Photosynthetic upgrading of biogas from anaerobic digestion of mixed sludge in an outdoors algal-bacterial photobioreactor at pilot scale

Abstract

Anaerobic digestion can biotransform the biodegradable fraction of sewage sludge into biogas, while the symbiotic action of algal-bacterial consortia can remove both the CO2 and H2S from biogas and nutrients from digestate. A 100 L anaerobic digester operated at 20 days of retention time coupled with a 180 L high-rate algal pond (HRAP) engineered to upgrade the biogas and treat the liquid fraction of the pilot digester was optimized along four operational stages: (I) operation with a greenhouse during winter; (II) operation without greenhouse; (III) process supplementation with NaHCO3; (IV) process supplementation with Na2CO3. The biogas produced was composed of 63.7 ± 2.9% CH4, 33.7 ± 1.9% CO2, 0.5 ± 0.3% O2 and 1.6 ± 1.1% N2. An average methane productivity of 324.7 ± 75.8 mL CH4 g VSin−1 and total COD removals of 48 ± 20% were recorded in the digester. The CH4 concentration in the biomethane gradually decreased to 87.6 ± 2.0% and 85.1 ± 1.3% at the end of stage I and II, respectively, attributed to the loss of inorganic carbon in the HRAP. The supplementation of NaHCO3 and Na2CO3 mediated an increase in the CH4 content to 90.4 ± 1.5 and 91.2 ± 0.7% in stages III and IV, respectively. Steady state CO2 removals of 90% and 88% in stages I and II, and 95.7 and 97.6% in stages III and IV, respectively, were recorded. A constant biomass productivity of 22 g m−2 d−1, set by daily harvesting 26.5 g dry algal-bacterial biomass from the bottom of the settler, was maintained concomitantly with a complete removal of the N and P supplied via centrate.