Enhanced Mesophilic Anaerobic Digestion of Primary Sewage Sludge

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.

Energy Balance of Turbocharged Engines Operating in a WWTP with Thermal Hydrolysis. Co-Digestion Provides the Full Plant Energy Demand

“…Equation ( 8) calculates the thermal efficiency (η thermal-eng ) based on the ratio between the useful thermal power of the engine (TP useful-eng ) and the thermal power contained in biogas and consumed by the engine (TP biogas-eng ). Equation ( 9) estimates the energy efficiency (η energy-eng ), and Equation (10) estimates the usable thermal energy factor of the engine (UTEF) considering the thermal power that has application in the WWTP, that is, the power used (TP usable-eng ) against the value of the thermal power susceptible of application. Equation (11) estimates the work/heat ratio of engine (W/Q) eng , as the ratio between the mechanical power of the engine (MP eng ) and TP useful-eng :…”

“…UTEF = TP usabel−eng × 100/ TP useful−eng (10) (W/Q) eng = MP eng × 100/ TP useful−eng (11) Equation ( 12) estimates the ratio of the stoichiometric mass of air (A stoich ) for biogas combustion considering molar fraction of methane in biogas (X CH4 ) and the molecular weight of biogas and air (MW biogas , MW air ). Equation ( 13) estimates the stoichiometric ratio (F stoich ) for biogas-air:…”

“…Several studies have focused on improving digestion performance of sewage sludge by the application of different techniques, through the addition of supplements [8][9][10], the application of pre-treatments [11][12][13], and co-digestion [14][15][16] as a way of enhancing biogas production and benefiting from the common utilization of plant equipment. The introduction of a pre-treatment unit in an existing digestion plant causes changes in the electrical and thermal demand because of the additional equipment installed.…”

Application of Biological and Chemical Processes to Wastewater Treatment

“…To tackle this problem and increase the sustainability of conventional WWTPs a movement towards zero and/or negative net energy facilities (i.e., energy produced during treatment is greater than the energy required for their operation) should be made. Clean energy as set out in SDG 7 can be produced during wastewater treatment by technologies such as anaerobic digestion [4] and microbial fuel cells [5].…”

“…Sakaveli et al [4] explored the anaerobic digestion of the primary, instead of the combined primary and secondary, sludge from a municipal WWTP and assessed its treatment and energy recovery potential. It was found that the overall yield in methane production during anaerobic digestion of primary sludge alone was higher (up to 40%) than that obtained by the anaerobic digestion of mixed primary and secondary sludge.…”