【正文】 Figure 8External cost analysis of seawater desalination Desalination process Energy costs Concentrated brine missionscosts The cost of desalination tapwater users Sewage treatmen Sewage cost Seawater Sewage of energy consumption costs： Energy consumption cost is calculated in accordance with the quantity of the resources consumed by unit electricity, which is generated by general coalfired power generation (Chen 2009). The formula is： ??1 = ?? ∑ ???????? (9) Where R is electricity consumption Per ton of desalination water (kW*h)。P iis the prices of resource (yuan)。 Qi indicates that the resources quantity consumed by unit of electricity is i (g/kWh). . Analysis of environmental costs： When the sewage charges is calculated by pollution equivalent, according to the type and quantity of the emission pollutants, charged standard of each pollution equivalent is Yuan. For each outfall, the type number of pollutants which are charged of sewage fees, in decreasing order of pollution equivalent amount, up to not more than 3. Pollution equivalent amount of some certain pollutant N is： Environmental costs of unit utilization: (11) Where ???? is the concentration of pollutant i, which is produced after the usage of desalinated water,(mg/L)；???? is the amount of sewage converted by unit freshwater；is the concentration of pollutant i in the freshwater,(mg/L)；???? is the pollution equivalent value of pollutant i, (kg). Among is the total amount of sewage emitted；??2 is the total water consumption. In the similar way， the external impact of concentrated brine emissions on the marine environment, can also be calculated by method of estimating sewage charges , of which the formula is: (12) Therefore， the costs of desalination can be calculated as: ?? = ??1 + ??2 + ??3 (13) where ??i is the cost of factor i. Example :solving of the desalination external costs：（taking desalination of Qingdao Baifa project as an example） on the energy external costs： The daily production of this plant is 100,000 m3 desalinated water, and energy consumption per ton of water is , which is generated by general coalfired power, are shown in Table 2. Table 2：Resources consumed by unit of bustion power generated electricity （g/kWh） resource iron ore raw coal chalk copper ore crude oil Price/(Yuan/t) 400 672 30 600 5500 Coal acquisition stage coal transportation production stages 0 total Notation: Take standard coal consumption of coalfired power as 350g/kWh, and the resource consumption of coal mining and transportation stages is calculated by the National 122departments inputoutput table in 2002. External costs of its resources can be calculated according to the formula： ??1 = ?? ∑ ???????? = .8 ( 400 + 5 672 + 0. 49 0 + 600 + 5500) 247。 106 = 1. 1 (yuan) .Calculation on the Environmental external costs, the data are shown as the fllows: Table 3 pollutantcontentsand their pollution equivalent value(2011) pollutant Contents in the reverse osmosis brine/(mg/L) Contents after usage/(mg/L) pollution equivalent value/kg 1 280 4 3 250 1 NH4 ? N 30 Tp The total water amount Qingdao used in 2010 is 943 million m3，and its wastewater emissions amount is million tons， then(2011): 8156 Then it can be estimated that the environmental costs of used desalinated water： Environmental costs of concentrated brine emission can be calculated according to table 4 : Table 4：The chemical ponents’ nature of pollutants Measuring Items Original seawater（μmol/L） Concentrated brine（ μmol/） Pollution equivalent value(kg) Mn u NH4 ? N Taking the current technology level into account, the efficiency of the desalination processing is approximately among 40% to 50%. Daily seawater requirement of BaiFa project is 260000t. It produces 100000 t freshwater every day, with the emission concentrated brine160000t every day, so the quantity of emissions concentrated brine for producing unit desalinated water Q is the environmental costs of brine emissions are: ?? = ??1 + ??2 + ??3 + ??4 = 1. 1 + + = treatment model Another idea to increase the available freshwater is recycle. After treatment of the sewage treatment plant, reuse the wastewater that meets the national emission standards. Cost Analysis： 1. The cost of processing and its ponents For municipal wastewater treatment plant, domestic wastewater is the main sewage source. And nowadays, its processing has been relatively mature, of which the core technology is biological activated sludge process. ?? = ??1 + ??2 + ??3 + ??4 + ??5 + ??6 (14) Where is the month processing cost of the sewage treatment plant。 ??1 is depreciation cost。2 is personnel cost。 ??3 is power cost 。 ??4 is maintenance cost。 ??5 is agent cost。 ??6 is other costs, which can be estimated as 5% of the previous five costs. 2. Analysis of Processing cost Take sewage treatment plant with the handling capacity of 30,000 t / d of North China as an example for analysis. a. Depreciation charges ???? Depreciation is a measure to recover the investment, which means extracting a certain amount of funds as a recovery of investment at the end of the year. According to the calculation method of the annuity, without extra consideration offinancial expense, the depreciation extracted every year is(Jin 2003) ??1 = ?? (1 ? ??)/(12 ??) (15) Where Z is total investment amount( 104 yuan)。 i is the discount rate（%）, which is calculated generally based on the loan interest rate。 n is depreciation life （year）. Assume that the total investment amount is 3000 104