A compost production system was employed to supply uniform material for controlled experiments of factorial design. Over a 96-h composting period, the cumulative amount of CO2 evolved was maximal at 56 to 60°C, an aeration rate that left an 02 residual of 10 or 18% in the exhaust gas and a moisture content of 60% wet weight. Carbon dioxide evolution was submaximal at 64°C and higher.
The effect of incubation time on most-probable-number estimates of autotrophic nitrifying bacteria was investigated by using waters, rooted aquatic plants, sediments, and slimes as inoculum sources. Maximum most probable numbers of the NH,+-oxidizing group were attained in 20 to 55 days (median, 25). Estimates of NO2-oxidizers were highest at termination (103 to 113) days.
Rational composting process control involves the interrelated factors of heat output, temperature, ventilation, and water removal. The heat is released microbially at the expense of organic material; temperature is an effect and, because it is a determinant of microbial activity, it is also a cause of heat output; ventilation supplies oxygen and removes heat, mainly through the vaporization of water, water removal results from heat removal. These relationships were implemented in a field-scale process of static-pile configuration, using a mixture of sewage sludge and wood chips. Heat removal was matched to heat output through a temperature feedback control system, thereby maintaining biologically favorable temperatures. The observations indicate that fundamentally there are two kinds of composting systems: those that are and those that are not temperature selflimiting. The self-limiting system reaches inhibitive temperatures (>600C) which debilitate the microbial community, suppressing decomposition, heat output, and water removal. In contrast, non-self-limiting temperatures (<600C) support a robust community, promoting decomposition, heat output, and water removal.
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