Effect of salinity on the physico-chemical characteristics of microalgal-bacterial granules treating shrimp farming wastewater

Abstract

This study explores the structural and functional response of microalgal-bacterial granules (MBG) to salinity stress up to 15 ‰ under minimal operational intervention—without aeration, pH adjustment, or artificial lighting. Elevated salinity improved granule settling ability but led to a decline in biomass quality, as indicated by a reduced MLVSS/MLSS ratio (from 0.9 to 0.7) and decreased protein content in bound extracellular polymeric substances (EPS). Salinity also altered microbial distribution within the granules, triggering morphological changes and a visible color shift from dark green to reddish-brown. Notably, moderate salinity (<10 ‰) enhanced granulation, increasing the proportion of large particles (>0.75 mm) from 33.7 % to 53.6 %. However, at 15 ‰ salinity, key physicochemical properties—particle size distribution, EPS secretion, and structural stability—were compromised, suggesting a salinity tolerance threshold for effective wastewater treatment. Interestingly, saline conditions also suppressed Red Midge Fly larval growth, reducing biomass loss risk. These findings offer insights into optimizing MBG performance under saline wastewater conditions with low energy input.