How to understand the addition of chemicals in water treatment

Alkalinity is an indicator of wastewater's ability to neutralize acids. Alkalinity is closely related to pH and is crucial for wastewater treatment plants using biological nitrogen and phosphorus removal processes.

Alkalinity consumption during nitrification leads to a decrease in wastewater pH. Chemical precipitation of phosphorus using iron or aluminum salts also causes a decrease in alkalinity.

A decrease in pH reduces the rate of nitrification; nitrification stops when the pH is around 6. When the pH is below 7, polyglucose bacteria compete with polyphosphate bacteria, affecting the polyphosphate bacteria's ability to utilize VFAs and thus impacting biological phosphorus removal. Additionally, alkalinity reflects the buffering capacity of wastewater, i.e., its ability to cope with variations in influent water pH.

Chemicals that can be used to supplement alkalinity include sodium hydroxide (NaOH), calcium hydroxide (hydrated lime) [Ca(OH)2], and calcium oxide (quicklime) (CaO).

Sodium hydroxide is more expensive but, compared to calcium hydroxide, is easier to handle and has lower annual operating costs for storage and dosing systems; calcium hydroxide is usually sold as a solid and must be made into a slurry before use; lime slurry tanks are prone to scaling; calcium oxide requires slaking, a process with harsh working conditions and high labor intensity, requiring significant manpower to maintain equipment operation.

When designing an alkalinity supplementation system, a target alkalinity of 50-100 mg/L (as CaCO3) in the effluent is generally adopted. Each plant must conduct an individual assessment during actual operation to determine the effluent alkalinity required to maintain stable effluent pH.

When determining the dosage, the impact of subsequent processes on effluent pH and alkalinity must be considered. Chlorine typically increases acidity, further lowering effluent pH; sodium hypochlorite increases alkalinity; when using iron or aluminum salts for phosphorus precipitation, excessive addition of aluminum or iron salts in the aeration tank can lead to the formation of hydroxide precipitates, increasing alkalinity consumption.

Typically, for aluminum salts, 5.56 mg of CaCO3 is consumed for every milligram of aluminum hydroxide produced. For iron salts, 2.69 mg of CaCO3 is consumed for every milligram of iron hydroxide produced.

Sodium hydroxide is a strong alkali; excessive addition will cause a significant increase in pH. Diluted sodium hydroxide solution must be stored frozen at below 0 ℃. The freezing point of a 50% sodium hydroxide solution is approximately 12.8 ℃; therefore, its storage tank and pipes must be heated and insulated. If the liquid temperature falls below 12.8 ℃, the sodium hydroxide will crystallize and precipitate out of solution. Crystallized sodium hydroxide is difficult to redissolve. Sodium hydroxide is diluted on-site using plant water supply or drinking water; scaling is likely to occur at the mixing point.

Therefore, the pipe connections at the mixing point of the dilution system should be designed to be easily cleaned; the sodium hydroxide dosing point is also prone to scaling; it is recommended to dose sodium hydroxide into the return sludge pipe, as the higher flow rate in the return sludge pipe can protect the pipeline from scaling.