In Germany, about a third of all drinking water is treated by slow sand filters. This traditional method uses sand and water as filter media, resulting in safe drinking water that is free of pathogens, microorganisms and other organic impurities. The sand filter is one of the most environmentally friendly methods for wastewater treatment. You can find more information about choosing a sandfilteranlageThe slow sand filtration process (SSF) is characterised by its high removal efficiency of both coliforms and enterococci in comparison to other water treatment technologies. In addition, sand filters do not produce toxic or harmful substances such as phenols or heavy metals. However, SSF requires a sand with specific properties regarding grain size diameter and uniformity coefficient that is often not available locally. To overcome this, sand is washed in special wet processing equipment. This produces a dewatered sand, known as CDE® Sand, that meets all the requirements for SSF. This sand is delivered to water treatment companies in a controlled manner and can be used in any slow sand filter.
Compared to the conventional sand, CDE® Sand has an even grain size distribution and better uniformity. In this way, it is less susceptible to clogging and provides the same removal of both coliforms and enterococci as conventional sand.
However, a sand filter is also subject to biofilm formation on the surface of the sand. This biofilm is referred to as the schmutzdecke (German for “dirt cover”) and typically comprises a gelatinous matrix of bacteria, fungi, protozoa, rotifera and annelid worms.
The schmutzdecke also prevents the sand from clogging by providing a continuous supply of nutrients and oxygen. However, if this microbial ecosystem is disturbed by abiotic factors, such as an interruption in water supply, it may be difficult for the filter to re-establish itself and recover its original efficiency.
For this reason, the researchers tested how two different abiotic stressors affect the schmutzdecke in two municipal WWTPs. Firstly, they simulated a pump failure by turning the pumps off for one week. Secondly, they performed weekly inoculations of the slow sand filter with Fusarium and Phytophthora.
Results showed that both abiotic stressors have an impact on the microbial composition of the schmutzdecke. The bacterial concentration in the effluents of the final clarifier and the sand filter before and after the pump failure was significantly lower than that measured after the inoculation. In addition, the presence of sand in the schmutzdecke before and after the pump failure increased significantly.
In the Frankfurt Sindlingen and Mannheim WWTPs, three samples each were taken from the final clarifier effluents and the sand filter outlet. The concentrations of the total microbial count (MPP+MPF) in the samples of the final clarifier effluent and the sand filter outlet were about 1.6 x 103 MPP m-3 and 3.0 x 102 MPF m-3, respectively. In the sample of the final clarifier effluent before and after the sand filter, PET was the only polymer identified. This is probably due to the fact that this type of plastic has a very low molecular weight and thus does not easily dissolve in the wastewater stream.
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