By Rafik H. Bishara, Steve Jacobs, and Dan Bell
In a number of water, wastewater and industrial process applications, pH is one of the most critical and highly sensitive analytical measurements. Examples of critical pH applications include: Reverse Osmosis (RO) systems in which a controlled feed of caustic solution is typically added to the feed stream in order to convert a portion of dissolved carbon dioxide into bicarbonate precipitate allowing for removal by the RO membrane. In wastewater treatment, online pH can be measured in the biological treatment (aeration basins) controlling the acidic or alkaline condition of the mixed liquor in order to provide optimum conditions for microorganism activity and therefore waste conversion. Drinking water plants also have a requirement for accurate and reliable pH control systems from inlet, to determine the appropriate treatment for incoming water to outlet, to ensure proper treatment has resulted in neutral pH water being circulated into the public water supply.
The majority of instrumentation in automated processes when it comes to mechanical measurements like flow, level, temperature and pressure offer relatively constant performance and response time independent of the instrument’s time in service. Being based on an electrochemical measurement, this however is not the case for a pH electrode as there are many process factors which can influence the response rate and accuracy of a pH measurement point. The three common methods to determine pH are the visual method using litmus paper, photometric method which uses a spectrophotometer to measure the wavelength and the potentiometric method which is an electro-chemical measurement, measuring the e.m.f created by a chemical reaction. The only method which can be used in process control for continuous in-line measurement is the potentiometric method. There are many factors which can cause a delay in accurate pH measurement with this principle. The response time can vary based on the thickness and composition of the probe gel layer, diaphragm porosity, reference contamination, age of probe, glass membrane integrity, and the length of the signal cable. To achieve the nominal tolerance of 0.01 pH units, a response time of 30 seconds is normal in ideal conditions. This delay in response time can have a negative impact on any of the pH critical processes mentioned earlier, the most costly and time sensitive of which being the RO system application. A delay in pH response can easily result in the fouling of an RO membrane when a chemical is being added for downstream consumption if the flow rate is not being monitored. The resulting overdose of caustic into the process stream if the flow rate were to drop and not be compensated for, would potentially cause the process stream to fall outside the specified pH range for the RO membranes. This is of particular concern when the concentrate (reject waste) is being recycled into the feed stream to improve overall efficiency. In this case the flow rate will be fluctuating and the pH will be decreasing due to the formation of carbonic acid. An ideal solution would be to correct the pH immediately based on flow. A program has been implemented in Burkert Fluid Control’s type 8619 multiCELL multifunction transmitter/controller to do just this. It is done using multiple logic blocks to allow for a great deal of flexibility.
In an inline pH control system it is essential to confirm the measurement line has the minimum flow velocity. This is because the chemical injection nozzle may be any distance away from the analytical measurement point and if the flow becomes static the pump will continue to dose until the pH adjusted process fluid reaches the measuring point, resulting in drastic overshoot beyond the set-point. This has been incorporated in the multiCELL program using a “System Switch” logic block which can be used to override the program if there is no flow detected. The override in this case would be to keep the pump turned off even if the pH set-point is not achieved. When trying to adjust a pH dosing pump based on attenuating flow, a ratio flow control loop can be used. This will need to be combined with a PID loop using pH as the process value. When the two outputs are combined using only a scaled value of fifty present on each, the result will be the desired full range. This has been done in the multiCELL using the “PROP”, “On/Off”, “PID”, and arithmetic “A+B” function blocks. Not only is this method effective for dosing chemicals in a safe and effective manner to protect RO membranes from damage but also to increase efficiency and cost control in a plant by minimizing excess dosing of expensive flocculants and neutralization chemicals.
With the sheer range of pH applications throughout all water and wastewater treatment industries and the criticality of each of these measurements on the process as a whole, it’s definitely worth taking the time to implement a well thought out pH control system which is designed to fit each unique application. Carefully considering the layout of injection points, pH measurement points and the potential need for integrated flow compensation between the two will lead to a more responsive pH loop, a more efficient chemical dosing scheme and better overall plant process control.
Sadiq Khan is with Burkert Fluid Control Systems, a part of Burkert GmbH & Co. KG. For more information, Email: firstname.lastname@example.org