J4 —-4 'w _U)\IOO ill“ L I QXYGfiN UPTAKE gAT-ES— MD. REfi’iM‘FW QUOTIEM’S OF AN AERQBICALLY fiECQM?0$ING WHEHC GARBAGE “nests §oz~ Hm Degree of M. S. MECEZSAN STA’E‘E Ui‘éiifERSETY Ailan R Moore 1958 In m |\Y'U\"G\T\\Y\’Wfii‘xlfi\i\i\'\ii\l\\\‘if\lWW 3 1293 10705 1744 This is to certifg that the thesis entitled AN Abn‘OLiCALLI LECOIv’uJOSlNG SINLHEJC anbhéb‘" presented by Mr. Allan F. Moore has been accepted towards fulfillment of the requirements for M" st ens degree 111W Enbineering -7 v 9"" r-~, (’5 Major professor Date April 21, 1958 0-169 OXYGEN UPTAKE RATES AND RESPIRATORY QUOTIENTS OF AN AEROBICALLY DECOMPOSING SYNTHETIC GARBAGE by Allan F. Moore AN ABSTRACT Submitted to the College of Graduate Studies of Michigan State University of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Civil and Sanitary Engineering 1958 p q 7 Approved jQéZf(J Xé' (égéie/>,4 2 ALLAN F. MOORE ABSTRACT An investigation was made on the oxygen uptake rates and respiratory quotients of an aerobically decomposing synthetic garbage. Preliminary work showed that composting in a cylin— drical metal container without mixing resulted in approx- imately 20 per cent of the material becoming so dry that little decomposition took place. To avoid these shortcomings, a rotating drum composter with a capacity of 0.75 cubic feet was built. The mixing was provided by rotating the drum at a speed of four revolutions per hour. The synthetic garbage used in this study consisted of freshly ground food and newsprint which had been proportioned to represent a typical garbage. Forced, room temperature air was used for aeration. The rate of aeration was regulated to provide a desired residual oxygen percentage in the exhaust gases (usually 2 to 5 per cent). Aeration rates used varied between 0.2 and 15.4 cubic feet of air per pound initial volatile matter per day. The exhaust gases from the rotating drum were analyzed for oxygen and carbon dioxide content. The oxygen content was measured with a Beckman Model D-2 magnetic oxygen analyzer, and the carbon dioxide content was measured with an Elliott Brothers Ltd., portable carbon dioxide meter. Oxygen uptake rates were computed by dividing the amounts of oxygen used by the original weight of volatile 3 ALLAN F. MOORE ABSTRACT matter. The amount of oxygen used was derived from the difference in oxygen concentration found between the air entering the drum and the exhaust gas leaving the drum. The activity in the aerobically decomposing organic material, measured as oxygen uptake rate, was found to be directly proportional to the compost temperature between 90 and 146OF, the range of temperature investigated in this study. The temperature coefficients were found to be Q10 (30 to 4000) = 2.60, Q10 (40 to 5000) = 1.80, and Q10 (50 to 600C) = 1.77. Under the conditions described in this study 18,000 to 20,000 cubic feet of air per day per ton of volatile matter were required to satisfy the maximum oxygen demand. The activity of finished compost was measured by the Warburg technique. The oxygen uptake rate varied from a maximum at 60.4 per cent moisture to an unmeasureable amount at 11.2 per cent moisture. As compared to oxygen uptake rates in the rotating drum, the maximum activity of finished compost was about 1/8. From general data on pH, R.Q. values, oxygen uptake rates and temperature, it was concluded that batch type composting can be separated into five phases: (1) fermen- tation, (2) assimilation of acids, (3) thermophilic or high temperature, (4) rapid temperature decline, and (5) decomposition of resistant materials. OXYGEN UPTAKE RATES AND RESPIRATORY QUOTIENTS OF AN AEROBICALLY DECOMPOSING SYNTHETIC GARBAGE by ( Allan F. Moore A THESIS Submitted to the College of Graduate Studies of Michigan State University of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Civil and Sanitary Engineering 1958 :§—-;;L-f;3' ér 5’44fi7 ACKNOWLEDGMENT This thesis was only possible with the sincerely appreciated support of an N.I.H. grant. The author wishes to express his sincere appreciation to Dr. Karl L. Schulze for his valuable guidance and assistance in connection with this thesis. Thanks are also extended to Dr. Robert F. McCauley and Dr. Charles E. Cutts for their interest in this work and reading of the thesis. TABLE OF CONTENTS Page ACKNOWLEDGMENT. . . . . . . . . . . . . . ii LIST OF FIGURES . . . . . . . . . . . . . v LIST OF TABLES. . . . . . . . . . . . . . Vi SECTION I. INTRODUCTION. . . . . . . . . . . . 1 II. LITERATURE REVIEW . . . . . . . . . . 2 A. Principal Variables in Composting . . 3 1. Temperature . . . . . . . . 3 2. Moisture Content . . . . . 4 3. Oxygen Supply . . . . . 5 4. Hydrogen Ion Concentration . . . 6 5. Ratio of Carbon to Nitrogen. . . 7 6. Seeding and Blending . . . . 8 7. Particle Size . . . . . . . 8 B. Other Considerations in Composting . . 9 1. Time Required for Composting . . 9 2. Criteria for Finished Compost . . 9 III. THEORETICAL CONSIDERATIONS . . . . . . . ll IV. EXPERIMENTAL APPARATUS AND MATERIAL . . . 13 A. Equipment . . . . . . . . . l3 1. Rotating Drum . . . . . . . l3 2. Air Supply . . . . . . 17 3. Gas Analyzing Equipment . . . . l7 4. Insulation . . . . . . . 18 B. Synthetic Garbage. . . . . . . . 19 SECTION V. EXPERIMENTAL PROCEDURE. . . . A. Preliminary Investigation B. Method of Procedure . C. Analysis of Compost . VI. EXPERIMENTAL RESULTS . . . Experiment No. 1 . . . Experiment No. 2 . . . . Experiment No. 3 . . . . Experiment No. 4 . . . . Experiment No. 5 . . . . Experiment No. 6 . . . Experiments No. 7 and 8 . Experiment No. 9 . . . VII. DISCUSSION OF RESULTS . . . . A. Oxygen Uptake Rates B. Oxygen Uptake Rates Measured Warburg Technique . . 1. Oxygen Uptake Rates of Various by the Samples of Finished Compost . 2. Oxygen Uptake Rates of a Homogeneus Sample with Varied Moisture Content C. Other Observed Data on Composting. Phases of Composting Moisture Content. . Matter, and Nitrogen U14: UOIUH 6. Continuous Operation VIII. CONCLUSIONS . . . . . . . BIBLIOGRAPHY. . . . . . . . . . . Time Required for Composting. . Daily Analysis of Total Dry Weight, Volatile Matter, and Nitrogen Losses in Dry Weight, Volatile iv Page 21 21 23 24 27 27 29 29 32 32 3A 3A 39 45 45 55 56 56 62 62 66 68 68 71 77 79 Figure l2. 13. 14. 15. 16. 17. 18. 190 20. 21. LIST OF FIGURES Experimental Apparatus. . . Flow Diagram for Rotating Drum.Composter. . Details End PlateConnection Some Microorganisms found in Composting Conditions of Finished Compost Experiment No. l. . . . Experiment No. 2. . . . . Experiment No. 3. . . . . Experiment No. 4. . . . . Experiment No. 5. . . . . Losses in Dry Weight, Volatile Matter, Nitrogen from Experiment No. 5 and Daily Weights of Ammonia, Organic and Total Nitrogen from Experiment No. Experiment No. 6. . .3 . . Experiment No. 7. . . . . Experiment No. 8. . . . Experiment No. 9. . . . . Relationship Between Oxygen Uptake Rate and Compost Temperature . . . Relationship Between Volume and Moisture Content Oxygen Uptake Rate of Various Finished Composts Oxygen Uptake Rate of a Finished Compost with Various Moisture Contents, 200C . Relationship Between Moisture and Oxygen Uptake Rate, Finished Compost, Temperature 2OOC . . . . . . . . Page 14 15 16 22 28 3O 31 33 35 36 37 38 40 A1 A2 60 61 LIST OF TABLES Page Constituents Used in Synthetic Garbage. . . . 19 Representative Composition of Synthetic Garbage. 20 Analytical Data for Experiments l--8 . . . . 43 Maximum Oxygen Uptake Rates from Literature . . 47 Relationship Between Oxygen Uptake Rate and Compost Temperature. . . . . . . . . . 52 Aeration Rates Reported by Wiley and Pearce(19). 54 Oxygen Uptake Rates of a Finished Compost at 20°C. . . . . . . . . . . . . . 62 Daily Weights of Dry Weight, Volatile Matter, Ammonia Nitrogen, Organic Nitrogen, and Ash from Experiment No. 5 . . . . . . . . . 70 Material Added to Drum . . . . . . . . . 73 Computed Daily Weights for Experiment No. 9 . . 74 Reduction in Total Material Used in Experiment NO. 9 o o o o o I o o o o o o o o 75 SECTION I INTRODUCTION Composting refers to the aerobic decomposition of putrescible organic matter to a relatively stable material. Different techniques have been used, such as: windrowing and various types of mechanical composters. In the windrow method, the shredded organic material is placed on the ground in piles 2 to 5 feet high and aeration is provided by periodic turning and mixing. Mechanical composters usually employ mixing in a closed unit with or without forced aeration. Mechanical composters are more costly but provide for better operational and nuisance control. Composting of organic material is essentially an aerobic biological process involving bacteria, yeasts, and fungi. The rate of decomposition is dependent on the environment in which the process takes place. Important environmental factors include: oxygen supply, moisture content, particle size, temperature, carbon to nitrogen ratio, and hydrogen ion concentration. The purpose of this study was to measure the oxygen uptake rates and respiratory quotients of an aerobically decomposing synthetic garbage in a batch type rotating drum composter with a forced air supply. SECTION II LITERATURE REVIEW The decomposition or stabilization of organic material by biological action has taken place in nature since life first appeared on the earth. Composting of organic waste materials has been practiced by farmers for centuries, but basic research on some of the fundamentals of composting has been accomplished only in recent years. Modern composting was first suggested by Sir Albert Howard (1). Recent extensive studies on composting have been reported by Scott (2), van Vuran (3), Gotaas and associates at the University of California (4, 5, 6, 7, 8), Michigan State University (9, 10, 11), the New Zealand Inter- Departmental Committee (12), and the United States Public Health Service (13). Composting of organic material is a biological process; therefore, the more suitable the environment the more rapid the microbial decomposition of the organic material. Snell (14) has listed nine essential factors to obtain maximum high-rate composting: (1) carbon to nitrogen ratio, (2) grinding, (3) moisture, (4) recycling of seed compost, (5) stirring, (6) oxygen supply, (7) temperature,(8) pH, and (9) continuous feeding. Gotaas (l5) mentioned the following fundamentals as important: (1) grinding or shredding, (2) carbon to nitrogen ratio, (3) moisture content, (4) place- ment of material, (5) temperature, (6) aeration, (7) seeding and blending, and (8) pH. A. Principle Variables in Composting 1. Temperature In composting, carbon compounds are oxidized to carbon dioxide liberating considerable quantities of heat. Due to the insulating properties of the compost material, a suffici- ently large composting mass will retain the heat of the exothermic biological reaction and high temperatures will be developed (15). Waksman (16) found the most rapid decomposition of horse manure at 650C. Gotaas (15) states that shorter time is required for stabilization of organic material in the thermophilic range (45-65OC) than in the mesophilic range. Wiley and Pearce (19) found that temperatures of 50 to 60°C produced good quality compost and that evolution of CO2 showed a marked increase at the higher thermophilic temper- atures. Michigan State University Studies (9) on oxygen uptake indicated that maximum oxygen utilization occurred at 450C. Ludwig (17) also found maximum oxygen uptake at 450C. Bek (18) reported largest reductions in volatile matter at 4000. The last three studies mentioned used water baths or cooling coils for temperature control. Most investigators (7, l2, l5, l6, 19) have stated that the optimum temperature for composting is between 50 and 700C, and that high temperature is desirable to "pasteurize" the waste material and to achieve rapid decom- position. Several workers have shown that temperatures above 700C (158OF) are harmful to essential microorganisms (12, 15). 2. MoBture Content Moisture content of the organic material to be decom- posed is one of the most important variables. Wiley (20) has pointed out that cellular absorption, secretion, dif- fusion and excretion could not proceed without water and that no decomposition could take place if the organic material was dry. On the other hand, water holds only 5.6 p.p.m. oxygen at l22OF and standard pressure. Thus if the voids in the material are completely filled with water the air-water interface surface is greatly re- duced and the benefits of aeration are lost (20). Gotaas (15) stated that aerobic decomposition can proced at any moisture content between 30 and 100 per cent if adequate aeration can be provided. Composting must be carried out somewhere between the two moisture extremes. The literature shows varied opinions as to the optimum moisture content. Waksman (16) recommended 75 to 80 per cent moisture for composting barnyard manure. Wiley and Pearce (19) found 55 to 60 per cent moisture most satis- factory. The University of California (8) reported 40 to 60 per cent moisture content to be the most satisfactory for aerobic composting. Scott (2) found 50 to 60 per cent moisture to be optimum. The Michigan State University reports (9, 10) listed 50 to 60 per cent moisture as optimum for composting garbage. This optimum moisture level varies with the type and the fineness of grind of material being composted. 3. Oxygen Supply McCauley (21) stated that solid organic material was poorly broken down by a predominantly anaerobic process and that an adequate oxygen supply was essential to successful composting. It was necessary, therefore, to provide a continuous supply of oxygen in both mechanical composters and windrow piles (21). Wiley and Pearce (19) used three different aeration rates expressed as cubic feet of air per pound volatile matter per day. Low rates were 4 to 6.4 cubic feet,medium rates 9 to 29 cubic feet, and high rates 32.8 to 77.8 cubic feet, with best results from the medium aeration rates. Bek (18) used 20 to 30 cubic feet per day per poundcry weight with good results. Michigan State University Studies (10) indicated that 2 to 4 cubic feet of air per day per pound dry solids was probably adequate to maintain aerobic conditions. Wiley and Pearce (14) found that excessively high aeration rates caused rapid cooling and dehydration and that very low rates produced anaerobic conditions. Ludwig (17) measured oxygen uptake rates of a decom- posing garbage at various temperatures and reported a maximum oxygen utilization of 9.6 mg oxygen per hour per gram dry weight using a moisture content of 85 per cent. Michigan State University Studies (9) found a maximum oxygen uptake rate of 13.6 mg oxygen per hour per gram dry weight. Braithwaite (22) reported a maximum oxygen uptake of 4.4 mg oxygen per hour per gram initial volatile matter at 60 per cent moisture and 400C. Stoller, Smith, and Brown (23) measured rates of carbon dioxide produced from horse manure and straw in a rotating drum. By assuming a respiratory quotient (R.Q.) of 0.9, their maximum oxygen uptake rate was computed as 7.86 mg oxygen per hour per gram volatile matter at 5900 and 67 per cent moisture. 4. Hydrogen Ion Concentration Another important variable that influences microbial activity is the pH of the organic material. The initial pH of garbage, refuse, manure, and other compostable material is usually between 5.0 and 7.0 unless ash or other highly alkaline materials are present (15). Gotaas (15) reported that an initial pH of 5 to 6 did not seriously retard the initial biological activity but that temperatures appeared to increase more rapidly when the pH was 7 or above. Both Michigan State Unrersity (9) and Wiley and Pearce (19) reported composting raw garbage to have an initial pH of 5 to 6, followed by a decline to a level of 4 to 5 and a subsequent steady rise to a final pH of 8 or above. According to Gotaas (15) and Snell (14) the pH of a finished compost is usually 8 to 9. McCauley (21) mentioned that dewatered raw sewage sludge with an initial pH of 10 or 11 can be satisfactorily composted with- out passing through an aCid stage. Gotaas (15) stated that the addition of alkaline material is rarely necessary and may do more harm than good because the loss of nitrogen through the evolution of ammonia gas will be greater at a higher pH level. 5. Ratio of Carbon to Nitrogen According to Gotaas (l5) microorganisms use about 30 parts of carbon for each part of nitrogen assimilated. If the C/N ratio is much above 30, the nitrogen must be re- cycled by the death of some organisms and the reutilization of the nitrogen contained in their cells. The University of California (8) reported a C/N ratio of 30 to 35 optimum for composting garbage or refuse. Scott (2) stated that CQN ratios of 38 to 40 were ideal for com- posting manure. According to the New Zealan‘d Report (12) a C/N ratio of 26 produced the most rapid composting. Snell (14) stated that a finely ground material with a C/N ratio of less than 50 should compost satisfactorily provided other variables are controlled. 6. Seeding and Blending The need and value of seeding compost with special inocula is debatable, and several composting studies have shown that seeding is unnecessary (2, 5, 12). Gotaas (15) found that the addition of partially dried finished compost can be used to reduce the moisture content of the raw material to a desired level. McCauley (21) mentioned that seeding with actively composting material may provide a large population of organisms, which would be immediately capable of breaking down the organic material. Snell (14) recommended the addition of 2 to 10 per cent of very active partially composted material to obtain maximum high rate composting. 7. Particle Size Gotaas (l5) mentioned that grinding improves the composting process by making the organic material more susceptible to bacterial invasion. In addition, oxygen has better access to smaller particles. It was Gotaas‘ (l5) opinion that a completely aerobic condition in com- posting is never attained even with adequate aeration because of the effect of particle size. McCauley (21) stated that composting is a combination of aerobic and anaerobic decomposition with aerobic organisms commonly being dominant. Gotaas (15) recommended a particle size of less than 2 inches. Wiley and Pearce (19) used a double grinding of 1 7/8 inches and 5/8 inch with satisfactory results. Batzer (25) found that excessively fine grinding was to be avoided because fine particles tended to pack together forming large dense clumps. B. Other Considerations in Compgsting 1. Time Required for Composting The time required for satisfactorystabilization de- pends primarily upon (1) the initial C/N ratio, (2) the particle size, (3) the maintenance of aerobic conditions, and (4) the moisture content. If all factors are optimum the material will be satisfactorily composted in 9 to 12 days according to Gotaas (15). The reported time intervals to produce a finished compost have varied from 3 days to 8 months; the longer periods usually including a "ripening" phase. The University of California (8) reported 9 to 21 days to produce a compost in windrow piles. Wiley and Pearce (19) produced a satisfactory compost in about 7 days. Bek (18) found that 9 to 10 days were needed to produce compost. 2. Criteria for Finished Compost The most widelyLBed criterion in judging the com- posting process is the temperature. When the temperature of a compost pile has returned to approximately ambient, the process iscpnsidered finished. Several investigators 10 have found that when finished compost was reground and the moisture content adjusted, the material would reheat to 100C above ambient temperatures (18, 19). A typical steep rise in temperature and a sustained high temperature plateau, followed by a decrease (which is not due to thermal kill, oxygen insufficiency, or low moisture content) is a good practical criterion for judging the composting process (15). Many other tests or checks have been suggested such as pH, reduction in volatile matter, oxygen consumed, C.O.D., B.O.D., odor and appearance. Thus far no reliable single test or check has been developed which will determine the degree of composting attained by any given process. Gotaas (15) considered a compost to be finished when it could be stored in large piles indefinitely without becoming anaerobic or producing appreciable amounts of heat. Such material may be placed on agricultural land with safety because of its low C/N ratio. Snell (l4) mentioned that a compost may be considered finished when it would not reheat, lose appreciable nitrogen or become offensive. CHAPTER III THEORETICAL CONSIDERATIONS The temperatures reached in a compost pile depend upon the amount of heat produced from the exothermic biological breakdown of the organic material and the heat loss. A sufficiently large composting mass will retain the heat of the exothermic biological reaction and high temperatures (6O-7OOC) will result (15). When small amounts of organic material are used, the heat loss becomes more significant. The amount of heat produced in this biological decom- position depends on the activity of the microorganisms involved. The more complete the oxidation the more energy is released. Glucose is often used as an example. When glucose is completely oxidized to carbon dioxide and water, 674 Calories per mole of glucose are released. If glucose is partially oxidized to form oxalic acid and water, only 493 Calories per mole of glucose are released, and if glucose is fermented anaerobically to an acid or an alcohol, only about 22 Calories per mole of glucose are released. The degree of microbial activity will depend on the available food supply and the environmental factors. In a raw organic material, such as garbage, carbon, nitrogen, 12 phosphorus, potassium, and other trace elements, which are essential to biological growth, are present in more than adequate amounts (12, 15, 19). The controllable environ- mental factors are mainly oxygen supply and moisture content. Heat is lost from the composting material by convection, conduction, and radiation. If the air temperature is lower than that of the compost pile a heat loss occurs. The air entering the compost pile is notsaturated with the moisture at the higher temperature, therefore, a certain amount of water is vaporized from the decomposing material. The amount of heat required to vaporize water is large; at 600C; 568.8 Calories per kilogram of water are required and 582.3 Calories per kilogram of water at 250C. SECTION IV EXPERIMENTAL APPARATUS AND MATERIAL A. Eguipment l. Rotating Drum All experiments of this study were conducted in a specially designed 0.75 cubicfbot capacity Plexiglass rotating drum with fixed ends as shown in Figures 1 and 2. The rotating drum was 19 inches long with an inside diam- eter of 10 inches and was equipped with four 1/4 inch by 1/2 inch Plexiglass strips 15 l/2 inches long located at the quarter points to insure mixing of the organic material when the drum rotated. The fixed ends were made of 3/4 inch Plexiglass and fitted with plastic expansion rings. A grease trap between expansion rings and the drum, shown in Figure 3, was filled with high vacuum silicone grease to insure an air tight connection. Openings for a temperature recorder element, a control thermometer, sampling and air inlet were drilled in one fixed end. The other fixed end had openings for exhaust air and for avater return to add moisture if this should become necessary. The air inlet and outlet were protected from plugging by a 1/4 inch length of 1 1/2 inch plastic tubing covered with plastic screen. The rotating drum was mounted on a 2" x 4" frame 14 .Hmapmpme afinmwpo can coapwHSmQH p50£pH3 anew msfipwpop mo mzoa> 039 mdpmhwmm< Hmpcoaapomxm . a @8me \ ’ 1 4’. o.“ \ -.. . .C‘I‘s’l a 15 mwaOdZOO 23mm. oz_._.<._.0m mo“. 2Jan5m ; _ E4 \ T. m NN>JA \\ \x. \t — I; |—I|| 4|! mazu ouxE It; 1:10 02:.(POK 16 zoflqomzzoo Mumqm Jam mqa¢.mq Mlmzbjah swam one ca 0 Ohm =w\H ca emaaam mean coamcemxm ‘ anon museum onoam cam.lll! 1L \1\4V\ “N“m“m\\nodcafiho , \\\ 17 base. The ends of the drum were fixed to the base, addi- tional support for the drum being provided by plastic rollers. The rotating drum was tilted at an angle of six degrees toward the front of the unit, so that water added for moisture control would be thoroughly mixed. The rotating drum was driven by a V-belt. The elec- tric motor, reducing gears, and differential pulley sizes rotated the drum at a speed of one revolution in 2 1/2 minutes. The electric motor was also connected to a G.E. time switch adjusted so that the drum rotated for five minutes each half hour. With the arrangement the rate of rotation was four revolutions per hour. 2. Air Supply Compressed air was passed into the unit through a cotton filter, pressure reducer and regulator, wet test gas meter, and 1/2" rubber tube. The exhaust air left the unit in 1/2" rubber tubing connected to a condenser, to remove excess moisture, then passed through a water trap and discharged under a hood. The air entered the unit through an opening 1 7/8" below the center of one fixed end and left through an opening 3/4" above the center of the other fixed end. 3. Gas Analyzing Equipment The exhaust air from the unit was analyzed for oxygen with a Beckman, model D—2, magnetic oxygen analyzer. The 18 oxygen analyzer was graduated from O to 25 per cent oxygen and had an accuracy of i 2 per cent of the full scale. Measurements were made with thesample tube of the oxygen analyzer extending into the unit through the air exhaust tubing. Percentage carbon dioxide in the exhaust air was measured with an Orsat gas analyzer using saturated KOH for the first three experiments. The remaining carbon dioxide analyses were made with an Elliott Brothers Ltd., portable carbon dioxide meter. Both oxygen and carbon dioxide analyzers were equipped with drying tubes. Air flow rates were measured on the inlet side of the unit with a wet test gas meter graduated in liters. Air flow rates were normally regulated to provide a reading of 2 to 5 per cent oxygen in the exhaust gas. Actual air rates varied between 1.5 and 115 liters per hour. 4. Insulation Insulation for the rotating drum was provided in two ways: (1) by placing 1 1/2" spun glass insulation around and on the ends of the drum, to the limit permitted by the belt drive and support rollers. Secondly, the whole unit was placed in a box covered on all sides with 1 1/2" of spun glass (rockwool bats). No significant differences in temperature were noted between the two methods of insulation. 19 Plexiglass material used in drum construction was a good insulating material with a thermal conductivity of lip 10 x 10-“ calories per second per square cm and degree C gradient at 1 cm thickness. B. Synthetic Garbage A synthetic garbage made of fresh ground vegetables, fruits, potatoes, bread, meat, coffee grounds and news print, was used to provide a material uniform both in composition and degree of grinding. The synthetic garbage was repre- sentative of typical East Lansing garbage (26); however, the percentage meat was reduced to give a C/N ratio of about 30. The amounts of constituents used are listed in Table 1. TABLE I CONSTITUENTS USED IN SYNTHETIC GARBAGE Per Cent Composition Food Constituent on Wet Wt. Basis Wet Weight,1b. Meat 3.8 1.0 Bread 11.3 3.0 Apples 18.9 5.0 Oranges 15.0 ‘ 4.0 Vegetable 18.9 5.0 Potatoes 18.9 5.0 Coffee grounds 1.9 0.5 News print 11.3. 3.0 Total 26.5 20 The fresh food was chopped and mixed manually then ground twice in an Enterprise Engine Company vertical mill grinder with 3/8" screen openings. Synthetic garbage after beingground was grey and re- sembled wet paper, the other ingredients being only slightly noticable. The fresh ground material had a fruity and acid but pleasant odor. The following is representative of the composition: TABLE 2 REPRESENTATIVE COMPOSITION OF SYNTHETIC GARBAGE Composition Per Cent MoBture 68 Volatile matter 94 Ash 5 Nitrogen 1 C/N ratio 28 pH 5 In experiment number one, two pounds of ground beef were used and the resulting mixture gave a C/N ratio of 19.5. Gotaas (15) stated that a C/N ratio of about 30 could be expected in a municipal garbage and that lower C/N ratios resulted in higher nitrogen losses. Therefore, one pound of ground beef was used in the remaining experi- ments, resulting in an average C/N ratio of 28.8. In experiment number six, three pounds of dry corn cobs were substituted for the news print. The material used in the last experiment was ground in a shredder, Model 2x BE,W-W Grinder Corp., Wichita, Kansas. SECTION V EXPERIMENTAL PROCEDURE A. Preliminary Investigations Preliminary investigations were conducted in a cylin- drical metal container, 8.5 inches in diameter, 21 inches in depth with a working capacity of four gallons or sixteen pounds of organic material at about fifty per cent moisture. The container was equipped with a plastic false bottom which diffused air into the material through numerous 1/16" holes. Air was supplied at a rate of 1.0 to 8.8 cubic feet per pound volatile matter per day to the bottom of the container, below the false bottom, and removed from the top of unit. The equipment was identical to that used by Zondorak (27), with the exception that the cylinder was completely covered with insulation. After adding the ground garbage mixture the unit was sealed with masking tape. The organic material reached a temperature of 14OOF (600C) in about 6 to 8 days. The digesting material was not disturbed until the run was finished, usually about 12 days. The end product tended to be too moist on top and too dry at the bottom. Approximately 20 per cent of the material had become so dry that little decomposition had taken place and it appeared to be still raw, see photograph in Figure 5. 22 l .__., -7 l,___-_ Microphotograph of yeasts that are predominant during the fermentation phase. Objective 43x and 10x eyepiece. Idicrophotograph of one type of fungi found in the white growth on finished compost. Same magnification Figure 4. Some Microorganism found in Composting 23 To avoid these shortcomings new equipment was designed and built to provide continuous mixing, as described in Section IV, A. All other experiments reported in this study were conducted in the rotating drum. B. Method of Procedure Twenty-six and a half pounds of synthetic garbage were ground twice through an Enterprise Engine Company vertical mill equipped with 3/8" screen openings. The moisture content of the fresh synthetic garbage was about 68 per cent. To reduce the moisture content to the desired level, the material was spread under a hood for three to four days until the excess water had evaporated. The material was then reground to produce a uniform moisture content and about 100 grams were removed for the initial analysis. The remaining portion was weighed and placed in the rotating drum. Usually approximately 15 pounds of raw material were added to the drum, the weight depending on the moisture content of the material. Dry weight of garbage added was about seven pounds. Aeration and rotation were started when the activity of the microorganisms was great enough to remove the oxygen from the air in the unit in about one hour. In most experi- ments the air supply was regulated so that the oxygen per- centages in the exhaust gas were between 2 and 5 per cent. A few experiments were conducted with higher aeration rates to maintain 10 to 15 per cent of residual oxygen. Actual 24 air rates varied between 1.5 and 115 liter per hour or 0.2 to 15.4 cubic feet of air per pound initial volatile matter per day. Measurements on the percentages of carbon dioxide and oxygen of the exhaust from the drum were taken about five times per day. The amount of air supplied to the organic material was measured with a wet test gas meter and tem- perature was recorded on an automatic recorder. The tem- perature recorder was checked by the control thermometer in the sampling hole. Daily 15 gram samples were taken and moisture and pH were determined. Moisture content was measured on a Cenco moisture balance. Each experiment was considered complete when the temperature of the organic material had returned to within 50F of room temperature. Material was then removed from the unit and weighed. A correction was made to the final weight to compensate for the samples removed. Approximately 100 grams of finished material were used for final analysis, and the remaining material was stored in several ways to study the effect of different storage conditions. C. Analysis of Compost. The laboratory analysis made on the raw and finished material included moisture, ash, total nitrogen, and pH. Tests were conducted as outlined in The Preliminary Report gn‘a Study of the Composting of Garbage and Other Solid 25 Organic Wastes, Michigan State University (9). Volatile matter was computed as 100 -% ash. The per cent carbon was estimated by % carbon = 100 - % ash /1.8 as suggested in the New Zealand report (12). All percentages of composition reported were on a dry weight basis except moisture. The _ carbon to nitrogen ratio (C/N ratio) was computed by dividing the estimated carbon content ( % dry weight) by the amount of nitrogen ( % dry weight). ,__.1 _..— - Finished compost without and with heavy white fungal growth. Clumps of compost found using preliminary equipment, note white growth on outside and raw appearing dry center portion. Figure 5. Conditions of Finished Compost SECTION VI EXPERIMENTAL RESULTS Experiment No. l As previously noted in Section IV, B, two pounds of ground beef were used in making the synthetic garbage. The synthetic garbage was dried to 42.6 per cent moisture and placed in the drum. The pH dropped from 6.3 to 4.8 during the second day. After a slight increase the temperature rapidly returned to near room temperature on the second day and biological activity was almost completely arrested. Lime and seed material were added but the temperature re- mained practically unchanged for five days. Only after the moisture had been readjusted to 54 per cent did the de- composition process start again. Temperature and oxygen uptake increased rapidly from the ninth day on, as shown in Figure 6. For the first seven days the drum was rotated once a day for 15 minutes. This probably caused the steep peaks and declines in the temperature during this.period. After the seventeenth day a timesmitch was installed and the material was mixed for five minutes each half hour with the drum rotating at 0.067 B.P.M. See Table 3 and Figure 6 for summary of data on experiment. _.OZ kszmmaxm um mmDGE m> .cofio: nub; 92E: 2.356 o c n a _ o - P p b b m .m 19 '8 17 E s n -s m R D E M .5 T E w R w w .4 A . W n o .3 u E .l fiz om. ow. h.o... om. ow 010 r o . . 00 O? On FIGURE 8- EXPERIMENT No.3 32 Experiment No. 4 The insulation was removed from around the drum and the entire apparatus was placed in a box covered with 1 1/2" spun glass insulation. Twelve and six tenthsgnunds of raw material at 49 per cent moisture were added to the unit. Air supply was regulated as in experiments 2 and 3. Actual aeration rates used were 1.4 - 8.6 cubic feet of air per pound initial volatile matter per day. The added insulation causedno in— crease in temperatures; in fact, the temperatures were lower than those found in experiments 1, 2, and 3. The lower temperature was probably caused by the reduced moisture content of raw material. See Table 3 and Figure 9 for summary of data. Odor and appearance of the end product was similar to that described in experiment No. 2. Experiment No. 5 Nineteen pounds of raw material at 67.3 per cent moisture were added to the drum and the entire unit was again placed in a box covered with insulation. Aeration rates were adjusted to provide residual oxygen readings of 5 to 10 per cent in the exhaustgas. Aeration rates ranged from 0.9 to 11 cubic feet per pound initial volatile matter per day. Daily samples were taken from the unit and analyzed for moisture, ash, ammonia, nitrogen, and organic nitrogen. 33 IF’_‘ It " _ 7'” 'BHI'N'A 'aarau'auu axudn uaoaxo Hd ¢ .02 kzmgmwaxm Im MSGE m>¢o o. S n. u. : o. m o s o n c n m . o o L . b p h k - n L p b p r h h | L '. 0 a a . .0 c. l .W I wt}. 9.5% 2356 t . am a. . an .71: um: 25..qu 0 Lu 0 .M .7 L o ..9 g l U h. 9 . I l 1. 4 I an .t . .1 6L fim 1?! T 0': Of 09 0. iflflLVUadNBL 34 Data from this run are presented in Table 7 and Figures 11 and 12. A discussion of the results is given in Section VII. Data for oxygen uptake rates, respiratory quotients (R.Q.), pH, and temperature are included in Table 3 and Figure 10. The end product had a more putrid odor than those described previously. The material showed considerably more balling with about 10 per cent of 1/2" to 1" balls. Experiment No. 6 Three pounds of dried corn cobs were substituted for the paper in the synthetic garbage. Nineteen and six-tenths pounds of material at 57.1 per cent moisture were placed in the drum. Aeration rates were regulated to maintain 2 to 5 per cent oxygen in the exhaust gas and varied from 0.2 to 5 cubic feet per pound initial volatile matter per day. The odor of the finished product was similar to that produced in experiment No. 5. The color was dark, almost black, in a moist condition. See Table 3 and Figure 13 for data. Experiments No. 7 and 8 A double batch of synthetic garbage was ground and placed in two separate rotating drums at 57.0 per cent moisture. Experiment.7 was aerated so that there was a residual oxygen content of 2 to 5 per cent in the exhaust gas and experiment 8 was aerated so that 10 to 15 per cent 35 mu '0‘ was rou‘aiva anion uaoxxo 'O'H m .02 PZMEEMQXU IO. wane—m w> Prom; >mo Z. mumeJazdi m>.:x0 .l 1 . a O 2 - 3% I'll u23h(o 0. o. .v. n. a. 2 O. a o n p mic n b h m P“ r- 9'I VI 21 0'! 8' UF<¢ u¥(ha: 2m0>x0 Ill ucabkcualmb 08 (do 03 m‘aamvamn fii OR 09 00 41 'aH/WA'aol'ou‘alva anvidn uaoxxo Hd 0.02 FngmameQ mmDGE I'IIIVI 9'I VI Z'I O'I n>4 [11 (00 m o 0 3'0 \OCI) N o o moo MON H o o \OCD H CO rha +360 HI: Cw-‘I HEP C O H 4.) H CO 0 0.4 E O 0 III 94 O—II'KO t~ux¢ muo “TONI—I NHKO * Initial Final % loss Moisture % HQ 00:: \O—II‘LH «1 Initial Final % loss Dry Weight,1b. \OM MOLD KOM—II' \Od) MCOO @000 (\IN r-IKOO 003m .3 NH [\an 10mm on OCO (\ICD-ZI' \0000— m 6.50 4.09 37.1 7.311 11.32. 40.5 Initial Final % loss Volatile matter, lb. .52 .52 .35 .37 .26 .26 .13 .11 15.4 .310 .312 .865 .892 Initial Final 1b. Ash, .071 .12 16.7 40.8 .12 .1O .120 .118 107 .13 .11 15.4 .10 .098 2.0 .135 .132 2.2 .210 .159 24.3 Initial Final % loss 1b. Nitrogen, um: s¢~ Hra LON [\N HCU 3'0 :I‘II' HO] and 2.55 0 Initial Final Nitrogen, % C/N ratio :- W 29.3 29.3 21.5 27.8 Initial Final 44 .H .02 pcoEHgomxm .H> Coauoom mom .UoUSHoQH poo 9:08pmzmpm ohdpmaoe opomon oEHB* 0.8 8.: m.a m.m 3.: m.e :.m a.m .arxaooooa mama a> anfipficfi .nw\ o .wa mama oxmpms Cowmxo .xmz m.m m.m o m.m m.» w.m m.© m.s ammo .omsmp madpm Iaanou ofiafism Ioahonp CH oEHB o.sa m.oa o.ma o.ofi s.ma o.HH m.HH *m.HH memo was mcfimeQEoo amp a.H m.H m.o m.o e.H w .H m.m o.a zoo .xz.>.H.ofl\.o.o a.ma o.w o.m o.HH o.m H.w s.m m.s swam moooa tea m N m m z m m H coapfimomsoo ucoefihoaxm Aooooaoooov m mamas SECTION VII DISCUSSION OF RESULTS A. Oxygen Uptake Rates Data on oxygen uptake rates during the process of composting are very limited so far. Experiments at Michigan State University (9), by Bek (l8), and Ludwig (17), had shown that the rate of decomposition was maximum at about 4500 and the rate decreased with increasing temperatures. This was in disagreement with the general concept that the activity of the microorganisms responsible for aerobic com- posting was higher in the thermophilic than in the meso- philic temperaturerange (8, 12, l4, l5, 16, 19). The oxygen uptake rates reported in the Michigan State University Studies (9) were measured under conditions which could not be compared directly with actual conditions in a compost pile. Relatively small samples of organic material in an atmosphere of 95 per cent oxygen were used, carbon dioxide was completely removed by KOH, and temperature control was by a water bath. It was difficult to prevent moisture losses in the organic material when the temperature was above 450C. Data by Ludwig (17) were collected under similar con- ditions using only 3 to 5 grams of organic material and 46 laboratory incubators. Ludwig also had trouble with moisture and pH control at temperatures above 45°C. Beck (18) did not measure oxygen uptake but found greater volatile matter reduction at 400C. In this work preheated air and cooling coils were used for temperature control. Braithwaite (22) measured oxygen uptake rates by checking the percentage of oxygen in exhaust gases, but data were sporadic due to insufficient control of aeration and moisture. Stoller, Smith, and Brown (23) measured the amount of carbon dioxide produced from composting horse manure and straw. Most measurements were taken after the aeration had been stopped for fifteen minutes. Only one set of readings was taken while the air was flowing. These data were used for the computation of oxygen uptake rates, under the assumption of a respiratory quotient of 0.9. Data from this literature are summarized in Table 4. The oxygen uptake rates measured in the rotating drum were, in general lower than those listed in Table 4. The maximum oxygen uptake recorded was 6.4 mg oxygen per gram initial volatile matter per hour at a temperature of 144OF (620C) in experiment No. 9. Most oxygen uptake rates for the higher temperatures, about 14OOF (600C),were in the range of 5 to 6 mg oxygen per gram initial volatile matter per hour. The amount of oxygen used was computed from the rate of air supply and the difference between the percentage of 47 mmmv Gaopm one madam: spasm.moHH0pm ompom no mm an\z.> pm\comzxo\we m©.> = __ mm __ NHH : Z. om 2. my 0 MW __ 2 62a : : 0.3 2 H o N. : :, mm : 0 @ owwnhmw C3 maroon oflofisnm mm om or}: Re . .888??? ma . m Ammv opfiwzsuamnm owmngmo hg\z>.aw\cowmxo\mz 3.: : :. mw : w H H : : ow : O N. Z 2 om : O“ HH 2 : m3 Z 6 HH : I. Z 0: Z W a 0 Amy phonon bmz.aoam owmnnow m+wm om an\p3 haw.hm\comzxo.wz 0.5 poumwfiumo>sH Hwfipopmz thpmHoz o oASpmpoQEoE opom oxwuob cowaxo 0 .mmbegmmBHq 20mm mme¢m mM¢BmD ZMUNNO SDSHNdS 3 HAM¢E 48 oxygen in the incoming air (assumed constant at 20.9 per cent by volume) and the measured oxygen percentage in the exhaust air. The accuracy of the Beckman model D-2 oxygen analyzer was i 2 per cent of the full scale. Oxygen uptake rates reported in this study were computed from the amount of oxygen used divided by the original weight of volatile matter. It was assumed in these computations that the volume of metered air flowing into the unit was equal to the volume of air leaving the unit. Changes in gas volume, and therefore, errors in calculated oxygen uptake, could be produced by the following factors: (1) Respiratory quotient (R.Q.) not equal to one, (2) faulty sampling, (3) channeling of air through the unit, (4) air leaks, (5) pro- duction of other gases, and (6) variations in pressure and temperature. 1. The respiratory quotient (R.Q.), computed by dividing the percentage of carbon dioxide in the exhaust gas by the percentage of oxygen used, was seldom exactly one; see Figures 6 through 16 for plots of actual data. The average R.Q. for the over-all composting process was close to 0.9. A R.Q. of less than one means that more oxygen was used than carbon dioxide produced, resulting in a smaller volume of gas leaving the unit. A smaller volume of gas leaving the unit would produce higher oxygen readings in the exhaust gas and therefore lower apparent oxygen uptake rates. A R. Q. greater than one means that more carbon dioxide was 49 produced than oxygen used, resulting in a larger volume of gas leaving the unit. A larger volume leaving the unit would produce lower apparent oxygen readings and therefore higher oxygen uptake rates. Since the average R.Q. was about 0.9 the majority of oxygen uptake rates reported would tend to be small. Assume the rotating drum when loaded with organic material had a free air space of 10 liters containing 2.1 liters of oxygen and 7.9 liters nitrogen. Assume further that two liters of oxygen were used by the microorganisms. The volume of nitrogen would remain constant at 7.9 liters but the volume of carbon dioxide produced would be only 1.8 liters due to the R.Q. of 0.9. The volume of gas leaving the drum would then be 9.8 liters (7.9 liters nitrogen plus 1.8 liters carbon dioxide plus 0.1 liters oxygen). The percentage of oxygen in the exhaust gas would be 1.02 per cent instead of 1.0 per cent if the R.Q. had been one. The variation caused by a R.Q. equal 0.9 is therefore well within the limits of error. 2. Errors in sampling would be caused by measuring exhaust gases mixed with air from outside the unit. Oxygen reading in exhaust gases would then be too high and oxygen uptake rates too low. To prevent air from becoming mixed with exhaust gases, a water trap was installed in the ex- haust line and the sampling tube of the oxygen analyzer extended into the unit through the air exhaust tubing. 5O 3. Channeling of air through the organic material would not provide an adequate oxygen supply for all the material. Some material would not decompose aerobically or use oxygen so the oxygen readings would be too high and the oxygen uptake rates too low. This source of error was minimized by rotating the drum for five minutes every half hour. 4. Air leaks from the unit to the atmosphere (most probable around the inlet and) would give low oxygen readings in the exhaust gas and too high oxygen uptake rates. The ends of the unit were fitted with expansion rings and a grease seal to help prevent the leaking of air out of the drum, see Figure 3. The error caused by this effect was considered to be small, since aeration rates and the pres- Isure in the drum were low. Chances that air from the outside could enter the drum were negligible due to the slight air pressure inside the drum. 5. The production of gases other than carbon dioxide would result in a larger volume of exhaust gas. A larger volume of exhaust gas would produce a lower oxygen percen- tage reading and a higher oxygen uptake rate. Ammonia and other volatile compounds were noted in the exhaust gas, but the amount was believed to be small compared to the volume of air used. 6. The oxygen uptake rates were computed for air supply at a temperature of 2000 and 750 mm.Hg. The room 51 temperature and air pressure varied above and below these average figures resulting in some too high and some too low oxygen uptake rates. Data for plotting temperature against oxygen uptake rates were obtained by using the portion of the temperature and oxygen uptake curves before the rapid final temperature decline. The oxygen uptake rate for each lOOF increase was read from the graphs, and tabulated in Table 5. The average values for oxygen uptake rates were then plotted against temperature, as shown in Figure 17. In order to show the degree of variation within each temperature level the largest and the smallest oxygen uptake were also plotted on Figure 17. A direct relationship between oxygen uptake rate and composting temperature was found. Figure 17 shows that as the temperature increased from 90 to lMOOF (about 30 to 600C) the oxygen uptake rate increased from 0.5 to 4.0 mg oxygen per gram initial voltile matter per hour. The curve for the relationship between oxygen uptake rate and compost temper- ature was not a straight line but a curve with a steeper slope for the lower temperature. The rate of many chemical reactiomsincreases with temperature and in general the velocity of a reaction doubles or trebles for each lOOC raise in temperature. Usually the increase of the reaction rate with temperature is expressed as the temperature coefficient, Q10 = 52_ . From the data Kt-lO plotted in Figure 17 several temperature coefficients were 52 computed. Q10 (50 to 6000) equaled 1.77, Qlo (40 to 5000) equaled 1.80, and Q10 (30 to 4000) equaled 2.6. TABLE 5 RELATIONSHIP BETWEEN OXYGEN UPTAKE RATE AND COMPOST TEMPERATURE* TemperatureOF 90 100 110 120 130 140 1.1 1.6 1.7 1.3 1.9 4.3 3.3 3.8 3.5 0.6 1.0 2.5 1.5 1.7 4.2 2.8 2.4 3.2 0.6 1.5 2.5 1.9 1.3 2.7 4.8 5.0 0.9 1.9 0.7 1.1 1.8 2.3 4.0 4.4 0.3 1.0 1.1 1.9 1.7 3.2 5.1 4.3 0.1 1.3 1.9 2.9 3.2 3.3 5.6 0.8 1.1 3.1 2.8 3.0 5.8 1.8 0.6 3.2 2.0 2.8 6.1 1.4 1.4 1.3 2.1 3.6 4.2 3.0 1.6 2.0 2.0 3.5 4.4 2.9 1.7 2.1 3.5 4.2 3.8 3.0 1.2 2.5 2.5 4.6 3.8 0.5 2.8 1.3 2.9 1.8 2.7 0.6 1.77 1.78 2.43 3.42 4.51 Average 44 *0xygen Uptake Rate in mg oxygen/gram initial volatile matter/hour Temperature coefficients were compared to data by Wiley and Pearce (19) on carbon dioxide produced. The values derived from Wiley and Pierce's curves were 010 (50 to 6000) Q10 (30 to 4000) 1.60, 010 (40 to 5000) = 1.65, and 1.70. The results agreed surprisingly well considering the differences in technique and in material composted. .l. £sz hwoazoo 024 w._.xo zumihwm d_Imzo_._.<4um It mmawfi £1525 x<¢o\.oz.m._.<¢ 92.5: 5956 n pl p e P b h 53 X mad) Fania-1 B m34<> kmugg 0 ”3.2) ue<¢u>< I 00' OH OBI OH I 09 02 09 l 09 o. ‘3unlvuadn31 54 Aeration rates required to provide the needed oxygen for maximum oxygen uptake rates found in this study were 8.0 to 9.0 cubic feet of air per pound initial volatile matter per day or 18,000 to 20,000 cubic feet of air per ton of volatile matter per day. This resulted in good agreement with data reported by Wiley and Pearce (19); see Table 6. TABLE 6 AERATION RATES REPORTED BY WILEY AND PEARCE (19) Type Rate* Remarks Low Aeration 4-6.4 Resumed in late peak temperature and incom- plete composting. Medium Aeration 9-29 Complete composting with no reheating. High Aeration 33-78 Not completely com- posted with reheating of material and some drying of material. *Cubic feet air/pound volatile matter/day. The difference in aeration rates between experiment N0. 7 and 8 did not result in any marked difference in activity; see Table 3 and Figures 14 and 15. In experiment No. 5 higher aeration rates were also used without a notice- able increase in the activity of the decompostion process. According to Figure 17 the oxygen uptake rate varied directly with the compot temperature, the oxygen uptake rate in mg oxygen per gram volatile matter per hour, increased 55 steadily from 0.6 at 400F to 4.6 at 1400F. The oxygen uptake rate would probably continue to increase with tem- perature until a temperature was reached that proved detrimental to the microbial population. Several workers have shown that temperatures above 70°C (1580F) are harmful toessential microorganisms (l2, 15). The maximum compost temperature 146OF (63.5OC) reached in this study was within physiological limits. B. Oxygen Uptake Rates Measured by_the Warburg Techniques An attempt was made to investigate the activity of the finished compost in terms of oxygen uptake rates as measured in a Warburg apparatus. The Warburg technique required a knowledge of the exact volume of the samples being tested. Since the volume of the compost changed with different moisture contents, it was necessary to deter- mine the specific gravity of the finished compost at various moisture levels. This was done by measuring the volume of benzyl ether (C6H5CH2)20 displaced by a known weight of finished compost of known moisture content. Benzyl ether was choosen because it is immiscible with water, not volatile, and its specific gravity of 1.0428 at 200C is close tocne. Samples of material used for this calibration curve ranged in moisture content from 10 to 56 per cent. The samples were passed through a 0.075 inch mesh screen, weighed, and placed in 10 ml. Erlenmyer flasks. The flasks were fitted with 56 ground glass stoppers and the weight of each flask empty and full of benzyl ether was determined. The compost samples then were placed in the flasks, and the flasks filled with benzyl ether and stored for 24 hours with fre- quent shaking to insure removal of air bubbles. The volume of compost could then be computed from the volume of benzyl ether displaced. Data are shown in Figure 18. This curve was used in computing the volume of material placed in the Warburg flasks. 1. Oxygen Uptake Rates of Various Samples of Finished Compost The samples used for this series had been stored under various conditions and stemmed from various experiments. The samples were passed through a 0.075 inch mesh screen, weighed, and placed in the Warburg vessels. They showed a variety of oxygen uptake rates ranging from 0.01 to 0.29 mg oxygen per gram volatile matter per hour as shown in Figure 19. The decline in oxygen uptake rates was probably due to insufficient time allowed for the material to reach equilibrium in the Warburg vessels before measurements were stated. It was noted that the oxygen uptake rates varied with the age and also the moisture content of the sample. 2. Oxygen Uptake Rates of a Homogeneous Sample with Varied Moisture Contents To obtain a more homogeneus sample, finished material from experiments3 and 4 was mixed, air dried, and stored 57 szP200 NEH—.902 oz< Minn—O) methm a_ImZO_.—.x0 oz< mePQOZ zwmgkmm Q.Iwzo_._.<1_um1_N.GE $95.56: ea 2. on on 2. on On O. F b P L L P film i? m 3 {O n O O Irv “ / O 3 to u .. m w I” m I. .o .3 c. n .9 I -n s .A tom 1. H .8 TO U 62 maximum oxygen uptake rates during the active decomposition in the drum the activity of the finished material was about 1/8. This explains the fact that this material when re- ground and readjusted to optimum moisture did not produce a raise of 100F above ambient temperature. TABLE 7 OXYGEN UPTAKE RATES OF A FINISHED COMPOST AT 200C Moisture Per Cent Average Oxygen Uptake Rate* 11.2 Not measurable 19.6 0.015 30.2 0.137 41.3 0.301 51.3 0.386 60.4 0.756 70.1 0.715 *Mg oxygen per gram volatile matter per hour A practical consideration to be drawn from these data is that the moisture content of the finished compost should be below 20 per cent if it were to bestored in paper containers. C. Other Observed Data on Composting l. Phases of Composting Data indicated that batch type composting could be broken down into five phases: (1) fermentation, (2) assimi- lation of acids, (3) thermophilic or high temperature, (4) rapid temperature decline, and (5) decomposition of resistant materials. 63 Fermentation was the first phase observed in the rotating drum composting. The organic material showed a rapid decline in pH, usually from 5.5 to about 4.4. The R.Q. during this stage was above one and temperatures ob- served were between 90-1100F. The fermentation stage lasted about two to five days; see Figures 6 to 16. Microscopic checks showed that the microbial population during this stage consisted predominantly of yeasts. Figure 4 is a microphotograph prepared from material taken from experiment No. 8 on the third day. In the fermentation stage, the soluble carbohydrates are fermented to carbon dioxide, alcohol and acids causing the rapid decline in pH. The organic material had a pleasant fruity and yeasty odor during this phase. The second phase usuaDy started about the third day and continued until the pH reached 7, usually on the fifth or sixth day. Apparently during this period the acids were assimilated and this caused the pH to rise. Under the microscope the microbial population showed a slight decline‘ in yeasts and anincrease in the number of bacteria. As the pH approached 7 the composition of the microbial population changed from a predominancy of yeasts to a pre- dominancy of bacteria. Subsequently, the pH showed a rapid increase from about 7 to well over 8. This rapid change in pH appeared to be caused by the liberation of ammonia as indicated by the nitrogen data shown in Figure 12. Simul— taneously the temperature increased to 14OOF (6000). The 64 R.Q. varied between 0.8 and 0.9. The odor of the organic material changed from fruity yeast to a slightly pungent, putrid odor with some ammonia. The time the material re- mained in the thermophilic range varied from 5.3 to 7.3 days and averaged 6 days (not including experiment 6 which never reached thermophilic temperatures); see Table 3. During the following 2 to 3 days the temperature dropped from a maximum of about 120 to 14OOF to within 50F of ambient temperature due to a decrease in activity as shown by the decreasing oxygen uptake rates. The initial decline in the hydrogen ion concentration followed by a pH rise to 8.0 or above was in good agreement with data reported by other workers in the field (9, 14, 15, 19). The lag period or length of time before the temperature reached the thermophilic range was longer than the periods generally reported in the literature. The finished material, when reground and adjusted as to moisture, showed little or no tendency to reheat, indi- cating that the more readily oxidizable organic material had been decomposed. The odor was not pleasant, but it could not be considered offensive. The ammonia odor disappeared in 5 to 7 days from the stored finished cOmpost and when it had dried to about 40 to 50 per cent moisture a heavy growth of white fungi began to appear, as shown in Figure 5. Figure 4 shows a microphotograph of one type of fungusfound. Due to the 65 loss in ammonia the pH dropped, usually below 8, and about 2 week after removed from the rotating drum the material assumed a rich moldy odor, similar to a good garden soil. The C/N ratio of the finished compost averaged about 20. This represents the value which had been recommended for compost. Gotaas (15) suggested that the C/N ratio of a compost should be measured with regard to the amount of readily available carbon. This would allow higher C/N ratios of the soil conditioner if it contained considerable amounts of cellulose. The synthetic garbage used in this study contained about 30 per cent paper on a dry weight basis. Most of this cellulose material was still present in the finished compost. Thus a C/N ratio of slightly above 20 would not prevent the material from being used as a fertilizer or a soil conditioner. 2. Moisture Content The aerobic decomposition of carbonacous material produces carbon dioxide and water. The room temperature air used for aeration removed moisture from the organic material as the air tended to become saturated at the elevated temperature inside the drum. However, in all experiments the percentage of moisture in the organic material had increased, see Table 3. The change in moisture content varied from a gain of 9.6 per cent to a loss of 4.6 per cent. The loss of 4.6 per cent moisture in experi- ment 8 can be explained by the fact that in this experiment 66 the aeration rate was twice the normal rate. Apparently with the aeration rates normally used in this study, the amount of moisture carried out by the stream of air was not great enough to remove all the water which was produced during the decomposition process and in addition to compensate for the loss in volatile matter. The experiments indicated that initial moisture con- tent of 42 per cent was too low and that an initial moisture content of 63.7 per cent was too high. Optimimiinitial moisture content appeared to be between 54 and 60 per cent. These values are in good agreement with optima reported for similar material by other authors (6, 7, 8, 9, 12, 14, 15, 19). In experiment No.6 where three pounds of dried corn cobs were substituted for three pounds of newspaper in the synthetic garbage, the temperature never went above 112OF and the end product had a very putrid odor. These were good indications of partial anaerobic conditions. The initial moisture content in experiment' No. 6 was 57.1 per cent which was within the limits of the optimum moisture content found for the regular synthetic garbage. Therefore, it appears that the optimum moisture content may vary with the composition of the material to be decomposed. This agrees with results reported by Gotaas (l5). 3. Losses in Dry Weight, Volatile Matter and Nitrogen Table 3 shows that the losses in dry weight in the first eight experiments varied from 34.7 to 42 per cent and 67 averaged 37.5 per cent. The reduction in volatile matter ranged from 37.1 to 44.5 per cent and averaged 39.9 per cent. The losses reported are similar to those found by various authors (7, 9, 15, 18). For the organic material used in this study it appeared that reductions of the magnitude reported were sufficient to provide a fairly stable end product. The first eight experiments showed an increase in the percentage of nitrogen for the finished product. Losses in total nitrogen varied from 2 to 40.8 per cent and averaged 14.8 per cent. In general they were too small to compensate for the loss in volatile matter and thus resulted in in- creased nitrogen percentages for the finished product. The higher nitrogen loss in experiment No. l was probably due to the narrower C/N ratio which resulted from the use of 2 pounds of ground meat. Experiment No. 8 showed the highest nitrogen loss. The composition of the garbage mixture used was identical with that of experiment No. 7. The difference betweenthe runs was that the aeration rate for run No. 8 was about double the rate used for run No. 7. It is doubtful whether there is a relationship between aeration rates and nitrogen loss. 4Waksman (16) found that in manure com— posting larger losses of volatile forms of nitrogen occurred whenever decomposition was delayed, owing to too high or too low a temperature. In general the data presented show that for the synthetic garbage used and at a C/N ratio of 68 about 30, the nitrogen losses will be small compared to losses in volatile matter. The net result is an increased nitrogen percentage in the finished product. 4. Time Required for Composting The time required for composting was measured from the time the synthetic garbage was placed in the unit until it was removed. In someexperiments aeration was started a day after the material was placed in the unit, Section V, B. The length of time required for composting varied from 11 to 19 days and averaged 14.8 days. The length of time the material remained in the thermophilic temperature range was fairly constant and averaged 6.1 days with the exception of experiment No. 6 where thermophilic temperatures were never reached, probably due to excessive moisture, see Section VII, B. The greater variation in the time required for composting could be attributed to the lag in some experi— ments in completing the acid assimilation phase. 5. Daily Analysis on Total Dry Weight, Volatile Matter and Nitrogen In experiment No. 5 daily samples were taken from the drum and analyzed for moisture, ash, ammonia nitrogen and organic nitrogen. The daily sampling gave resulting per- centages of dry weight composition, but the dry weight of organic material was being reduced as the microorganisms decomposed it to carbon dioxide and water. To compute the daily total weights of dry and volatile matter and nitrogen, 69 a constant weight of ash was assumed. This was justified since the total ash for eight runs showed little or no change during composting (Table 3). The total weight of the ash of the initial synthetic garbage mixture was-com- puted from the original analysis. The daily amount of total dry weightwas computed by dividing the original weight of ash by the daily measured per cent of ash. From this the daily weight of volatile matter, ammonia nitrogen, and organic nitrogen was computed, as shown in Table 8. These daily weights were plotted on Figures 11 and 12. On Figure 12 the pH was plotted together with the nitrogen curves. The curve shows that as soon as ammonia nitrogen was found in the organic material, the pH rapidly rose to above eight. Figure 11 indicates that the reduction of volatile matter and dry weight was progressing at a fatrly uniformrate during all phases of the decomposition process. To establish the effect of temperature on the rate of de- composition in the manner described, more data would be needed. The nitrogen data show an increase in the total weight of nitrogen followed by a steady decrease1nti1 the final weight was reached. The temporary increase of nitrogen could be explained in two ways: (1) nitrogen fixation, and (2) the production of some volatile compound that would interfere with the nitrogen determination. The second possibility appeared to be more probable 7O mm.m mod. omo. mmo. Hw.m : :a mac. meo. :mo. mm.m sm.m e ma maa. meo. Heo. :o.m mo.: = ma Hma. mmo. mmo. ms.m ma.: = Ha 60H. mwo. Hmo. Ho.e 64.: = OH sea. mmH. Hmo. mm.e ms.: : m Hma. moa. mHo. 64.: om.: e w mwfi. spa. mac. mo.: eo.m = A mam. mam. zoo. :m.: mm.m : o mma. mma. teddy so.m Hm.m : m 36H. 46H. 0. Hm.m mo.m : : 04H. 04H. 0. :H.m wm.m e m mma. mmH. o. mm.m om.m = m mma. mma. o. ::.m mm.m = H Hmfi. Hma. o. ss.m Hm.o H83. 0 .QH.:mw6tsz .oH.dmm6tsz .na.dmmoouaz .oa.pmppmz .oH.panmz .QH memo Hmpoe Ofiommpo mficose< oaapmao> ham . «cmd lIllIl IIIII El? E m .02 ezmszmmxm some mma oza .zmwomeHz 0Hz .emonz wmo mo memchz mQHao m MAM¢E 71 6. Continuous Operation Experiment 9 was an attempt to see if the rotating drum could be used for composting on a continuous basis. The synthetic garbage for this experiment was ground in a W—W shredder as mentioned in Section II, B. The experi- ment was started with 12.6 pounds of synthetic garbage of a moisture content of 56.7 per cent in the rotating drum. The aeration rates were adjusted to provide a residual oxygen reading in the exhaust air of 2 to 5 per cent oxygen. Actual aeration rates varied from 2.0 to 9.0 cubic feet of air per pound initial volatile matter per day. The temperature of the organic material increased rapidly and in three days 134OF was reached. The pH also increased rapidly without the usual lag period as shown in Figure 16. On the eighth day the temperature and oxygen uptake rate started to decrease, indicating that the decom- position process was entering the final phase. Aeration and rotation were stopped, one fixed end was removed from the rotating drum and approximately one-fifth of the material in the drum was taken out (about 0.42 pounds dry weight). This was replaced by raw synthetic garbage equal to about one-fifth the original dry weight. The raw garbage was mixed thorougly with the remaining material in the drum. The fixed end was replaced and aeration and rotation started. The material removed from the drum was tested for moisture content, dry weight, and ash. 72 The analysis showed that the moisture content had increased to 70.3 per cent. To avoid an excessive build up of moisture all following additions of raw garbage were at a moisture content of 35.6 per cent. Twice more the additions of raw garbage were made every other day, providing an average detention period of ten days for the composting process. Each addition of raw organic material caused an increase in activity as shown by temperature and oxygen uptake rates in Figure 16. The pH dropped slightly after the addition of raw garbage (from 8.6 to 8.2), but in a day or less rose back to 8.6. The temperature in the unit dropped when the raw material was added and mixed, but usually did not go below the thermo- philic range and always showed a rapid increase. After three such additions the averagecbtention time was reduced to five days by removing one—fifth of the con- tents of the drum and by adding raw garbage equal to one— fifth of the original dry weight every day. The result was an increase in temperature and oxygen uptake rates as shown in Figure 17. This operation was continued for five days. The activity in the unit appeared to be declining at the end of the experiment. The moisture decreased also gradually to 57 per cent, but this could not be considered as a reason for the declining activity since it was well within the optimum range. The amounts of material added during the experiment are shown in Table 9. 73 TABLE 9 MATERIAL ADDED TO DRUM Dry Volatile Loading Day Wt,lb. Matter,lb. Ash Moisture 1 7th 5.46 5.31 ‘.15 57.6 2 9th 1.04 1.01 .03 " 3 llth 1.09 1.06 n 35.6 4 12th II -II .II II 5 13th II .II II .II 6 114th .H II II II 7 15th II II .II .II 8 16th N 3" II II Total 14.13 13.74 0.39 An attempt was made to compute the daily amounts of dry weight from the amount of ash in the same manner as was described in Section VII for experiment No. 5. The results are given in Table 10. It should be noted that the computed amounts of dry weight, volatile matter, and ash differed ‘ somewhat from the actual amounts found in the drum when the experiment was finished. The values of ash were small and errors in ash determinations would produce large errors in dry weight and volatile matter. At the end of the experiment, all of the material removed from the drum was mixed together and checked for moisture, dry weight, volatile matter, and ash; see Table 11. COMPUTED DAILY WEIGHTS FOR EXPERIMENT NO. 9 TABLE 10 L f Reduction,% Dry Volatile Ash, Volatile Dry Wt. Days Weight,1b. Matter,lb. lb. Matter 0 5.46 5.31 0.15 50.0 48.6 7 2.80 2.65 0.15 50.0 48.6 added 1.04 1.01 0.03 removed 0.42 0.40 0.02 total 3.42 3.26 0.16 9 2.79 2.63 0.16 19.3 18.4 added 1.10 1.07 0.03 removed 0.45 0.43 0.02 total 3.44 3.27 0.17 11 2.80 2.63 0.17 19.6 18.6 added 1.10 1.07 0.03 removed 0.46 0.43 0.03 total 3.44 3.27 0.17 12 2.81 ‘2.64 0.17 19.3 18.3 added 1.10 1.07 0.03 removed 0.38 0.36 0.02 total 3.53 3.35 0.18 13 2.96 2.78 0.18 17.0 16.2 added 1.10 1.07 0.03 removed 0.53 0.50 0.03 total 3.53 3.35 0.18 14 3.05 2.87 0.18 14.3 13.6 added 1.10 1.07 0.03 removed 0.55 0.52 0.03 total 3.60 3.42 0.18 15 3.15 2.97 0.18 13.2 12.5 added 1.10 1.07 0.03 removed 0.43 0.41 0.03 total 3.82 3.63 0.19 16 3.19 3.00 0.19 17.4 16.5 added' 1.10 1.07 0.03 removed 0.42 0.40 0.02 total 3.87 3.67 0.20 19 3.18 2.98 0.20 18.7 17.8 Actual wt.found 4.02 3.77 0.25 in unit 75 TABLE 11 REDUCTION IN TOTAL MATERIAL USED IN EXPERIMENT N0. 9 ‘* Initial Final % Reduction Dry Weight, lb. '14.13 7.25 . 48.7 Volatile matter,lb. 13.74 6.81 50.4 Ash, lb. 0.39 0.44 The fifty per cent reduction in volatile matter was higher than found in the batch experimentscbscribed previously. The organic material which was removed from the drum during the experiment showed a tendency to reheat. After about a week of storage in an open can, the material removed from the drum was covered with the usual growth of white fungi and the odor had changed from a slightly pungent, putrid odor (similar to that noted in batch experiments) to a rich earthy odor. About 15 to 20 per cent reduction in volatile matter per day could be expected in the drum when operating in the thermophilic temperature range. The material removed from the drum showed approximately 50 per cent reduction in volatile matter as compared to the raw synthetic garbage. The organic material remained in the thermophilic temperature range during five daily additions of raw garbage, but the activity appeared to be declining at the end of the experi— ment as indicated in Figure 16. It appears that if the feeding of raw garbage would have been continued the 76 temperature of the organic material may have dropped below the thermophilic range. Additional work is planned to evaluate the factors involved in continuous composting. SECTION VIII CONCLUSIONS 1. The activity in aerobically decomposing organic material, measured as oxygen uptake rate, increased with the temperature within physiological limits. Temperature of 146OF (63.500) reached in this study were within physio- logical limits. Under the conditions described, l8,000 to 20,000 cubic feet of air per day were required to satisfy the maximum oxygen demand of a ton of volatile matter. The temperature coefficients (Q10) computed from the direct relationship between oxygen uptake rate and compost temperature were Q10 (50 to 6000) = 1.77, Q10 (40 to 5000) = 1.80, and Q10 (30 to 4000) = 2.6. 2. Batch type composting can be broken down into five phases: (a) fermentation, (b) assimilation of acids, (0) high temperature, (d) rapid temperature decline, and (e) decomposition of resistant organic material. 3. The activity of finished compost can be satisfac- torily measured by the Warburg technique. The activity of a finished compost varied from a maximum when the moisture content was 6.4 per cent to an unmeasurable amount at 11.2 per cent moisture content. Compared to the maximum oxygen 78 uptake rates during the active decomposition in the drum, the maximum activity of the finished compost was about 1/8. 4. A ripening period of 7 to 14 days is required to produce a satisfactory end product. 10. 11. 12. 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Zondorak, John B., Use of Restricted Air Supply for Temperature Control in the Aerobic Decomposition of Solid Organic Wastes, Master's thesis, Michigan State University, Department ofCivil and Sanitary Engineering, December, 1956. >41»: I“;,‘L/. fl 1‘ '9' A ' . . . Q ' 7. l “W ’0'." . memnm “WWI-fir- . ' _ l 0'0 ’3» carafe A1 V1» C1 IQSC? MM (p/flp/Oé 1 Learn