Use of freshwater macroalgae Spirogyra sp. for the treatment of municipal wastewaters and biomass production for biofuel applications

Highlights: • The cultivation of a naturally collected freshwatermacroalgae, Spirogyra sp., has been investigated. • Two different reactor types were used, cylindrical columns and rectangular flat-plate aquariums. • A low mixing rate, as controlled by the air flow rate, resulted in the most productive growth. • Spirogyra sp. demonstrated potential for the treatment of municipal wastewater. • Biochemical components were suitable for many applications. - Abstract: Freshwater macroalgae has competitive advantages compared to microalgae and marine macroalgae, such as lower separation and drying cost requirements and an abundance of available freshwater media. Municipal wastewater containing large quantities of nutrients (particularly nitrogen and phosphorus) is a valuable and underutilized resource. In this study, the cultivation of the naturally isolated filamentous freshwater macroalgae Spirogyra sp. was investigated in three different types of municipal wastewater including primary (PW), secondary (SW) and centrate (CW) wastewaters. Two different types of reactors including closed column photobioreactors and open rectangular aquarium reactors were operated under no and low air flow rates of less than (18 ± 2) cm⁻³·min⁻¹, respectively. The SW, PW diluted with water to a 20 % volume fraction and CW diluted with water to a 2 % volume fraction appeared to promote ash-free biomass productivities of (2.17-6.68) g·m⁻²·d⁻¹ and specific growth rates of (16.4–29.7) %·d⁻¹. Nitrogen and phosphorus removal efficiencies ranged from (50.6–90.6) % and (60.4–99.1) %, respectively. Based on ultimate analysis, the biomass produced a higher heating value of (12.4–17.1) MJ·kg⁻¹, and also showed relatively consistent protein ((16.7–19.5) % of the dry mass fraction), carbohydrate ((41.5–55.0) %) and lipid ((2.8–10.0) %) contents. These results indicate the feasibility of using Spirogyra sp. to recover nutrients from multiple municipal wastewater sources with the simultaneous production of biomass that contains value-added biochemical components for bioenergy and biofuel applications.