Growth and nutrients removal characteristics of attached Chlorella sp. using synthetic municipal secondary effluent with varied hydraulic retention times and biomass harvest intervals

Advanced nutrients removal from low strength municipal secondary effluent is of urgent need, while microalgae-based approaches can achieve satisfactory effluent polishing without extra chemicals and energy consumption, and the obtained biomass could be processed into value-added products. Conventional suspended systems are either faced with algae-water separation issue, or with the requirement of membrane modules and low scale-up probability. The widely applied attached systems for water remediation such as Algal Turf Scrubbers can resolve the algae-water separation issue, but the biofilm is often dominated by filamentous microalgae, which are of high ash content and low commercial value. Attached systems dominated by selected microalgal strains, however, are mostly fed with high strength wastewater, and the growth and nutrients removal characteristics of these microalgae using low strength effluent is so far unclear. This study explored the potential of using attached Chlorella sp. for synthetic municipal secondary effluent polishing and biomass production, and investigated the effects of hydraulic retention time (HRT) and biomass harvest interval on microalgal growth, extracellular polymeric substances (EPS) content, harvested biomass composition, and nutrients removal. When only HRT was concerned, highest microalgal growth rate (0.77 g/m(2)/d) and EPS content (80.8 mg/g dry cell weight, DCW) were found at HRT = 2 d, meanwhile total nitrogen and total phosphorus were respectively reduced to 1.2 mg/L and 0.1 mg/L. Attached microalgal growth increased to more than 1.2 g/m2/d at 4 d harvest interval, generating biomass with 27.9-35.2% protein, 20.6-23.4% polysaccharides, and 21.6-26.4% lipid. The biomass EPS content was significantly positively correlated to biomass productivity, reaching highest level (156.8 mg/g DCW) at 4 d harvest interval. Nutrients removal performances, however, were inferior to those without biomass harvest. The mismatch between attached microalgal growth and nutrients removal highlighted the need to balance biomass production and effluent polishing performances when using low strength effluent as culturing medium.