Anaerobic digestion restricted to phase I for nutrient release and energy production using waste-water grown Chlorella vulgaris

Abstract

Anaerobic digestion (AD) can be used to recover nutrients from waste streams by isolating its phases. Here, we present a phase-restricted AD system (phase I only) for Phosphorus (P) release, production of Volatile Fatty Acids (VFA), ammonia (as NH4+) and biogas by digesting Chlorella vulgaris grown on agricultural waste-water using sewage sludge as inoculum. In our phase I AD system, we observed that changing the digester hydraulic retention time (HRT) altered the P, NH4+ and VFA concentrations in the effluent and biogas production. The reactor operated at 10 days HRT released maximum inorganic P (orthophosphate) in the effluent with a value of 199 ± 1 mg/L, the highest amongst considered HRTs. Maximum N in the phase-restricted AD system was observed at 14 days HRT with a final concentration of 118 ± 1 mg NH4+/L. However, the value was exceeded by a single-stage two-phase positive control batch reactor with a value of 350 ± 0.2 mg NH4+/L. The Gas Chromatography (GC) analysis indicated that VFA were present in all phase I AD reactors, with the 10 days HRT digester featuring the highest concentration at 233 mg/L. Furthermore, the phase-restricted AD approach had a positive impact on biogas generation. The 14 days HRT digester exhibited the greatest biogas production at 522 ± 1 mL in the digester’s headspace, followed by the 10 days HRT. The results of the study have established that anaerobic digesters operated at phase I with microalgae could be a promising approach for harvesting nutrients and generating biofuels in a green bioremediation system.