2.5.4 What is composting?

Compost is a mixture of biodegradable organic matter such as straw, husks, tree bark, animal waste products and organic animal/plant matter (excluding sludge and fish organs) that are accumulated or mixed, and decomposed by heat. However sludge and fish organs may be considered compost if properly processed.

2.5.5 Basic principles of composting

Composting is the process of accumulating, mixing and aerating organic matter to decompose them with aerobic bacteria inside the material, evaporate the moisture generated by the heat from decomposition, and sterilize or make the harmful microbes or weed seeds inactive, to render the compost mixture safe and hygienic. Figure 5.7.1 shows the composting process. Composting offers the advantages of: (1) being easy to handle by hygienically-minded users since it eliminates noxious odors and the sense of handling biological waste products, (2) producing nutrients including the correct amount of elements for safe and high quality fertilizer for soil and crops, and (3) contributing to create a society that recycles its resources.

Evaporation

Fig. 5.7.1. Concept image of composting process.

2.5.6 Basic elements of composting

Composting is basically made up of (a) preprocessing, (a) fermentation processing, and (c)

product forming processing.

(a) Preprocessing

Preprocessing requires equipment for adjusting factors such as the moisture and organic matter, particle sizes, and aeration to make compost with the desired properties. When starting the composting, the moisture must usually be adjusted between 55 to 70 % and good aeration must be provided. This preprocessing includes additive methods (addition of adjustment materials such as shell, husks, sawdust, and chips), return methods (returning the product compost and mix with composting feedstock), and drying methods (drying using external energy).

(b) Fermentation

Fermentation requires a fermenting tank, aeration equipment and hydrolysis equipment. The fermentation tank decomposes organic matter and emits heat to raise the temperature of material accumulated in the fermentation tank so that conditions to produce safe and hygienic compost is attained by raising the temperature of the entire compost material to 65°C or higher and maintaining that temperature for 48 h or more. Fermentation methods are broadly grouped into accumulation methods and mechanical return methods. In the accumulation methods, materials such as compost, adjustment materials, and returned compost are accumulated on the floor and then repeatedly turned as needed with a shovel loader, etc. In the mechanical return method, an agitating device having a material loading slot and drainage slot for mixing the material is installed on the upper side wall of the fermentation tank. The aeration equipment maintains the material at a uniform aerobic state and provides ventilation to make the moisture evaporate from the material simultaneous with causing fermentation. The hydrolysis equipment supplies water to the material to ensure continued aerobic fermentation since microbial activity in the material ceases when the moisture in the material falls below 40%.

(c) Product forming process

The product forming process includes mechanical sorting and bagging/packing equipment for enhancing the product value and making the product compost easier to handle. Other facilities may include deodorizing equipment as part of environmental measures.