Municipal wastewater effluent characterization and variability analysis in view of an ozone dose control strategy during tertiary treatment: The status in Belgium

Abstract

Ozonation is known for removing trace organic contaminants (TrOCs) from secondary wastewater effluent. However, its implementation and overall efficiency on a broad scale depends on effluent characteristics, which can differ both in time as well as between different treatment plants (nowadays referred to as water resource recovery facilities (WRRFs)). Therefore, water quality was assessed over time at 15 different Belgian sampling locations to increase the understanding of effluent variability in view of online control of the tertiary ozonation step. Conventional and surrogate parameters as well as those specifically related to tertiary ozonation (e.g. instantaneous ozone demand) were assessed. Little differences between the different locations were found for spectral measurements (e.g. UVA254 or fluorescence). The small amount of observed outliers was clearly site or event dependent. A lower variability (for spectral measurements) is advantageous in simplifying the development and application of a generic control framework based on these spectral measurements. In addition, also variations in TrOC concentration levels seemed to be small, as the concentration of most individual compounds resided within one order of magnitude over multiple sampling events at two different WRRFs. The combination of this low variability in TrOC levels in the effluent before ozonation with a control strategy using a TrOC removal efficiency set-point, allows to indicatively assess absolute TrOC levels after ozonation. In contrast, significant variations between different plants (especially smaller sized plants) were observed and could be related to the conventional water quality parameters alkalinity (correlated with the electrical conductivity) and pH which are both known to have an influence on the ozonation process. This confirms that a differential dosing control strategy (i.e. accounting for the matrix reactivity) should be applied instead of one solely based on the (organic) effluent load before ozonation.