In primary treatment, what are the pH process controls for treating wastewater?

Oct. 21, 2014

The measure of free hydrogen activity in water — called “pH” for “potential for hydrogen” — is undoubtedly the most important and most-used wastewater treatment …

The measure of free hydrogen activity in water — called "pH" for "potential for hydrogen" — is undoubtedly the most important and most-used wastewater treatment measure. Dependence on pH as a measure relevant to water quality and its control occurs in practically every phase of water supply and wastewater treatment, including acid-base neutralization, water softening, precipitation, coagulation, disinfection and corrosion control.

The best pH target for a given wastewater treatment process depends on the use to which the water will be put and how that use impacts pH.

The pH is the controlled parameter because it has direct influence on wastewater treatability — regardless of whether treatment is physical/chemical or biological. Because it is such a critical measure of wastewater makeup, it is critically important to treatment.

As you assume control

In taking control of pH, first identify the constituents — the pollutants or impurities — that are actually in the wastewater. Once those pollutants are identified, determine the starting and the ending pH values, along with a specific treatment procedure. Then select the chemicals best suited to the prescribed treatment.

The starting and ending pH values in large part determine the treatment procedure. The pH adjustment requires residence or contact time during the wastewater treatment for the pH to change as needed. Traversing a very narrow pH range (i.e., 7.0 to 8.0) requires less contact or residence time compared to a wider pH range (i.e., 7.0 to 10.0).

Chemical selection is important because of varying reaction times, which in turn affect pH adjustment and control. Therefore, what treatment chemicals are chosen impacts equipment residence or contact time.

The pH cannot be changed instantly. The change rate depends on chemical reaction times, in turn associated with tank volume, the amount of mixing, and other treatment aspects. Often, the pH can change even after the wastewater leaves the treatment tank. In those instances, the reaction time exceeds residence or contact time.

Equipment & devices

Treatment often requires rigorous pH control precision. It is only seldom that manual pH control is attempted. Multiple constraints make this impractical, so automation is recommended.

The equipment — including tank size and mixer, as well as location and use — instrumentation and specific procedures all profoundly influence the outcome. For instrumentation, including controllers, the type and settings are paramount.

Widely used, PID-type controllers can regulate the volume of chemical pumped, to maintain a requisite value regardless of variations of wastewater and its pH value. Finding the right PID settings is easy so as to optimize wastewater treatment performance with today’s modern process controllers.

PID stands for Proportional-Integral-Derivative, referring to the three parameters used to produce a control signal based on the measured error from the setpoint.

Proportional control gives a change in output that is proportional to the deviation of the process variable from the set point. The range over which the output is adjusted from 0 to 100% is called the proportional band. The proportional band is expressed as a percentage.

Ultimately, rigorous pH control improves wastewater treatment and simultaneously reduces chemical usage and the associated cost, therefore increasing profitability. Rigorous control can be achieved implementing a pH adjustment system in a wastewater tank using either a continuous or a batch pH adjustment system.

Continuous and batch

In a continuous pH adjustment system, the pH treatment tank operates full at all times. Consequently one gallon entering the tank influent location displaces one gallon through the tank discharge location. A major advantage to this type system is its simplicity and capability of handling relatively high flows.

In a batch pH adjustment, the layout includes a treatment tank, mixer, acid and caustic metering pumps, pH probe and controller, level sensor and discharge valve. Influent flow enters the tank anywhere convenient and exits the tank near the bottom, or wherever the tank effluent location can be operational.

The advantage of a batch system is that no effluent is removed from the tank until the effluent meets the control-discharge criteria. Batch systems are far more suitable for smaller treatment volumes and effluents that may be characterized by large swings in influent pH, concentrated discharges or erratic flow rates.

Industries requiring pH adjustment during primary wastewater treatment include food production, chemical processing, textiles, pulp and paper, metal fabricating/plating/painting and others.

Certain general steps for controlling pH (Potential for Hydrogen) has been described above, however if you have specific pH or other wastewater queries, please submit a question.

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