Assessing the potential of soil cyanobacteria for simultaneous wastewater treatment and carbohydrate-enriched biomass production

Abstract In this study, a mixed nitrogen-fixing soil cyanobacterial culture was tested to treat municipal wastewater and produce total carbohydrates in a one-stage operation. Four photobioreactors were operated in semi-continuous mode for 30 days, evaluating the effect of different hydraulic retention times and nutrients and carbon loads on nutrients and organic matter removal, biomass composition, and carbohydrate production. One photobioreactor operated at a hydraulic retention time of 10 d with diluted 2:1 wastewater with distilled water, and other photobioreactors worked at 10 d, 8 d, and 6 d of hydraulic retention times with undiluted sewage. The results evidenced that high hydraulic retention time and low nutrients load achieved the highest removal efficiencies in total nitrogen >95%, total phosphorus 35–78%, total organic carbon >93%, and total inorganic carbon >82%. These high removals led to nitrogen limitation that stimulated a continuous carbohydrate accumulation of up to 48%. Also, a biomass production of 0.05–0.07 mg L−1 d−1 dominated by easy-settling flocs of filamentous nitrogen-fixing cyanobacteria was achieved. Otherwise, lower hydraulic retention time and thus high nutrients load promoted carbon depletion, which led to lower nitrogen and phosphorus removals, low biomass production and contamination of green algae species. The results of this study highlight the nutritional and operational mechanisms of soil microorganisms and strategies to simultaneously clean waste streams and produce valuable by-products.