2018 Volume 89 Issue 4 Pages 295-301
Sewage sludge is generated 2.26 million Mg annually in Japan, and its dealing is one of the social problems. This study aimed to promote the application of dried sewage sludge in farmlands for recycling purposes. We composted sludge and bamboo chips in the ratios of 5 : 5, 7 : 3, and 9 : 1. Characteristics of hot water extracts during composting were analyzed approximately every week until the end of composting process (41 days) by testing inhibition of germination of Japanese mustard spinach (Brassica rapa var. perviridis) and by chemical analyses such as those using UV-vis spectroscopy and those involving dissolved organic carbon measurement. The concentration of ammonia, which is one of the strongest odor sources, decreased by about 60％ in the first 2 weeks of composting. The pH, electrical conductivity, and E2/E3 tended to decrease, whereas specific UV absorbance (SUVA254) tended to increase during composting, suggesting the progression of humification and maturation of composts. Although the 7 : 3 and 9 : 1 compost extracts demonstrated considerable germination inhibition in the early stages of composting, the inhibition was alleviated as maturation progressed to 28 days. Inhibition was not observed in the 5 : 5 compost extract, and heavy metals did not exceed the safety standard, so the compost did not exhibit harmful effects on crop growth or safety. We also performed three replicates of a field growth experiment using Tanbaguro black soybean (Glycine max) with and without addition of 50-day-old compost (5 : 5, 7 : 3, and 9 : 1). Results showed that addition of the composts significantly increased the main stem length and the number of pods (p<0.01). Soybean yield was the highest in the 9 : 1 compost addition group, although this difference was not significant. These results suggest that composting of dried sewage sludge and bamboo chips is effective in agricultural production as it recycles local resources, reduces odor, and does not inhibit germination.
