Processing of Biowaste by Anaerobic Composting to Plant Growth Substrates
Kitchen derived biowaste was fermented in a 40 1 digester at 55°C with a loading of 5.2 g VS/l.d and an average residence time of 42 days. This resulted in an average biogas production rate of 3.6 1/l.d. Different types of paper were investigated for their anaerobic biodegradability; the conversion on VS-basis ranged from 17 % for a regular newsprint to 100 % for a bleached cellulose. A reactor, fed with biowaste supplemented with paper, was operated at 55°C and a loading rate of 12.6 g VS/l.d; this corresponded with an average residence time of 20 days and an average biogas production rate of 7.14 1/l.d. The addition of paper lowered the NH4+−N level from 3.5 g/kg wet material to 1.65 g/kg wet material. Compared to MSW-digest, the biowaste-digest was found to be a NPK rich organic fertilizer with low levels of heavy metals. The moist digest was supplemented with 40 % peat(w/w) to attain 25 % dry matter and subsequently allowed to compost aerobically during 7 days at ambient temperature to a mature endproduct. Horticultural plants were grown on various endproduct/peat mixtures. The latter exhibited no phytotoxicity and performed in some cases better than optimal reference substrates. The blending of the wet digest with peat thus allows to obtain in a very short process time an excellent plant substrate. Furthermore, peat also demonstrated its quality as odour absorbant. It instantaneously reduced the odour units of digested biowaste by a factor of 50. These tests indicate that biowastes can be elegantly transformed into plant growth substrates of high quality by supplementing them with paper waste in the anaerobic digestion phase and by peat in the subsequent aerobic maturation phase.