The sand filter system in Germany is really a common and proven method for the treating drinking water. This filtration method is characterized by its high level of pathogen removal and economical efficiency. It’s certainly one of the most effective means for pathogen removal in water treatment plants (WWTP).
The slow sand filter system functions introducing polluted water to the sand in a way that will not disturb the sand. This is performed by using a diffuser plate that controls the flow of the water to prevent disturbing the layer of sand at the the surface of the filter. The polluted water is then filtered through the sand bed before desired amount of purity is reached. At the end of the filtration cycle, the filter is backwashed with water. Throughout the backwash process, the pores of the sand are enlarged, which allows for an efficient cleaning of the sand bed. This enables the sand to be regenerated and the purification process can be repeated. Ovalpools
A slow sand filter is often equipped having an aeration system. In Denmark, this is usually in the shape of stairs or multiple slat tray aerators, while in Germany, waterfall and spray aerators are more commonly used. The aeration system increases the surface area of the sand and promotes a highly effective distribution of prokaryotes and eukaryotes through the filter bed. This results in a much better inactivation of enteric microorganisms, such as for instance E. coli. Additionally, it results in a better aerobic bacterial load, also called heterotrophic plate counts (HPC).
The HPC can be an indicator for the entire microbial activity in the wastewater and indicates whether the wastewater has been successfully treated. The results of several studies demonstrate that slow sand filters achieve substantial mean removal for enteric bacteria (E. coli) and Clostridium perfringens spores. In addition, these filters can effectively eliminate cyanobacterial hepatotoxins, also called microcystins.
Despite this excellent pathogen removal potential, the sand filter is not without its shortcomings. The main problem is so it releases small plastic particles, so-called microplastic particles (MPP) and microplastic fibers (MPF), to the effluent during backwashing. This is particularly so for sand filters with a coarse gradation and large gaps in the sand bed.
The MPP and MPF are generally made up of polyethylene (PE), polypropylene (PP) and polyethylene terephthalate (PET). These particles have diameters of 10-50 m. In the sample of the final clarifier effluent from the Mannheim WWTP and in the sample from the Frankfurt Sindlingen WWTP, these polymers were detected. The sand filter of the PVC manufacturer eliminates these polymers normally in about 99.2% +/- 0.29% of the total MP discharged from the plant. A higher proportion of those polymers was also identified in the types of the final clarifier effluents from the municipal WWTPs. These findings suggest that the municipal WWTPs need certainly to boost their monitoring of MPP and MPF in their sand filter plants. The authors wish to thank the operators of the Mannheim and Frankfurt Sindlingen WWTPs for their cooperation in this study, in addition to the PVC manufacturer for providing on-site support and the samples for analysis.
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