Design And Application Study Of Photovoltaic Driven Integrated Treatment Device For Rural Domestic Sewage

The treatment of rural domestic sewage is an important part of the rural ecological revitalization.In 2021,Shandong Province discharged 1.352 million tons of rural domestic sewage per day,but only 50%of the administrative villages have completed the treatment of rural domestic sewage.Some rural domestic sewage is discharged directly without treatment,which pollutes and damages the rural water environment and seriously threatened human health.Due to the high cost of drainage pipeline construction,difficult operation and maintenance of sewage treatment stations,and high cost of power in actual operation,the existing rural sewage treatment facilities can not meet the treatment needs of rural domestic sewage in most northern hilly areas.Based on the full investigation of rural domestic sewage treatment in S County in north China,the photovoltaic driven integrated treatment device for rural domestic sewage was constructed based on characteristics of decentralized discharge of rural domestic sewage in hilly and mountainous areas in the north China and the high energy consumption of traditional aeration and reflux methods.The photovoltaic driven integrated treatment device for rural domestic sewage is mainly based on the biological contact oxidation process and is suitable for the decentralized treatment of rural domestic sewage,which is through photovoltaic driven aeration and nitrification liquid reflux.The purification effect of the photovoltaic driven integrated treatment device for rural domestic sewage in treating simulated rural domestic sewage was studied.The influence of microbial community on pollutant removal was discussed.Furthermore,the economic cost and ecological value of the photovoltaic driven integrated treatment device for rural domestic sewage were analyzed.The purpose of this paper was to provide a practical basis for the application of photovoltaic driven integrated treatment devices for rural domestic sewage in most northern hilly areas.The main results are as follows:（1）According to the investigation results,the photovoltaic driven rural domestic sewage integrated treatment device is designed to be divided into three treatment units:primary anoxia,secondary anoxia,and biological contact oxidation,with a total effective volume of 94.5 L.The design-rated daily treatment capacity is 100 L.The device is equipped with a photovoltaic system driving device for aeration and reflux,which flows back from the biological contact oxidation tank to the primary anoxic tank.The photovoltaic driven integrated treatment device for rural domestic sewage used suspended filler and realized the rapid start-up by artificial inoculation and film hanging after 29 days daily under the processing capacity of 23.3-28.1℃.According to the operation of the photovoltaic driven integrated treatment device for rural domestic sewage under different nitrification liquid reflux ratios,nitrification liquid reflux ratio had an impact on Chemical oxygen demand（COD）removal.When the nitrification liquid reflux ratio is 300%,the COD removal was prompted.However,the improvement effect was not obvious.When the reflux ratio of nitrification solution was 100%,it was conducive to the removal of NH₃-N in the contact oxidation tank.When the reflux ratio was 200%,it was most conducive to the removal of TN by the photovoltaic-driven integrated treatment device for rural domestic sewage.Considering the pollutant removal and the cost of operation,it was recommended that the operation parameter of nitrification liquid reflux ratio was 200%.（2）When the reflux ratio was 200%and aeration rate was 8 L·min^-1,the removal of COD,ammonia nitrogen（NH₃-N）,total nitrogen（TN）and total phosphorus（TP）was 90.48%,88.05%,59.83%,and 33.64%,respectively,under the daily treatment capacity of 100 L at 20.1-25.3℃.The effluent concentrations of COD,NH₃-N,and TN,except TP,could meet the class I standard of"the discharge standard of water pollutants for rural domestic sewage treatment and disposal facilities"（DB 37/3963-2019）of Shandong Province.When the daily treatment capacity was 50 L and 150 L,the photovoltaic driven integrated treatment device for rural domestic sewage still had a good operation effect under the above conditions.However,when the daily treatment capacity was 150 L,there was too much biofilm proliferation and the concentration of dissolved oxygen decreased at the late stabilization stage,which resulted in poor treatment effects of NH₃-N,TN,and TP.This study also discussed the impact resistance of the photovoltaic driven integrated treatment device for rural domestic sewage by increasing COD concentration(300 mg·L^-1).The results showed that the photovoltaic driven integrated treatment device for rural domestic sewage could adapt to the change of COD concentration within 7 days.（3）During stable operation,the microorganisms had high biodiversity in each pool of the photovoltaic driven integrated treatment device for rural domestic sewage.Through microscopic examination,it could be found that there were bacterial micelles with filamentous bacteria as the skeleton on the filler,and a small number of rotifers and other protozoa in the contact oxidation tank.Through 16S r RNA high-throughput sequencing,it was found that the alpha diversity in the primary anoxic pool was higher than that in the secondary anoxic pool and contact oxidation pool.There were Planococcus,Exiguobacterium,and Simplicispira in the primary anoxic tank which could remove carbon and phosphorus.There were nitrogen-fixing microorganisms,such as Rhodocyclaceae,Simplicispira,and Hydrogenophaga,and a small part of Planococcus and Azospira in the secondary anoxic tank.There were dominant aerobic nitrification bacteria in the contact oxidation tank,mainly including Chryseobacterium,Yersinia,Pseudomonas,Hydrogenophaga,Exiguobacterium,and Verrucomicrobium.（4）According to the market standard price estimation,the total cost of the photovoltaic driven integrated treatment device for rural domestic sewage is 3007 yuan,including costs of material and labor.Compared with the traditional sewage treatment station mode,the treatment of each ton of water can reduce the cost of electricity and medicine by 1.06 yuan and 1.11 k W·h of electric energy.Compared with the traditional sewage treatment station mode,taking Q Village,L Town,and S County as an example,the decentralized treatment mode by using the photovoltaic driven integrated treatment device for rural domestic sewage can save large costs of capital construction and transportation and maintenance.It can save costs（10090.35 yuan）of electricity and reagent every year,electric energy（10566.3 k W·h）,glucose（637.79 kg）,polyaluminium chloride（495 kg）and indirect carbon emission（12632.93 kg CO₂）,which has higher economic and environmental benefits.The photovoltaic driven integrated treatment device for rural domestic sewage has a stable removal effect on rural domestic sewage.The photovoltaic driven integrated treatment device for rural domestic sewage could realize the diversion of rain and sewage,and save the cost of infrastructure,pipe network,and operation and maintenance.It had low requirements for environmental sensitivity.For the hilly areas with a low level of economic development and difficulty in laying pipe networks in a large area,the photovoltaic driven integrated treatment device for rural domestic sewage could be widely used,which could bring technical,economic,and ecological advantages.