Determination of Minimum Required Water Flow Rate for Diluting Wastewaters before Their Discharge, Taking into Account the Summation of Effects of Harmful Substances
Procedures of reducing waste water discharges into water bodies to safe levels are described. A technique of evaluating the minimum required water flow rate for the dilution of waste waters containing several harmful substances before their discharge into a water reservoir, as well as evaluating maximum allowable discharges (MADs) of wastewaters, is given. This technique makes it possible to take into account classes of hazard and the summation of effects of substances with the same limiting harmful indices. Expressions for determining the minimum required water flowrate for wastewater dilution were obtained from the condition of admissibility of the effect of several harmful substances on reservoir waters as well as from equations of substance balance in a mixer and the mathematical description of the process of eddy diffusion of substances in the flowing water reservoir. The conditions of admissibility of the effect are presented in the form of logical products of inequalities in which the sums of relative concentrations should not exceed the value of 1. An analytical expression of the dependence of relative substance concentration in the most contaminated jet of the control point of the river on the water flow rate for dilution has been obtained. It is shown that the minimum required water flow rate for wastewater predilution can be calculated only on the basis of a numerical method. An example of numerical calculation of the water flow rate for the predilution of technical facility waste waters containing two substances, polysulfide oil and nitrites, is given for illustrating and testing the technique. In addition to these two harmful substances, the reservoir upstream the point of wastewater discharge contained chrome and fluorine. For nitrites, chrome, and fluorine, the summation of their harmful effects was taken into account. For the example under consideration, it is shown that nitrites can be safely discharged without predilution if the unidirectionality of effects is not taken into account. If the summation of effects of harmful substances is taken into account, it is necessary to predilute waste waters at a dilution multiplicity of 6.4 and a decrease in oil and nitrite concentration by 6.8 and 7.3 times to ensure environmental safety.Keywords: environmental safety, wastewaters, limiting harmful index, class of hazard, predilution, minimum required dilution multiplicity, numerical method