Characteristics And Removal Technology Of The Fine Grit In Sewer System Of Mountainous Urban Area

High concentration and small particle size grit in the urban sewage of Chongqingin Three Gorges Reservoir （average particle size <250μm can be classified to theultra-fine grit） led to widespread badly worn of power equipments, low level of organiccomponents of the activated sludge, water quality fluctuations and higher excess sludgeand other series of problems in wasterwater treatment plants, posing a serious threat onthe Reservoir Area’s fragile aquatic environment. There are no much researches onsewage fine grit, especially the relevant researches on sewage fine grit of Chongqing inThree Gorges Reservoir. This dissertation combines the characteristics of Chongqingmountain landforms, the seasonal rainfall and drainage pipe network, treats sewage finegrit as the research object, through setting sampling point, underbrush and pilot-plants,carries out the researches on sewage fine grit, including spatial and temporaldistribution, the first flush effects, EMC （event mean concentration）, particle sizefeatures, distribution, settling performance, mineral composition, migration process,removal technology and such, drawing the following conclusions:In dry weather, the varying patterns at various sampling points are basically thesame. There are some differences in the hourly fluctuations, changes in water gritcontent of the sewage treatment plant lag behind that in the drainage network. The gritconcentration of Fuling wastewater treatment plant is183.9mg.L^-1, and the gritconcentration of the rest sampling points is less than50mg.L^-1. The annual average gritconcentration and daily inlet grit amount of the wastewater treatment plant in the firsthalf average were180mg.L^-1, and10507kg.d-1respectively, higher than the second halfof the year, which is the major degritting period.For the initial combined sewage grit concentration of sampling points in the incombined sewer systems, Jiangnan residential area is469.3mg.L^-1, the traffic area394.5mg.L^-1, and Jiangdong residential area356.4mg.L^-1; the rainfall intensity is11.7mm.h^-1,5.6mm.h^-1and1.6mm.h^-1, the grit peak value appearance time is10min,15min and20min. As for the grit content and appearance time of the sampling points inthe in separate sewer system, commercial area sewage pipe is425.0mg.L^-1and greaterthan20min, the commercial area rainwater pipe is199.1mg.L^-1and less than5min.Higher rainfall intensity leads to earlier appearance time of grit content. The longerlasting of the dry days before rain, high population density, higher traffic volume and poor sanitation shall lead to higher concentration of grit content. As for the sewage pipein separate sewer system, the flush exist due to a small amount of rain mixed, theappearance time is later than that in combined sewer systems, but its higherconcentration of grit content can not be ignored. Overall, the grit content of the first rainand sewage sand are subject to the combined influences from length of dry days beforerain, land type and drainage network system.The first flush effects evaluation M（V） curve and the power function“b”parametermethod analysis indicate that the rainfall intensity>5mm.h^-1rainfall, the grit contentload percentage curve is higher than45°angle bisector, b parameter values are lessthan1, determination coefficient r2is greater than0.8, the initial50%rain sewagecarries about80%-90%pollution load, and there exists a first flush effect. The rainfallscreenings pollutant concentration （EMC） evaluation shows that the early combinedsewage grit content EMC value is333mg.L^-1, about6times than that in dry weather.Analysis on the sewage fine grit particle size features shows that the averageparticle size range of fine grit of the sewage in each sampling point during dry weatheris150μm--210μm, the average particle size in the early rain sewage fine grit is between180μm-240μm,80%of grit particle is less than325μm,60%less than225μm, and40%less than150μm. Compared with that in North America and the southeastern UnitedStates, the fine grits in Chongqing of Three Gorges Reservoir have similar particle sizeand difficult to be removed due to the average particle diameter of less than250μm.Sewage fine grit particle size characteristics and distribution in dry weather is mainlyaffected by the impact from land-use types, the rainfall intensity, dry days before rain,land-use type and drainage network system affects the sewage fine grit particle sizefeatures and distribution in the first combined sewage. When designing the fine gritdegritting system, if the grit with particle size of100μm is the degritting targets, andthese grits degritting results are considered as the system efficiency standards, thedegritting system can ensure that70%-95%of the fine grit on the cumulativedistribution characteristic curve will be effectively removed.Mineral composition and origin analysis shows that54%of quartz and27%ofcalcite in sewage fine grit minerals are similar with the surface and pipeline depositedgrit samples, which is related with cement road surface weathering and wear and otherfactors and the main sources of sewage fine grits. The mineral quartz in first combinedsewage fine grit is more than70%, similar with soil samples, which is related with soilerosion and the sandy soil is the main sources of sewage fine grits. About49%of quartz and24%of feldspars in grit chamber deposited grits are similar to the mountainweathered rock samples. It shows that some grits in the sewage come from thedeposited fine grits after the rocks are weathered which is also the major source ofsewage fine grits. The mountain urban sewage fine grits migration of the Three GorgesReservoir region is related to varied factors including rock weathering, soil erosion,surface runoff and type of collecting sewage through ditches, creeks, streams andgullies.Fine grit natural subsidence performance and impact tests show that given HRT1.46h meter of the primary settling tank of the sewage plant, the removal efficiency offine grits is only53.4%. So only relying on primary settling tank cannot bring idealeffect. These influencing factors of pH, water temperature, Na+and Mg2impact little onthe removal efficiency. Test on excess sludge’ impact on fine grit removal shows thatwhen the excess sludge dosing is than5%, the removal efficiency is good at73.7%. Dueto the remaining high content grit in the excess sludge, the removal results are unstable.The rotational flow grits and coagulating sedimentation tests show that thedegritting efficiency of the improved rotational flow grit chamber is limited, only standsat35%. Coagulating sedimentation has better degritting efficiency with the removal rateof up to91.6%, COD removal efficiency62.5%, but the carbon source losses are huge.So it is need to supple the carbon source for meeting the carbon source demands of latenitrogen and phosphorus removal and relieving the carbon source competitions.