Hollow fiber membranes successfully removed sulphate from a very hard groundwater during the operation of a pilot plant without the need of antiscalants.
Wassergewinnung Würzburg-Estenfeld GmbH (WW-E) uses very hard groundwater for drinking water supply in the Würzburg-Estenfeld area. The sulfate concentration is well over 500 mg/L and therefore well above the drinking water regulation limit of 250 mg/L. In order to comply with legal requirements, the water has to be mixed with water from an external supplier before distribution. For the supply area of the city of Würzburg, an exemption was granted by the responsible health authority, which allows sulfate concentrations in drinking water of up to 500 mg/L.
During pilot tests to determine the technical possibilities of sulfate removal using low-pressure reverse osmosis (LPRO), it was found that, in addition to high hardness and high sulfate contents, the well waters also occasionally had increased turbidity levels. Therefore regular flushing of the wells and cleanings of the water chambers in short intervals have to be carried out.
In order to be able to use the well water without restrictions, the implementation of a large-scale UF system is in preparation. The process engineering parameters for the design of the UF system were determined as part of a pilot test between March and October 2018.
Since the WW-E continues to examine the process engineering options for sulfate removal, the TZW: DVGW Water Technology Center in Karlsruhe (TZW) has proposed carrying out a pilot test with an innovative process of direct nanofiltration via hollow fiber membranes. Those membranes show high retention for sulfate and may be operated without the use of antiscalants. Moreover, they can handle particulate matter as well, which means, that they can fulfill both tasks, turbidity removal and sulfate removal in one system. Another advantage would be the antiscalant-free operation, which could significantly simplify the problem of concentrate disposal at the site.
The hollow fiber membranes used are ultrafiltration membranes, which are covered with alternating layers of polycations and polyanions using the so-called layer-by-layer process in order to achieve nanofiltration properties. These membranes can retain divalent ions very well, while they have little retention for monovalent ions. They also have a high retention capacity with regard to different organic water ingredients.
NX Filtration has provided a pilot system with an NF module for piloting at the well location in Estenfeld, as is also used in large systems. The system is housed in a 20-foot container and can be operated via remote control so that process parameters can be changed immediately. The module with 43 m² of membrane area was operated in feed-and-bleed mode for approximately 6 months, with approximately 1.2 m³/h being supplied to the system and continuously producing approximately 0.9 m³/h of permeate and 0.3 m³/h of concentrate. A tangential flow was maintained over the membrane surface (recirculation of approx. 4-5 m³/h across the module) in order to minimize the effects of concentration polarization (CP). The CP layer is created due to the retention of the divalent water ingredients (calcium, magnesium, sulfate) on the membrane surface. The less crossflow there is, the thicker the layer becomes, which has a direct negative effect on retention. As part of the piloting, the influencing variables such as crossflow, yield and surface loading were systematically examined and analyzes of the partial flows were carried out.
No antiscalant was dosed during the entire period of piloting. The goal of reducing the sulfate concentration was fully met in every setting tested with an average value of <100 mg/L sulfate in the permeate. No decrease in permeability was observed during the individual test series under constant conditions. Since the yield was increased gradually, a slight decrease in permeability could be observed over the entire operating time, which could be related to the increase in osmotic pressure, but could also indicate scaling effects. However, chemical cleaning was not necessary.
Publication:
Lipp, P.; Janietz, O.; Sewerin, T.: Antiscalantfreier Betrieb von Hohlfaser-Nanofiltrationsmembranen zur Sulfatentfernung aus einem Grundwasser – Ergebnisse einer Pilotierung. Beitrag zur 15. Aachener Tagung am 25. und 26. Oktober 2023 (in German)
