Abstract
Processing of the produced primary and secondary sludge during sewage treatment is demanding and requires considerable resources. Most common practices suggest the cotreatment of primary and secondary sludge starting with thickening and anaerobic digestion. The aim of this study is to investigate the anaerobic digestion of the primary sludge only and estimate its impact on sludge treatment and energy recovery. Within this context, the performance of the anaerobic digestion of primary sludge is explored and focused on practices to further enhance the methane production by using additives, e.g., a cationic polyelectrolyte and attapulgite. The results showed that the overall yield in methane production during anaerobic digestion of primary sludge alone was higher than that obtained by the anaerobic digestion of mixed primary and secondary sludge (up to 40%), while the addition of both organic polyelectrolyte and attapulgite enhanced further the production of methane (up to 170%). Attapulgite increased the hydrolysis rate of biosolids and produced relatively stabilized digestate, though of lower dewaterability. Moreover, the results suggest that single digestion of primary sludge may accomplish higher methane production capacities at lower digestors’ volume increasing their overall efficiency and productivity, while the produced digestates are of adequate quality for further utilization mainly in agricultural or energy sectors.
Highlights
Primary sludge from municipal wastewater treatment plants (MWWTPs) is usually codigested anaerobically with secondary sludge to produce biomethane and stabilized sludge that can further be utilized as a secondary material in various economy sectors in the context of circular economy
Higher methane production potential is indicated by the results of VFA analysis, expressed as mg/L of acetic acid, where primary sludge fraction demonstrates more than 1.5 times higher VFA concentration against PSS samples
The comparison of the hydrolysis rate between the two substrate to inoculum ratio (SIR) indicates that the k value decreased as the SIR increased, which agrees with the results reported by Filer et al [1]
Summary
Primary sludge from municipal wastewater treatment plants (MWWTPs) is usually codigested anaerobically with secondary sludge to produce biomethane and stabilized sludge that can further be utilized as a secondary material in various economy sectors in the context of circular economy. These two fractions of sewage sludge are of different quality. Primary sludge contains higher quantities of biodegradable organic material compared to secondary sludge, anaerobic digestion (AD) is expected to be more efficient with regards to energy capacity and overall methane production. Anaerobic digestion of primary sludge is expected to yield higher methane production compared to secondary sludges
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