Low-retention operation to enhance biomass productivity in an algal membrane photobioreactor

A detailed and systematic study was conducted to investigate the role of sludge retention time (SRT) on biomass production in an algal membrane photobioreactor (MPBR) treating synthetic tertiary wastewater. At 10-350 days SRT, biomass accumulation was independent of SRT and steady state biomass concentration was stable at about 1.9 g/L. At lower SRTs of 2-10 days, biomass concentration in MPBR decreased proportionately, and reached a minimum of 0.54 g/L at 2 days SRT. On the other hand, biomass productivity increased monotonically from 5.9 mg/L-day at 350 days SRT to 318 mg/L-day at 2.5 days SRT, although these changes had lithe influence on nutrients removal in the MPBR. In contrast, biomass production in the MPBR was independent of hydraulic retention time (HRT), although nutrient removal decreased at lower HRTs. Changes in light intensity affected both biomass accumulation and nutrients removal. By operating the MPBR at high light intensity, 95% NO3--N and 73% PO43--P removal were obtained even at a shorter HRT of 0.5 days, while biomass concentration and productivity were 3.13 g/L an\d 313 mg/L-h, respectively. Statistical analysis indicated that the influence of SRT and light intensity on biomass productivity in the MPBR were comparable. These results indicate that SRT is not merely an operating parameter to control biomass removal rate, but it can serve as a tool to alleviate light limitation and enhance biomass production in MPBRs.