How to calculate the amount of citric acid to neutralise the pH within wastewater? We have an effluent tank, with a consent to discharge at between 5.5 - 11 pH, the tank holds 100000&nbsp;liters of dirty water.

When the pH reaches 10.5 we add citric acid to bring the pH down. How many liters of citric acid with a pH of 2.2 should we add to the system to bring the pH down to around 7. We always end up putting too much in then add caustic to bring it back up.
Gotta start with my favorite formula:
$$Q = CV$$

$$C_\textrm{eff}V_\textrm{eff}+C_\textrm{acid}V_\textrm{acid} = C_\textrm{out}V_\textrm{out}$$

$$(10^{-10.5}~\mathrm{mol/L})(10^5~\mathrm{L})+(10^{-2.2}~\mathrm{mol/L})V_\textrm{acid} = (10^{-7}~\mathrm{mol/L})(10^5+V_\textrm{acid})$$

$$V_\textrm{acid} = 1.51~\textrm{L}$$