Greater versatility for sodium hypochlorite dosing

The outgassing issues associated with remote water treatment control stations have been largely overcome by developments in dosing pump technology, writes Andrew Armytage of ProMinent Fluid Controls

The treatment of potable water with sodium hypochlorite solution has been the preferred method of disinfection for water companies in the UK for many years. Disinfection of the UK water supply is, of course, a national requirement, and maintaining water quality to EU directive standards in remote rural areas can present problems not experienced in towns and cities.

The installation of local water treatment control stations in rural areas gives local disinfection capabilities and ensures that the correct disinfection levels are maintained. But establishing such stations is not a trivial exercise, for a variety of different reasons. Suitable sites for these control stations can be difficult to locate and there are frequently no existing concrete buildings.

In addition, power supplies can be temperamental, exposed sites can be prone to lightning strikes with associated power interruption, access can be difficult in winter, and the quality of incoming water may vary considerably. Also, due to the remote locations, there is a requirement for residual chlorine levels in the treated water to be monitored at a central office to ensure that there are no problems.

Another difficulty faced by these remote stations is the fact that sodium hypochlorite is not the easiest chemical to use due to the fact that it can outgas easily. Dosing is achieved through the use of precision dosing pumps but, if gas is formed in the system, dosing is interrupted and disinfection efficacy is impaired.

Scottish Water is one of the authorities that has responsibility for ensuring water quality across a number of remote areas, including Foula - nearly 50km off the Atlantic coast of the Shetland Islands. Seeking to address all the problems associated with unmanned water treatment stations, Scottish Water Solutions has developed the Sodium Hypochlorite Disinfection Signature Solution. This involves the construction of dedicated dosing kiosks that can be assembled, equipped and tested before transportation to the particular location.

The stations (of which there are expected to be more than 60 across Scotland) are equipped with full sodium hypochlorite dosing systems, including dosing pumps, residual chlorine sensors, chemicals, battery back-up, PLC controller plus telemetry systems to relay residual chlorine level and alarm signals back to a central monitoring station. The station consists of an all-weather GRP kiosk split into two compartments.

A dry-side compartment houses the PLC control panel, heating, lights and emergency lights, lightning protectors, telemetry equipment and power supplies. The wet side-compartment contains the dosing pumps, day tanks, residual chlorine sensor, housing and buffer system, sample pump and carbon dioxide pH correction system.

One of the dosing pumps approved for use in these kiosks is the Delta Pump from ProMinent Fluid Controls. It features a controlled solenoid drive system that the company says extends the pump's capability to include continuous dosing and addresses the problem of outgassing, both through a gentle pumping process and an integrated hydraulic monitoring function.

The Delta Pump has two methods of addressing the outgassing phenomenon. First, the drive mechanism gives a smoother flow and reduces the impact of the diaphragm on the hypochlorite solution. This significantly reduces the likelihood of the gas being formed, says ProMinent.

The solenoid dosing pump is controlled using a method called optoDrive. This uses a position sensor to detect the diaphragm displacement and reports it to a microprocessor. During the stroke, the microprocessor permanently compares the diaphragm position with the predefined target data, while the solenoid drives the diaphragm.

The drive also automatically compensates for possible back-pressure fluctuations, which otherwise would have a negative effect on accuracy. The arrangement allows a slow pressure output stroke to be set for almost continuous dosing, or a fast stroke to be set for pulsed dosing. In addition, the pressure of the suction stroke can be adjusted to give greater control over a range of applications.

The combination of different suction and output stroke lengths means that the pump can be easily adapted to the properties of the dosing medium, and to the process conditions. The diagram above shows the range of output flows that can be achieved using these combinations.

Slowing down the suction stroke is used for outgassing fluids such as sodium hypochlorite solution. The pump also features the optoGuard integrated injection control system, which detects air or gases locked in the liquid end of the pump that would prevent dosing of the correct quantities. The pump display shows the relevant error messages in full text (see right), and the system allows automatic de-aeration by sending a signal to a special relay module and solenoid valve attached to the coarse/fine bleed valve of the pump.

Another feature of the pump is the turn-down ratio of 40:1. The turn-down ratio determines the range of operation for which dosing meets the accuracy specified for the pump. Although a dosing pump can be adjusted to dose at any flow rate between zero and its maximum capacity, most have a turn-down ratio of 10:1.

This means that the pump is within its accuracy rating anywhere between 10% and 100% of capacity. But the Delta pump meets its accuracy rating anywhere between 2.5% and 100% capacity. This gives a greater dosing range using a single pump capacity. This is important because, depending on the location and incoming water quality, dosing requirements could vary significantly. The high turn-down ratio avoids the need for additional dilution of the sodium hypochlorite solution.

The Delta pump has almost no wear because the drive has only few movable parts. Low maintenance and high durability mean low operating and life cycle costs. And the high turndown ratio means that the same capacity pump can be used in a variety of locations, keeping down the cost of replacements and spares. The minimising of outgassing and automatic de-aeration is also of benefit for remote operation.

ProMinent Fluid Controls, working in co-operation with its subsidiary in Scotland, ProShield, can take responsibility for the assembly and testing of the complete kiosk. It is currently supplying three such systems to Scottish Water.

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