Water treatment processes are used to remove impurities, both biological and chemical, as well as balance pH levels. In the vast majority of installations, on-site staff can monitor and maintain the equipment to ensure continuous operation. However, in remote locations, regular on-site interventions are not practicable, so reliability has to be built-in to the equipment.
Water quality is determined by standards and specifications and it is the responsibility of the local water authority to deliver these standards. Significant geographical areas within the UK experience high pH levels within their surface water, due to organic matter within the soil, so there is a need for a simple and reliable control process.
RSE uses carbon dioxide (CO2) to reduce the pH of the water to increase the mineral content through reaction with limestone/remineralisation media. It is a gas that is easy to handle, non-corrosive and its most appealing feature is that it will not lower the pH of water below 4.0. In addition, the only maintenance required for the dosing system is to replenish the gas cylinders periodically.
The control structure for this dosing system needs to cope with variable flow as well as decreasing gas pressure as the volume in the cylinders decreases. It is very important for the integrity of the process to use a mass flow controller that is calibrated for the gas and delivers accurate measurements independent of temperature and pressure. The MFC is precalibrated by Bürkert for the gas being controlled before it is delivered to the customer.
Many engineers will read a pH sensor after the dosing point and use this information to adjust the gas flow rate. This reactive process can be optimised by adding a pH sensor to the input side, and using the readings from this sensor to set the CO2 dosing rate. The second sensor then acts as validation of the process setting. This offers a quicker response to changes in the pH levels at the input.
The design and parameter settings for the pH control process are determined by the local water authority. On each site the process control loop consists of a pH sensor, some solenoid control valves and a mass flow controller (MFC) supplied by Bürkert. A Bürkert Type 8711 MFC is used in each application. In contrast to a simple flow control device, the measurement is independent of pressure and temperature, making it more accurate in varying conditions. When used with CO2, the Type 8711 has a nominal flow range from 0.02 LN/min up to 40 LN/min.
RSE is planning to to use this system on six future water treatment projects in 2019. Paul McCloskey, senior technical manager at RSE, concludes: “We design these compact treatment plants for optimum performance with minimal maintenance. The Bürkert mass flow controller is central to maintaining the correct pH and it is delivered as a ‘plug & play’ component, which minimises our installation and configuration time.”