《专业英语》课件Unit-29.pptx

1、TextSewage is created by residences,institutions,and commercial and industrial establishments.Raw influent(sewage)includes household waste liquid from toilets,baths,showers,kitchens,sinks,and so forth that is disposed of via sewers.In many areas,sewage also includes liquid waste from industry and co

2、mmerce.The separation and draining of household waste into greywater and blackwater is becoming more common in the developed world,with greywater being permitted to be used for watering plants or recycled for flushing toilets.A lot of sewage also includes some surface water from roofs or hard-standi

3、ng areas.Municipal wastewater therefore includes residential,commercial,and industrial liquid waste discharges,and may include stormwater runoff.Sewage systems capable of handling stormwater are known as combined systems or combined sewers.Such systems are usually avoided since they complicate and t

4、hereby reduce the efficiency of sewage treatment plants owing to their seasonality.The variability in flow also leads to often larger than necessary,and subsequently more expensive,treatment facilities.In addition,heavy storms that contribute more flows than the treatment plant can handle may overwh

5、elm the sewage treatment system,causing a spill or overflow.It is preferable to have a separate storm drain system for stormwater in areas that are developed with sewer systems.TextAs rainfall runs over the surface of roofs and the ground,it may pick up various contaminants including soil particles

6、and other sediment,heavy metals,organic compounds,animal waste,and oil and grease.Some jurisdictions require stormwater to receive some level of treatment before being discharged directly into waterways.Examples of treatment processes used for stormwater include sedimentation basins,wetlands,buried

7、concrete vaults with various kinds of filters,and vortex separators(to remove coarse solids).Process overviewSewage can be treated close to where it is created(in septic tanks,biofilters or aerobic treatment systems),or collected and transported via a network of pipes and pump stations to a municipa

8、l treatment plant(see sewerage and pipes and infrastructure).Sewage collection and treatment is typically subject to local,state and federal regulations and standards.Industrial sources of wastewater often require specialized treatment processes(see Industrial wastewater treatment).Conventional sewa

9、ge treatment may involve three stages,called primary,secondary and tertiary treatment.TextPrimary treatment consists of temporarily holding the sewage in a quiescent basin where heavy solids can settle to the bottom while oil,grease and lighter solids float to the surface.The settled and floating ma

10、terials are removed and the remaining liquid may be discharged or subjected to secondary treatment.Secondary treatment removes dissolved and suspended biological matter.Secondary treatment is typically performed by indigenous,water-borne micro-organisms in a managed habitat.Secondary treatment may r

11、equire a separation process to remove the micro-organisms from the treated water prior to discharge or tertiary treatment.Tertiary treatment is sometimes defined as anything more than primary and secondary treatment.Treated water is sometimes disinfected chemically or physically(for example by lagoo

12、ns and microfiltration)prior to discharge into a stream,river,bay,lagoon or wetland,or it can be used for the irrigation of a golf course,green way or park.If it is sufficiently clean,it can also be used for groundwater recharge or agricultural purposes.Pre-treatmentPre-treatment removes materials t

13、hat can be easily collected from the raw wastewater before they damage or clog the pumps and skimmers of primary treatment clarifiers(trash,tree limbs,leaves,etc).TextScreeningThe influent sewage water is strained to remove all large objects carried in the sewage stream.This is most commonly done wi

14、th an automated mechanically raked bar screen in modern plants serving large populations,whilst in smaller or less modern plants a manually cleaned screen may be used.The raking action of a mechanical bar screen is typically paced according to the accumulation on the bar screens and/or flow rate.The

15、 solids are collected and later disposed in a landfill or incinerated.Grit removalPre-treatment may include a sand or grit channel or chamber where the velocity of the incoming wastewater is carefully controlled to allow sand,grit and stones to settle.Primary treatmentIn the primary sedimentation st

16、age,sewage flows through large tanks,commonly called primary clarifiers or primary sedimentation tanks.TextThe tanks are large enough that sludge can settle and floating material such as grease and oils can rise to the surface and be skimmed off.The main purpose of the primary sedimentation stage is

17、 to produce both a generally homogeneous liquid capable of being treated biologically and a sludge that can be separately treated or processed.Primary settling tanks are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the

18、 tank from where it can be pumped to further sludge treatment stages.Grease and oil from the floating material can sometimes be recovered for saponification.Secondary treatmentSecondary treatment is designed to substantially degrade the biological content of the sewage which are derived from human w

19、aste,food waste,soaps and detergent.The majority of municipal plants treat the settled sewage liquor using aerobic biological processes.For this to be effective,the biota require both oxygen and a substrate on which to live.TextThere are a number of ways in which this is done.In all these methods,th

20、e bacteria and protozoa consume biodegradable soluble organic contaminants(e.g.sugars,fats,organic short-chain carbon molecules,etc.)and bind much of the less soluble fractions into floc.Secondary treatment systems are classified as fixed-film and suspended-growth.Fixed-film OR attached growth syste

21、m treatment process including trickling filter and rotating biological contactors where the biomass grows on media and the sewage passes over its surface.In suspended-growth systems,such as activated sludge,the biomass is well mixed with the sewage and can be operated in a smaller space than fixed-f

22、ilm systems that treat the same amount of water.However,fixed-film systems are more able to cope with drastic changes in the amount of biological material and can provide higher removal rates for organic material and suspended solids than suspended growth systems.Roughing filters are intended to tre

23、at particularly strong or variable organic loads,typically industrial,to allow them to then be treated by conventional secondary treatment processes.Characteristics include typically tall,circular filters filled with open synthetic filter media to which wastewater is applied at a relatively high rat

24、e.TextThey are designed to allow high hydraulic loading and a high flow-through of air.On larger installations,air is forced through the media using blowers.The resultant wastewater is usually within the normal range for conventional treatment processes.Activated sludgeIn general,activated sludge pl

25、ants encompass a variety of mechanisms and processes that use dissolved oxygen to promote the growth of biological floc that substantially removes organic material.The process traps particulate material and can,under ideal conditions,convert ammonia to nitrite and nitrate and ultimately to nitrogen

26、gas,(see also denitrification).Surface-aerated basinsMost biological oxidation processes for treating industrial wastewaters have in common the use of oxygen(or air)and microbial action.TextSurface-aerated basins achieve 80 to 90%removal of Biochemical Oxygen Demand with retention times of 1 to 10 d

27、ays.The basins may range in depth from 1.5 to 5.0 metres and use motor-driven aerators floating on the surface of the wastewater.In an aerated basin system,the aerators provide two functions:they transfer air into the basins required by the biological oxidation reactions,and they provide the mixing

28、required for dispersing the air and for contacting the reactants(that is,oxygen,wastewater and microbes).Typically,the floating surface aerators are rated to deliver the amount of air equivalent to 1.8 to 2.7 kg O2/kWh.However,they do not provide as good mixing as is normally achieved in activated s

29、ludge systems and therefore aerated basins do not achieve the same performance level as activated sludge units.Biological oxidation processes are sensitive to temperature and,between 0 C and 40 C,the rate of biological reactions increase with temperature.Most surface aerated vessels operate at betwe

30、en 4 C and 32 C.TextFilter beds(oxidizing beds)In older plants and plants receiving more variable loads,trickling filter beds are used where the settled sewage liquor is spread onto the surface of a deep bed made up of coke(carbonized coal),limestone chips or specially fabricated plastic media.Such

31、media must have high surface areas to support the biofilms that form.The liquor is distributed through perforated rotating arms radiating from a central pivot.The distributed liquor trickles through this bed and is collected in drains at the base.These drains also provide a source of air which perco

32、lates up through the bed,keeping it aerobic.Biological films of bacteria,protozoa and fungi form on the medias surfaces and eat or otherwise reduce the organic content.This biofilm is grazed by insect larvae and worms which help maintain an optimal thickness.Overloading of beds increases the thickne

33、ss of the film leading to clogging of the filter media and ponding on the surface.TextBiological aerated filtersBiological Aerated(or Anoxic)Filter(BAF)or Biofilters combine filtration with biological carbon reduction,nitrification or denitrification.BAF usually includes a reactor filled with a filt

34、er media.The media is either in suspension or supported by a gravel layer at the foot of the filter.The dual purpose of this media is to support highly active biomass that is attached to it and to filter suspended solids.Carbon reduction and ammonia conversion occurs in aerobic mode and sometime ach

35、ieved in a single reactor while nitrate conversion occurs in anoxic mode.BAF is operated either in upflow or downflow configuration depending on design specified by manufacturer.Membrane bioreactorsMembrane bioreactors(MBR)combine activated sludge treatment with a membrane liquid-solid separation pr

36、ocess.The membrane component uses low pressure microfiltration or ultra filtration membranes and eliminates the need for clarification and tertiary filtration.TextThe membranes are typically immersed in the aeration tank;however,some applications utilize a separate membrane tank.One of the key benef

37、its of an MBR system is that it effectively overcomes the limitations associated with poor settling of sludge in conventional activated sludge(CAS)processes.The technology permits bioreactor operation with considerably higher mixed liquor suspended solids(MLSS)concentration than CAS systems,which ar

38、e limited by sludge settling.The process is typically operated at MLSS in the range of 8,00012,000 mg/L,while CAS are operated in the range of 2,0003,000 mg/L.The elevated biomass concentration in the MBR process allows for very effective removal of both soluble and particulate biodegradable materia

39、ls at higher loading rates.Thus increased Sludge Retention Times(SRTs)usually exceeding 15 days ensure complete nitrification even in extremely cold weather.The cost of building and operating an MBR is usually higher than conventional wastewater treatment.Membrane filters can be blinded with grease

40、or abraded by suspended grit and lack a clarifiers flexibility to pass peak flows.TextThe technology has become increasingly popular for reliably pretreated waste streams and has gained wider acceptance where infiltration and inflow have been controlled,however,and the life-cycle costs have been ste

41、adily decreasing.The small footprint of MBR systems,and the high quality effluent produced,make them particularly useful for water reuse applications.There are MBR plants being built throughout the world,including North Librty,Iowa,Georgia,and Canada.Secondary sedimentationThe final step in the seco

42、ndary treatment stage is to settle out the biological floc or filter material and produce sewage water containing very low levels of organic material and suspended matter.Rotating biological contactorsTextRotating biological contactors(RBCs)are mechanical secondary treatment systems,which are robust

43、 and capable of withstanding surges in organic load.RBCs were first installed in Germany in 1960 and have since been developed and refined into a reliable operating unit.The rotating disks support the growth of bacteria and micro-organisms present in the sewage,which breakdown and stabilise organic

44、pollutants.To be successful,micro-organisms need both oxygen to live and food to grow.Oxygen is obtained from the atmosphere as the disks rotate.As the micro-organisms grow,they build up on the media until they are sloughed off due to shear forces provided by the rotating discs in the sewage.Effluen

45、t from the RBC is then passed through final clarifiers where the micro-organisms in suspension settle as a sludge.The sludge is withdrawn from the clarifier for further treatment.A functionally similar biological filtering system has become popular as part of home aquarium filtration and purificatio

46、n.The aquarium water is drawn up out of the tank and then cascaded over a freely spinning corrugated fiber-mesh wheel before passing through a media filter and back into the aquarium.TextThe spinning mesh wheel develops a biofilm coating of microorganisms that feed on the suspended wastes in the aqu

47、arium water and are also exposed to the atmosphere as the wheel rotates.This is especially good at removing waste urea and ammonia urinated into the aquarium water by the fish and other animals.sewagen.污水；下水道；污物residencen.住宅，住处；居住institutionn.公共机构；制度；建立；（社会或宗教等）习俗sinkn.水槽；洗涤槽；污水坑sewern.下水道；阴沟；污水管mun

48、icipaladj.市政的，市的；地方自治的dischargevi.排放；卸货；流出stormwatern.雨水；暴雨水overwhelmvt.压倒；淹没；受打击spilln.溢出，溅出；溢出量；摔下；小塞子overflown.充满，洋溢；泛滥；超值；溢值sedimentn.沉积；沉淀物sedimentation basin澄清池；沉淀池greasen.油脂；贿赂jurisdictionsn.司法权，审判权，管辖权；权限，权力wetlandn.湿地；沼泽地；塘地filtern.滤波器；过滤器；筛选；滤光器vortexn.涡流；漩涡；旋风septic tank 化粪池biofilter n.生物

49、滤池New Words and Phrasesaerobicadj.需氧的；好氧的quiescentadj.静止的；不活动的；沉寂的indigenousadj.本土的；土著的；国产的；固有的disinfectvt.将消毒lagoonn.泻湖；环礁湖；咸水湖whilstconj.同时；时时，有时；当的时候landfilln.垃圾填埋地；垃圾堆gritn.粗砂，砂砾sludgen.污泥；泥状雪；沉淀物detergentn.清洁剂；去垢剂solubleadj.可溶的，可溶解的；可解决的activated sludge 活性污泥denitrificationn.脱氮；反硝化作用ammonian.氨ni

50、triten.亚硝酸盐nitraten.硝酸盐surface-aerated basin表面曝气池filter bed滤床；滤水池biological aerated filter曝气生物滤池membrane bioreactor膜生物反应器rotating biological contactor 生物转盘New Words and PhrasesNotes（1）Sewage is created by residences,institutions,and commercial and industrial establishments.污水是由居民住宅，机关机构，商业和工业活动产生的。（