Investigation Of Reclaimed Water Quality In Wastewater Treatment Plant And Study On Its Reuse In Thermal Power Plants

In recent years,the global urbanization,acceleration of industrialization and sustainable growth of the population have expanded the demand for water resources.The reuse of sewage after regeneration is an effective way to reduce water pollution and alleviate the shortage of water resource.Using recycled water as cooling water for thermal power plants,not only achieves the resource utilization of sewage,but also promotes the benign water cycle in nature.However,in use of recycled water as a circulatory water cooling system,domestic and foreign power plants generally encountered corrosion and scaling,microbial breeding and other problems.Taking the reclaimed water treatment system of third sewage treatment plant of Xi’an’as an example,this study investigates and analyzes the water quality characteristics of recycled water effluent in the past six years,and combines with the requirements of major industrial users such as Xijiao Thermal Power Plant,Baqiao Thermal Plant and Weihe Plant.Facing with the problem that alkalinity and hardness out of limits under the condition of water quality fluctuation,this study probes into the effect of two methods,adding lime and adding activated carbon,and draws the following conclusions:(1)In the rainy season,the influent water quality of sewage plant is affected by upstream reservoir,which makes influent exceed the treatment capacity,and eventually leads to the excess of ammonia nitrogen,total alkalinity and total hardness.The existing reclaimed water treatment system has limited effect on the removal of high alkalinity and hardness water.The removal rate of alkalinity is 6.08%,and the removal rate of hardness is 4.02%.,after testing,alkalinity is always greater than hardness;(2)Adding lime and activated carbon can play a role in reducing alkalinity.However,the effect of lime is obviously better than that of activated carbon and it is more suitable to choose lime as alkali lowering agent in daily production.In addition,if activated carbon is addedd in high alkalinity water for a long time,the brightness in water is obviously improved;(3)The treatment effect of lime alkali reduction method is related to the original alkali degree.In the case that original alkali degree is 350-400mg/L,400-450mg/L and450-500mg/L respectively,the 18.1%,28.9%and 26.8%of toal alkalinity can be removed by adding 0.6g limen each time;(4)The results of the pilot show that under the condition that the average total alkalinity of the influent is 426mg/l,a total of 26200m~3 water was treated,and 3144 kg of lime is consumed to configure a lime milk solution with a concentration of 1:3.The treatment cost per ton of reclaimed water increases by 0.12 Yuan,which can make total alkalinity of the effluent meet the standard and operation cost can be saved by 30654Yuan per year;(5)In practice,different lime dosing methods can be adopted.The cost per ton of water treated by the full-course dosing method and the interval jump dosing method will be increased by 0.063 yuan and 0.037 yuan respectively,that is,the actual cost is about0.313 yuan and 0.287 yuan,respectively.Compared with the full-course dosing method,the interval jump dosing method can reduce the production operation cost;(6)The advanced treatment of the reclaimed water by the double membrane method can further improve the water quality(for example,the total hardness is less than 50 mg/L,and the total alkalinity is less than 50 mg/L),but the treatment cost is high.According to estimates,the double-film system with a water production of700m~3/h has a treatment cost of about 1.22 yuan per ton of water,which is much higher than the treatment cost of 0.313 yuan per lime dosing.Therefore,after comprehensive comparison,the method of lime dosing should be selected as the treatment method for the high alkalinity and low hardness water of the reclaimed water plant.