Adjusting pH of wastewater & a bit about alkalinity too

Biological wastewater treatment usually works best in a pH range from 7.0 - 8.0. Remember that this is the "best" range in a general sense. In making bioaugmentation products, we have used strains with pH ranges from 3.0 (Thiobacillus) to 11.0 (alkanophilic Bacillus). 

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.

Buffering refers to how the pH tends to remain stable once adjusted. Organisms such as AOB & Nitrite Oxidizing Bacteria (NOB) like a slightly alkaline pH while also consuming significant alkalinity (usually expressed as calcium carbonate). Additional alkalinity is required to buffer against organic acids, carbon dioxides (from respiration), and other biochemical processes. This is why we often run alkalinity tests on influent during initial characterization to see if we have enough buffering at the inlet for desired biological activity. To add alkalinity we have several choices:

• 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.