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Sensible and self-monitoring
Online process for soft water control

Dipl. Ing. Winfried Schellbach
OFS Online Fluid Sensoric GmbH

For the generation of soft water are mostly used ion exchanging devices with generation of sodium chloride since the beginning of the Sixties. The generated soft water quality and soft water quantity are depending thereby on the static and dynamic characteristics of the softening plant in operation.

The static characteristics are given from the type of the plant by quality and amount of the resin as well from the conditions of   the special process of regeneration and creating a planned and calculated value of capacity of soft water and the required resources of regeneration. The dynamic characteristics are given by the hardness concentration of the raw water that has to soften. This is subject to natural variability.

The dynamic and static characteristics are terminating the technical quantity of a soft water plant that is planned for a maximum raw water value in practice. With the achievement of a planned quantity independent of the technical softening capacity (stated capacity) of the softening plant the regeneration of the plant will be started already. The hereby used softening capacity (benefit of capacity) is less then the technical possibilities allow.

Picture 1: Schematic structure of the  Softcontrol  measuring method.

This operational mode, named as quantitative operational mode, works independent from the concrete hardness conditions at  the soft water outlet, where normally the technical capacities of the softening plants can not work to capacity because of the absent of continuously, process secured and economical soft water monitoring systems.

During the last 40 years as technical standard for the soft water monitoring has been proven the cyclical photometric measuring system to spot check the technical reliability performance of the softening plant and to report in case of trouble.

A operational mode of a softening plant regarding quality demands a selective continuously monitoring of the complete process of soft water withdrawal. Not before the attainment of a fixed soft water concentration the regeneration of the softening plant will be started up, where a more important soft water quantity can be reached then with the operational mode regarding quantity, shown as benefit in the following.

Documentation of a water hardness breakthrough at a softening plant with quantitative control of regeneration of the pendulum system( Soft water hardness in °dH).

With the changeover of a softening plant from an operational mode regarding quantity to a quality based one in practice a benefit of quantity of 40 to 120 % can be achieved, what is especially interesting for plants with a high quantity of soft water.

For an operational mode regarding quality of softening plants the photometric water hardness measuring system was not successfully. A continuous, solid and economic measuring solution of soft water hardness was not given anymore: Harmful have been affected growing demands to the devices and the high consumption of indicators.
Water hardness can not be measured by photometric but also by potentiometer measuring systems, similar to a pH-measurement. The measurement method with ion selective Calcium- or Magnesium electrodes requires high demands for calibration and measuring operation. That why it was used in chemical laboratories only. Until now it was unsuitable for practical use in the industry and further applications, like a qualified soft water monitoring.

With the development of an innovative equipment family  for soft water monitoring , named “Softcontrol”  by the company OFS GmbH,  a measurement and control device  with potentiometer measuring operation for continuously soft water monitoring is introduced. Previous problems of qualitative soft water monitoring will be solved by very simple construction of the device and economic continuous measuring over long time periods. The simple principle of operation is based on, that a calcium-magnesium selective electrode is defining the water hardness difference between raw and soft water and rated to a fixed limit.

According to this basic principle no In-line measuring similar to a pH-electrode measurement is possible. That process needs a bypass measurement.

An external flow rate measuring cells is filling automatically controlled with raw resp. soft water. After measurement a pressured new filling is replacing the predetermined water.
This special process is working with a simple construction of the device with smallest test hoses (inside diameter 2 mm), a small filling volume of the flow rate measuring cell (app. 5 ml) in very short measuring cycles (15 to 30 s/measurement). The operating time of the sensor is independent from the measuring cycles. No need for indicators. First time this principle allows an effective virtual-continuous monitoring of softening plants under economical conditions.

The applied difference process allows to exclude sensor drift and to determine sensor quality. Not only soft water hardness will be monitored but measuring function additionally. Softening and measuring operation will be safer for process and control because of this. According to these possibilities, optimal conditions for the qualitative control of softening plants are given. With permanent monitoring of soft water hardness during the whole process of water withdrawal the increasing soft water hardness can be indicated and specified at a very low concentration. Furthermore the problem of stand-hardness in the softening columns will be sensible specified and considered according the drain of capacity during the whole process of hardness exceeding. Untimely beginning of regeneration can be prevented according the trend of hardness development of the stand hardness problems, and the soft water quality can be used optimal.

Picture 2 shows with Softcontrol the so called hardness breakthrough at a softening plant, independent of soft water withdrawal.   With deactivation of soft water withdrawal the adverse ion-effect of each higher hardness level can be seen well. According to high sensitivity and selectivity of the sensor also threshold values below 0, 1 dH can be controlled all the time. Like at other chemical sensor systems the operating time of the sensor is limited. It will be negative influenced from softening plants by increasing of hardness limit and disturbing ingredients like push-chlorination and brine-shears.

Picture 2

This senescence of sensor will be automatically monitored by a measuring system and signalized in time to the operator in the case of a lower deviation of a fixed quality for a necessary change. The change of the sensor is the only maintenance at the measuring system and can be done independently and easily by the operator. Only in this case of service is during the control of a softening plant the quantitative operational mode automatically switched and keeps in operation until the change of the sensor is recognized by the measuring system.

Conclusion

The new measurement method is offering, independent from existing raw water matrices and very high raw water hardness’s, a new alternative possibility for more efficient, continuous soft water monitoring and for a transition for a qualitative operational mode of softening plants. The application is optimizing the softening operation at steady soft water quality and is increasing the security of process and operation.