The most pH sensitive process tends to be ammonia removal or nitrification. Ammonia Oxidizing Bacteria (AOB) do best at a pH of 7.2 - 8.2 where the free ammonia (NH3-N) is present but is still soluble in water.
Now for pH adjustment - most systems use strong chemistry:
- To lower pH acids such as sulfuric acid (H2SO4) can be used. A newer technology is to use carbon dioxide to adjust pH without the chance to over shoot your target. The correct choice all stems from how much adjustment is required.
- Raising pH is usually done using sodium hydroxide (caustic soda) (NaOH). As with sulfuric acid, sodium hydroxide immediately raises pH. It does not buffer the solution & buffering is a key concept that we need to consider in biological systems.
- Lime (calcium carbonate) - the oldest method but is highly insoluble and somewhat messy
- Magnesium hydroxide - effective but also has solubility issues
- Sodium carbonate - much more soluble, but can be "too strong" a base
- Sodium bicarbonate - soluble, tends to max out pH at 8.3 - so low overdosing potential.
So remember that you have choices in adjusting pH and buffering the system. Alkalinity or buffering capacity is a key consideration in wastewater treatment especially if you require ammonia oxidation. AOB/NOB activity consumes approximately 7.1 mg alkalinity as CaCO3 per mg ammonia fully oxidized. Other processes in the nitrogen cycle can release alkalinity back into the water.