ROTARY DRUM COMPOSTING OF MUNICIPAL SOLID WASTE

Abstract

One of the major environmental problems faced by India is inappropriate solid waste management. India requires appropriate strategies, methods for mobilization of financial resources and necessary infrastructure for organized waste management. This widespread practice leads to significant health risk for the population and causes rapid degradation of the urban environment. The solid waste characteristics reveal that in India the organic fraction of the waste makes upto 40-85% of the waste depending upon income and life style of the population. By composting this fraction, a significant contribution could be made towards waste utilization. In view of this centralized, large-scale composting plants in urban areas were promoted in the 1970s but proved to be uneconomical (Dulac, 2001) and currently few installations are still operational (United Nations Development Programme, 1991). Due to high operating and transport costs and the poorly developed market for compost, the expected profits could not be realized as planned. Therefore, UNDP study (UNDP/WB RWSG-SA 1991) recommended that instead of setting up single large mechanical compost plants, it would be beneficial and more effective to set up several small composting plants. Decentralized composting at a neighborhood or community scale provides small groups (Housing, Dairies, Nurseries, Schools and Institutions etc.) to pursue it at a relatively low cost. One of the promising decentralized high rate composting techniques could be rotary drum composter which provides agitation, aeration and mixing of the organic waste material, to produce a consistent and uniform end product (Compost). Several small and medium scale drums can be installed at various sites of sizeable organic waste generation such as peri-urban areas of large cities, institutions, vegetable markets, large dairies and demand driven places such as garden/park/nurseries. This not only reduces the burden of waste management of municipalities, but also produces valuable by-product (Compost). It can be designed to handle continuous (daily) inflow of waste and can be applied for composting diverse organic wastes such as cattle manure, swine manure, municipal biosolids, brewery sludge, chicken litter, animal mortalities and food residuals. However, the available information/literature generally dealt with the rotary drum type composting of a specific kinds of wastes. Information on operational aspects and compost dynamics for the mixed municipal organic wastes in a rotary drum composter is rather limited. Additionally, limited investigations have been made on rotary drum composting in controlled and repeatable conditions. Therefore, the study deals with the rotary drum compost dynamics and operational aspects for various types of municipal organic wastes generated. The intention was to obtain a deeper understanding of the transformation processes in rotary drum composting process. In order to accomplish the objectives, studies were carried out on performance evaluation of rotary drum composters under batch and continuous operating conditions. The size of the batch reactor was decided based on waste generation by single household and can be easily turned by manual rotation. The composting period of 20 days was decided for both active degradation and stabilization based on the performance of earlier in-vessel composting reactors. Two to three rotations were fixed at every 24 hours. The experimental work was carried out in five phases i.e. performance under combination of different kind of waste materials, performance under different combinations of same kind of waste materials, performance under different turning frequencies, performance under amended (cattle manure and leaves) municipal solid waste (MSW), performance under composting of various combinations of MSW and cattle manure. The second attempt dealt with the composting studies on the relatively large-scale continuously operated rotary drum. The size of the batch reactor was decided based on waste generation by a community of 1000-2000 persons, vegetable market, institution (maximum 120 kg/day) which can be easily operated and managed by single person. One rotation at the rate of 2 rpm decided before and after the feeding of waste into drum. The experimental work was divided mainly into two phases. The first phase dealt with the start-up conditions and performance evaluation under different seasonal conditions for various types of waste materials. The second phase dealt with the maturation of primary stabilized compost under seasonal conditions by using traditional composting (Windrow and Vermicomposting) methods. Results indicated that mixed organic waste composting in pilot-scale rotary drum at the initial C/N ratio of 22 would be able to produce high quality, hygienically safe and stable compost compared to lower C/N ratio 16 and higher C/N ratio 30-38 within 20 days of composting period. It was observed that mesophilic phase was reduced to 1-2 days with further 7-10 days of active-stabilization period. In warm, moist environment with ample amount of oxygen and organic material available, aerobic microbes flourish and decompose the waste at a quicker pace. Study indicated that rotary drum composting was not affected by the different organic waste combination, if the initial C/N ratio of 22 maintain during composting. Studies on the effect of turning frequencies revealed that once a day turning would yield better quality compost. It provides longer thermophilic phase (4 day) with VI highest temperature rise (58.4°C) for microbial degradation. However, higher turning frequencies cause short duration thermophilic phase and unfavorable conditions for the proliferation of important thermophilic actinomycetes and fungi. Studies on MSW alone (Directly obtained from dustbin-C/N>32) and MSW amended with cattle manure & leaves indicated that final compost quality ofMSW amended with cattle manure and tree leaves to achieve initial C/N ratio of 22 is much higher than MSW alone. The final compost had lower pH, EC and NH4-N, C02 evolution and OUR with a Solvita® maturity index of 7. Furthermore, compostingof MSWmixed with cattle manure in 1:0.67 ratio producedhigher final total nitrogen and final total phosphorus compared to other mixtures (1:1 and 1:1.5) after 20 days of composting in a rotary drum composter. Long term monitoring (150 days) of full-scale rotary drum composter suggested that most of the organic waste combinations were composted successfully within 7 days period. Adequate air supply was provided in the form of exhaust fan in the drum, as the biological degradation process is aerobic in nature and the drum was maintained at a temperature of approximately 60 to 70°C even in cold weather conditions (Temp:4-6°C). A steep temperature gradient exists horizontally inside the drum. Very high degradation takes place at inlet zone resulting high thermophilic temperature (68-75°C), subsequently the temperature reduces gradually in middle portion and lowest in the outlet zone. Sustained higher temperature at inlet zone has transformed the quality of waste material immediately after feeding into the drum. The increase in ash content (28 to 35%) and decrease in TOC (34 to 46%) at outlet within 7 days indicates the robustness of rotary drum composter. Higher final total nitrogen (2.6%) and final total phosphorus (6 g/kg) bring out high compost quality. However, average number of fecal coliforms, BOD and COD were quite high at the end of composting, indicating the requirement of further maturation for hygienization. Solvita® results proved that compost enters into the maturation phase and required further few more days for reduction in CO2 values. Maturation studies were conducted using traditional windrows and vermicomposting techniques. It was found out that vermicomposting yielded better results during all seasons in term of TN (2.64-3.22%) and total phosphorus (5.2-7.25 g/kg). In addition, EC (0.43-2.89 dS/m), NH4-N (0.12- 0.51%) and TOC concentrations (0.12-0.19%) were least. Apart from this, it has produced more hygienic compost with COD (718-844 mg/L), BOD (90-320 mg/L), C02 evolution rate (1.21-1.82 mg/g VS/d) and lower number of conform bacteria. Vll