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In-Vessel Composting A Technology Assessment

Introduction
Composting is a thermophilic, aerobic decomposition process whereby complex organic constituents of waste sludge are broken down microbially into simpler compounds. The composting reaction can be illustrated as follows (2):

Fresh Organic Waste + O2 —Microbal Metabolism— Stabilized Organic Residue + CO2 + H2O + Heat

Heat generated during the process reduces the number of pathogenic microorganisms in the sludge. The stabilized organic residue or end product of the composting process possesses physical and chemical properties which make it useful as a soil amendment in landscaping, reforestation, land reclamation and land development projects.

Until recently, composting in the U.S. has been carried out only through non-enclosed, windrow, or aerated static pile methods. Problems related to odor control and land area requirements, and the reduced efficiency of these methods during adverse weather conditions have prompted the use of in-vessel, or enclosed, composting (IVC) methods.

A schematic comparison of IVC and non-enclosed composting methods is presented in Figure 1 - Relative to non-enclosed methods, IVC provides enhanced odor control, reduced land area requirements, and better operations control during adverse weather. IVC also offers greater potential for recovery and subsequent reuse of the heat generated during the composting process.

Process Controls
Efficient composting requires waste with a solids content of 18-30%, a volatile solids content exceeding 50%, a pH of 6-9, and a carbon to nitrogen ratio (C:N) of 25-35:1. The moisture content and pH are often dependent on sludge processing prior to composting (1,3,4).

The C:N ratio and moisture content of sewage sludge are commonly altered through the addition of bulking agents or amendments prior to composting. Bulking agents provide a carbon source to increase the C:N ratio and also increase the solids content and subsequently the porosity of the composting mass. A sludge/bulking agent ratio to provide an infeed mixture in the range of 35-40% solids is desirable for optimal

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