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Modern reverse osmosis system with retentate treatment

Households in Baden-Baden have been supplied with softer, largely PFC-free drinking water since August 2018. To make this possible, a low-pressure reverse osmosis system with retentate treatment was installed in the Oberwald groundwater plant after a complete renovation and extension. Activated carbon filtration eliminates PFC from the concentrated retentate before the retentate is fed into the discharge system. This is probably the first time that this process technology has been used in such a configuration in Germany and probably also in Europe.

PFC problem

In summer 2013, a study was carried out into per- or polyfluoroalkyl chemicals (PFCs) drinking water treatment plants in response to a directive from the Rastatt and Baden-Baden Public Health Department. During the study, PFC was detected in different deep groundwater catchment wells. Treatment measures became essential due to the steadily increasing contamination levels in the drinking water wells.

Water treatment in the groundwater plant

The municipal utility company needed to expand its existing concept plan for a groundwater softening facility to include a low-pressure reverse osmosis system to reduce PFCs. Retentate disposal posed a particular problem. In a normal situation, retentate was discarded into a receiving water course via a direct discharge. In Sandweier, the Sandbach brook, about 1.8 km away, was the only option in this case. Discharge of this type was not possible or was not approved for PFC-contaminated retentate.

Low-pressure reverse osmosis system (LPRO system)

Treatment in the nanofiltration  system has been designed to achieve a hardness within a range between 1,5 and 2 mmol/l after processing and complete PFC removal downstream from the system with a permeate treatment capacity of 600 m³/h. Variations in volumes of up to about 20% also needed to be taken into account. The treatment capacity was divided between six racks, each with a permeate capacity of 100 m³/h and a maximum feed of 125 m³/h. Optimally efficient vertical centrifugal pumps have been fitted and operate via frequency converters.

Retentate treatment

The activated carbon filters for retentate treatment have been installed outdoors (Fig. 10). The approval under water resources law for retentate discharge requires strict compliance with the discharge levels. This also applies to the mandatory anti-scalant and its concentration.
Three activated carbon filters have been installed, although a maximum of two are operated at the same time. The third filter is not switched on until one filter is no longer effective. The water volumes channelled via the filters are measured and recorded on a continuous basis as are the retentate volumes released into the Sandbach brook. The filters feature thermal insulation and are equipped with connections to flush out used activated carbon and jet in the new carbon.

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