What is pH Exactly?
pH means “potential of hydrogen,” and refers to how much hydrogen is mixed with the water. The pH scale is used to measure the acidity or basicity of an aqueous solution. pH is approximately the negative of the base 10 logarithm of the concentration (in moles/liter) of hydrogen ions.
The range goes from 0 – 14, with 7 being neutral. An acidic solution has a pH below 7, and a basic (also, “alkaline” or “caustic”) solution has a pH above 7. Often, these pH units are abbreviated “s.u.,” for “standard unit.” Because it is based on a logarithmic scale, each s.u. represents a 10-fold change in the solution’s acidity/basicity. A solution with a pH of seven is ten times more acidic than a solution with a pH of eight.
The pH of water determines the solubility and biological availability of chemical solutes, such as nutrients and metals. For example, pH affects how much and what form of phosphorus is most abundant in the water, and whether aquatic organisms can use it. Metals tend to be more toxic at lower pH because they are more soluble.
In addition to the potential to harm organisms, solutions deviating too far from a neutral 7.0 value may damage solids with which they come into contact. For example, acid solutions can damage metal, for example. Alkaline solutions can damage engineered stone.
Normal rainfall has a pH of about 5.6. It is slightly acidic because of the carbon dioxide gas in the atmosphere. The human body has a natural pH of 7.4.
pH Adjustment and Water Quality
An integral part of maintaining clean water, pH monitoring ensures final water streams are roughly neutral. In order to protect surface and groundwater, regulations often require pH to be monitored and maintained within a specified range, typically 6.0 or 6.5 to 8.5 or 9.0 s.u.
Depending on the nature of a project and initial water characteristics, it may be necessary to add appropriately-selected acid or caustic solutions to adjust the pH to meet quality requirements. Raising hydrogen (H+) ion concentrations by adding acid-containing solutions decreases pH levels. Conversely, addition of hydroxide (OH-) groups increases pH levels. With proper monitoring, pH adjustments and precautions, appropriate chemical addition returns water bodies to appropriate pH levels.
pH and Water Treatment Systems
In addition to helping meeting appropriate pH discharge standards, pH adjustment can be used within a system to maximize its efficacy. Below are examples of water and wastewater treatment applications that require pH adjustment.
The pH of water often rises by as much as 1.0-2.0 s.u. after the water passed through granular activated carbon (GAC) adsorbers. The pH increase is caused by the release of hydroxyl ions of water molecules adsorbed on the GAC, which is replaced with anions such as sulfate. The pH of the GAC effluent often must be reduced in order to meet a discharge limit.
GAC’s adsorptive capacities for acidic organic compounds such as perfluorinated compounds (PFCs) or phenolic compounds significantly decrease when the pH of water is above 9.0. It is beneficial to maintain the pH control of water below 9.0 when GAC is used to remove those types of compounds.
In air stripping processes, the groundwater pH may rise due to the loss of acidic carbon dioxide through the air stripping. The increased pH causes precipitation and scaling of minerals such as calcium, magnesium, iron and manganese. Injection of an acid solution may be needed to prevent the rise in pH and subsequent mineral precipitation.
Removing gaseous contaminants such as hydrogen sulfide (H2S) and ammonia (NH3) from water by air stripping requires pH adjustment. The pH must be reduced to below 5.0-6.0 in order to convert sulfide ions (S-) into the strippable gaseous form of H2S. On the other hand, the pH must be raised to above 11.0-11.5 in order to convert ammonium ions (NH4+) into the strippable gaseous form of NH3.
In flocculation to remove suspended solids or heavy metals through co-precipitation, the pH should be kept within the optimum ranges. The optimum pH for aluminum salts such as alum (aluminum sulfate) is between 6.0 and 7.0. The best pH for iron salts such as ferric chloride is between 5.0 and 7.0.
Heavy metals can be removed from water by raising the pH. At high pH’s, heavy metals precipitate in hydroxide forms. However, the optimum pH for precipitation of each metal can be different. The optimum pH’s are in a range of 8.0-9.0 for chromium, copper, lead and zinc, and in a range of 10.0-12.0 for nickel, iron, and cadmium.
Iron and manganese can be removed from water by chemical oxidation using oxidizing agents such as chlorine, sodium hypochlorite, and potassium permanganate. These processes require pH to be above 7.0.
Two-step processes are used to remove hexavalent chromium and amenable cyanide from water. The chemicals are first changed to different forms by pH control and small adjustments. The new chemicals can then be precipitated or converted to a less-toxic form by adjusting the pH a second time.
ProAct Can Handle Your pH Adjustment and pH Control Needs
ProAct has equipment designed to either manually or automatically adjust pH in a water stream. In addition to equipment rental, ProAct specialists are proficient in chemical selection and verification of a proposed system’s ability to function with a given body of water.
Based on water treatment applications, contaminant concentrations, and desired flowrates, pH adjustment and selection of appropriate equipment can easily be integrated into an existing or new water treatment system. Our trained technicians are proficient in performing pH adjustment measures safely.
Call a ProAct specialist today to discuss how best to meet your water treatment needs.