Optimized Operation at the Largest Drinking Water Nanofiltration Plant in Germany

To ensure the water supply for the city of Dinslaken (Germany), the water treatment company – Wasserwerk Dinslaken GmbH – has since 1961 operated the Löhnen plant which is located approximately 2.5km from the river Rhine. The water treatment plant (unit 1), fed by 6 vertical wells and driven by large pumps (150 m3/h each), was in 1989 expanded by the addition of a second more flexible treatment unit (based on 3 speed-controlled pumps – 50 to 150 m3/h each). Since the expansion of the process treatment plant in 2003 by the addition of a highly flexible duty Nanofiltration plant with two rapid cold softening reactors and 6 downstream turbidity filters, the plant reached a minimum processing capacity of 400 m3/h and a maximum treatment capacity of 1,100 m3/h. According to the existing water agreements, the two production units (I and II) can allow the production of a total of 5.6 million m3/a. In the city, the daily highest water consumption is 19,000 m3/d. The new filtration technique became necessary in anticipation of future coal mining projects in the area of Mommbach causing deterioration in feed water quality due to risk of over extraction of water from the local water table resulting in ingress of Rhine river bank filtrate (with it all unwanted impurities). After prefiltration of the well water by passing through gravel and sand filters, it feeds to the Nanofiltration Plant to remove any residual particulates and dissolved but undesired minerals and organics substances.

Energy supply of the Löhnen Plant

The water treatment plant is connected to two 10-kV electrical power lines however a full-load operation can be achieved with just one electrical line which will deliver the needed 2.000 kVA power. The second electrical line gives the ability to add an additional 700 kVA power. In regards of the background previously detailed, and due to the fact that the membrane system is operated over a wide range of frequency, 2 air-cooled emergency electrical generators were installed with a capacity of 1.450 kVA each. This extra set-up ensures that during a power supply failure, the Nanofiltration Unit, both water production plants as well as the water pumping station can still function fully.

High retention

The retention / rejection efficiency for the water constituents is determined by the type of membrane used. In the Löhnen plant, Toray TMH20-430 composite polyamide membrane elements were chosen following a pilot study at the Rheinisch-Westfalische Institute for Water (IWW), which showed this membrane’s superior retention / rejection capability. The resultant water quality was close to that which could be achieved by conventional reverse osmosis membranes but with an inlet operating pressure of fewer than 8 bars major energy savings could be made; hence the commonly used term – low-pressure / low energy reverse osmosis membranes. The salt rejection of the selected membrane is 99.3 percent. The plant design enables a treated water capacity of 1.100 m3/h by having a parallel arrangement of 11 processing lines, each with a capacity of 110 m3/h (one line is kept as a reserve). Each line is operated with a speed-controlled feed-pump (Capacity = 110 m3/h; Pel = 46.8 kW) and protected by two pre-filters connected in series (5 um, then 1 um). In each line there is a two-stage Nanofiltration array having 15 vessels, 10 in the 1st stage and 5 in the second. The concentrate of the first-stage feeds the second-stage. Standby lines are kept preserved in sodium meta bisulfite solution.

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RPI Hardness Stabilizer (or Antiscalant)

The operating performances are correlated to the site specificity. In the Löhnen case, the RO membranes are smoothly operated with a flux of 25 l/m2/hr, a transmembrane differential pressure of about 5.5 bars and a maximal yield of 87 percent. Reaching such high recovery would not be feasible at all, without the use of a –crystal growth inhibitor – also named anti-scalant. As a safe and environmental friendly substance, sodium poly-Phosphonate substances are used. They will prevent for example the immediate damage of the membranes due to the formation of abrasive crystals like CaCO3 or CaSO4. In Löhnen, the ROPUR sodium poly-Phosphonate only needs to be dosed at 2 mg/l (or 2ppm). The family of sodium poly-Phosphonates are included in the list of “treatment substances and disinfection methods according to paragraph §11 TrinkwV 2006”.

More specifically why must an antiscalant be used here?

The raw water used is pumped up from a depth of 17 meters. It is then pre-treated before it flows into the Nanofiltration banks. As the natural product that comes from the Löhnen sources, is “very hard”, it contains many minerals such as Calcium, Magnesium or Carbonate cations and anions. During the desalination process, using nanofiltration / low pressure reverse osmosis membranes, the water molecules pass rapidly through the membrane while the ions (anions and cations), metal oxides, and silicate particles will be filtered and thus concentrated and later be discharged with the concentrate. In this situation of high recovery, the natural solubility of one or more salts is immediately exceeded resulting in rapid formation of a crystals that precipitate onto the membrane surface.

“The plant design enables a treated water capacity of 1.100 m3/h by having a parallel arrangement of 11 processing lines, each with a capacity of 110 m3/h”

The consequence of the deposition, apart from the required higher inlet pressure for the same filtrate flow, the progressive increase in filtration resistance (higher pressure is needed) and the corresponding increase in power consumption, there is the immediate risk of irreversible membrane damage from crystal surface abrasion leading to poor permeate quality and loss of membrane performance. In this eventuality, the only technical solution is the replacement of the membrane elements, resulting in an increase in replacement frequency and operating costs.

To avoid such damage, Toray Membrane Europe AG developed and produced a range of nine different antiscalants. Each of these products has been developed to prevent a specific scaling risk whilst complying with strict national and international safety and quality standards (NSF60, KIWA, ISO9001 and ISO14001). Historically, the first RPI antiscalant was sold in 1987 by the company ROPUR located in Münchenstein (Switzerland). The ROPUR AG company was taken over later on by the Japanese Toray Group to become Toray Membrane Europe AG. Since 1987, the ROPUR RPI products are exclusively manufactured and sourced in Germany to ensure the highest quality standards.

The RPI ingredients currently used are from the latest generation of poly-phosphonate salts. They have a 100 times higher efficiency than comparable products such as ATMP and only needs to be used in surprisingly low concentrations making it highly cost effective. It prevents or delays the formation of CaCO3, CaSO4, barium and strontium sulfate in crystalline form. Thereby, the membrane surface is kept free of inorganic and organic components and thus stabilizes the filtration flow through the membrane layer at optimum rates. This allows for a constant high flow rate at a moderate and stable pressures resulting in a low energy consumption and extended membrane life.

“In this eventuality, the only technical solution is the replacement of the membrane elements, resulting in an increase in replacement frequency and operating costs”

Conclusion

The operation, the energy efficiency as well as the reliability of the Water Treatment Plant Dinslaken are the result of the association of trained engineers, designers, managers and high performance elements and Antiscalant. The daily plant operation relies on the fine tuning of dozen of parameters like, flow rates, hydraulic pressures, pH, recovery rates…etc. The Toray Membrane Europe team, with over 30 yrs experience, is proud of its contribution to this success story.

Dr. Lagref Jean-Jacques
Business Development & Product Manager
Norman Bischoffberger
Robert Reisewitz
Toray Membrane Europe
Marco Binder
Michael Hörsken
Stadtwerke Dinslaken GmbH