Microbiological parameters as an additional tool to improve wastewater treatment plant monitoring

The operation of an activated sludge wastewater treatment plant (WWTP) was assessed by determining the removal efficiency of treatment processes. For that purpose, different physicochemical and microbiological parameters, including total, active and culturable bacteria, were measured along a year. Values of both physicochemical and microbiological parameters were significantly reduced by biological treatment but no relation between these groups of parameters was obtained. Besides, results suggest that microbiological parameters were more sensitive than physicochemical ones to changes in WWTP conditions (i.e. influent quality). Additionally, high percentages of noncultured (>88%) and noncultured remaining active (until 50%) cells were detected before and after treatment. Therefore, ISO standards adopted to enumerate indicator bacteria by Spanish legislation, and based on culturability, underestimate the bacterial population present in wastewater. We have compared two culture-independent methods for bacterial enumeration, microscopy and flow cytometry (FCM) and results indicate that FCM provides a rapid and automated quantification in WWTP. We conclude that microbiological parameters are essential to monitor the correct WWTP operation and we propose quantification of active bacteria as indicator of wastewater microbiological quality.