Biomethane production from algae biomass cultivated in wastewater

To recover the internal energy potential of wastewater (1.6–2.3 kWh/m3), this chapter explores algae growth and digestion as alternative to conventional wastewater treatment (WWT). Raw WW is treated in High Rate Algae Ponds (HRAPs), either directly or after anaerobic pretreatment (UASB), comparing biomethane yield of the biomass to energy needed in mixing and separation, depending on cultivation and digestion parameters. As oxygen is provided by algae, HRAPs replace activated sludge (AS, including nutrient removal to meet the EU directive 91/271/CEE and tertiary standards) and avoid aeration (0.3–2.1 kWh/m3), needing only mixing (<0.1 kWh/m3) and harvesting using DAF (<0.1 kWh/m3) while including partial solar disinfection at 3 d HRT. While HRAP provides full biological treatment, DAF also thickens the algae biomass to >40 g/L for direct anaerobic digestion (AD), which was evaluated under different conditions (psychrophilic, mesophilic, and thermophilic) to reach energy self-sufficiency. First, HRAP reactors of 30 m2 were fed with UASB effluent and operated at 7 d HRT to yield algae growth around 10 g TS/m2/d in winter and 20 g TS/m2/d in summer. The biomass grown was digested in thermophilic anaerobic pilot reactors (V = 4.5 L at 14 d HRT), reaching methane yields of 0.15 m3 CH4/kg VSS/d. In a second phase, two larger HRAPs of 500 m2 were operated at 3 d HRT directly on raw screened WW, and the richer biomass (including primary solids) reached methane yields between 0.18 m3 CH4/kgVS/d for ambient temperatures (25°C and 30 d HRT) and 0.28 m3 CH4/kgVS/d for thermophilic AD (55°C and 20 d HRT). Energy and mass balances for a WWT of 25,000 PE (5000 m3/d) to meet EU limits show that, comparing algae to an optimized AS system consuming 0.26 kWh el/m3, HRAP + DAF + thermophilic digestion would be a net energy producer, yielding biomethane with a thermal energy of 0.75 kWh th/m3 while needing around 0.15 kWh el/m3 for pumping, mixing, and DAF.