Use of photocatalytic nanomaterials for volatile fatty acid removal from anaerobic digestate leads to improved algal growth

Abstract

Abstract. Cultivating algae in anaerobic digestate has the potential to produce valuable bio-products with economic and environmental benefits. Growing algae in wastewater will immobilize the nutrients as well as produce biomass suitable for biofuel and animal feed production. However, culturing algae on high-strength anaerobic digestates faces technical challenges. One of the major challenges is the presence of inhibitory compounds in digestate that suppress algal growth. Volatile fatty acids (VFA) are commonly found in the effluent of anaerobic processes. VFA concentrations can exceed 1 g/L in digesters operating under unstable or overloaded conditions. VFAs present in digestate can significantly harm downstream algal growth, potentially leading to culture crashes. We tested the inhibitory effects of two model VFAs (propionic acid and butyric acid) on two different strains of green microalgae. We observed that the VFAs had a strong inhibitory effect on algal growth with EC50 concentrations of ~400 mg/L. Therefore, removing VFAs from the digestate would be beneficial for algae cultivation. Here, we report a pretreatment process for the effluent that uses a titanium oxide-based nanomaterial designed to oxidize VFA and other organic molecules through photocatalysis. Two model green algae, Chlorella sorokiniana and Auxenochlorella protothecoides were cultivated in the presence of VFAs (originally 1g/L for both propionic acid and butyric acid) with and without nanomaterial treatment. With the nanomaterial treatment, we observed a 20-30% reduction in VFA concentrations and X% faster algal growth. Further research is needed to understand more inhibitory compounds in digestate and to develop nanomaterials with more efficient removal capability.