Reported removal efficiencies of micropollutants at wastewater treatment plants vary a great deal for the same substance, with negative removal efficiencies even being reported and simply averaged out.
Often, to reduce cost and effort, sampling studies are very short at one to two days. The efficiency is then based on a calculation of mass balancing the sample loads in influent and effluent, using the flawed assumptions that the volume of each is equal and that micropollutant concentrations are in steady-state conditions at all times.
Researchers at the Resource Center for Environmental Technologies in Luxembourg saw this need to systematically examine this method and the accuracy of it with regard to the sampling method and conditions. This HOT article describes the application of hydraulic modeling to match up the influent and effluent loads, reducing uncertainties.
Hydraulic residence times are often used in chemical engineering but not in evaluation of wastewater treatment sampling. In previous modeling work, the team has concluded that the load carried by an effluent sample taken over the course of one day is made up of influent load from the days before. This study takes this and attempts to calculate how much of one day’s micropollutant influent load ends up in a day’s effluent sample. The number of influent sampling days was gradually increased until 80% of the effluent sample was accounted for.
They provide a model that can be adapted for use in other wastewater treatment plants and recommend best practice for taking inevitable errors ranges into account. This article is free to access for the next 4 weeks* so you can read it now at:
A case-study on the accuracy of mass balances for xenobiotics in full-scale wastewater treatment plants
Marius Majewsky, Julien Farlin, Michael Bayerle and Tom Gallé
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