Nitrate removal by reverse osmosis

Increasing nitrate concentrations in groundwater force water supply companies with the challenge of complying with the nitrate limit value specified in the Drinking Water Guideline. If desirable preventive measures in the catchment area not successful, waterworks are required to implement other solutions. The example of city of Groß-Umstadt in the Federal State of Hessen shows how reverse osmosis can be used as a tailor-made and operationally stable alternative for the removal of nitrate in waterworks.

To provide drinking water from local groundwater sources with elevated nitrate levels, the town of Groß-Umstadt decided to upgrade the water treatment technology of their waterworks. In 2017, the city's water supply initiated a collaboration with TZW: DVGW-Technologiezentrum Wasser (German Water Centre) to design a reverse osmosis system for nitrate removal. In Germany, about 100 reverse osmosis systems for inland desalination are currently in operation in the public water supply - in many cases TZW was involved in the conception and therefore has extensive practical experience.

Within the design process of the reverse osmosis, various load cases were considered to determine the required drinking water flow rate. The possible decommissioning of separate groundwater wells and the drinking water demand for normal and peak delivery were taken into account. As a result, two reverse osmosis lines were planned, each with a drinking water production of 63 m³/h, which cover varying drinking water demands without long downtimes and ensure redundancy. Reverse osmosis permeate was re-mineralized by mixing with an untreated bypass. The mixing ratio of RO-permeate and untreated by-pass determines the composition of the drinking water. In this case the ratio was chosen so that a nitrate concentration of approx. 23 mg/L results in the drinking water. This comparatively low concentration was desired by consumers. By nature, the removal of nitrate is linked with a decreaseof hardness. In this case the hardness in drinking water is 8.7 °dH (1.6 mmol/L).

The RO-plant has been running smoothly since it was commissioned in 2021. Chemical cleaning of the membranes has not yet been carried out because both pressures and salt retention remain constant. Manpower required for operation, including maintenance, averages about 16 hours per month. This drinking water treatment plant is a good example of how advanced technology may be adapted to local requirements to solve even unusual challenges.

Publikation:

Mattheß, B.; Müller, U.: Nitratentfernung mit Umkehrosmose: vom Konzept zur Umsetzung. DVGW energie | wasser-praxis 09/2023, 20-24 (2023) (in German)