With regard to wastewater, the pH refers to the acidity and alkalinity of influent sewage with the actual value depending on the hardness of the supply water (soft water catchments generally have a pH range of 6.7–7.5 and hard water a range of 7.6–8.2).
All organisms have an optimum growth pH at which the organism’s maximal specific growth rate can be achieved. Most organisms have a pH optimum within the neutral pH range of approximately 7, however some organisms require extremes of pH in order to function optimally.
Within wastewater systems, the majority of bacterial populations function optimally at neutral pH and are inhibited by extremes in pH, thus pH within wastewater systems should be monitored and adjusted.
In the treatment of industrial wastes, it is necessary at least to provide initial adjustment of pH, particularly if anaerobic biological treatment is to be successful. One method for restoring the pH balance is to increase alkalinity by adding chemicals such as lime, anhydrous ammonia, sodium hydroxide, or sodium bicarbonate.
"Hard water" is water that contains a high concentration of dissolved minerals- particularly calcium and magnesium cations. As water moves through soil and rock, it dissolves very small amounts of minerals and holds them in solution. Calcium and magnesium dissolved in water are the two most common minerals that make water “hard.” The degree of hardness, as well as the pH becomes greater as the calcium and magnesium content increases.
Hard water is not harmful, however it can cause problems to precision equipment or machinery in industry.
Nitrification is favoured by mildly alkaline conditions with an optimum pH between 8.0 and 8.4. The rate of nitrification becomes completely inhibited below pH 5. Decrease in pH will not only inhibit nitrification but also heterotrophic activity, reducing BOD5 removal. Thus, when this occurs a characteristic cycle in effluent quality can be identified, with high nitrate and low BOD5 concentrations alternating with low nitrate and high BOD5 concentrations.
The optimum pH in the aeration tank is 7–7.5. pH values below 6 may favor the growth of fungi (e.g., Geotrichum, Candida, Trichoderma) and cause filamentous bulking.
Most methanogens function optimally at a pH range of 6.7–7.4, but optimally at pH 7.0–7.2, and the process may fail if the pH is close to 6.0. Acidogenic bacteria produce organic acids that tend to lower the pH of the bioreactor. Under normal conditions, this pH reduction is buffered by bicarbonate produced by methanogens.
• Bacteria have optimum pH near 7 (in a range from 5-9).
• Fungi prefer acid environment (pH < 7).
• Algae prefer neutral environments (~pH 7)
• Most protozoa have a pH optimum range from 5 – 8.
Low pH also affects the PAO and GAO population dynamics by favouring GAO enrichment over the PAO population thus affecting orthophosphate removal.