Microbiological changes along a modular wastewater reuse treatment process with a special focus on bacterial regrowth

Water reuse is becoming an increasing necessity due to depleted water resources or increased water demand. A treatment process on a pilot scale was designed to produce different water qualities for different applications in industry or agriculture. We report heremicrobiological changes along themodular process using treated municipal wastewater effluent as raw water. Treatment technologies included coagulation, ultrafiltration (UF), reverse osmosis (RO), quartz sand, activated granular activated carbon (GAC) filtration and disinfection. Elimination of traditional hygiene indicator bacteria was already achieved by ultrafiltration as the first barrier. Profound changes by each treatment step also applied to the microbiome. Total and intact cell concentrations as quantified by flow cytometry underwent a strong decline after UF and RO, whereas biological stabilization was achieved through quartz sand filtration and GAC passage. Interestingly assimilable organic carbon (AOC) was still present even after RO at levels that allowed substantial regrowth of bacteria. Overall, UF and RO led only to a 0.43 and 0.78 log decrease in intact cells concentrations in stagnated water after regrowth compared with 6.5 log intact cells/ml in the stagnated rawwater. Temperaturewas shown to be an important parameter determining themicrobiome of the regrown population. Regrowth could, however, be efficiently suppressed by monochloramine.