Composting of Urban Sewage Sludge and Its Application in Quarry Soil Reclamation: A Field Case Study

Mining activities often result in ecosystem degradation and landscape destruction. The restoration of abandoned mine lands is particularly challenging due to the poor physicochemical properties and low fertility of the soil, which necessitate the use of soil amendments. Sewage sludge, which contains abundant nutrients, has potential for use in mine soil restoration. Four separate piles of sewage sludge, each weighing 5 tons, were subjected to aerobic composting and then applied at different rates (0%, 2%, 5%, 10%, and 20%) to reclaim an abandoned mine land site (500 m2). During the composting process, the pH, moisture content, organic matter (OM), and dissolved organic matter (DOM) of the sewage sludge all decreased, while electrical conductivity (EC) and germination index (GI) increased. The sewage sludge compost reached maturity after 83 days. Soil pH and bulk density decreased with increasing application levels, whereas soil fertility, such as OM, alkali-hydrolyzable nitrogen, available phosphorus (AP), and available potassium (AK), significantly improved after application of sewage sludge compost. Vegetation coverage (ryegrass and alfalfa) reached 100% after 2 months at the 20% application level. Fresh biomass and plant height were significantly higher at all application levels compared to the control (p < 0.05). Results from Pearson’s correlation, redundancy analysis (RDA), and the random forest model indicated that soil fertility, particularly AP, OM, and alkali-hydrolyzable nitrogen, rather than soil physicochemical properties, was the key factor influencing the restoration success of the mine site. The use of sewage sludge compost as a soil amendment for reclaiming abandoned mine lands is feasible and can help reduce the ecological restoration costs of mining.