How does urban wastewater treatment affect the microbial quality of treated wastewater?

The design of wastewater treatment plants (WWTP) includes facilities for the removal of suspended solids, organic matter, nitrogen and phosphorous, as required in current legislation, while removal of microorganisms is not literally emphasized. Although the different unit processes have some effect on microbial populations, disinfection is not achieved in many cases unless a specific step is included to regenerate wastewater. This study assesses the effect of the unit processes of six WWTPs that treat municipal wastewater on five microbiological parameters (total coliforms, E. coli, Enterococcus sp., Staphylococcus aureus and Pseudomonas aeruginosa), as well as their contribution to receiving waters. Bacterial concentration in the inlet of WWTPs varies from the lowest concentration of Staphylococcus aureus (10(4) CFU/100 mL), to the highest for total coliforms (10(8) CFU/100 mL), independently of the industrial activity discharging to the WWTP. Considering log bacterial removal of each step of treatment processes, trickling filters reduced 2.6 and 1.5-log Enterococcus sp. and Pseudomonas sp. respectively, secondary decanters achieved 2.5-log average removals and ponds reduced bacteria in a range from 1 to 2.2-log, being the processes with the highest bacteria removal, although in the case of secondary decanters this depends on design parameters such as hydraulic residence time, loads of solids, or types of previous biological treatments. The average of the bacteria concentration in the outlet of the six WWTPs was between 10(3)-10(6) CFU/100 mL. The effect of discharging these bacteria on the quality of receiving waters was assessed by analyzing microbial concentration upstream and downstream of the discharge point. In almost all cases, the bacterial concentration upstream of the discharge point was similar to downstream concentration because the effluent is rapidly diluted in the receiving river, not representing a risk to ecosystems. (C) 2019 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.