13.3: Other Composting Techniques

High-temperature (thermophilic) piles or windrows account for most composting, but other methods are also used. Instead of making piles, small farmers in developing countries often dig pits for composting (Figure 13.4), especially in dry and hot climates. The pits can be covered with soil material to prevent animals from getting into them, and they retain moisture in the compost material better. Many home composters prefer using vessels to facilitate the turning process, to have better control over temperature and moisture conditions, and to keep out rodents. But these systems are generally not economical for large-scale commercial operations.

Vermicomposting involves the use of earthworms—typically red worms—to perform the decomposition process. The method is, in a way, still mostly bacteria based, but the process occurs in the gut of the worm. The end product is worm casts, coated with mucus consisting of polysaccharides that make them into somewhat stable aggregates. The system requires bedding materials like newspaper strips, cardboard, hay and similar carbonaceous materials that mimic the decaying dried leaves that worms find in their natural habitat. The process is fast and efficient: worms can process half their weight in organic material in one day. The final product has an attractive feel and smell, and is appealing to consumers.

Vermicomposting is most often used to process kitchen scraps and can be done indoors in small bins. Vermicomposting methods are also used in large commercial operations. Two main approaches are used: windrows or raised beds. With windrows, new materials are added on one side of the bed, and the other side is harvested for compost after about 60 days. With the raised-bed or container system, which is preferred for indoor operations in colder climates, the worms are fed at the top of the beds and the castings are removed at the bottom. Some vermicomposting operations are connected with livestock farms to process manure for export of excess nutrients off the farm as a value-added product.

Fermenting composting, or bokashi, is an anaerobic composting methodology developed in Korea and Japan. The organic feedstock is inoculated with Lactobacilli bacteria that generate a fermentation process under anaerobic conditions, converting a fraction of the carbohydrates to lactic acid. The process is similar to the making of silage and fermented foods (like kimchi and sauerkraut). It is mostly done on a small scale, with food scraps as the primary source material and using a sealed container, but some large-scale bokashi is done with tightly covered windrows. The preserve can be soil applied after a few weeks of fermentation or stored for later use. The process also releases some of the feedstock’s water content, which is high in nutrients. The advantages of the bokashi process are that it is fast and that it produces less odor and fewer greenhouse gas emissions. Disadvantages are the need for sealed containers and ways to capture the liquid discharge, the purchase of fermentation bacteria, and the need to bury the compost into the soil (i.e., not use as topdressing).