Nutrient removal in an algal membrane photobioreactor: effects of wastewater composition and light/dark cycle.

Graesiella emersonii was cultivated in an osmotic membrane photobioreactor (OMPBR) for nutrients removal from synthetic wastewater in continuous mode. At 1.5 days of hydraulic retention time and under continuous illumination, the microalgae removed nitrogen (N) completely at influent NH4+-N concentrations of 4-16 mg/L, with removal rates of 3.03-12.1 mg/L-day. Phosphorus (P) removal in the OMPBR was through biological assimilation as well as membrane rejection, but PO43--P assimilation by microalgae could be improved at higher NH4+-N concentrations. Microalgae biomass composition was affected by N/P ratio in wastewater, and a higher N/P ratio resulted in higher P accumulation in the biomass. The OMPBR accumulated about 0.35 g/L biomass after 12 days of operation under continuous illumination. However, OMPBR operation under 12 h light/12 h dark cycle lowered biomass productivity by 60%, which resulted in 20% decrease in NH4+-N removal and nearly threefold increase in PO43--P accumulation in the OMPBR. Prolonged dark phase also affected carbohydrate accumulation in biomass, although its effects on lipid and protein accumulation were negligible. The microalgae also exhibited high tendency to aggregate and settle, which could be attributed to reduction in cell surface charge and enrichment of soluble algal products in the OMPBR. Due to a relatively shorter operating period, membrane biofouling and salt accumulation did not influence the permeate flux significantly. These results improve the understanding of the effects of N/P ratio and light/dark cycle on biomass accumulation and nutrients removal in the OMPBR.

Tertiary wastewater treatment in membrane photobioreactor using microalgae: Comparison of forward osmosis & microfiltration.

Discharge of wastewater with high nitrogen and phosphorus content is a major cause of eutrophication. In this study, a microfiltration-based membrane photobioreactor (MPBR) and forward osmosis-based osmotic membrane photobioreactor (OMPBR) have been operated with Chlorella vulgaris for continuous tertiary wastewater treatment. Both the bioreactors exhibited good biomass accumulation (over 2g/L), although the OMPBR achieved better nutrients removal due to high rejection properties of the membranes. At 2days HRT, the OMPBR achieved nitrogen and phosphorus removal efficiencies of 86-99% and 100%, respectively, whereas the corresponding values in the MPBR were 48-97% and 46%, respectively. Based on the energy input, the total operating costs for OMPBR were 32-45% higher than that of the MPBR, and filtration cost for OMPBR was 3.5-4.5 folds higher than that of the MPBR. These results indicate that the integration of membrane filtration with photobioreactors is promising in microalgae-based tertiary wastewater treatment.

Nitrogen and phosphorus removal from tertiary wastewater in an osmotic membrane photobioreactor.

An osmotic membrane photobioreactor (OMPBR) was designed and operated for 162days for nitrogen and phosphorus removal from wastewater using Chlorella vulgaris. The removal efficiency for NH4(+)-N, NO3(-)-N and PO4(3-)-P reached as high as 95%, 53% and 89%, whereas the maximum removal rates were 3.41 mg/L-day, 0.20 mg/L-day and 0.8 mg/L-day, respectively. The microalgae exhibited high tendency to aggregate and attached to the bioreactor and membrane surfaces, and total biomass accumulation in the OMPBR was over 5 g/L. Salt accumulation and biofouling had adverse effects on membrane filtration, but the performance could be recovered through periodic backwashing of the membranes. Extracellular polymeric substances characterization indicated higher fraction of polysaccharides as compared to proteins. The biomass in the OMPBR accumulated higher levels of carbohydrates and chlorophyll. These results indicate the suitability of OMPBR in wastewater treatment and in high-density microalgae cultivation.