The calculation of the pH value at the discharge location of the receiving water is necessary for the calculation of the NH3-NH4+ equilibrium. The alkalinity must also be known for this calculation. The following formula from Jordan (1989) is based on the assumptions that (1) the CO2 exchange with the atmosphere can be neglected and (2) the alkalinity of the water is mainly determined by HCO3 ions. So, the following relation between alkalinity, the concentration of dissolved carbonates and the pH for a highly buffered water mixture can be established:
By means of this equation the pH value can be calculated for any combination of alkalinity and cT value. The procedure for the pH calculation of a water mixture is as follows:
•For each part of the water mixture (in our case receiving water, discharges from combined and separate sewer system) pH and alkalinity must be known.
•cT values for all parts are calculated with the above equation.
•Alkalinity and cT value for the water mixture results from a simple volumetric mixing calculation as for the determination of the NH4-N concentration.
•The resulting pH value in the receiving water at the discharge location can be computed by applying the above equation with alkalinity and cT value of the mixture.
REBEKA computes the pH value at the discharge location from the mixture of the receiving water (part 1) and the discharged water (part 2). The second part is the sum of the discharges from combined and separate sewer system. It is assumed that pH and alkalinity of these two discharges are identical.