Conductivity measurement in a water reuse plant: Precise measured values even in low measuring ranges

The production of frozen vegetables requires a significant amount of water.In order to save water, Belgium-based Pasfrost, which has beenproducing and distributing high-quality frozen vegetables since1977, decided to reuse it. The water is purifiedin a water reuse plant, which includes a reverse osmosis system (RO). The coreof the RO system is a membrane through which the water is filtered. A goodcondition of this membrane is a prerequisite for the proper functioning of theplant. With the help of Memosens conductivity sensors, the membrane conditioncan be monitored both reliably and easily.

Benefits of Memosens conductivity sensors

Customer challenge

Pasfrostprocesses fresh local vegetables into frozen vegetables. Therefore, many water-consuming process steps are necessary, such as washing vegetables and blanchingthem. Since the company is continuously growing, it built a new wastewatertreatment plant. In addition, Pasfrost also has a water treatment plant where it purifies a large part of the process wastewater into drinking water, which isthen reused. For purification, Pasfrost relies on technologies likeultrafiltration, UV disinfection and RO. Currently, multiple RO installationsare running to purify a large part of the wastewater.

Water for reuse

Reverseosmosis is a water purification process that uses a partially permeablemembrane to remove unwanted substances — such as ions, molecules or particles — fromdrinking water. Pressure is applied to overcome osmotic pressure. Unwantedsubstances are retained on the pressurized side of the membrane (concentratedwastewater), and the pure water can pass to the other side. The membranes’status needs to be monitored to ensure pureness and to achieve high waterquality. Therefore, conductivity is measured at the inlet of the RO and after amembrane. To fulfill the high hygienic requirements regarding drinking water, clean-in-place (CIP) is performed regularly at the RO installations. They are cleaned every fewweeks with caustic soda or acid at 40°C. The conductivity sensors must be able to withstand this tough cleaning routine.

The solution

Thechallenge was to select suitable sensors that cover the broad range ofconductivity values from incoming and outcoming water in the RO plant. At theRO inlet, a CLS21D sensor was installed, whereas at the outlet, the choice was a Memosens CLS82D. With the CLS82D, even lower conductivity values of purifiedwater after the partially permeable membrane can be measured precisely. Bothsensors, the CLS82D and CLS21D, are low maintenance and easy to usedue to Memosens. Via the Liquiline CM44 transmitter, values are transmittedto the PLS that controls the membrane regeneration and CIP cycles automatically.

Conductivity values in the reverse osmosis installation

The results

With the help ofthe conductivity sensors, Pasfrost is able to monitor the proper functioning ofits RO system. Thanks to water reuse, high-quality drinking water can beproduced from recycled effluent constantly and reliably. Pasfrost takes onlyone-fourth of the total amount needed of freshwater from a well. The remaining three-fourths of the water demand are covered by the production plant’s ownrecycled water. Thus, Pasfrost is almost independent of the municipal watersupply. The solution is both sustainable andcost effective.

“Endress+Hauser demonstrated again that they are able to provide the suitable product for each application. With the CLS82D we found the optimal sensor to ensure the drinking water quality,” said Pasfrost's Alexander Wallays.

Endress+Hauser is aglobal leader in measurement and automation technology for process andlaboratory applications. Endress+Hauser devices,solutions and services can be found in many industries. Customers use themto gain valuable knowledge from their applications, and this enables them toimprove their products, work economically and protect peopleand the environment.

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