The pH drop you're ...

The pH drop you're experiencing in the aeration tank of your Sewage Treatment Plant (STP) is indeed unusual, as typically the pH in the aeration tank tends to increase slightly due to biological processes (such as nitrification). A pH drop from 7.5 to 6.0 could be caused by a few factors:

Potential Causes for pH Drop:

1. Increased Biological Activity :

   • In the aeration tank, the nitrification process (conversion of ammonia to nitrate by nitrifying bacteria) can produce acidic by-products , particularly nitric acid . This can lead to a slight drop in pH.

   • If the ammonia concentration in the influent is high, the nitrification process will be more intense, potentially lowering the pH more than usual.

2. Excessive Organic Matter :

   • High concentrations of organic pollutants in the influent could lead to increased microbial activity. As microbes metabolize organic matter, they produce carbonic acid , which can lower the pH of the water.

3. Inadequate Oxygen Supply :

   • Inadequate aeration or insufficient dissolved oxygen (DO) could limit the efficiency of aerobic processes and push the system towards anaerobic conditions in some areas. Anaerobic bacteria produce acidic by-products , such as hydrogen sulfide or organic acids, which can lower the pH.

4. Chemical Imbalance :

   • If there is an issue with the chemical dosing in the STP, such as an excess of coagulants or flocculants containing acidic components, this could lead to a decrease in pH during aeration.

5. Contamination or Change in Influent Composition :

   • A change in the composition of the incoming wastewater (e.g., a shift towards more industrial effluent or high concentrations of acidic chemicals) could contribute to a lower pH after aeration.

Solutions or Steps to Investigate:

1. Monitor Ammonia Levels :
   Check the ammonia concentration in the influent. If high, the nitrification process could be contributing to the pH drop. You may need to adjust aeration or consider adding alkalinity (such as sodium bicarbonate) to buffer the pH.

2. Optimize Aeration Efficiency :
   Ensure that aeration is adequately maintained to promote aerobic conditions. If DO levels are low, increase aeration to maintain stable aerobic conditions and prevent anaerobic processes.

3. Check for Chemical Inputs :
   Review any chemicals being added to the system (e.g., coagulants, flocculants, or disinfectants) to ensure they aren’t contributing to the pH drop. Consider adjusting chemical dosing as needed.

4. Alkalinity Addition :
   If nitrification or organic acid production is lowering pH excessively, adding alkalinity (e.g., sodium bicarbonate) can help buffer the pH and maintain a stable level within the desired range.

5. Check Influent Quality :
   Investigate if there has been a change in the influent characteristics. The presence of industrial effluents, acids, or large amounts of organic material could contribute to the pH drop.