Lactic Acid Production from Filamentous Algae grown using Aquaponics Wastewater

Abstract

Algal Turf Scrubber (ATS) systems are a known method of treating nitrogen and phosphorous rich wastewater by cultivating green filamentous algae biomass. One novel source of this wastewater is from an aquaponics system, where fish and vegetable crops are grown in an aquatic environment connected by the movement of the water from the fish to the plants for their nutrient sources. The wastewater from the fish is potent in nitrates, phosphates, and ammonium that the plants thrive on by pulling these compounds from the water. However, often there are still lingering nutrients in the water afterwards, so ATS lanes can be attached at the end of the system for further nutrient recovery, while also potentially producing additional value products from algal biomass. The filamentous algal species typical in ATS systems are often rich in carbohydrates. As such, this algal biomass has the potential to be used as feedstock for lactic acid fermentation from lactic acid producing bacteria (LAB). The fermentation process typically requires both a nutrient source, usually composed of forms of nitrogen, and a carbohydrate source, in the form of monosaccharides or simple sugars. The nitrogen serves as a building block for the growth of the LAB while the sugars provide the energy for the LAB to conduct the fermentation. In this study, algal biomass cultivated on aquaponics wastewater on laboratory-scale indoor ATS units was studied for use as the carbohydrate source for lactic acid fermentation production. A dilution experiment was first performed to investigate the growth yield of higher carbohydrate concentration algae over a range of concentrations of the aquaponics wastewater. Results showed that the half wastewater, half tap water dilution yielded the highest yield of green filamentous algae with a mean ash free dry weight productivity of 7 g/m2/day, based upon weekly harvests. Production of the algae biomass at the half dilution iii yielded an additional 1 kg of the dried biomass. The fermentation experiment tested the suitability of the different algal biomasses harvested as a carbohydrate source by measuring both the concentration of lactic acid present within the system and the yield of lactic acid from the available sugars from samples taken over the course of the fermentation. Results of the fermentation experiments indicated that algae grown while under half dilution ATS conditions and without a customary heat and pressure pretreatment produced lactic acid concentrations of 20 g/L and a yield of 80% lactic acid from the available sugars. These results outstripped the other fermentations trials ran, with the lower quality green algae biomass from the dilution experiment at 15 g/L lactic acid concentration and algae biomass from the half dilution production that were pretreated with heat and pressure at 17g/L concentration. Further comparisons to predominantly cyanobacteria algae biomass initially grown from the ATS system at 8 g/L and cucumber residues at 18 g/L lactic acid concentrations were favorable, while waste paper mill sludge had near double the concentration with up to 45 g/L lactic acid produced. These results suggest that there is potential for filamentous algae for use as a carbohydrate feedstock for lactic acid fermentation due to high carbohydrate availability that merits continued research.