Increased harvest frequency improves biomass yields and nutrient removal on a filamentous algae nutrient scrubber

Abstract

Filamentous algae nutrient scrubbers (FANS) are artificial flow-ways, used for the treatment of a range of aquatic pollutants, and their performance is dependent on the rate at which the algae can both assimilate nutrients from the water and produce biomass. As the filamentous algae mat develops and thickens with age, algae in the lower layers becomes both light and nutrient limited, negatively impacting on performance. Periodic harvesting therefore plays a central role in the performance of FANS. This paper investigates how the frequency of harvesting affects filamentous algae performance treating anaerobically digested food-waste centrate, and if strip harvesting (the removal of half the algal biomass on each sampling occasion) improves performance. The daily production rates for both ash free dry mass and chlorophyll-a, decreased with increasing time between harvesting events. For all three harvest treatments, the daily production rate peaked at Day-2 and significantly declined (p < 0.01) over the rest of the growth cycle. Nitrogen and phosphorus removal rates followed a similar pattern to biomass production, peaking at Day-2 and declining significantly (p < 0.01 for nitrogen and p < 0.05 for phosphorus) after that. Normalised biomass productivity (g m−2 d−1) and nitrogen removal rates were significantly higher (p < 0.01) when FANS were harvested weekly compared to fortnightly or strip harvested. Based on the summertime measurements, estimated annual areal productivity and nitrogen removal was enhanced by approximately 60% when the FANS were harvested weekly, compared to fortnightly or strip harvesting. Strip harvesting did not increase nutrient removal or productivity. This study has demonstrated that increased harvest frequency improves both biomass yield and nitrogen removal rates in filamentous algae grown on anaerobically digested food-waste centrate. With harvesting costs estimated to be <5% of total annual operating costs, higher yields generated through more frequent harvesting are likely be cost effective, while achieving a higher level of nutrient removal.