The ionic balance is done in the same way as for drinking water. As you probably know, the calculations are based on the principle of electro-neutrality. The principle of electro-neutrality requires that the sum in eq/L or meq/L of the positive ions (cations) must equal the sum of negative ions (anions).
The wastewater contains mineral salts which must be analyzed. Certain mineral salts will surely be found in greater quantity.