The elimination of micro-pollutants is a rather new topic in wastewater treatment and a new challenge. The substances, e.g pharmaceuticals, occur in very low concentrations and are only marginally eliminated with conventional treatment steps. Two process alternatives are currently used for disposal: ozonation and adsorption. But not all questions are solved yet. Therefore, a research program will be performed at the waste water treatment plant Weißenburg. Especially the posttreatment after an ozonation will be examined. It is planned to compare the effects of sand filtration and filtration with granular activated carbon (GAC) as filter material. If GAC was used as the main treatment step for the elimination of micropollutants the carbon had to be reactivated after 2 to 3 years. For the post-treatment after ozonation the focus is on biological processes. The carbon will therefore stay in the filter considerably longer. The wastewater treatment plant Weißenburg was selected as a particularly suitable location for the pilot project due to the pollution situation and its location in a sensitive and low-rate receiving watercourse river catchment area.
Implemetation and first research results
For economic reasons a partial flow treatment was selected for the elimination of trace substances in Weißenburg. The selected design can treat about 86 percent of the annual wastewater volume in the fourth treatment stage. Based on the elimination rate of twelve indicator substances, the purification performance of the 4th treatment stage was determined. The selected indicator substances include: Amisulpride, Benzotriazole, Candesartan, Carbamazepine, Citalopram, Clarithromycin, Diclofenac, Hydrochlorothiazide, Irbesartan, Metoprolol, Tolyltriazole and Venlafaxine.
The trace substance reduction of the Weißenburg sewage plant without the fourth purification stage is around 20 percent on average. With the additional oxidative and adsorptive treatment total reduction rates of about 80 percent in average can be achieved for the year. During undisturbed operation, the process combination of ozonation and activated carbon filter achieved an average elimination rate of about 92 percent (92 percent mean value from the averaged elimination rates for the twelve indicator substances over eight sampling days). In contrast, the average elimination rate with ozonation and subsequent sand filtration is significantly lower, between 76,1 and 89,8 percent. The sand filter functions as a post-treatment after the ozonation with a biological process which will be further investigated. The elimination rate of the ozonation ranges from 73,7 percent to 89,5 percent. As expected, the additional elimination capacity in the activated carbon filter is clearly higher than in the sand filter. This is mainly due to the strong adsorptive effect of the activated carbon filter.
In regard to the cleaning performance within the fourth treatment stage the combination of ozonation and a downstream granulated activated carbon adsorption stage seems most effective.