Wastewater polishing by consortia of Chlorella vulgaris and activated sludge native bacteria

The increase of human population and anthropogenic activities has resulted in an excessive discharge of wastes into water bodies, contributing to nutrients enrichment and consequent degradation of aquatic environments and freshwater resources. The need for cost-effective systems able to reduce nutrients concentration in wastewaters is therefore a major concern. Microalgal-bacterial consortia can be effectively applied in wastewater polishing, since co-cultivation of these microorganisms can promote mutual growth and nutrients removal from wastewaters. In this study, dual-species cultures of the microalga Chlorella vulgaris and a bacterium isolated from a municipal wastewater treatment plant (Enterobacter asburiae, Klebsiella sp. or Raoultella ornithinolytica) were performed. Studying these consortia aimed the evaluation of their potential in nutrients (nitrogen, phosphorus and chemical oxygen demand) removal from a synthetic medium that mimics a secondary-treated effluent (nitrogen, phosphorus and chemical oxygen demand loads of about 45 mg N L−1, 10 mg P L−1 and 300 mg O2 L−1, respectively). The results have shown that the consortia containing E. asburiae and R. ornithinolytica have significantly improved C. vulgaris growth: when in single cultures maximum cell concentration achieved by C. vulgaris was (8.27 ± 0.79) × 106 cells per mL, whereas in these consortia maximum cell concentrations achieved by C. vulgaris were (19.8 ± 2.0) × 106 and (14.4 ± 2.3) × 106 cells per mL, respectively. The three studied consortia have also contributed to higher nutrients removal, since the time required to achieve the limits in discharged effluents (established by European Union legislation) was reduced to at least half of the value determined for the single C. vulgaris culture. Due to the effectiveness demonstrated by the studied consortia, these microalgal-bacterial systems should be considered as a viable approach for application in the tertiary treatment step of wastewater treatment processes.