Integrating Flow and pH Control

By itself, a pH controller is blind to variations in flow, which can easily lead to under-treating or over-treating a wastewater stream. A flowmeter on the other hand, has no idea if pH changes occur. A treatment system that can allow for changes in either of these parameters would be ideal. For most customers this goal seems unreachable due to the sophistication and expense of the necessary equipment. This typically includes individual pH and flow measurement systems that can communicate to either a Programmable Logic Controller (PLC) and a linked computer, or a Proportional Integral Derivative (PID) Controller. These instruments can integrate the pH and flow signals to produce a single pump output to effectively correct pH for both variables.

Create a Simple Dual Control Treatment System using Flowmeters

Furrow Pump offers customers a simple treatment approach to accomplish the integration of flow and pH — one that minimizes the equipment required while still providing an acceptable level of pH control. All this calls for is an existing pulse or 4 to 20 mADC flow signal (or a new flowmeter to provide the same), an on/off pH controller with an electrode, and an LMI externally controlled pump. The flowmeter sends its proportional signal directly to an acid and caustic feed pump, which responds if it is enabled by the pH controller. The pH controller allows the pump(s) by switching on the power whenever the wastewater stream pH is outside the chosen set point range. Thus, you get a flow of proportional pH treatment whenever the pH is outside of acceptable parameters. The pH goes untreated only when the pH is within its acceptable range, or there is less than a minimum measurable flow. That low flow being untreated seems like a reasonable trade-off to economically achieve the flow and pH proportional treatment of all your measurable wastewater flow. If the application requires greater accuracy then there is more intricate equipment available.

Example 1: Short Form

Q: A customer has a wastewater pH that varies between 3 and 11, and a discharge window of 5 to 10. His flow ranges from zero to 500 gallons per minute, with an existing flowmeter that has a 4 to 20 mA output. How can this treatment system be set up for both pH and flow proportional treatment?

A: Connect the flowmeter signal directly to a chemical pump, setting it for 4 mA at 0 flow and 20 mA at 500 gpm. Any flow above the minimum measurable amount will start the pump, then the output will increase until it reaches its maximum of 500 gpm. Next, using an on/off pH controller put its low and high set points at 6 and 9, and plug the pump power into the output relay. This works for either an acid or caustic pump, or even both simultaneously. The result of this arrangement is chemical pumping that increases in proportional to flow, but only if the measured pH is either below 6 or above 9 since the pump has no power to operate within the 6 to 9 pH window. That makes this treatment system both pH and flow proportional.

Example 2: The Rest of the Story

A local tool manufacturer uses acid to clean and prepare the surfaces of its components and caustic cleaner for its tanks and other maintenance purposes. Along with their process and cooling water, their water flows can vary from 0 to 500 gallons per minute. Acid and caustic deliveries are planned and done at the same time so that they neutralize each other, minimizing the amount of pH treatment chemical needed to keep the plant discharge pH within the allowed window of 5 to 10. It is difficult to always do this so the wastewater pH varies from as low as 3 to as high as 11. A treatment system was called to ensure that the plant effluent always complies with environmental standards.

There is no need to treat water within the acceptable discharge range, but when sudden surges of acid or caustic are detected by the pH probe the system must respond instantly in proportion to that moment’s flow to get the treatment right. This is possible due to the plant’s effluent flowmeter, which has a 4 to 20 mA output signal. With the chemical pumps connected directly to this control signal, they are ready to respond immediately at the appropriate level whenever the pH goes beyond its programmed set points. For this application, having set points at 6 and 9 should prevent the discharge from going below 5 or above 10. This does depend on adequate mixing and reaction time which need to be engineered into the system.

The system described here works by controlling power to the pumps about the measured pH load. The pumps are “asleep” inside the 5 to 10 discharge window, because they have no 120 VAC power to operate with. The flow signal might be telling one of the pumps to go, but with a neutral pH there is no need to treat the water and they can’t work without power. As soon as the pH wanders, a relay turns on the power and the pump responds in proportion to the demand. This is an efficient way to operate. The only time that water could use treatment doesn’t get treated is when it is below the minimum measurable flow (which is neutralized by the bulk of the water already in the system). The tool manufacturer gets an efficient and economical package to solve his discharge problem.