Paddy straw–sewage sludge co-composting: a microbial solution for sustainable waste use

Abstract

This study investigated the co-composting of paddy straw and sewage sludge under pit house conditions over two consecutive years (2018–19 and 2019–20) to assess the quality of the resulting compost. The experiment was performed using six treatments: T1, paddy straw alone; T2, sewage sludge alone; T3, paddy straw + sewage sludge at a 1:1 ratio; T4, paddy straw + sewage sludge at a 1:1 ratio + microbial culture; T5, paddy straw + sewage sludge at a 1:2 ratio + microbial culture; and T6, paddy straw + sewage sludge at a 1:3 ratio + microbial culture. Key physicochemical parameters—including temperature; pH; electrical conductivity (EC); moisture content; and total carbon (C), nitrogen (N), phosphorus (P), and potassium—were monitored throughout the composting process. Additionally, phytotoxicity was evaluated using the germination index, and the heavy metal concentrations in the final compost were analyzed via atomic absorption spectroscopy (AAS). The results indicated that the treatment combining paddy straw with sewage sludge at a 1:1 ratio and using microbial consortia (T4) achieved the lowest carbon-to-nitrogen (C/N) ratios, recorded as 17.73 in 2018–19 and 15.02 in 2019–20 after 90 days. In contrast, paddy straw alone demonstrated higher C/N ratios, highlighting the positive impact of sewage sludge on the composting process. Throughout composting, the potassium and phosphorus levels ranged from 1.05% to 4% and 0.158% to 0.391%, respectively, while the heavy metal concentrations remained within the permissible limits. These findings suggest that incorporating sewage sludge significantly enhances the paddy-straw-based composting process, yielding high-quality compost.