Accelerated Humification in Wastewater Sludge via Addition of Iron – Manganese Slag and the Prospect for Land Utilization of Composting Products

Abstract

Iron–manganese solid waste was used to enhance the efficacy of sludge composting, and the humification process and physicochemical properties of the compost were systematically investigated. Iron–manganese slag was added to the composting reactor, the temperature was increased to 69.5°C, and the thermophilic phase was extended to 110 h. Meanwhile, the composting products maintained relatively higher dissolved organic carbon, NH₄⁺-N, protein, and polysaccharide. After composting for 15 days, the humic and fulvic acid ratios in the system with the addition of iron–manganese smelting slag (named as R2) were at least 25% higher than those in the control group, the seed germination index increased to 108%, and the nutrient composition of the composting product reached 8.2%, which was higher than the 6.8% in the raw sludge. Except for arsenic, other heavy metal contents in the R2 system were lower than the Class A limits stipulated by the Chinese Agricultural Sludge Pollutant Control Standard. When iron–manganese slag was added to the sludge piles, microbial metabolism facilitated the release of mineral elements, which enhanced the activities of enzymes such as polyphenol oxidase and alkaline protease, and abundant organic matter was released due to cell lysis and extracellular polymeric substance disruption. Some of the substrates were converted into small-molecule products by microbial actions; meanwhile, precursors such as polysaccharides and proteins were maintained at high levels in the sludge compost, facilitating the formation of humic substances.