Research To The Transformation Process Of Landfill Leachate Treatment Works In Jiangcun Gou, Xi’an City, Shanxi Province

Nowadays, about90%of China’s cities use landfill for waste disposal. The maineffect of life rubbish filling area on environment is the large producing of landfillleachate, whose component of this water is complicated with the characteristics of highCOD content, high NH₃-N content and high heavy metal ions. Once the landfill leachateentry into the environmen， the soil and groundwater will be badly damaged. Therefore,we must seek for a economical and rational technological process to meet the need ofthe landfill leachate treatment in China, which is also a urgent problem for thedevelopment of landfill leachate technology. The main conclusions were as follows:The landfill leachate treatment works in Jiangcun gou had been established inSeptember,2004and then put into operation. However, owing to its low disposalcapabilities and poor quality water, the leachate has been transported to the municipalsewage treatment plants which significantly impact the regular operation of themunicipal sewage treatment plants.With the high requirement of landfill leachate discharge standard, thetransformation of the landfill leachate treatment works in Jiangcun gou is quitenecessary to meet the requirement of first-rate emission standard. As for what kind oftechnics can meet the requirement, there stiil needs futher research.2、The scale of the6th Municipal Wastewater Treatment Plant of Yinchuan: recent,50000m³/d; long-term,150000m³/d.Based on the category analysis of the landfill leachate in Jiangcun gou of Xi’an andwater quality monitoring, the paper has studied the design capacity, inflow water qualityand upgrading&modification technology of the landfill leachate treatment works in Jiangcun gou of Xi’an. It has also put forward the main technical parameters, layingtechnical foundation for the implementation of the project.Main results in the paper as follows:1) Treatment capacity of leachate is1200m³/d. After years’ investigation of thelandfill leachate in Jiangcun gou, confirm the inflow water quality as follows: COD_Gr:30000mg/L, BOD5:8000mg/L, suspended matter:2000mg/L, the total of nitrogen:3000mg/L, ammonia nitrogen:2500mg/L, the total of phosphorus:15mg/L,coliform:2.4x10⁵/L, the total of mercury:0.016mg/L, the total of cadmium:0.4mg/L, the total of chromium:0.16mg/L, hexavalent chromium:0.02mg/L, the total ofarsenic:0.01mg/L, the total of lead:0.4mg/L, chromacity:2000（diluted times）.2) In the landfill leachate in Jiangcun gou of Xi’an, BOD/COD=0.26, with poorwastewater biodegradability and a high level of ammonia nitrogen always maintainedwithin2000²500mg/l, thereby resulting in difficult normal functioning ofbiochemical processing. With the increase of landfill time, the leachate will be more"aging": the reduction of COD_crand BOD5concentration, the increase of ammonianitrogen content, the rise of carbon and nitrogen ratio, and the drop of biodegradability,which can exacerbate the difficulty of leachate treatment. The treatment key is toremove the ammonia nitrogen and COD_cr.3) The original design of the landfill leachate treatment works in Jiangcun gou usesbiological treatment processes, namely, leachate by ammonia nitrogen stripping ispoured into the anaerobic and aerobic biological filter ponds, to dispose up to thestandard after the primary treatment through the combined effects of micro-organisms in2pools. The leachate after primary treatment is sent into Xi’an No.3sewage treatmentplant through a dedicated pipeline for the secondary treatment. After putting intooperation, in order to improve the treatment level and ability, a large number ofactivated sludge and cow dung from municipal wastewater treatment plant have beenthrown into the anaerobic and aerobic biological filter ponds to increase the bacteria andimprove the living environment. In accordance with the aeration requirements, the SBRreaction tank, fan and inlet pipe have been for a number of transformations. Throughefforts, the removal rate of the anaerobic pond and SBR reaction tank have reached50%and70%respectively, however, the complex composition of landfill leachate, extreme value range of influent COD between11000mg/L and48000mg/L and too highindexes have resulted in extreme value of effluent indicator COD_crhovering in the rangeof2200mg/L to3800mg/L, deviated from the designed standard dischargerequirement of1000mg/L. Moreover, the volume of the existing adjusting pond is only10000m3, and can not play a regulatory role, which has brought some difficulties to thelandfill leachate treatment works in Jiangcun gou.4)In the general conventional biological treatment processes （including the MBRtreatment process）, the leachate COD_crafter treatment can only reach600⁸00mg/L,which is difficult to reach100mg/L or less. If you want to achieve better water quality,membrane treatment-nanofiltration or reverse osmosis needs to be adopted. Thenanofiltration and reverse osmosis can meet the requirements of COD_crto reach100mg/L or less. Landfill leachate contains refractory organic matter of high concentration,especially for the leachate produced by the aging landfill; its organic material is difficultto achieve the desired treatment effect through individual and combined biologicaltreatment.5)The use of one landfill leachate treatment method can not meet water qualityrequirements of first-grade standard, to achieve significant effect with2methods ormore. Recharge method can only handle a small amount of generated leachate, and thelong-term recharge of pollutants in the leachate will not result in thorough treatment, butwill continue to accumulate, which is not a long-term effective method. Chemicaloxidation method is greatly influenced by changes in water quality, and the treatmenteffect is not stable enough yet for further research and development. Membranefiltration method has a significant effect on the retention of refractory organic matterand requires pre-processing, otherwise it will cause serious membrane fouling andreduce membrane life; biological method can handle easily degradable organic mattereffectively, but the treatment effect of refractory organics is poor; therefore, membranetreatment-nanofiltration or reverse osmosis is required to adopt, which can make uptheir deficiencies and play to their strengths so as to improve the treatment effectsignificantly to achieve better water quality.6)Initially identify two common schemes: flocculent settling+ammonia stripping+anaerobic+membrane bioreactor+Nanofiltration; flocculent settling+anaerobic+ external membrane bioreactor+nanofiltration+reverse osmosis, to analyze andcompare. Combining with project-related factors such as water quality, water quantity,leachate treatment plant site, technical reliability of leachate and sludge treatmentprocess plan and investment estimation, economic rationality and analysis andverification of feasibility of implementing a comprehensive multi-program, selecttreatment process of flocculent settling+anaerobic+MBR+NF+RO finally.7)Through cases analysis of Heilongjiang sanitary landfill leachate treatmentworks, Shandong TengZhou domestic garbage landfill leachate treatment station,domestic garbage treatment works in Lingchuan county of Guilin of Guangxi andcomprehensive waste treatment plant in Jiaojiapo of Mentougou district of Beijing, theresults show that: flocculent settling+anaerobic+MBR+NF+RO treatment processhas a good effect, and outlet water after treatment can achieve emission standard.8)The expansion project should adopt the following design:2regulation ponds are set, and the volume of the existing pond is10000m³, with asemi-underground-type reinforced concrete structure. The volume of the newly-builtpond is30000m³, with a ground type structure. With polystyrene foam board as the heatpreservation of the pond wall, pond top closure uses double-layer color steel sandwichpanel, to meet requirements of on-site containment and insulation.Sedimentation tank is used to remove precipitable suspended solids in the sewage.In vertical flow sedimentation tank, the surface hydraulic loading is1.00m³/m2.h, ponddiameter is8.00m, and hydraulic retention time is3h, with reinforced concretestructure. Suspended solids concentrations of influent and effluent are2000mg/L and800mg/L respectively, removal rate of suspended solids is60%, sludge concentrationof sedimentation tank is97%, and sludge volume is48m³/d.In view of the high concentration of pollutants in engineering wastewater andcomplex water quality, anaerobic process uses upflow anaerobic sludge blanket （UASB）technology, which consists of three-phase separator, to maintain anaerobicmicro-organism of high activity and precipitation in the reactor.The project selects external membrane bioreactor, which has advantages of highconcentration of sludge, small floor space and high membrane flux. Anaerobic effluentafter a pre-aeration enters into the membrane bioreactor MBR through the filter from the middle pond by the MBR feed pump. Membrane bio-reactor is designed withdenitrification, nitrification and ultrafiltration system. Total effective volumes of thedenitrification and nitrification tanks are2400m3and6000m3respectively. Aeration innitrification tank uses special equipment for jet flowing and blast aeration, and themajority of organic pollutants in sewage tanks has been degraded in nitrification tankthrough the highly active aerobic microorganisms action, while ammonia nitrogen underthe effect of nitrification and micro-organism action is oxidized to nitrate. Mixturereflux （nitrate backflow） is set in nitrification tank to the pre-denitrification tank, andnitrate backflow is reverted to nitrogen to discharge under the oxygen-deficientenvironment in the denitrification tank, to achieve the purpose of biologicaldenitrification.As the need to consider the state enactment of new standards requiring the leachatetreatment effluent to achieve the total nitrogen discharge limit of40mg/L, two-gradenitrification and denitrification are designed. When the pre-denitrification andfirst-grade nitrification are incomplete, depth denitrification is conducted in thesecondary denitrification and nitrification reactors, to control the total nitrogen ofeffluent by controlling the reaction of nitrification and denitrification. Whendenitrification and nitrification denitrification are complete, the secondarydenitrification and nitrification can be overcome. Effective capacities of nitrificationand denitrification tank are960m3.MBR effluent ammonia nitrogen, total metal ions, SS and other indicators havereached an emission standard, but some still can not remove the biodegradable organicmatter; using nanofiltration can further separate larger molecular refractory organiccompounds, for a further advanced treatment.Reverse osmosis system as safeguard measure to reach the total nitrogen standardis designed to use as well as the nanofiltration system, which uses spiral reverse osmosismembrane, only in the case of the incompleteness of biological denitrification inexternal membrane bioreactor.The project will produce240m³/d concentrate, In leachate treatment plant in theposition of the reserve concentrate processing, concentrate temporarily into third xiancity sewage treatment plant. Two sets of deodorant facilities should be set in the project. One is set on the top ofnewly-built regulation pond, responsible for regulating the original pond and the ozonetreatment of newly-built regulation pond, with a handling capacity of92000m³/h;another pond is set in the anaerobic tank top, responsible for the ozone treatmentbetween biological tank and sludge dewatering, with a handling capacity of36000m³/h.