BioCycle June 2004, Vol. 45, No. 6, p. 58
Once considered useless, filter cake and distillery spentwash – following solid state aerobic composting technology – become value-added products for sustainable agriculture.
C. Senthil and K.C. Das

THE SUGAR INDUSTRY follows textile manufacturing to be the second largest agriprocessing sector in India. A typical sugar and distillery complex generates large quantities of wastes like bagasse, filter cake, molasses and distillery spentwash. Bagasse is a solid waste of the sugar mill that is typically used as fuel in boilers for power generation. Molasses is a liquid by-product that is utilized as a fermentation substrate for production of ethanol for industrial and potable uses. Until recently, the filter cake and distillery spentwash were considered as “wastes” and had no known beneficial use. The scope for recycling these wastes in agriculture is vast by any standards, and the enormous potential that they offer in the production of compost has only been recently realized.
Recognizing the importance of organic additives in sustainable agriculture, a commercially viable, large-scale composting technology based on principles of “Aerobic Microbial Solid State Fermentation” was developed to convert sugar industry wastes into nutrient rich ecofriendly compost. In 1994, Rajshree Sugars & Chemicals Limited (RSCL) implemented this process for managing its sugar complex wastes and commissioned a state of the art Compost Production Unit at Varadaraj Nagar in the Theni District of Tamil Nadu, India. The Compost Production Unit of Rajshree Sugars was the first and remains the only ISO 9001 certified unit in India that adopts the highest quality controls in the production process of compost.
In the emerging industrial climate with increasing need to conserve natural resources and prevent pollution problems, the innovative use of sugar industry wastes such as filter cake and distillery spentwash for the production of compost assumes greater importance.
Filter Cake
A by-product from clarification of sugarcane juice, filter cake consists primarily of sugar, fiber, and coagulated colloids including cane wax, albuminoids, inorganic salts and soil particles. As the constituents of filter cake have been derived only from sugarcane, it does not contain any toxic substances that may be harmful to soils. In India, around 220 million tons of sugarcane are produced annually and sugar mills crush 61.6 percent of this cane to produce sugar. Typically filter cake production is 3.5 percent of total cane crushed resulting in an annual production of 4.7 million tons of filter cake.
Filter cake is a finely pulverized organic material with high nutrient concentrations (Table 1). The composition and properties of filter cake vary depending upon the quality of cane and the process followed for the clarification of cane juice in a sugar factory. Filter cake has in the past been used mainly as (1) a source of plant nutrients and (2) an amendment to marginal soils. However, direct application of filter cake to the soil has been criticized because of the high wax content (8-15 percent by weight) in it that could impact soil physical properties. Composting of filter cake before application to the soil could remove this drawback and has been suggested as a beneficial alternative process.
Distillery Spentwash
There are about 329 distilleries in India producing a combined total of over a billion liters of alcohol and approximately 15 billion liters of effluent (distillery spentwash) annually. Spentwash is a dark colored, acidic, high strength (BOD = 40,000 to 80,000 mg/L) liquid consisting primarily of biodegradable organics and some inorganic constituents. Because of its high strength and acidic nature, environmental regulations require intensive treatment of spentwash before discharge or reuse. Spentwash has been a cause of concern in many parts of the world for the past few decades and various disposal techniques have been developed and evaluated over the past 40 years. This body of research work has shown that distillery spentwash can be a valuable by-product generated from distilleries because it is essentially a plant extract derived from sugarcane and contains nutrients and organic matter that could be returned to the soils. Spentwash is a rich source of organic matter and nutrients like nitrogen, phosphorus, potassium, calcium and sulfur. In addition it contains sufficient micronutrients such as iron, zinc, copper, manganese, boron and molybdenum (Table 2).
In contrast to most industrial effluents, distillery spentwash does not contain any toxic heavy metals or hazardous constituents. After realizing the value of the distillery spentwash and its importance as an organic input for sustainable agriculture, a large-scale, commercially viable “Composting Technology” was developed. This technique uses the principles of solid-state fermentation for treating distillery spentwash comprehensively and economically. Through the utilization of this approach, many complexes within India’s sugar industry achieves zero liquid discharge while returning to the earth those elements that were absorbed by sugarcane during its growth.
The Composting System
In this process, a mixed population of microorganisms decomposes organic matter in a moist aerobic environment within a solid matrix. The process developed at Rajshree Sugars has two unique features distinguishing it from conventional windrow composting. First is the use of a specially developed enriched microbial consortium containing bacteria, actinomycetes and fungi that rapidly degrade spentwash. The second is the use of a special purpose windrow turner. This machine agitates, aerates and shreds the composting mass and facilitates spreading the microbial inoculum uniformly over the entire length of windrows.
Filter cake is received from the sugar mill in trucks and formed into windrows that are 1.5 m high and 4.0 m wide at the base. Windrows are sprayed with measured quantities of distillery spentwash where the filter cake acts as a solid matrix absorbent. The ratio of distillery effluent to filter cake in the composting mixture is usually about 2.5 to 3.5 : 1 (wet weight basis). Windrows are inoculated with the microbial starter culture (0.1 percent by weight) and the application of distillery spentwash over windrows is carried out at specific intervals for 9 to 10 weeks. The windrow turner tills the windrows at least five times a week followed by spentwash application. The tilling operation breaks clumps of filter cake into a uniform fine particle size thereby increasing the surface area and assisting spentwash absorption. Windrow mixing also provides oxygen to microorganisms and helps dissipate the heat and moisture that exist in the windrows. In addition, mixing serves to remove toxic gases such as carbon dioxide and ammonia. The entire composting process requires 12 to 13 weeks depending on ambient conditions (Table 3).
Typically during composting, windrow temperatures rise to 70°C during the initial weeks and remain between 50 and 70°C for a period of 9 to 10 weeks. The rate of temperature increase and the length of the period where temperatures are above 50°C are used in process management as indicators of compost activity and final stability. During the process, the C:N ratio decreases significantly (Table 4), while the moisture content is maintained between 50 and 60 percent by the application of spentwash. A stable compost product is obtained after 12 weeks of processing and has typical characteristics as shown in Table 5. The maturity of the final product is evaluated to assess the state of decomposition and the suitability of the compost as a medium for plant growth.
Advantages and Constraints in Composting Technology
Composting is an integrated treatment technology to convert organic wastes into a value-added product. Despite the heavy organic loading of spentwash, the process allows sustained abatement of pollution through “Zero Liquid Discharge “and eliminates the need for primary treatment of distillery effluent. The end product formed in this process is no longer a waste but is a nutrient-rich compost that can improve soil fertility and agricultural productivity. Implementation of this technology has promoted the 4R strategies of reduce, reuse, recover and recycle wastes to conserve resources and prevent pollution.
Although the technology has been successfully demonstrated, there are issues faced by other sugar complexes that may not allow them to immediately implement composting. The authors identified the following six issues as limitations to implementing this approach to all sugar complexes nationwide are:
Availability of adequate quantity of filter cake (or alternate compostable absorbent); Availability of adequate land area for composting; Ability to locate the compost yard away from public dwelling places; Cost constraints to installing an impervious flooring for compost yard; Management of the composting operations during monsoon seasons; and Provision of leakproof storage tanks for storing spentwash and rainwater leachate.
Benefits of Filter Cake Based Compost
Filter cake based compost is used as a soil conditioner for agricultural and horticultural crops. Apart from improving organic matter content of the soil, it provides nutrients that were depleted from agricultural fields due to extensive cultivation of sugarcane. The compost improves soil tilth and water-holding capacity thereby reducing irrigation needs. It is reported to increase crop yields by 15 to 20 percent over a period of 3 years.
Compost reduces chemical fertilizer requirements by trapping and holding minerals from fertilizers which otherwise get leached out of the soil. Farmers using the compost have indicated that application of compost reduces chemical fertilizer needs by 10 to 30 percent without loss in productivity. It is also rich in natural plant growth stimulants and enriched with beneficial micro flora. The filter cake based compost is consistent in quality and completely free from pests, pathogens, nematodes and weed seeds.
Long-Term Effects
After the Green Revolution, Indian farmers started using large quantities of chemical fertilizers, pesticides, herbicides and fungicides despite warnings from agricultural scientists about long-term effects on soils and the environment. Because of this continued practice, the sustainability of agricultural production has been jeopardized by the harm inflicted upon the physical, chemical, and biological fertility of soils. In this scenario, it is imperative to add adequate quantity of organic matter to soils in order to increase soil fertility and achieve crop production sustainability.
Sustainable agriculture is based on the continued recycling of soil nutrients. Sugar industry filter cake and distillery spentwash, which were once considered useless wastes can now be used as a means to support sustainable agriculture. The successful implementation of solid-state aerobic composting technology for the conversion of filter cake and distillery spentwash into a value-added ecofriendly compost has shown that this option is a commercially viable, environmentally acceptable, and practically enforceable option for sugar distillery complexes.
C.Senthil, Ph.D. is Chief Scientist (Research & Development), Rajshree Sugars & Chemicals Limited in the state of Tamil Nadu, India. He is a composting specialist and over the past seven years has focused a significant effort in the bioconversion of wastes from sugar mills and distillery by composting technology. E-mail: csenthil_kolathur@yahoo.com. K.C. Das, Ph.D., P.E. is Assistant Professor in the Faculty of Engineering at the University of Georgia. He is Research Leader in the Bioconversion Research and Education Center that provides technical assistance in the implementation of solid matrix biological technologies for waste management in the United States. He can be reached at kdas@engr.uga.edu.