The Research On Scheme Of The Location Of Sewage Outfall Based On EFDC Model

The pollutant discharge to sea is the principal way of tail water disposal by sewage treatment plant in coastal areas.However,the unreasonable layout of drainage outlet may also have adverse effects to the Marine environment and the sensitive area.Therefore,how to scientifically and reasonably select the drainage outlet location is an important subject in the design of sewage treatment plant project.The design capacity of the newly planned western sewage treatment plant in Dalian Changxing Island Economic Zone plan is 80000 t/d,the treated sewage in which will execute 'Discharge standard of pollutants for municipal wastewater treatment plant'Level A standard,and the design scheme preliminarily selects three outlet positions with different coastal and offshore distances from the four types of functional areas.This dissertation chose a commercial software,'Environmental Fluid Dynamics Code'(EFDC)that has been widely-used in ocean hydrodynamics and water quality simulation in recent years to predict the water quality influence at three tail water discharge positions,and build the comprehensive evaluation system for the outlet location suitability,being combined with relevant index,to select the best outlet position scheme.First,the EFDC model was applied to numerically simulate the hydrodynamic conditions of the coast near Changxing Island,and the calculation results are consistent with the measured tide and tidal level from the monitoring sites.Based on the design position of three sewage outlets to the sea,then the EFDC model was applied to simulate and calculate the envelope area of the maximum concentration of representative pollutants in two tidal cycles.It was obtained that at the normal condition,the envelope areas of the maximum concentration of each pollutant from the three outlets exceeding the water quality standard ? are COD:0.04,0,0.03 km~2,inorganic nitrogen(DIN):1.57,0.76,0.47 km~2,active phosphate(AP):0.15,0.09,0.08 km~2;at the accidental situation when untreated sewage is directly discharged,the envelope areas of the concentration of each pollutant exceeding the water quality standard are COD:2.49,3.78,2.30 km~2,DIN:9.73,7.32,9.50 km~2,AP:12.2,5.91,7.09 km~2,and at a rising tide may cause the exceeding standard of concentrations of different pollutants,so that the direct discharge should be avoided at the rising tide period.For the comprehensive comparison among different positions of outlet of the environmental impact and economic cost,based on contaminated degree of the waters and the cost related indicators of the engineering construction,the comprehensive evaluation system of site suitability of outlets was constructed.The weight was determined by the ordered binary contrast method which is more simple and practical than the analytic hierarchy process,which obtains the weight by the results of fuzzy comparison between two adjacent indices in order,on the basis of qualitative ranking of all indices through comparison by their importance.The results of the comprehensive analysis of the three candidate outfalls show that the relatively optimal overall suitability in environmental impact and cost can be achieved when the one with moderate offshore distance and long distance to functional zone I is selected as the outlet.In this paper,the proposed research method and system of the location optimization for the outlet to sea has a high application value.