Factors impacting the effectiveness of biological pretreatment for the alleviation of algal growth inhibition on anaerobic digestate

Abstract

Algal growth is often inhibited in full-strength anaerobic digestate. The objective of this study was to investigate conditions under which digestate pretreatment using bacteria is effective in promoting algal growth, nutrient removal, and favorable changes in algal biomass composition. Batch culture experiments were carried out using low- and high-strength municipal sludge anaerobic digestate, two algae strains of varying sensitivity to digestate inhibitors, short and long pretreatment periods, and axenic vs. non-axenic algal cultures. Pretreatment of digestate increased algal growth up to 40%, N assimilation (up to 29%), and P removal (340%) by Chlorella sorokiniana (resilient algae) when grown on high-strength digestate. Pretreatment did not increase algal growth or nutrient assimilation when Chlorella sorokiniana was combined with low-strength digestate. The more sensitive strain, Auxenochlorella protothecoides, benefitted from pretreatment on both low- and high-strength digestate, even preventing complete cell death in the latter. Pretreatment increased starch content but not lipid content of algae.