Effect of temperature on bacterial species diversity in thermophilic solid-waste composting

Continuously thermophilic composting was examined with a 4.5-liter reactor placed in an incubator maintained at representative temperatures. Feed was a mixture of dried table scraps and shredded newspaper wetted to 55% moisture. One run at 49°C (run A) employed a 1:4 feed-to-compost ratio, while the other runs used a 10:1 ratio and were incubated at 50, 55, 60, or 65°C. Due to self-heating, internal temperatures of the composting mass were 0 to 7° C hotter than the incubator. Two full-scale composting plants (at Altoona, Pa., and Leicester, England) were also examined. Plate counts per gram (dry weight) on Trypticase soy broth (BBL Microbiology Systems) with 2% agar ranged from 0.7 x 10⁹ to 5.3 x 10⁹ for laboratory composting and 0.02 x 10⁹ to 7.4 x 10⁹ for field composting. Fifteen taxa were isolated, including 10 of genus Bacillus, which dominated all samples except that from run A. Species diversity decreased markedly in laboratory composting at 60°C and above, but was similar for the three runs incubated at 49, 50, and 55°C. The maximum desirable composting temperature based on species diversity is thus 60°C, the same as that previously recommended based on measures of the rate of decomposition.