Effects of hydraulic and solids retention times on productivity and settleability of microbial (microalgal-bacterial) biomass grown on primary treated wastewater as a biofuel feedstock.

High biomass productivity and efficient harvesting are currently recognized challenges in microbial biofuel applications. To produce naturally settleable biomass, combined growth of native microalgae and bacteria was facilitated in laboratory sequencing batch reactors (SBRs) using primary treated wastewater from the Christchurch Wastewater Treatment Plant (CWTP) in New Zealand. SBRs were operated under a simulated, local, summer climate (i.e., 925 μmol/m(2)/s of photosynthetically active radiation for 14.7 h per day at 21 °C mean water temperature) using 1.4- to 8-day hydraulic retention times (HRTs) to optimize growth. Solids retention times (SRTs) were varied from 4 to 40 days by discharging different ratios of supernatant and completely mixed culture. Biomass productivity up to 31 g/m(2)/day of solids was obtained, and it generally increased as retention times decreased. Biomass settleability was typically 70-95%, and the microbes aggregated into compact flocs as cultures aged up to four months. Due to a low lipid content of 10.5%, anaerobic digestion appeared to be the most appropriate biofuel conversion process with potential to generate 19,200 m(3)/ha/yr of methane based on settleable mixture productivity.