污水处理方法与技术.doc

Sewage treatmentAbstract：Sewage treatment, or domestic wastewater treatment, is the process of removing contaminants from wastewater and household sewage, both runoff (effluents) and domestic. It includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants. Its objective is to produce a waste stream (or treated effluent) and a solid waste or sludge suitable for discharge or reuse back into the environment. This material is often inadvertently contaminated with many toxic organic and inorganic compounds.Key words: Sewage treatment, fixed-film and suspended-growth, Activated sludgeOrigins of sewageSewage 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 commerce. 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-standing 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 thereby 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 overwhelm 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.As rainfall runs over the surface of roofs and the ground, it may pick up various contaminants including soil particles 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 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 municipal 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 sewage treatment may involve three stages, called primary, secondary and tertiary treatment. Primary 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 materials 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 require 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 lagoons 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 that 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).ScreeningThe influent sewage water is strained to remove all large objects carried in the sewage stream. This is most commonly done with 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 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 stage, sewage flows through large tanks, commonly called primary clarifiers or primary sedimentation tanks. The 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 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 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 waste, 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. There are a number of ways in which this is done. In all these methods, the 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 asfixed-film and suspended-growth. Fixed-film OR attached growth system 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-film 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 treat 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 rate. They 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 sludgeMain article: Activated sludgeIn general, activated sludge plants 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 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. Surface-aerated basins achieve 80 to 90% removal of Biochemical Oxygen Demand with retention times of 1 to 10 days. 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 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 2.7kg O2/kWh. However, they do not provide as good mixing as is normally achieved in activated sludge 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 between 4 C and 32 C.Considerations for the choice of MBR technologySince membrane filtration allows raised sludgeconcentrations, the activated sludge tank volumecan be significantly reduced. In combination withthe option to convert the secondary clarifier, thatis no longer required as a sedimentation tank, asan additional activated sludge tank, the treatmentcapacity of the existing plant can be largely extended.That way it is possible to upgrade existingwwtps from simple carbon removal to BNRsystems just using the already existing volume.Therefore favourable conditions for the choiceof MBR technology are given, where retrofittingof the existing plant by the conventional activatedsludge (CAS) process would demand for substantialextension of the activated sludge volume. Aswell, where limitations due to insufficient efficiencyof the secondary clarification basin exist,particularly however, where both problems haveto be solved.MBR technology should also be considered ifhigh effluent criteria such as removal of suspendedsolids or absence of pathogens have to be met.Examples are discharge into small creaks as wellas into bathing water or other sensitive areas.Due to the small space requirement the MBRoffers special advantages if the given locationholds no or only a limited amount of area in reserve.Moreover, the small footprint allows a completeindoor installation in a building designed toblend in with its surrounding environment andsuch to address issues of visual amenity, odour ornoise.Another distinct advantage of MBR technologyis direct utilisation of the effluent for reusepurposes. The water reuse potential includes irrigation of agricultural land, recharge of aquifersor river flow replenishment. On several occasionsthis was the major decision criteria to opt for MBRtechnology.With the choice of the MBR special attentionshould be paid to the fact that the investment costsare largely correlated with the hydraulic peak flow.This parameter determines the total membranesurface area which needs to be installed. Therefore,accompanying measures to minimize wetweather flow or to harmonise resultant wastewaterlargely contribute to cost effectiveness of theMBR approach. One option for dealing with highwet weather peaks is to use the former secondaryclarifier as storage volume. Another option is hybridsystems where the conventional system isused as a backup to treat the inflow volume thatexceeds the hydraulic membrane capacity. An alreadyrealised hybrid concept designs the MBR line to treat the dry weather flow at the maximum.The inflow volume beyond dry weather conditions is treated conventionally.With upgrading of wwtp as a pure MBR process the question is to be cleared whether existing sedimentation tanks can be included sensibly in the future concept of utilisation. According to the structural state it can be possible to use these tanks as an additional biological volume. Also the suitable installation and operation of the membrane modules either in activated sludge tanks or in separated filtration chambers depends on the quality of the respective structural situation. At this point wide engineering space is given. Fig. It shows some possible variations.Beside the combination with the CAS process also combinations with other wastewater treatment procedures are possible, e.g. with SBR technology or with pond technology. Today SBR technology integrating membrane technology as separation process is used at industrial applications or at package plants. Applications combining pond technology and MBR technology are operated at wwtp St. Peter Judenburg, Austria and at wwtp Ihn, Germany. Filter beds (oxidizing beds)Main article: Trickling filterIn 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 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 percolates 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 thickness of the film leading to clogging of the filter media and ponding on the surface.Biological 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 filter 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 achieved 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 treatme