Microalgal photobioreactors for carbon-efficient wastewater treatment

Abstract

Algae-based wastewater treatment technologies are gaining popularity because of their sustainable treatment capabilities, coupled with their ability to capture carbon and consequently reduce the carbon footprint of the overall treatment process. Research was undertaken to develop a low-cost hybrid mixed microalgae-activated sludge municipal wastewater treatment system coupled with CO2 sequestration. Red light-emitting diodes were used as light source to illuminate 1 L and 21 L microalgal photobioreactors. Three phases of laboratory experiments (I, II and III) were conducted to treat real or synthetic municipal wastewater using batch and continuous modes of operation, either with or without CO2 addition. Phases I and II experiments were conducted in batch mode using a mixed microalgae-bacteria culture as inoculum, while Phase III was conducted in continuous mode using a mixture of microalgae and activated sludge as inoculum. The added gas in Phases I and II had O2 supplementation whereas the gas in Phase III had no O2 but a substantial amount of CO2. Average ‘optimal’ irradiance (582.7 μmol.s-1.m-2) was used in Phases II and III, while Phase I investigated a range of lower light regimes (i.e. 25.3 to 234.3 μmol.s-1.m-2). Results showed high wastewater treatment efficiency, in terms of soluble chemical oxygen demand (SCOD) and ammonium-nitrogen (NH4-N) removal. SCOD removal efficiency greater than 70% was achieved in all the three experimental phases. Furthermore, NH4-N removal efficiencies greater than 90, 70 and 40%, were achieved in Phases I, II and III, respectively. However, nitrite accumulation was observed in Phases I and II, indicating that NH4-N removal was due to partial nitrification. Furthermore, low phosphate removal efficiencies were achieved in Phase III. Results confirmed that considerable reduction of operational costs could be achieved by satisfying bacterial oxygen requirement through photosynthetic oxygenation in the hybrid microalgae-activated sludge (HMAS) photobioreactors, with considerable energy savings possible whilst maintaining high levels of SCOD removal. Typically, a dissolved oxygen concentration > 2 mg.L-1 could be maintained in the HMAS photobioreactors without external aeration. Microbial analyses of samples collected from Phase II and III photobioreactors revealed a dominance of bacteria over microalgae. In order to prevent system failure, it was recommended that HMAS photobioreactors be set-up with an initial microalgae-bacteria ratio of at least 90:10, as determined by flow cytometry. Overall, this study demonstrated the potential for achieving high treatment efficiency by coupling wastewater treatment with carbon capture in HMAS photobioreactors. The potential for realising cost savings in wastewater treatment through use of HMAS photobioreactors at full-scale are discussed.