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

• Vermicomposting of pre-compost material admixture of activated sludge and corn stalk residue.
• Role of microbial inoculation on different substrates to improve vermicompost quality.
• The effect of active sewage sludge to biodeteriorate potential environmental hazards.

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.