Assessment of the bioenergy and bioremediation potentials of the microalga Scenedesmus sp. AMDD cultivated in municipal wastewater effluent in batch and continuous mode

Municipal wastewater is a major source of nutrients and pollutants to freshwater and marine ecosystems and current treatment technologies are either expensive or only partially effective at removing them. We cultivated the alga Scenedesmus sp. AMDD at different pH in chemostats and batch photobioreactors in wastewater obtained from a local treatment plant. In batch mode, biomass productivities averaged 130 mg dry weight L− 1 d− 1, with no significant effect of growth pH detected. Maximum nitrogen and phosphorus removal rates were equivalent to 7% and 0.7% of the biomass productivity rates. Average hydraulic retention times for 90% N and P removal ranged from 6.55 to 6.65 days and 6.50-6.56 days, respectively. Recovered biomass yields ranged from 0.23 to 0.65 kg m− 3 wastewater, equivalent to approximately 5-15 MJ bioenergy m− 3 wastewater based on an average calorific value of 23 MJ kg− 1 d.w. algal biomass. Approximately 65% of energy equivalent could potentially be recovered from the biomass through anaerobic digestion to methane. Cellular N and P content varied, with cells held longer in stationary phase showing higher C:N and C:P ratios indicative of N- and P-limitation, respectively. Analysis of trace metals in the algal biomass indicated near total depletion of Fe, Zn and Cd from the wastewater, and lower, but substantial, uptake and/or adsorption of seven other elements. Cultivation in 2 L continuous chemostats containing wastewater was also conducted. Biomass productivities in chemostats were almost 2-fold greater than the maximum rates in batch cultures. Dissolved N and P in chemostats were both either undetectable or > 99% reduced compared to the wastewater. Production of bioenergy from the chemostats was estimated to be roughly 5.3-6.1 MJ m− 3 d− 1, significantly higher than in batch culture.