As pH and Alkalinity are two parameters that are commonly monitored and controlled in wastewater treatment, this article serves to describe what they are and the importance of controlling them. Keep reading to find out more.
pH is simply the measurement of the amount of hydrogen ion (H+, acid) in the water. As a water becomes more acidic, the H+ concentration increases and the pH goes down. On the pH scale, a water that is perfectly neutralized has equal amounts of H+ and OH– (hydroxide ion), but both are so low in concentration (each at 0.0000001 mg/L or 1 x 10-7 mg/L) that they are practically not present. Inserting the H+ concentration into the pH equation gives a pH of 7 for neutral water (pH = -log[1×10-7] = 7). With 7 being neutral and at the center of the scale, the pH can range from 0 to 14.
If acid (H+) is added to the water so that the H+ concentration is now 0.001 mg/L or 1 x 10-3, then the pH will be -log[1×10-3] = 3. As water becomes more acidic, the pH value goes down. Weak acids like vinegar and orange juice have pH values around 2 to 4, while the addition of a strong acid like sulfuric acid (H2SO4) to water can drop the pH to <1.
If base (OH–, hydroxide) is added to the water, then the H+ concentration will decrease, perhaps down to 0.000000001 mg/L or 1 x 10-9 mg/L). In this case, the pH will be -log[1×10-9] = 9. As water becomes more basic, the pH value goes up. Weak bases like magnesium hydroxide and hand soap can have pH values around 8 to 11, while the addition of a strong base like caustic soda (NaOH) can drive it up to >13.
It’s important to control pH for two primary reasons, starting with industrial discharge, where controlling the pH above neutral (>7) helps to prevent harm to the infrastructure. A water with an acidic pH is corrosive to concrete and carbon steel pipe and surfaces in the collection system. Therefore, industrial dischargers are required to do pretreatment to control their pH within an optimum range near neutral or slightly basic. When the water reaches the WRRF (water resource recovery facility), maintaining that neutral or slightly basic pH is ideal for microorganism activity, health, and biodiversity. In this way, the performance of the bacteria within the treatment plant is optimized, and when the final effluent water is discharged back into the environment it will have a positive effect on the native life in the receiving water system (river, ocean, etc.).
M Alkalinity is simply a measure of a water’s ability to resist changes to the pH. In actuality, it’s a measure of the bicarbonate ion (HCO3-) concentration. The higher the HCO3- concentration, the higher the alkalinity, and the stronger the buffering strength of the water. In practice, the alkalinity lets you know how much acid can be added to the water before you would see a drop in your pH level.
The key parameter for controlling pH is alkalinity, and microorganisms perform best when they are controlled at neutral to slightly basic pH. Maintenance of sufficient alkalinity buffers the effects of any incoming wastewater pH fluctuations. Likewise, by holding a strong alkalinity, any acids produced via WRRF microbiological processes (the most notable is Nitrification) will not result in a low pH upset condition. However, when the wastewater entering a WRRF is low in alkalinity, a supplemental additive will be needed to provide the alkalinity buffering necessary to maintain stable microbiological processes.
The four most utilized alkaline additives are sodium hydroxide, sodium carbonate, calcium hydroxide, and magnesium hydroxide. The most common additive is Sodium Hydroxide (NaOH or caustic soda). NaOH is a very strong base that is highly hazardous. It can rapidly raise the pH but is a weak buffer and does not add much alkalinity to your system. Sodium Carbonate (Na2CO3 or soda ash) provides the bicarbonate ion for improved alkalinity. However, it requires a much higher dose to raise the pH to the desired level. Calcium Hydroxide (Ca(OH)2 or lime) gives a good pH increase and alkalinity. However, it is hazardous and difficult to feed and tends to cause calcium scale formation in your system and high levels of inert solids in the sludge. Magnesium Hydroxide (Mg(OH)2 or milk of magnesia) is the safest and strongest alkaline additive known, providing the highest pH increase per dose of treatment, along with strong alkaline buffering and improved sludge dewatering. For these reasons, it is known to be the most cost-effective alkaline additive for wastewater treatment processes.
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