Ashish Sahu

Abstract BACKGROUND: In the presence of light, micro‐algae convert CO 2 and nutrients to biomass that can be used as a biofuel. In closed photo‐bioreactors, however, light and CO 2 availability often limit algae production and can be difficult to control using traditional diffuser systems. In this research, a hollow fiber membrane photo‐bioreactor (HFMPB) was investigated to: (1) increase the interfacial contact area available for gas transfer, (2) treat high nutrient strength (412 mg NO 3 − ‐N L −1 ) wastewater, and (3) produce algal biomass that can be used as a biofuel. RESULTS: A bench scale HFMPB was inoculated with Spirulina platensis and operated with a 2‐15% CO 2 supply. A mass transfer model was developed and found to be a good tool to estimate CO 2 mass transfer coefficients at varying liquid velocities. Overall mass transfer coefficients were 1.8 × 10 −6 , 2.8 × 10 −6 , 5.6 × 10 −6 m s −1 at Reynolds numbers of 38, 63, and 138, respectively. A maximum CO 2 removal efficiency of 85% was observed at an inlet CO 2 concentration of 2% and a gas residence time (membrane‐lumen) of 8.6 s. The corresponding algal biomass concentrations and NO 3 removal efficiencies were 2131 mg L −1 and 68%, respectively. CONCLUSION: The results show that the combination of CO 2 sequestration, wastewater treatment and biofuel production in an HFMPB is a promising alternative for greenhouse gas mitigation. Copyright © 2010 Society of Chemical Industry

Proof-of-concept has been demonstrated for a process that will utilize nutrients from sludge liquor, natural light, and CO2 from biogas to grow microalgae at wastewater treatment plants. This process will reduce the impact of returning side-streams to the head of the plant. The produced algae will be fed to anaerobic digesters for increased biogas production. Dewatering of anaerobically digested sludge in centrifuges produces reject water with extremely low transmittance of light. A pretreatment procedure was developed that improved light transmittance for reject water from the FREVAR, Norway, wastewater treatment plant from 0.1% T to 77% T (670 nm, 1 cm path). Chlorella sp. microalgae were found to be suitable for growth in this pre-treated reject water. Typical nitrogen removal was 80-90 g N/kg TSS of produced microalgae. The microalgae were successfully harvested by chemically assisted flocculation followed by straining through a 33 microm sieve cloth, achieving up to 99% recovery. Harvested algae were anaerobically co-digested with wastewater sludge. The specific methane gas production (mL CH4/g VS fed) for the algae varied from less than 65% to 90% of the specific methane gas production for the wastewater sludge, depending on digester temperature, retention time and pre-treatment of the algae biomass.

Integration of algal biofuel production to wastewater anaerobic digestion infrastructure has the potential to increase biogas production, decrease high and variable internal nitrogen loads, and improve sludge digestibility and dewaterability. In this research, two species of microalgae, Spirulina platensis and Chlorella sp., were grown on sludge centrate and a centrate and nitrified wastewater effluent mixture. Harvested algae were co-digested with waste activated sludge (WAS) at varying ratios. High-growth (6.8 g m(-2) x d(-1)), nitrogen (36.5 g m(-3) x d(-1)), and phosphorus (6.5 g m(-3) x d(-1)) uptake rates were achieved with Chlorella on centrate. No growth was observed with S. platensis under the same conditions; however, both organisms grew well on the centrate and effluent mixture. Co-digestion of algae with WAS improved volatile solids reduction. Although co-digestion with S. platensis improved biosolids dewaterability, Chlorella had a slight negative effect on dewaterability compared to WAS alone. The efficiency of energy conversion from photons to biogas generated from Chlorella was estimated at 1.4%.