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The “potential of hydrogen” or “pH” is the measure of free hydrogen activity in water and thereby indicative of the measure of its free acidity or free alkalinity. Gauged on a numeric scale of 0-14, solutions with a pH of less than 7.0 are acids. Solutions with a pH greater than 7.0 are bases and solutions with a pH of 7.0 are neutral.
The pH is the most used measurement in wastewater treatment. Dependence on pH is found at every phase of water supply and wastewater treatment, including acid-base neutralization, water softening, precipitation, coagulation, disinfection and corrosion control.
In the simplest terms, bases are used to neutralize acids, while acids are used to neutralize alkalis. The terms “alkali,” “alkaline,” “caustic” and “base” are often used interchangeably.
The pH of common solutions vary dramatically such as lime juice having a pH of <2.5 to Milk of Magnesia having a pH of >10.0.
The best pH for a wastewater treatment process depends on the water’s ultimate use.
For example, the pH value for discharging treated wastewater into an environmental water stream, be it ocean, river or creek, often requires regulated pH ranges between 6.0 and 9.0. In comparison, pre-treated wastewater being discharged into a municipal sewer systems requires regulated pH ranges between <6.0 and >9.0.
Optimum pH also varies dependent upon wastewater treatment requirements, especially when treatment process tanks receive untreated wastewater from production processes or upon the available wastewater treatments available, and the need for tightly maintained set points.
For certain wastewater treatment processes a pH as low as 3.0 is maintained, while others specify a precise pH value as high as 11.0.
Consider the following in regards to controlling pH in treatment tanks to sustain unique wastewater treatments during a continuous or batch process.
Continuous pH adjustment
In continuous pH adjustment, the treatment tank operates full at all times. Consequently, one gallon entering the tank displaces one gallon exiting the tank discharge. As the influent flow enters the treatment tank it mixes with the tank contents. If the influent pH varies from that of the tank contents, which is likely, then the influent flow will be pH adjusted through the resultant chemical reaction that occurs as the influent mixes with the contents.
An equal and opposite reaction takes place within the tank contents. This opposite reaction is sensed by the pH probe which delivers a continuous pH feed-back signal to the pH controller. The controller triggers the appropriate metering pump to bring the tank water level back into set-point range.
If the influent flow was alkaline, for example, the result would be a steady rise in the tank pH as measured by the pH probe at the tank discharge location. The pH controller would then signal to operate the acid metering pump at an appropriate rate to return the pH to set-point range.
A major advantage to this layout is simplicity and relatively high flows. However, since the tank is always full there is no guarantee, regardless of tank size or control-system proficiency, that the effluent will always be in set-point range. Recognize, the pH control uses a feed-back loop, which does nothing until an out of set-point value is sensed.
If influent flow and chemistry are high enough or strong enough then the effluent pH diverge from and remain out of the pre-programed set-point ranges. Therefore, a pH control backup measure such as regular monitoring may be advised.
Batch adjustment of pH
Here there is 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.
In batch pH adjustment, untreated influent enters and fills the tank to the high tank-level point. For the untreated waste, the pH adjustment process occurs much in the same way that a continuous system performs. The difference, however, is that a large volume is treated in one cycle. Once the tank contents are within the discharge range for a minimum working period of time the effluent discharge valve opens thereby allowing the tank to empty. Once the tank is empty, the cycle repeats.
The batch advantage is that no effluent is removed from the tank until the discharge criteria is met. 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.
The throughput of many designed pH neutralization / adjustment systems is limited by several major drawbacks. These flaws pertain to pH probe response time, mixing efficiency, tank design, chemical metering precision, chemical reaction times and pH control interaction.
So-called “advanced-procedure” pH controls address each of these deficiencies individually and harmoniously. With the use of advanced procedure pH controls, consistent, reliable results are achievable.
Certain general steps for controlling pH have been described. If you have specific pH or other wastewater queries, please submit a question.