Effect of Ca(OH) dosing on thermophilic composting of anaerobic sludge to improve the NH recovery.

The primary biological treatment method for organic sludge is composting and/or anaerobic digestion, but their product (compost or biogas) is of little economic benefit; therefore, an improved process to produce a high-value product is required to make sludge management more sustainable. Maximizing NH gas recovery during composting processes has the potential benefit of producing high-value microalgal biomass. However, the majority of produced ammonia does not evaporate as NH gas but retains as NH-N in the compost after fermentation. The present study investigates the effects of the timing of Ca(OH) dosing (on days 2, 5, and 9), and the Ca(OH) dose (1.1-2.6 mmol/batch), on lab-scale thermophilic composting of anaerobic sludge. The effects on NH recovery, organic matter degradability, and microbial activity are evaluated. Ca(OH) dosing immediately improved the emission of NH, with yields 50-69% higher than those under control conditions. The timing of the dosing did not influence NH recovery or organic matter degradability. Higher Ca(OH) doses resulted in higher NH recovery, while microbial activity was temporarily and marginally inhibited. The pH of the compost reached 10-11.5 but quickly dropped to 8-8.5 within a day, probably because of neutralization of Ca(OH) by the emitted CO and release of NH, which maintained the microbial activity. The present study indicated that Ca(OH) dosing would be useful to apply during thermophilic composting for NH recovery to cultivate high-value microalgal biomass, which enables this process to obtain a more economic benefit.