Microbial biodegradation of waste materials for nutrients enrichment and heavy metals removal: An integrated composting-vermicomposting process

Abstract

The aim of present study was to improve the quality of vermicompost through different substrates and adding active sewage sludge as a source of N-fixing and P-solublizing bacteria in a shorter time than conventional composting process. The experiment setup included 15-L reactors used for pre-composting, a vermicomposting mixture of activated sewage sludge (control, 2000, 4000 and 6000 mg L−1) and corn stalk residue (40, 60 and 80%). The physico–chemical changes in vermicompost caused by the microbial biodegradation and their combinations were measured over a period of 70 days. The results showed that the values of total organic carbon (TOC), total volatile solid (TVS), total Kjeldahl nitrogen (TKN) and carbon to nitrogen ratio (C/N) decreased in all treatments, while those of electrical conductivity (EC), total phosphorous (TP), nitrate and heavy metals increased. A minimum C/N ratio of 13.16% obtained in the 40% corn stalk waste substrate with 4000 mg L−1 activated sludge treatment while it was 23.44% in the 80% corn stalk waste substrate without activated sludge treatment. Results indicated that with the increase in 6000 mg L−1 sewage sludge and with the decrease of 40% corn waste substrate lead a decrease in TKN and an increase in nitrate, viz. 1.36–2.06% and n.d.−1889 mg kg−1 respectively. However, in comparison to decrease in TKN nitrogen, decrease in TOC (39.94–27.32%), TVS (63.48–43.48%), C/N ratio (63.48–13.43) and pH (7.33–3.15) and increase in EC (1.55–3.15 mS cm−1) and TP (2.395–3.31 g kg−1) was obtained. The decrease of heavy metals in the final vermicompost materials was detected by noting a low heavy metals concentration in the corn residue.