High robustness of attached Chlorella sp. on semi-continuous low strength effluent polishing under axenic and xenic conditions

Chlorella is widely applied in various wastewater streams of different strengths. Utilization of attached Chlorella biofilm for satisfactory high strength wastewater treatment have been frequently reported, yet the performance of Chlorella biofilm for low strength effluent polishing is still ambiguous. This study first explored the preference of relatively low concentrations of nitrogen sources by attached Chlorella sp. under batch cultivation, then compared the growth and nutrients removal of the biofilm under axenic and xenic conditions in semi-continuous mode, i.e. running the attached system at hydraulic retention time of 2 days and biomass harvest interval of 4 days, treating synthetic effluent with 15 mg/L NO₃⁻-N as the sole nitrogen pollutant representing the nitrogen profile of municipal secondary effluent. Results showed that NO₃⁻-N and NH₄⁺-N were preferred over others under batch cultivation, with up to 99.2 % N removed from the synthetic effluent. EPS content and particularly EPS polysaccharides content were highly significantly correlated to Chlorella sp. biofilm growth, with EPS polysaccharides to protein ratio being >1.8 when fed with NO₃⁻-N in batch culture. Throughout the 48-day semi-continuous operation, because of the high robustness of attached Chlorella sp. biofilm, the biomass harvest rate, EPS content, biomass composition, and nutrients removal were similar under axenic and xenic conditions, yielding 1.2–1.6 g/m²/d attached biomass, >73 % nitrogen removal and > 90 % phosphorus removal. A new finding of three-fold extracellular protein content increase was observed under semi-continuous operation comparing with batch culture, leading to mostly <1.0 EPS polysaccharides to protein ratios, while intracellular lipid content significantly reduced (from 20.8 to 23.8 % to 13.6–17.0 %). Overall, results of this study suggested that biofilm of the ubiquitous Chlorella sp. biofilm could be applied not only for nutrients rich wastewater treatment, but also for advanced nutrients removal from low strength effluent under xenic conditions.