Windrow composting of agricultural and municipal wastes

Transcription

1 Resources, Conservation and Recycling, 4 ( 1990) I Elsevier Science Publishen B.V./Pergamon Press plc - Printed in The Netherlands Windrow composting of agricultural and municipal wastes L.R. Kuhlman Resource Recovery Systems of Nebraska, Inc., Sterling, CO (U.S.A.) (Received July 18, 1989; accepted August 27, 1989) ABSTRACT i' Kuhlman, L.R., Window composting of agricultural and municipal wastes. Eur. Symp. on Integrated Resource Recovery from Municipal Solid Wastes. Resour. Conserv. Recycl., 4: Many wastes are produced from agricultural and municipal facilities that are not suitable for direct land application. Composting these wastes converts them to a humus-containing organic material advantageous for crop production. Major advantages are to stabilize the wastes, substantially reduce the carbon-to-nitrogen ratio (in the case of agricultural residues and municipal solid wastes), reduce and virtually eliminate odors, weed seeds, and pathogens and to produce a product easily handled mechanically. Comporting is accomplished under aerobic conditions developing temperatures of 55 "C or higher. The windrow technique is simple and accomplished easily with standard equipment. Specialized windrow aeration equipment has been developed for use when large amounts of waste are involved. Municipal sludge is readily composted in combination with finished sludge compost or other products in windrows aerated twice or more per week with virtually complete pathogen destruction. INTRODUCTION Composting is as old as the world itself but is now being rediscovered, so to speak, because of the present need to conserve landfill space, to handle wastes in a more environmentally acceptable manner and the further realization that the end-product is a valuable resource. Humus production is the end-result of composting. Humus is the lifeblood of the soil and must be present if the soil is to be fertile. A total of only 1 to 2% humus is needed to differentiate between fertile and non-fertile soils. Humus and its related acids are important for soil tilth, structure, and water and nutrient holding capacity. Microorganisms of the soil use the humus as a substrate. Most nutrients in soil minerals remain unavailable to plants in soils devoid or lacking in humus. Many soils are now chemically dependent upon commercial fertilizers. Large chemical oitrogen appliraths t:, the soi! are betiiixxitg! far the eoiiversioii /90/$ Elsevier Science Publishers B.V./Pergamon Press plc

2 n. I52 L.R. KUHLMAN of organic matter of roots and plant residues to humus. Optimum yield and the maintenance of soil fertility can be obtained by a balanced program of composts and other suitable products such as manure and mineral fertilizers. Compost, because of the humus and other properties, is more valuable to the soil than are manures and raw organic wastes. Manures and other wastes are applied with the intent to increase the humus content but often this does not happen because of fertilization practices. Manures are often allowed to lay on top of the soil for a period of time causing many nutrients to be lost. Many agricultural crop residues are contaminated with insects, weed seeds and disease and have a high carbon-to-nitrogen ratio thus should not be returned to cropland in their raw state. Wastes containing high carbon-to-nitrogen ratios which are added to soil, require additional nitrogen fertilization so that the crop does not suffer negative nitrogen balance. Many economic and health factors favor composts compared to raw wastes for land application. A major issue is the high cost,of transportation of raw wastes, especially those high in water content or of low bulk density, versus the resultant compost much reduced in water and volume. Composts are virtually free of both weed seeds and the organisms causing disease in plants and animals as well as humans. THE PROCESS OF COMPOSTING Composting is a biological process. It is the aerobic, thermophilic, selfheating, biological decomposition of biodegradable organic materials. During the composting process microorganisms convert the raw material to humus and related compounds. Proper composting generates sufficient heat to kill weed seeds, pathogenic bacteria and helminths and reduces the moisture content for handling or stockpiling. The process does not attract flies, rodents and birds, or cause objectionable odors. Composting is a simple process that must remain within the limits of the biological system and is affected by the basic environmental conditions that affect all biological activities. Only biodegradable organic materials can be stabilized by composting. In addition to bacteria and fungi, actinomycetes are involved. There are several important criteria required for composting. Although there are optimums, the range is quite wide. Initially the water content may be between 45 and 75% with 50 to 65% as optimum. The material will be stabilized at various moisture levels depending upon several factors including initial moisture and volatile solids content, turning frequency, and water added from rainfall. Finished compost should contain less than 40% moisture for application with mechanized equipment. Composting requires the incorporation of air. Void or pore spaces for holding air are not readily formed in windrows containing over 75% moisture. Aeration and reduction of particie size is accompiisnea by using standard firm

3 WINDROW COMPOSTING OF AGRICULTURAL AND MUNICIPAL WASTES 153 equipment or a specialized composting machine (Fig. 1 ). Equipment capable of aerating in excess of 1000 tonnes per hour is required for composting projects producing tonnes or more of waste per year. Examples of these projects would include the composting of sludge for municipal wastewater treatment plants, municipal solid waste, animal manures from large production units and various crop residues from centralized processing facilities. Turning frequency may be daily to weekly depending upon conditions. Experience with the waste enables the operator to judge turning frequency which may vary with the season. Temperatures below 0 C to - 10 C hamper composting but do not stop it. In these cases lower initial moisture content is beneficial for initiating the process. The carbon-to-nitrogen ratio affects the speed of the composting process and the volume of material finished. The optimum carbon-to-nitrogen ratio is given by Golueke as to 1 [ 1 1; however, the experience of this author indicates that a carbon-to-nitrogen ratio of 50: 1 is well within the optimum range and excellent results are obtained with even higher ratios as demonstrated by successful composting of leaves, sugar cane bagasse, cotton wastes, etc. When there is a severe nitrogen deficiency in the waste, addition of small amounts of urea or other nitrogen sources may be required. Different kinds of wastes are often mixed together to provide the desired carbon-to-nitrogen ratio and to adjust the moisture content. Because the process is biological in nature there is a practical limit as to the speed and extent of the composting process. Environmental factors such as temperature and rainfall must be con- Fig. 1. Specialized windrow straddling composting machine capable of aerating 2000 tonnes waste per hour.

4 154 L.R. KUHLMAN Fig. 2. Site for composting sewage sludge showing windrows nearing completion. Site is functional and attractive and surrounded by a wall with dwellings in the background. sidered in order to determine when to aerate. The temperature in the windrow will reach 55 to 65 C and be maintained for several weeks with proper and timely turning. Temperatures higher than 65 C are undesirable and can be lowered by aeration. WINDROW COMPOSTING Windrows may be constructed by several methods, however, it is usually done by truck and front-end loader. Windrows can be from 2 to 6 m in width at the base and 1 to 3 m in height and of any length. The most practical windrow size is 3 to 5 m at the base and 2 to 3 m in height and somewhat triangular in shape. Optimum size will vary due to weather, turning equipment utilized, and initial characteristics of the waste. Windrows which are too small are vulnerable to weather conditions, especially rain, and require considerably more land area for an equal amount of waste compared to larger windrows. Excessively large windrows, if not aerated at the proper times, readily form anaerobic cores with the resultant release of odors when aerated. Figure 2 shows a sewage sludge composting site with windrows of approximately 4.3 m in width at the base and 1.75 m in height. f COMPOSTING OF SEWAGE SLUDGE Composting virtually eliminates pathogens from sewage sludge [ 2,3 1. Composting of anaerobically digested sewage sludge reduced fecal coliform

5 WINDROW COMPOSTING OF 4GRICULTURAL AND MUNICIPAL WASTES \ \ FECAL COLIFORM MPNlg BOD I 2 Mg'g I T E M. PW E R $ 40, U R E30 ( 'C) 20 ' I 1 I I ---_ I lo DAYS Fig. 3. Effect of composting time upon temperature (solid line), BOD (dots) and fecal coliform (dashed line). levels from 2.3 x 1 O6 MPN/g (most probable number per gram) to 1.4, Salmonella sp, from 63 MPN/g to less than 0.2, and viable Ascaris lumbricoides from 1.7 ova/g to less than 0.4 [ 21. In other experiments sludge from the same plant showed values after composting to be < 0.5 MPN/g, < 0.2 MPN/ g, l/g and <8 CFU/g (colony-forming units per gram) for fecal coliform, Salmonella sp., viable Ascaris ova and Aspergillus sp. (including A. fumigatis), respectively [ 31. Data from both experiments were highly repeatable by both researchers. Seventeen percent dry matter anaerobically digested sewage sludge cake was mixed with compost derived from sewage sludge on a volumetric basis of 2 : 1 to form a windrow containing 65% moisture [3]. The windrows were mechanically aerated on three consecutive days after forming and twice per week thereafter. Temperatures, BOD and pathogen destruction were monitored at various intervals over a 35 day period. The BOD was reduced from 114 Mg/g of dry weight to 45 in 28 days and to 3 1 by day 35. Temperatures reached or exceeded 60 C for 9 of the first 20 days and 17 of the first 28 days. Total coliform and fecal coliform were reduced to 50 and 43 MPN/g, respectively, from 2.3 x lo6 and > 2400 MPN/ g, respectively, by the 20th day of composting. After 28 days the values were 10 and 3, respectively, and no further reductions occurred by the 35th day. Salmonella sp. were inactivated by the 20th day of composting. The relationship of temperature, BOD and pathogen inactivation is shown in Fig. 3.

6 156 L.R. KUHLMAN TABLE I Typical values for various measurable parameters in composted sewage sludge Total solids (I) Volatile solids (W) Total coliform (MPN/g)" Fecal coliform (MPN/g) Salmonella, sp. (MPN/g) Total Ascaris ova (Number/g) Viable Ascaris ova (Number/g) Aspergillus sp. ( CFU/g)b Biological oxygen demand (Mg/g dry wt. ) 60 to to 38 < 10 < I < I < 2 < I < 'MPN: most probable number per gram, dry weight basis. bcfu: colony forming units per gram, dry weight basis. In this experiment 28 days were required to complete the composting process. Experience with the waste is needed to determine the length of time required for complete stabilization of the waste and complete pathogen destruction. The single best way to monitor the composting process is by observing temperatures at points along the windrow, Coliform organisms are more resistant to inactivation than Salmonella sp. and thus are good indicator organisms. Many researchers have shown that Escherichia coli and Salmonella sp. are killed in 20 min at 60 C. The thermal death points of most disease causing organisms including Salmonella typhosa, Mycobacterium tuberculosis, Mycobacterium diptheriae and Brucella abortus or sius are well within the temperature range achieved in composting if exposed to these temperatures for at least a few minutes. Organisms are exposed to high temperatures for many hours and/or days during composting. Composting is an excellent method of rendering a pathogen containing waste into a safe product when good mixing, proper aeration and fine particle size is achieved. Typical solids and pathogen values at the conclusion of the sewage sludge composting period are shown in Table 1. Regulatory standards are met and exceeded when these values are obtained. COMPOST USES Compost applications on lands are basically of three types: ( 1 ) cropland; (2) ruined or disturbed soils (i.e., mine spoils or roadsides); and (3) land used for horticulture, golf courses, parks, landscaping, etc. Composts containing heavy metals are not to be used on food and vegetable crops. Composts likely to contain heavy metals are derived from sewage sludge produced in industrialized cities. Guidelines for application of composts containing heavy metals are available from state and federal authorities.

7 WINDROW COMPOSTING OF AGRICULTURAL AND MUNICIPAL WASTES 157 The nutrient and humus content of composts is variable. For economic reasons composts should be low in inorganic contaminants such as sand and also as low as possible in moisture content. The range in nitrogen content of composts will be from 0.5 to 2.75Yo. Those low in nitrogen are derived from certain agricultural crop residues (sugar cane bagasse, straw, cotton burrs, etc.) and municipal solid waste. Composts at the upper range of nitrogen are derived from animal manures (without bedding), sewage sludge, etc. Analyses for phosphate will be in a range similar to nitrogen while potash can be found to be above 3.0%. The mineral content of wastes will vary depending upon the origin of the waste. The value of a compost can be based both upon its nutrient and humus value. The nutrient value is comprised of nitrogen, phosphate, potash, sulfur and trace minerals and determined by laboratory analysis. The value of humus is more difficult to determine and is often established arbitrarily. Humus is the most important component. Farmers in the United States are increasing the usage of composts in the conventional systems and in conversion to organic farming. The general program followed in the conventional system is to apply 4 to 6 tonne of compost per hectare per year and reduce the purchased chemical nutrients by the amount (or a percentage of the amount ) found in the compost. The expressed goal of the farmer for the first year or two is to reduce input costs, maintain yields and to begin to restore fertility to the soil. As the program continues, the amount of chemical nutrients are gradually further reduced and some nutrients such as phosphate and potash completely eliminated while at the same time yields are gradually increased. This author has monitored several programs which are highly successful (L. Schilz, 1988, pers. commun.; s. Kissinger, 1988, pers. commun.). Holden Farms, a commercial composting and farming corporation, has conducted extensive research for the past several years on the use of compost made from turkey manure and pine shavings on crop production [4]. The TABLE 2 Effect of level of cattle manure compost upon corn yield Compost b/ha Yield b/ha IO I

8 158 L.R. KUHLMAN.2: c-.9 " ' e-;. Fig. 4. This sequence of photos shows the reclamation of a salt polluted soil by the application of 2.21 tonnes of poultry manure compost per hectare [4]. The lower photo shows the grass growing in the second year. This is a very dramatic effect showing the ability of a compost to correct a soil so that it will support vegetation. Disturbed soils are highly variable and may requii e higher levels of compost to restore partial or full productivity.

9 WINDROW COMPOSTING OF AGRICULTURAL AND MUNICIPAL WASTES 159 general practice of the farmers in the area is to apply both compost and commercial fertilizer. Average yield of several different crops over a four year period and a large number of comparisons showed a consistent yield advantage favoring compost treatments. The economic return due to the treatments was shown to be advantageous and profitable. Research has shown increased corn yields in response to increased levels of cattle manure compost (Table 2) (P.H. Grabouski, 1977, pers. commun. ). The compost applied contained 1.459/0 nitrogen, 1.60% phosphate and 3.20% potash. The plots for the experiment had received no nitrogen fertilizer for three prior years and indicated the presence of 28 kg of nitrogen per hectare. Soil tests should be an integral part of the management of soil fertility. Specific recommendations should be made only after soil testing due to the variability of soils and the previous cropping program. Composts are valuable to aid in returning disturbed soils to greater productivity and also to reclaim ruined soils upon which nothing will grow. The effect of a poultry manure compost applied to a salt polluted soil is shown in Fig. 4 [5]. Although land spreading of sewage sludge is practiced, the trend is toward composting. Disadvantages of spreading non-composted human excrement on soils include pathogens living in the soil for weeks or months, pollution of streams from run-off, potential odor problems and seasonal problems of appiicat ion. CONCLUSION Every composting project has similarities but in its own way is unique. In each case the many variables must be identified and analyzed. Composting is, to a large degree, an art which implies good management practices and technique. The environmental and economic aspects must be studied and the product must be marketed or used in a productive manner. This brief review and discussion of composting presents basic information so that one can better understand the process and make a judgement as to whether or not composting is a viable option for handling nuisance wastes. The end-results to be obtained is twofold. The nuisance wastes are eliminated and become a product of value. There is a critical and definite need to improve soils in every part of the world. There is insufficient organic waste to satisfy the potential demand. REFERENCES I Golueke, C.G., Composting, a study of the process and its principles. Rodale Press, Inc. Emmaus, PA, 1 10 pp.

10 I60 L.R. KUHLMAN 2 lacaboni, M.D., Livingston, J.R. and LeBrun, T.J., Windrow and static pile composting of municipal sewage sludges. Los Angeles County Sanitation District Rep. to Municipal Environmental Res. Lab., U.S.E.P.A. 3 Kuhlman, L.R., Composting of municipal sewage sludge using the windrow method. Presented at the Third Int. Symp. on Industrial and Hazardous Wastes, Alexandria, Egypt. 4 Holden Farms, Inc., test and demonstration plots on Agri-Brand compost. 5 Hileman, L.H., Using poultry manure compost to reclaim salt polluted soils. Compost Sci., March/April: