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

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% O-2 and 1.6 +/- 1.1% N-2. 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.

Automated summary

Anaerobic digestion coupled with algal-bacterial consortia can transform sewage sludge into biogas and remove CO2, H2S, and nutrients. The optimization of operational stages improves methane productivity and COD removal, and the addition of alkaline substances enhances methane content and CO2 removal.