The breakdown of a quantity of typical American solid waste by volume is as follows: 34 corrugated cardboard 7% newspapers 14% miscellaneous papers 25% plastic film 2% leather, molded plastic, and rubber 2% garbage 12% grass, leaves, and dirt 10% textiles 3% wood 7% glass, ceramics, and stone 10% metallics 8%12 Paper does not compost well and the majority of it should be removed before the process begins. Plastic is non-biodegradable and is considered a contaminant. However, if the quantities of plastic are very small they can be left in the compost. Metal, glass, ceramic, stone, and the classification which includes leather, molded plastic, and rubber are all non-compostable. They must be screened, hand picked or ballistically separated from the material to be composted. This leaves garbage, grass, leaves and dirt, textiles, and wood to be composted. These together with a portion of the paper refuse, most of the plastic film, and any ashes which may be present are what enter the composter. They will total between 30 per cent and 50 per cent of typical municipal solid waste. In Europe where composting is commonly practiced, the percentages of paper, plastic, metal and glass in refuse are much lower and the percentages of garbage and other compostables much higher. 12Jones & Henry Enginees Limited, Prpposals £95 a Refuse Disposal System for Oakland CountyL Mich., 1970, p. 9. 35 As a result composting can be truly effective as a waste disposal process as well as a means of producing a useful product. Because less than half of United States refuse is compostable, the problem of what to do with the remainder continues. When non-compostables are separated from compostable material they are usually segregated by the separation process into paper, glass, and stone, ferrous metal and nonferrous metal. This separation process, which requires a combination of hand picking and machine sorting, costs money. In order to recoup the sorting costs, most of the sorted material must be sold. A compost market does exist as can be seen by the large number of soil conditioners available commercially. The city of Milwaukee has for years sold its dried sewage Sludge under the brand name "Milorganite." Whether a large enough market exists to handle the great quantities of compost that would be generated by a number of cities composting their solid waste cannot be determined easily. However, the problem of finding markets for the 60 per cent of solid waste which is non-compostable is the larger problem. The difficulties inherent in the salvage method of refuse disposal will be discussed later. The appeal of composting will remain because it offers the possibility of disposing of solid waste while actually producing a useful material. It is likely that 36 in the future easily compostable substances such as sewage sludge, abbattoir wastes, and institutional garbage may be gathered together and composted. It is also possible that improved sorting methods will lower the cost of composting to make it competitive, even without satisfactory markets, with other methods of solid waste disposal. Until that time, it does not have great potential as a refuse disposal method. Incineration There are two basic types of incinerators, on-site and central. On-site incinerators are located at the point of refuse production and central incinerators have wastes brought to them from a wide generating area. On-Site.--On-site incinerators range from back- yard burners often found in suburban homes to full-scale high-temperature incinerators found in hospitals and large office buildings. On-site incinerators have served well for many years. They are able to reduce the bulk of most refuse to 25 per cent of its original volume and render it innocuous so that it can be spread on the ground or hauled away and dumped. If wastes are segregated into burnables, garbage, and non-burnables, as is the case in some insti- tutions and some cities, incinerators can effectively deal with half of the rubbish and leave almost no residue. 37 The major problem with on-site incinerators is that wastes do not burn cleanly and the equipment is generally too small to be fitted with effective air pollution devices. Los Angeles in the past required its residents to burn their burnable trash but had such great air pollution problems that the city government was forced to stop home incineration. Larger incinerators such as the one used for years by Michigan State University (closed March, 1972) burn refuse more cleanly than do back-yard burners. Trash is agitated while burning and auxiliary fuel is added when the refuse is wet and not burning hot enough for complete combustion. However, these instal- lations still release large quantities of particulate matter into the air and in cities are rapidly falling out of favor. In rural areas, though, on-site incineration will probably remain an important method of refuse disposal. Central Incineration.--Central incinerators are used to reduce the bulk and nuisance problems of solid waste and prepare it for disposal in a landfill site. For larger municipalities which are now experiencing a shortage of landfill space, incineration will probably be the method used to handle solid wastes during the next few decades. Many large cities presently use incinerators and have been using them for years. The problem of air pollution long associated with incinerators is rapidly being solved, 38 thus eliminating the major drawback to this technique of refuse disposal. The advantages of incinerators are: Incineration requires much less land than does the landfill method of disposal. The incinerator itself can be situated on a relatively small parcel of land. Some of the incinerators in Detroit occupy parcels of less than five acres. Residue disposal requires less land than required for landfilling raw refuse--a significant contribution to conserving land resources. A more central location is possible for an inciner- ation plant. The incinerator can be located close to the service area in an industrially zoned, or in some instances a commercially zoned area. A well designed building with attractively landscaped grounds surrounding it will make the operation acceptable in many neighborhoods. Locating a plant near the center of the refuse shed reduces the hauling cost. An incinerator produces a residue that contains small quantities of organic material and is less nuisance than raw refuse. It is often mis-stated that residue from an incinerator is sterile. Recent examinations of incinerator residue indi- cates that the ash abounds in biological life, but that most of the pathogenic organisms are destroyed by incineration. Residue produced by an inciner- ator must be covered like raw refuse because of the minute quantities of organic matter. An incinerator plant can burn many kinds of refuse. It will burn most combustibles to an ash and can even reduce the bulk of some non—combustible components of a mixed refuse. The residue of a modern incinerator represents approximately one- third the weight of material introduced to the furnace. At the same time that the weight is being lessened, the volume of material is reduced to approximately one-fifth of the delivered volume. An incinerator cannot, however, handle large objects, those which cause excessive smoke, or explosives. The operation of an incinerator is generally not affected by climate or unusual weather conditions. Some flexibility exists in the incinerator for handling varying amounts of refuse. A plant can operate 8, 12, 16, or 24 hours per day, for example. The operation can also be carried out on a 5, 6, or 7 day burning week. Grate speed in the must be 1. 39 furnace may be adjusted within a limited range to regulate the time material remains in the com- bustion chamber. Waste heat can sometimes be sold to nearby insti- tutions or industry in the form of steam. The incinerator has several disadvantages which weighed against the advantages outlined above. The incinerator is expensive in capital cost as well as operating cost. Depending upon the type and size of unit constructed, the initial cost will vary from $4,000 to almost $10,000 per ton of rated daily capacity, whereas the operating cost can range from $5.00 to $9.00, including residue disposal and amortization. Skilled employees are required to operate, repair, and maintain an incinerator. These men are more in number and generally higher paid than employees at a sanitary landfill. Maintenance and repair costs are also higher because of the type of equipment involved in the furnaces. Equipment is often damaged by wire, . fusable metals, abrasives, or explosive objects entering the furnace with the refuse. The combined high capital investment and the costly maintenance and repair for incinerators create a per ton cost for refuse disposal considerably higher than for sanitary landfill. It is often difficult to obtain the best site for an incinerator because refuse disposal operations are not acceptable to many people. Because of heavy truck traffic, the possibility of noise, or other real or imagined nuisances, incinerator locations are frequently confined to industrial areas. Even this precaution will not prevent nuisance complaints from nearby residents. Incineration does not complete the job of disposal of the community's waste. Residue and flyash must be transported to a landfill site for burial.13 Incinerators are divided into two major types with numerous minor variations. One constant which holds for all types of incinerators is that they are most efficient when constructed in the 200-400 ton per day capacity range. 13Ibid., p. 21. 40 Assuming that each person in the service area produces about five pounds of solid waste per day and that the minimum operating time of an incinerator is eight hours per day, five days a week, the minimum service population for an efficient incinerator is 20,000-25,000. Smaller incinerators can be and have been built but they are not nearly as efficient in terms of cost per ton as the larger incinerators. The two major types of central incinerators are refactory-lined and water wall. These terms refer to the lining of the combustion chamber, the most costly and most easily damaged part of the incinerator. Some classifi- cation schemes divide incinerators further into batch-fed and continuous-feed type furnaces. In the batch-fed type incinerator, refuse is placed in the combustion chamber, incinerated, and the ashes removed before more refuse is added. The continuous-feed incinerator performs all processes on a continuous basis. Refuse moves into the combustion chamber on a moving grate and ashes are dumped off the end and removed, all without stopping. The classification system used here does not refer to batch—fed furnaces, however, because in the large furnaces under discussion, the batch-feed system is not used. Refractory-Lined Furnaces.--The refractory-lined furnace is the type of furnace commonly in use in America today. It is most economical in sizes under 250 tons per 41 day, but can be constructed in sizes up to 300 tons per day rated capacity. Refractory-lined furnaces are lined either with refractory brick or refractory clay. The bricks are made of high quality clay and are able to withstand high temperatures. The clay lining of a clay- 1ined incinerator is applied like plaster and is heat cured in place. Relining a furnace costs $20-$30 per square foot ($100,000 for a 300 ton per day incinerator) and must be done every two to three years. Refractory linings deteriorate due to rapid heating and cooling, moisture expansion, and slagging. Refuse is never uniform in quality and its burning produces uneven temperatures inside the combustion chamber. In addition, when the unit is allowed to heat and cool during start-up and shut-down, stresses are created in the brick and furnace frame. These stresses cause movement of the furnace lining, which in turn can chip or break the bricks. Condensation of moisture also occurs when the furnace is cooled. Moisture is absorbed by the refractory lining and when the furnace is brought back to operating temperature, turns to steam. This rapid expansion spalls off the face of the lining. Gases produced during refuse burning can, at high temperatures, cause corrosion to the refractory lining. This is known as slagging. In order to cool the refractory as well as to insure complete combustion, overfire air is blown into the 42 combustion chamber. The amount of air blown in is usually expressed in terms of the amount needed to burn the refuse plus whatever extra air is blown through the chamber. A refractory furnace usually operates at 150 per cent to 200 per cent excess air. In addition to overfire air, underfire air is blown under the grate in order to cool the metal and insure complete combustion. This underfire air contributes to the total excess air figure. Just as all the water that passes through the sewage system of a city becomes contaminated, all the air that passes through an incinerator becomes contaminated with fly ash and smoke. Since central incinerators are usually located in populated areas, air pollution control equipment is a necessary addition to the installation. Below is a list of common air pollution control devices and their percentage efficiencies in removing particulate matter from flue gases.14 Baffled spary chamber 50% Wet scrubber 96% Cyclone 78% Electro-static precipitator 95% The first two devices utilize water while the last two are dry. Very good air pollution control is possible with present technology. The cost varies with the tech- nique used, the amount of excess air coming through the furnace, and the individual installation. It can range 14Ibid., p. 34. 43 from $.40 to $2.00 per ton processed. Pollution control devices will be discussed fully below as the type selected has a direct bearing on the type of furnace installed. Water-Wall Furnaces.--The water-wall furnace eliminates one of the major drawbacks of the refractory- lined furnace in that it does not require huge amounts of excess air to cool the lining. Consequently, air pollution control is easier and cheaper because the scrubbers do not have to process as much air. Water-wall furnaces have not been used extensively in this country although their use is well established in Europe. At the present this type of furnace is operated in Norfolk, Virginia and New York City. The principle of the water-wall furnace is that the furnace lining itself conducts heat away and thus stays cool. The lining is constructed like a tube boiler with the tubes either welded solidly together or connected by metal fins. Water circulating through the tubes conducts heat away and keeps the tubes from burning through. The waste heat can be sold in the form of steam. Two additions which are often found on incinerators should also be discussed, rotating kilns and waste heat boilers. Rotating Kiln.--The rotating kiln is an addition to an incinerator which insures almost complete burnout of combustible materials. After leaving the combustion 44 chamber, the incinerator residue enters a revolving drum through which the hot flue gases are passed. The rotation exposes all remaining unburned material to the hot gas and completes burnout. Since modern incinerators are usually equipped with agitation grates in the combustion chamber which help expose and turn the refuse, little added benefit results from the inclusion of a rotating kiln to the operation. Wagte Heat Boilers.--The flue gases leaving the combustion chamber of a furnace are extremely hot. In order to cool these gases before they pass through air pollution control devices and to utilize the waste heat, waste heat boilers are often installed in the flue between the combustion chamber and the base of the stack. The waste heat boiler consists of tubes through which water is circulated. The water conducts the heat away and cools the gas. As stated before, air pollution control equipment is capable of high levels of particulate matter removal from flue gases. Each type has certain operating charac- teristics and requirements and should be carefully matched to the furnace to insure efficient operation. The following is a brief description of the devices mentioned previously. Baffled Spray Chamber.--The baffled spray chamber forces flue gases through a series of baffles which slow the gases and bring them into contact with water spray. 45 The spray scrubs the gases and removes much of the particulate matter. Wet Scrubber.--The wet scrubber removes dust particles from the gas stream by bubbling it through a water bath or injecting water into the gas stream. This method is essentially a much more efficient version of the baffled spray chamber. Cyclone.--Mechanical cyclone collectors are a dry system which rely upon centrifugal force to separate suspended particulate matter from the gas. The cyclone causes the gas to spiral, throwing solid particles to the outside and leaving a core of clean gas which is allowed to escape. The dust falls down the walls of the cyclone and is removed. Electro-static Precipitator.--The electroestatic precipitator is a highly efficient dry dust collector. The precipitator consists of a series of plates between which wires are suspended. Dust particles receive an electrical charge from the wires, and are attracted to the oppositely charged plates and removed. Rappers vibrate the plates and dislodge the dust which falls into receiving hoppers. Gas temperatures for electro-static precipitators are critical and must fall within the range 350-570 degrees Fahrenheit. Gas velocities are also 46 critical and cannot exceed eight feet per second.15 Electro-static precipitators should be preceded by cylones which remove the large particles from the gas stream. This will prevent possible short-outs. In general, then, incinerators fitted with effective air pollution control devices will continue to be a major method of solid waste disposal for medium to large cities in the future. As collection costs increase and open land becomes more scarce the incinerator may become even more economical and favored than it has been in the past. Swine Feeding The feeding of garbage to swine is an ancient practice that was probably begun when swine were first domesticated. Garbage was simply thrown in the streets and the pigs were permitted to consume it there. After the advent of organized garbage collection, private contractors often collected garbage from a variety of sources and sold it to nearby swine raisers. In the early part of this century in America the garbage portion of solid waste was often disposed of in this fashion. Old paper was burned in the furnace and bottles and metal were reused. Consequently, for many years municipalities lsIbid., p. 34. 47 concerned themselves only with the collection and disposal of ashes and demolition wastes. The connection between feeding raw garbage to pigs and the incidence of pork infection with trichina worms has long been known. However, it was a swine disease, vesicular exanthema, which brought about regulation requiring that garbage be cooked before it was fed to swine. This disease spread rapidly between 1953 and 1955 and killed over 400,000 pigs.16 Since then, all garbage fed to swine has been required to be cooked and this has made garbage feeding less economical than it was in the past. The reason for the decline of garbage feeding, however, has been due more to labor costs, zoning, and technology than to regulations requiring cooking. Home garbage grinders and municipal collection of mixed solid wastes has eliminated domestic garbage as a source of feed. It is not economical to separate the garbage portion of solid waste from mixed wastes after they have been collected. This has left institutions such as schools and hospitals and restaurants as sources of unmixed garbage. Oftentimes today these establishments grind their food wastes and flush them into the sewer system. Also, the costs of collection and hauling garbage have 16American Public Works Association, Municipal Refuse Disposal, p. 243. 48 gone up, labor costs have risen, and pig farms have moved further out from the city. Strict nuisance laws and zoning have forced swine raisers far into the country where odors will not disturb suburban residents. In the past, swine herds could often be found close to the edge of town. In general, then, there is no future in swine feeding as a method of garbage disposal. This does not mean that municipalities should discourage the practice where private enterprise is willing to carry it on. However, the method should not be relied upon to dispose of any significant portion of the solid wastes generated in a community. Garbage Grinding The home garbage grinder has been in use for forty years and a popular item in new house construction for twenty years. It will grind any food wastes except bone, feathers, husks, and gristle. When used properly it almost eliminates the nuisance factor of household solid waste. The institutional garbage grinder has become universal in hospital, restaurant and hotel construction and quite popular with grocers and produce handlers. Large grinders eliminate garbage as fast as it is produced, ending the need for storage facilities and garbage pickup. 49 The garbage grinder works either on the shredding or hammermill principle. In the shredder type grinder garbage is shredded by rotating knives and flushed through a screen. In the hammermill type shredder, rotating swinging hammers tear up the garbage and pulp it against a side plate. Both types produce evenly ground particles that will pass a quarter inch screen. The effect on sewage treatment facilities is the most important problem associated with garbage grinders. Normal sewer lines easily transport ground garbage without clogging. In fact, with sufficient gradient, ground garbage can have a scouring effect on sewer lines. If all garbage were ground at the point of origin and flushed through the sewer system, the suspended solids content would increase by up to 100 per cent, with an average of 50 per cent for any one system. The bio- chemical oxygen demand (B.O.D.) of the sewage will increase up to 65 per cent, with an average of 30 per cent. Ground garbage may increase the amount of grit in sewage by about 40 per cent where home grinders are used and by 80 per cent where central grinding stations are used. . . . In primary tank operation, the volume of scum and sludge can be expected to increase up to 100 per cent, partly as a result of the increase in suspended solids in the raw sewage and partly as a result of an apparent increase in removal efficiency caused by the addition of garbage solids. Primary effluent may increase in suspended solids by 5 to 10 per cent and B.O.D. by 15 to 30 per cent, requiring corresponding enlargement of secondary treatment facilities. Increased facilities for sludge digestion, sludge handling, and digester-gas collection are necessary if ground garbage is added to either the sewage or the digester directly.l7 l7Ibid., p. 225. 50 Garbage grinding, then, can have a significant impact both on solid and liquid waste disposal needs. In areas where universal home garbage grinder installation occurs, such as new subdivisions, solid waste collection can be reduced from twice to once a week with little fear of a nuisance problem. This can mean a great savings in collection costs. There may be a corresponding doubling of the sludge load upon the sewage treatment plant and this cost must be taken into account. Garbage grinders are not municipally installed but should be municipally controlled. Where sewer pipes are of adequate width and gradient and where grinders can be established in a large percentage of homes and institutional buildings, they should be encouraged or even required. Nearly universal grinder installation can save more money in collection costs then it will require in expanded sewage treatment facilities. Where sewage treatment facilities are already overloaded, however, and no collection cost savings can be gained by requiring grinder installation, municipalities should consider banning their use. Salvage Any economic reuse of solid waste is salvage. For instance, incineration, if the resulting heat is sold, is a form of salvage; reclaiming land with refuse is a form of salvage. However, "salvage," when used as a waste disposal term, means the direct recycling of the raw 51 materials in solid waste. Salvage today is an organized, large scale enterprise which usually has little to do with municipal wastes. There are markets for clean, separated paper, rags, metals, glass, rubber tires, some chemicals, and some plastics. In order for the costs of cleaning, separation, and hauling to be below the price that the material will bring on the open market, the sources themselves must be relatively uncontaminated and unmixed. Unfortunately, municipal refuse is both mixed and con— taminated. This has made salvage uneconomical for any materials except metal. Since the quantity of metal that can be hand picked from municipal refuse at a disposal site is very limited, the prospects of salvage as a disposal method are very dim. Since most of the components of urban waste can be recycled, however, a discussion of how to increase the percentage of salvaged material is in order. As with composting, a salvage system of disposal would not have to make a profit, it would only have to be competitive with alternate systems in order to become attractive. Efforts in the recycling field have been directed in two opposite ways, production site separation and disposal site separation. Production Site Separation.--Most salvage that takes place today occurs at the site of production. Factories regularly reuse as much of their wastes as possible and sell others that have market value. Paper, 52 plastic, glass, and metal scraps are produced in large quantities but never leave the confines of their production sites. For years beverage companies have included a deposit on glass containers sold in order to encourage their return in a separated and undamaged condition. Recently this idea has spread and many communities have initiated cooperative recycling programs in which refuse is sorted at home into components with market value and carried to collection points. Although this system relies on countless hours of unpaid labor in order to function and often works only sporadically, it can reduce the amount of waste collected by a municipality significantly. In the future it may be possible to organize and institution- alize this salvage system and rely upon it to dispose of up to half the solid waste produced in a community. Disposal Site Separation.--Cities have in the past let contracts to scavengers who pick through wastes at the disposal site and remove either manually or mechanically any with market value. This reduces the amount of refuse which must be disposed of and profitably recycles some material. The basic problem with scavenging as a disposal system is that disposal sites are usually not organized or set up to facilitate recycling. They are designed to dispose of refuse by burial or incineration and do not take into account the possibilities of separation and reuse. In the future it may be to the advantage of some 53 municipalities to design refuse disposal systems around salvage. Mechanical sorters can effectively screen refuse for metal and glass and air separation can remove paper. Incinerator residue can be washed and sorted profitably and this may become the most popular technique for recycling in large cities. A ton of incinerator residue contains on the average $15.76 worth of ferrous metal, aluminum, cooper and zinc, and glass. A plant which processed 1,000 tons per day of incinerator refuse could theoretically operate at $1.80 per ton,18 producing a handsome profit. This quantity of incinerator refuse, however, would only be produced by a large city; smaller cities and towns would have to accept a lower profit or not process their incinerator wastes at all. It is possible that a disposal site separation process could be combined with a simple home separation into burnable and unburnable fractions. This home separation might enable a large portion of the waste paper to be recycled, as well as the metal and glass. Salvage, then, does not presently have much impact upon the municipal disposal scene. However in the future it could play a significant role as over half of the refuse now produced can be recycled. If salvage becomes 18P. M. Sullivan and M. H. Stanczyk, "Economics of Recycling Metals and Minerals from Urban Refuse," Bureau of Mines Technical Progress Report, April, 1971, p. I0. 54 widespread as a disposal method the market demand for recycled material will undoubtedly change dramatically. There will be an immediate drop in prices paid for such material as the market is flooded. Following this, however, the demand for recycled raw material should rise as new industries move to take advantage of this cheap source of supply. While these trends cannot be detailed, some account of the changing market prices of paper, metal, and glass should be made before any large-scale municipal recycling project is undertaken. Experimental Systems In addition to the above solid waste disposal systems, all of which have been used many times at plant scale, there are a number of experimental projects underway to develop new methods of refuse disposal. Three of these show enough promise to be taken into consideration by municipalities developing new waste disposal systems. Recycling and Heat Recovepy Systems.--Recently a number of private firms in addition to the Federal Govern- ment have put money into developing waste disposal systems which would recycle the unburnable portion of solid waste and burn the flamable portion to produce usable heat. These systems are essentially highly technical versions of the salvage method described previously. The Bureau of 55 Mines describes a system for profitably "mining" inciner- ator residue.19 The Aluminum Association (a group of sixty-six manufacturers) are constructing a plant in which trash is shredded, burned for heat, and the remainder recycled.20 Combustion Power Co., Inc. has produced an incinerator which shreds all trash, air sorts it and burns everything except glass, metal, and stone. The gas drives a turbine and waste heat can be used for a variety of purposes. The remaining metal and glass is recycled.21 These systems show definite promise for two reasons. First, the thermal content of solid waste has been going steadily upwards for several decades, and by now it constitutes a good source of fuel. Second, the glass and metal percentages of solid waste are far above the per- centages of these materials found in nature. If adequate separation techniques can be developed it may become much simpler and more profitable to "mine" refuse than to extract the minerals from the ground. Pyrolisis Systems.--The Bureau of Mines has recently developed a technique for pyrolyzing (reducing lgIbid. 20"A Solid Waste Recovery System for All Munici- palities," Environmental Science and Technology (March, 1968), p. 52. 21"Outlook: Converting Solid Wastes to Electricity," Environmental Science and Technology (August, 1970), p. 17. 56 with heat in a closed container) the burnable portion of solid waste into combustible gas, tar, light oil, liquor (mostly water), and ammonium sulfate. The energy from the gas is more than sufficient to provide the heat for 22 The pysolisis, making the system self-sustaining. unburnable portion of the refuse can be recycled. This system looks promising because it converts heterogeneous refuse into a few simple substances in one self-sustaining process. Hercules, Inc. has also developed a system using pyrolisis and composting to handle refuse. This system is to be tested on a plant scale in Delaware.23 Conversion Systems.--The Bureau of Mines had developed a process which holds great promise for disposing of all organic wastes. Cellulose, lignin, other carbo- hydrates, proteins, and fats can all be converted to oil under 4,000 pounds pressure per square inch at 350-400 degrees C. and in the presence of carbon monoxide and water. The oil yielded is low in sulphur and is suitable for use as fuel or conversion to gasoline or diesel oil. The Bureau of Mines estimated that 2.5 billion tons of organic waste are produced in the United States every year. 22W. S. Sanner, et al., "Conversion of Municipal and Industrial Refuse Into Useful Materials by Pyrolisis," Bureau of Mines Technical Progress Rgport, August, 1970, p. 11. 23"Reclaiming Solid Waste for Profit," Environmental Science and Technology (August, 1970), p. 61. 57 If two billion tons of this waste were converted to oil it would yield approximately two billion barrels of oil, or 24 In other words, solid waste half the nation's needs. may eventually become an important energy source if the conversion process of the waste to a usable form can be done cheaply enough. Although this technique has not been tried on a plant scale and the economics have not been explored, the potential is so great that municipalities should watch this system for possible use in the future. Rural Disposal Disposal of solid waste in rural areas is an increasing problem in many states as people who work in the city buy land and live in the country. Waste disposal in low density areas is a problem in collection rather than disposal. The needs of the city for better refuse disposal techniques have almost overshadowed the need of rural areas for better collection practices. Rural dwellers used to be able to easily dispose of their solid wastes. Garbage was composted or fed to the chickens and what little burnable material there was disappeared into the furnace. Today, however, rural dwellers, particularly those who work in the city, produce, and have problems disposing of, just as much waste as the city dweller. 24H. R. Appell, et al., "Converting Organic Wastes to Oil, A Replenishable Energy Source," Bureau of Mines Technical Progress Report, 1971. 58 The major problem in rural areas is that it is prohibitively expensive to regularly collect trash from houses which are spread over a wide area. Left to their own devices, rural dwellers have disposed of their refuse with mixed results. Old appliances, car hulks, and open dumps dot the landscape contributing to rural blight and creating potential pollution problems. Two solutions to the rural disposal problem were posed in a study done for Oakland County, Michigan. Convenience Centers.-- A convenience center will provide a sightly place for rural residents to dispose of their wastes. The center would be enclosed to prevent blowing paper and roofed to keep the weather from the user and the deposited material. All-weather roadways would allow easy traffic movement in all seasons. The convenience center would contain 20 cubic yard to 40 cubic yard portable sanitary containers in which the public could deposit their refuse. . . . Convenience centers would be visited three times daily throughout the week by county crews. A pick up vehicle would collect filled and partly filled containers and take them to a transfer station or a disposal site.2 These convenience centers would be located such that they would be within easy reach of all rural residents. Because they would not be manned, the operating costs would be low, and since these would not be actual disposal sites, the acreage needed to construct one would be minimal. Frequent collection would eliminate odor and pest problems. 25 Ibid. 59 Sanitarprandfills.-- Traditionally, open dumping has been the method of solid waste disposal in rural areas. Consideration of sanitary landfills to replace these open dumps is a natural outcome of the passage of Public Act 87. . . . Sites could be kept open for public convenience over a 12-16 hour daily period. An attendant would serve each site, directing traffic, picking up blowing papers and doing other general site maintenance during the day. Cover and compaction would be provided at each fill by a crawler tractor front end loaded unit suitable for short haul of cover. Each tractor could serve all sites in a township by being hauled from site to site on a low-boy semi-trailer driven by the loader operator. These disposal sites would essentially be small sanitary landfills, again located within easy reach of all rural dwellers. They would be inexpensive to operate because the heavy equipment needed to compact the refuse and cover it would be hauled from site to site, thus keeping it in almost constant use. Municipalities must keep rural areas in mind when developing solid waste disposal systems, and these two possibilities for organ- izing rural waste disposal would be good to consider. Ligpid Wastes Liquid waste, or sewage, is about 1 per cent solids and 99 per cent contaminated water. The contaminants are chiefly organic solids and various nutrient minerals such as nitrates and phosphates. If industries are pumping wastes into the sewage system there may also be a variety 26Jones & Henry Engineers Limited, Proposals for a Refuse Disposal System in Oakland County, Michigan, 1970, pp. 53-55. 60 of poisonous compounds such as chrome from chrome plating works, salts, and acids such as those used to pickle steel. Where storm sewers and sanitary sewers are combined, sewage may also contain a variety of large solids such as dead rats and pieces of lumber and paper. It will also contain significant amounts of oil from the street and grit. All sewage contains some grit, but the amount is greatly increased in combined systems. The treatment of sewage has traditionally been divided into three areas, primary, secondary, and tertiary treatment. Although the treatment of sewage is actually more of a continuum than a staging process, the three-part classification is useful and will be used below. In addition to sewage treatment, two related problems, sludge disposal and rural sewage disposal, will be discussed. PrimarygTreatment The term "primary treatment" encompasses a number of treatment techniques from raw sewage dumping to highly effective Imhoff tanks. As the name implies, primary treatment is often the first process that raw sewage undergoes as it is being treated. However, since some one- stage techniques can impart tertiary treatment to sewage, "primary" actually refers to the degree to which the sewage has been treated. Primary treatment removes the heavier solids and grit from sewage by allowing them to settle out. Depending 61 on the nature of the sewage and the length of settling time allowed, anywhere from 20 per cent to 90 per cent of the solids can be removed in this fashion. One of the most successful primary treatment devices ever built, one which is still in common use in smaller towns and in large cities as a first stage process, was developed by Karl Imhoff about 1900. Imhoff tanks are used to remove settleable solids suspended in sewage, and in turn, digest these solids in the lower portion of the same unit. In a properly designed and operated Imhoff tank, 90 per cent of the settleable solids in raw sewage can be removed.27 The tank is wide at the top and slopes steeply inwards towards the bottom where a slot permits the solids to settle into a continuous digesting compartment below. Raw sewage is fed in from the top, water is extracted from the center, and digested sludge is extracted below. The process is continuous. Digestion is anaerobic; this means that the organisms decomposing the sludge work without oxygen and produce as their main by-products methane and C02. These bubble up to the surface and are either burned or piped away. The sludge is pumped into drying beds and when dry either burned or disposed of on land. The water is generally piped to a receiving stream, although in some cases it may be used for irrigation. Prior to discharge, 27Water Pollution Control Federation, Operation of Wastewater Treatment Plants, 1961, p. 89. 62 the water is often aerated to control odor. Anaerobic bacteria produce very unpleasant odors, and when they are present, sewage is said to be septic. Aeration kills these bacteria and eliminates the odor. If the water in the receiving stream is used as a drinking or bathing source, sewage effluent is often required to be chlorinated before being discharged. This is done primarily as a health measure although some odor control also results. Pathogenic organisms are destroyed, leaving the water safe for swimming. Although primary treatment is far better than no treatment at all, it fails to remove from the effluent, many nutrients which can be harmful to the receiving stream. The best-run Imhoff tank removes only 90 per cent of the suspended solids in sewage, and very few plants in oper- ation actually approach this level of efficiency. These remaining solids, when¢ xwuunx \ l / \gJ-a/ . \ -L- / \ -._- /4 mauoo 0.»:qu can ocfiumaxm nouusomom Alllv maugomom Pap—C van ufiunwxm Snow mama: 3:03 §3%Hq8 EU 309w UZHX‘tIZOH m HUNG 102 systems best suited to its needs. Of course a great deal of engineering data will have to be collected before a community will know exactly what form the plants will take, but the flow chart should lead to a decision about the general design of the plants and what they will be able to achieve. Below is a hypothetical example of a community seeking imaginative answers to its waste disposal problems. City: Ashbrook, New Jersey, population 50,000 consisting of a small central core and numerous middle class suburbs. The city has a moderate growth rate, fair tax base, and average civic pride among its citizens. Solid Waste Situation: At present, solid wastes are disposed of in a sanitary landfill located in marsh land on the estuary of the local river. This landfill is destroying marsh land and threatening the river with leacheate pollution. The city decides to reassess its solid waste situation and discovers the following facts: (1) New residential development is taking place away from the river and garbage trucks presently are making some runs of over fifteen miles to the disposal site. (2) The only acceptable new landfill sites are even further from town and would have to be carved out of prime agriculture land. The city notes that its population size is well beyond the threshold needed to support an incinerator and so the decision is made construct a water wall incinerator with electrostatic precipitator pollution control equipment. 103 A bond is approved by the voters for construction after an extensive publicity campaign and a site is found adjacent to the city's community college. Steam generated by the incineration process is sold to the college, thus reducing Operating costs. It is also found that the more central location of the disposal facility means a savings in hauling costs and brings the operating cost of the incin- erator almost in line with the costs of the landfill. Next the city applies for and receives federal funds for a pilot incinerator residue recycling plant which further reduces operating costs as the city is able to sell the recycled material for a profit. Liquid Waste Situation: The city's present waste water treatment plant has a primary settling tank and a chlorination tank. The effluent is discharged into the local river and the city has been cited by the state and federal governments for pollution. When a reassessment of liquid waste disposal takes place the following facts are noted: (1) At present all development within the city limits has sewerage but there is a trend towards large lots with septic tanks outside the city. (2) Soil con- ditions are such that large-scale use of septic tanks will cause problems in the future. (3) The city has been considering building a golf course near the present treatment plant. (4) Local farmers pump water out of the river during the warm months of the year to irrigate their crops. 104 As a result the city coordinate with the county government to restrict residential development and prevent the use of septic tanks in subdivisions. Plans are made to go ahead with the golf course and to install a sprinkling system linked to the present sewage treatment facility. Local farmers are contacted about using sewage effluent instead of river water during the warm months of the year. In addition, the city applies for and receives a federal matching grant to expand and modernize its facilities and eliminate pollution. To do this, a chemical secondary treatment facility is added to the present primary treatment facility which brings BOD to within state limits and significantly reduces phosphate levels. During the warm months the chlorinated effluent from the primary treatment facility is used to irrigate the golf course and piped to local farmers. During the cold months when no irrigation water is needed, the secondary chemical treatment facility is used to treat the effluent before it is discharged into the river. Chemical rather than biological secondary treatment is used because it can be started up or shut down within a day's time whereas biological treatment requires up to two weeks to start. The sludge collected during primary settling is digested anaerobically, dried, and distributed for a nominal charge to local farmers. It is also bagged and sold to local residents as soil conditioner. Its popularity has grown due to a 105 publicity campaign and because it is priced lower than commercial preparations. Of course, solving community waste disposal problems is usually much more difficult than has been indicated above. However this hypothetical example does illustrate some of the factors which should be considered and some of the possibilities for imaginative solution which do exist. CHAPTER VI CONCLUSION Two major questions were asked at the beginning of this discussion about the problems of waste disposal and their technological solutions. The first was "Can wastes be disposed of without causing damaging pollution to the environment." The answer is definitely "yes" with available technology. Properly operated and sited sanitary landfills will cause almost no pollution. More complex solid waste disposal techniques such as composting, incin- eration, pryolisis, and oil conversion can all be designed with effective anti-pollution controls. Non—polluting liquid waste disposal is more a matter of degree than an absolute. Waste water can be treated successively with finer and finer techniques until the water is essentially pure. All that is needed is sufficient funds to build and operate the treatment facilities and the motivation to do so. Where waste water will not be discharged into a receiving stream, but rather on land, treatment can be less complete with no fear of causing pollution damage. With both liquid and solid wastes, effective, safe, 106 107 disposal appears to be well within financial reach of most municipalities. The second question was "Can we begin to reuse some of the resources now being lost through the waste disposal process?" The answer again is "yes" with available technology. Some materials, particularly metal and glass can be easily recovered with present technology. Heat, too, can be salvaged either through the use of waste heat boilers in incinerators or by converting solid wastes into fuels through pryolisis or oil conversion. These last two techniques hold a great deal of promise but will have to be supplemented with further research. Paper and plastics are more difficult to recover from mixed refuse intact but can be salvaged as heat. This is particularly true of plastic which does not presently have a salvagE‘ market. Since plastics do not break down in nature it may in fact be a desirable ecological goal to dispose of plastics by incineration, pyrolis, or oil conversion. At present these are the only techniques available for returning the constituent elements of plastics to the environment. The barriers to solid waste reuse appear to be (1) the high capital investment needed for recycling equipment; (2) developing markets for recycled material, and (3) the high expense, relative to present economic return, of operating recycling processes. Liquid waste reuse appears to be even more promising than solid waste reuse. Waste water can either 108 be treated to the point that it is reusable or it can be partially treated and used for irrigation or aquifer recharge. The solids produced during waste water treatment have proven soil conditioning and fertilizer value. Barriers to waste water reuse appear to be (1) an insti- tutional inertia which inhibits the search for alternatives to pumping waste water into the nearest receiving stream; (2) the high capital investment costs of effective treatment facilities; and (3) the problems associated with dispersing the water and solids back to the land after they have been concentrated by the collection and treatment process. The third major question, implicit throughout this discussion, is "What can municipalities do now to improve their waste disposal processes?" The answers to this question are not as clear-cut as they were to the preceding two questions. However, the beginnings of answers suggested by Chapter V "Choosing the System" are as follows: 1. Municipalities must reorder priorities to put clean air and water ahead of new residential, industrial, and commercial development and ahead of some other local capital improvement programs. 2. Local governments must reorient their thinking from viewing waste as something which is a nuisance to be disposed of and being to see it as a resource to be reused. 109 3. Creative techniques for disposing of waste must be developed. This may mean for instance piping waste water out to nearby farms for irrigation or packaging and selling dried sewage sludge for soil conditioner as Milwaukee now does. Or it may mean actively seeking federal demonstration grants for solid waste recycling plants. 4. And finally municipalities must begin to completely reasses all waste disposal programs using a process similar to the previously presented flow diagram. 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