pH is a measure of the how acidic or basic water is, with a range from 0 to 14. More exactly, it is the measurement (negative logarithm) of hydrogen ion concentration [H+]. The scale ranges from 0 (most acidic) to 14 (most basic). The term pH refers to “pondus hydrogenii” or power of hydrogen.
pH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity, aH+, in a solution.
Wastewater Discharge Requirements
Neutralization of wastewater that is highly acidic (low pH) or highly basic (high pH) is required for discharge to municipal sewer systems or to rivers and to streams. The allowable pH range for discharge is generally 6 to 9 standard units (S.U.) but can be 5.0 to 11.0 depending on the source of discharge. There are four main reasons to control the pH in wastewater:
- Protect human health and the environment; fish and other living organisms in rivers and streams.
- Protect the wastewater infrastructure from corrosion, especially from acidic wastewater.
- Protect the biological treatment process incorporated at most publicly owed treatment works (POTW).
- Many pretreatment systems require that the pH be adjusted as an initial step in the treatment process, such as:
- Coagulation and flocculation processes for the removal of oil and grease, phosphates, and suspended solids.
- Precipitation of metals in the metals removal treatment process.
There are many types of industries with multiple sources of wastewater that, if discharged untreated, will violate pH requirements. Acids and bases are used throughout plant production and in cleaning operations.
Wastewater with a low pH is generally neutralized using sodium hydroxide (NaOH), lime (CaO), magnesium hydroxide (Mg(OH)2), or calcium hydroxide (Ca(OH)2). Wastewater with a high pH is generally neutralized with sulfuric acid (H2SO4), hydrochloric acid (HC1), or carbon dioxide (CO2). The most common chemicals used are H2SO4 and NaOH.
Neutralization can be accomplished by batch treatment or continuous flow processes. Batch treatment is usually for lower flow volumes.
Most systems are continuous flow type systems consisting of one or two neutralization tanks, depending on the flow rate and magnitude of pH swings. With some systems, the pH must be raised or lowered for other removal processes to function properly, and then the pH adjusted back prior to discharge. A simple single tank continuous flow pH neutralization process is shown in the diagram.
A pH sensor, located in the tank, measures the pH and, if necessary, the pH indicator-controller system activates the appropriate metering pump, which automatically doses acid or caustic based on the pH of the wastewater.