Nitrogen transformations during co-composting of herbal residues, spent mushrooms, and sludge

Sewage sludge composting is an important environmental measure. The reduction of nitrogen loss is a critical aim of compost maturation, and the addition of spent mushrooms (SMs) and herbal residues (HRs) may be helpful. To evaluate the nitrogen transformations during co-composting of sewage sludge, SMs, and HRs, windrows were constructed in a residual processing plant. Dewatered sewage sludge and sawdust were mixed with SMs and HRs at two proportions on a fresh weight basis, 3:1:1 (sewage sludge:sawdust:SMs or HRs) and 3:1:2 (sewage sludge:sawdust:SMs or HRs). The mixture was then composted for 40 d. Changes in the physicochemical characteristic of sewage sludge during composting were recorded and analyzed. Addition of SMs and HRs accelerated the temperature rise, mediating a quicker composting maturation time compared to control. The addition also resulted in lower nitrogen losses and higher nitrate nitrogen levels in the compost products. Among the windrows, SM and HR addition improved the nitrogen status. The total nitrogen (TN) and nitrogen losses for SM and HR treatments ranged from 22.45 to 24.99 g/kg and from 10.2% to 22.4% over the control values (18.66-21.57 g/kg and 40.5%-64.2%, respectively). The pile with the highest proportion of SMs (3:1:2 (sewage sludge:sawdust:SMs)) had the highest TN level and the lowest nitrogen loss. The germination index (GI) values for all samples at maturity were above 80%, demonstrating optimal maturity. The addition of SMs and HRs augments sewage composting.