Evaluation Of The Effect Of Wastewater Reuse Based On Thermodynamic Entropy Calcuation

Urban sewage discharge will cuase impacts on a water environment,the existing impact indicators of pollutants are mostly qualitative or semi-qualitative evaluation.It is lacking quantitative methods for evaluating pollutants in the water environment.According to the law of thermodynamics,the thermodynamic entropy increase of pollutants in urban sewage can be calculated.The reduction of sewage entropy implies less energy dissipation in the form of waste heat which results in less negative impacts on the environment.Thus,thermodynamic entropy can be used as a comprehensive indicator to quantitatively assessment the impact on water environment.The entropy increase in urban sewage is mainly caused by water quantity and water quality.It is found that the entropy increase caused by its quantity is mainly due to the temperature difference,while the entropy increase caused by its quality is mainly due to the degradation of pollutants.According to the electronic equivalent equations of different biochemical reactions of organic matter,nitrogen and phosphorus,which are major pollutants in the effluen,the thermodynamic enthalpy change of the biochemical reaction for different pollutants can be calculated,and then the corresponding thermodynamic entropy increase of pollutants biodegradation in the water environment can be obtained.The main conclusions in this thesis are as follows:(1)The entropy increase in the process of urban water cycle is mainly caused by water quantity and water quality.The entropy increase caused by the disturbance of water quantity is mainly due to the water temperature change in the process of water intake and wastewater discharge.The formula for the entropy increase due to disturbance on water quantity(?S1)was obtained.(2)According to the biomass yield coefficient of 14 kinds of organic substances in aerobic culture,the corresponding biochemical reaction equation were obtained,then the equivalent BOD and the enthalpy change of the reaction(?H)were calculated.It was found that there was a linear relationship between BOD and ?H as-?H = 13.66 ?BOD(R2= 0.915).As the enthalpy change of the biochemical reaction for organic matters can be used to represent the energy change of the thermodynamic system in the water environment,the entropy increase due to the biodegradation of the organic matter in the water environment(?SC)can be obtained.(3)In the water environment,the inorganic nitrogen is found mainly as ammonia nitrogen(NH4+-N)and nitrate nitrogen(NO3--N).Ammonia nitrogen is first converted to nitrate nitrogen by nitrification and then removal through the denitrification.So the entropy increase for biochemical removal of ammonia nitrogen in the water environment(?SNH4+)can be obtained.Denitrification process requires organic carbon source,through the denitrification reaction equation for 12 kinds of organic,the average enthalpy change of the reaction and the amount of organic matter consumption were obtained.The entropy increase of the denitrification process in the water environment(?SNO3-)was obtained.(4)The effect of phosphorus discharge on water environment is mainly related to algal proliferation,that can lead to eutrophication.Phosphorus was uptaked by algae,then the algae were oxidized by aerobic microorganisms,these processes would cause entropy increase in the water environment.According to the energy transfer efficiency of aerobic microorganisms(? = 0.2 ~ 0.4),the reaction equation of decomposition of algae biomass was obtained.So the entropy increase of phosphorus uptake by algae in the water environment(?STP)can be obtained.(5)Through case analysis regarding effluent quality control in China,the effect of the national wastewater reuse was evaluated.The results indicated that under the current condition of water supply and wastewater treatment,if 15% of the treated wastewater could be reused,the total entopy increase(?S)would be decreased by 13.0% from the current level,while if the percent of treated wastewater reuse could be increased to 30%,?S would be decreased by 25.7%..Thus,the reuse of sewage can effectively reduce the entropy increase of the water environment.Based on analysis of entropy increase in the process of urban water cycle caused by water quality,it was found that the process of inorganic phosphorus uptake by algae had a great influence on the water environment,while the biodegradation of inorganic nitrogen had little effect on water environment.