The phase equilibrium gas-liquid plays a great role in the heat-diffusion processes which are widely used in the treatment of industrial as well as communal waste water. The general thermodynamic model describing equilibrium in the system H2O-NH3-CO2 has been analyzed in this work. Some calculation to determine the limits of possible simplifications were carried out. The analytic formulae for fast calculation of approximate values of equilibrium concentrations were derived on the base of some assumptions. Some test calculations were carried out basing on the simplified model. The results of the calculations point to the leading role of electrolyte dissociation of CO2 and NH3 in the phase equilibrium. The effect is particularly profound in the range of low concentrations.