| Urban wastewater discharge could cause a certain effects on urban waterenvironment, whereas the existing assessment methods are difficult to quantify andcompare the environmental impact of the pollutants discharge. If the wastewatercontaining extra pollutants is discharged into an urban water environment, the waterwill undergo a process of self-purification under physical, chemical, or biologicalactions which bring about a recovery of the bulk water quality to the background level.Based on the second law of thermodynamics, such a process was virtually accompaniedby energy consumptions through various ways, and thus results in entropy increase inthe water body. This entropy increase reflects the impact of the pollutants discharge onwater environment, the smaller the entropy increase, the smaller the impact of pollutantsdischarge on water environment. In order to quantitatively assess the thermodynamicimpact of different pollutants discharge on water environment, this study takes threemain pollutants in urban sewerage discharge as research objects, including organicmatter represented by total organic carbon (TOC), nitrogen and phosphorus. Accordingto the possible natural purification process and the required energy or heat afterpollutants discharge into water environment, a corresponding method was proposed forcalculating the entropy increase ΔS of water environmental system using the SecondLaw of Thermodynamics. In the method, entropy increase was used as a quantitativeindicator for the water environmental impact assessment. The main studies andachievements in this thesis are as following:(1) Based on the Second Law of Thermodynamics, the basic calculated method ofentropy increase in water environment was studied, along with pollutant decompositionprocesses. The research object of water body was view as a tiny part of the natural water cycle system, under the equilibrium condition of natural water cycle process. Thisprocess can be regarded as a reversible process where entropy increase is equal to zero.Then the entropy increase ΔS due to external interruption (extra discharge of pollutants)assuming that a water body would recover its quality to the background level throughself-purification (decompose the receiving pollutant thoroughly), can be calculated byconsumed heat of water body. Based on the relationship between reaction heat andenthalpy change of reaction and loss of molar reactional exergy, the basic calculationformula of entropy increase of water environment was established by the standard molarenthalpy change of reaction and the standard molar exergy loss.(2) The quantitative calculation method of entropy increase due to organicsdischarge represented by TOC was studied. After the residual TOC in urban drainagewas discharged into the water body, considering that the main degradation process waschemical oxidation reaction. By using201organic substances of known molecularstructures and available thermodynamic data relating to their decomposition byoxidation reaction, a good proportional relation was found between the standard molarenthalpy change of reaction, ΔrH0(or the standard molar exergy loss, Δbx) and thetheoretical organic carbon contents (ThOC) with the proportional coefficient askH=53.7360kJ/g (R2=0.9670). By analysing the TOC and ThOC of32model organicsubstances of sufficient solubility in aqueous solution, an almost equal-valuerelationship of ThOC=TOC was indicated. Then the calculation formula of the chemicalreaction heat which approximate calculation by TOC value is ΔrH0=53.736TOC (thecoefficient of Δbxis closed to ΔrH0, indicating the ΔrH0and Δbxapproaches were almostidentical). Based on the equivalence relation between the chemical reaction heat and thestandard molar enthalpy change of reaction, the entropy increase of chemical oxidationof TOC under a certain temperature can be calculated.(3) The combustion heat of5pure organic substances and6organic mixtures weredetermined by the RBC-II type rotating-bomb calorimeter. Based on the equivalencerelation between the combustion heat and the chemical reaction heat for the completeoxidation of organic matters, the calculation of entropy increase by TOC was verifiedand corrected. The results showed that a good proportional relation was also foundbetween the combustion heat and the TOC of5pure organic substances with the proportional coefficient as55.098kJ/g (R2=0.9843). This coefficient is nearly closed tothe proportion coefficient of calculation formula for chemical reaction heat, and then thecalculated relation is tested and verified to be applicable for organic substances. For the6organic mixtures, a good proportional relation was also found between the combustionheat and the TOC (R2=0.9843), but the proportional coefficient is69.277kJ/g, it isabout1.26times of the organic substance. The difference is attributed to chemical forceof organic substances in the organic mixtures, and some additional chemical reactionheat is required for the breakdown of the chemical force. The TOC of actual urbandrainage is organic mixtures, it is more appropriate to use measured coefficient tocalculate the entropy increase.(4) For the conversion process of nitrogen and phosphorus in water environment,considering the two cases: firstly, the water body does not have the properties foraquatic plant growth, and nitrogen and phosphorus were removed by biochemical orchemical reaction; secondarily, the water body have the properties for aquatic plantgrowth, and the transformations of nitrogen and phosphorus were mainly plant uptake,such as the eutrophication of water body. For the first case of water body, theconversion of inorganic nitrogen (nitrate, nitrite and ammonia) into gaseous nitrogen(N2) usually depends on biochemical reactions such as nitrification and denitrification.Phosphorus and calcium can be converted to calcium phosphate precipitation, whichpossibly results in the removal of phosphorus by chemical precipitation. The abovementioned heat changes are directly proportional to the mass of nitrogen or phosphorus,and then based on the equation, the corresponding proportional coefficient was obtained.For the second case of water body, the main conversion of nitrogen and phosphorus isplant uptake by photosynthesis. Based on the chemical reaction of algae photosynthesis,4typical algae general formula were selected to calculate the reaction heat, then linearformula of reaction heat with mass of nitrogen and phosphorus were found in theprocess of algae growth. The entropy increase due to nitrogen and phosphorusconversion and uptake of two cases are compared, the results showed that for thenitrogen, the entropy increase due to algae uptake unit mass of ammonia is10times ofthe entropy increase due to biochemical remove unit mass of ammonia. While for thephosphorus, the entropy increase due to algae uptake unit mass of phosphorus is640 times of the entropy increase due to chemical precipitation of unit mass of phosphorus.Therefore, the worst case for the impact of nitrogen and phosphorus on water body iswater eutrophication.(5) In order to verify the applicability of the theoretical equations of entropyincrease for plant uptake of nitrogen and phosphorus, Microcystis flos-aquae, Anabaenaflos-aquae, and Chlorella vulgaris were selected for element analysis and determinationof the combustion heat. Based on the measurement results, the stoichiometric relation of3algae were established, then the combustion heat for unit mass of TN and TP wereobtained. which were used to obtain this entropy increase. Results showed that for thenitrogen, the differences between the theoretical value and the experimental value wasless than11.5%, while for the phosphorus, the differences between the theoretical valueand the experimental value was less than20.8%. Thus the established entropy increaseequation can be used for assessment of the entropy increase of water environmentcaused by plant uptake of nitrogen and phosphorus.(6) Taking one upgrading of the existing WWTP in Xi’an as an example andconduct case studies, the entropy increase due to extra discharge of organics, nitrogenand phosphorus were calculated by TOC chemical oxidation method, plant uptake ofnitrogen and phosphorus method. When the quality of the secondary effluent hasimproved from Class I-B to Class I-A, the improve effect of receiving waters andemission reduction of TOC, TN, TP were compared. The assessment results showed that,the values of total entropy increase for receiving water body could be reduced3.15×108kJ/K after upgraded the WWTP with1×105m3/d emission to the receiving water bodymeeting with Grade III of Chinese Standard on Surface Water Quality. Among them, thecontribution rate of TOC, TN and TP emission were11.1%,52.7%and36.2%,respectively. Considering the reduced concentration of three pollutants in the drainagewere4.0mg/L,5mg/L and0.5mg/L, respectively, the most remarkable improved effectof water environmental quality was unit mass of TP emissions. |
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