Abstract
Food industry sludge has low C/N for composting. In this investigation, composting tests of food industry sludge were attempted without addition of any organic materials for bulking agents. The relationship between chemical properties and plant growth inhibitory actions during the composting process was investigated in order to find out the proper criterion for the maturity of the compost. The sludge mixed with foam silicate particles (25% w/w) and solidifying agent ( 5 % w/w) was composted in a laboratory-scale reactor (36 L) under aeration conditions for 50 d. Plant growth inhibitory action of the compost was estimated by the komatsuna (Brassica campestris var. rapifera group) growth test. Ignition loss and concentration of ammonia gas in the exhaust from the reactor decreased with the lapse of composting. C/N of the compost increased after a temporary decrease. The NH_4^+ concentration in the water extract of the compost increased at early stage of composting while NO_3^- was not detected at all. Fatty acids with low molecular weight were detected only at the early stage of composting. The 〓logK of alkaline extract of the compost increased gradually. Chemical properties of the compost became stable after about 20 to 30 d of composting and the plant growth inhibitory action disappeared earlier than that. The main inhibitory factor seemed to be ammonia gas originating from compost. The change patterns of ignition loss, ammonia gas and 〓logK during composting process could be represented as simple functional curves and they were closely correlated with the plant inhibitory action. Therefore, they could be useful indexes for the maturity criteria of compost from food industry sludge.
