Performance of retention soil filters for the reduction of hygienically-relevant microorganisms in combined sewage overflow and treated wastewater.

Environmental quality standards for surface waters have been significantly expanded through recent amendments to German regulations. Limit values are only established for applicable regulations if the water is indicated for certain uses, e.g. abstraction of irrigation water. Nevertheless, surface water bodies are often used for hygiene-sensitive purposes. In the course of climate change, stronger precipitation events will occur, which may lead to more frequent loading and discharge of combined sewer overflow (CSO) into surface water bodies. Retention soil filters (RSFs) are attracting attention as an extensive treatment technology for CSO and additional wastewater treatment. This study examined large-scale RSFs for CSO treatment, as well as the effectiveness of RSFs as a fourth purification stage. An RSF test facility was established at a municipal wastewater treatment plant (WWTP), consisting three semi-technical RSFs that were fed exclusively with treated water from the WWTP. The reduction of microorganisms mostly occurred within the first centimeters of the RSFs. For most hygienic-microbiological parameters, a 1-2 log unit reduction could be detected in addition to the reduction within the WWTP. Antibiotic-resistant bacteria were reduced to the same extent. Investigation of the large-scale RSFs showed that a flow rate reduced by half corresponded to better reduction performances.

Retention of pharmaceutical residues and microorganisms at the Altendorf retention soil filter.

A study has been conducted on a retention soil filter (RSF) to test its effectiveness in removing pharmaceutical residues and microorganisms from combined sewer overflows (CSOs). Efficient removal of solids, nutrients and heavy metals has already been proven. The possibility that organic micropollutants and microorganisms are also retained by the use of RSFs has been identified, but data are lacking. Results obtained in this study, in which testing for removal by a RSF of numerous micro-pollutant substances was performed, are most promising. The pharmaceuticals diclofenac and ibuprofen are presented in detail as examples of such micropollutants. Both showed a reduction in positive samples of more than 55% as well as a significant reduction in median and maximum concentrations. For microorganisms such as Escherichia coli, coliphages and Giardia lamblia (cysts), an average reduction in concentrations by three logarithmic steps (99.9%) was achieved. These results add to the evidence that using a RSF in the advanced treatment of wastewater from CSOs reduces the exposure of water-courses to pharmaceutical residues and microbial contamination.