Elimination von Phosphor mittels Flockungsfiltration und Erweiterung des Verfahrens zur parallelen Elimination von organischen Mikroschadstoffen in einem GAK-Filter

The discharge of phosphorus into natural waters is one of the main causes that impede the achievement of the „good ecological status“ of the European Water Framework Directive (WFD 2000) in many river catchments. Therefore, the requirements for numerous wastewater treatment plants are being increased. One of the technical options for an additional treatment is the installation of a rapid filter with flocculation. Beside the removal of phosphorus, the increasing concentrations of micropollutants in receiving waters is a current subject. These pollutants e.g. pharmaceuticals and industrial chemicals are released from anthropogenic sources into domestic wastewater and thus transported to the effluent of treatment plants. They are considered problematic because they may harm the aquatic environment and the supply of drinking water. They can be removed by activated carbon processes. The processes for phosphorus and for micropollutants elimination overlap if granular activated carbon (GAC) is used as filter bed material. Thus, the filter bed may be used for the removal of the precipitated and flocculated phosphorus and simultaneously for the adsorption of micropollutants. Adsorption takes place by diffusion of the molecules from the water bulk into the pores of the activated carbon. This work presents the results of the examination of phosphorus removal in a filterbed of anthracite and sand in downflow mode. The feasibility of the process regarding the achievement of very low concentrations of total phosphorus was proven. The effective use of the filterbed was shown to be strongly dependent on the operation conditions which must be chosen with great consideration. Subsequently the filter column was filled with activated carbons of different grain diameter. Removal of total phosphorus was also well achieved by flocculation and filtration. The elimination of micropollutants was only poor with a GAC of coarse grains, but in a filterbed of fine GAC good results compared to other studies were achieved. The filtration process is mainly influenced by the load of suspended solids. On one side they must be impeded to reach the filter effluent, on the other hand the volume of the filter bed should be used as completely as possible. The suspended solids of a great number of filter cycles were calculated in terms of a surface load that included not only the suspended solids from the plant effluent, but also the flocs, that were formed through flocculation. The surface load permits an estimation of the filter run in dependence on the filter bed material. No evidence was found of reduced adsorption efficiency caused by suspended solids, neither in composite samples nor in concentration profiles along the height of the filter bed. Beyond that, the concentration profiles showed that the filter bed was repeatedly mixed through the flushing so that the load of the filter bed with adsorbed micropollutants was redistributed each time. The combination of the flocculation filtration process with the elimination of micropollutants in a filter bed of GAC may be the process of choice. The design of the filterbed has to be adjusted to the requirements. The installation of a measurement and control technology for an intelligent filter operation and monitoring of the effluent concentration is mandatory.