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1、附录：Wastewater Biological Treatment ProcessesThe objective of wastewater treatment is to reduce the concentration of specific pollutants to the level at which the discharge of the effluent will not adversely affect the environment or pose a health threat. Moreover , reduction of these constituents ne
2、ed only be to some required level.For any given wastewater in a specific location , the degree and type of treatment are variables that require engineering decisions . often the degree of treatment depends on the assimilative capacity of the receiving water . DO sag curves can indicate how much BOD
3、must be removed from wastewater so that the DO of receiving water is not depressed too far . The amount of BOD that must be removed is an effluent standard and dictates in large part the type of wastewater treatment required .To facilitate the discussion of wastewater , assume a “ typical wastewater
4、 ”and assume further that the effluent from this wastewater treatment must meet the following effluent standards :BOD15mg/LSS15mg/LP1mg/LAdditional effluent standard could have been established , but for illustrative purposes we consider only these three . The treatment system selected to achieve th
5、ese effluent standards includes 1. Primary treatment : physical processes that nonhomogenizable solids and homogenize the remaining effluent .2. Secondary treatment : biological process that remove most of the biochemical demand for oxyen .3. Tertiary treatment : physical , biological , and chemical
6、 processes to remove nutrients like phosphorus and inorganic pollutants , to deodorize and decolorize effluent water , and to carry out further oxidation .Primary treatment The most objectionable aspect of discharging raw sewage into watercourses is the floating material . Thus screen were the first
7、 form of wastewater treatment used by communities , and they are used today as the first step in treatment plants .Typical screens consist of a series bars that might be about 2.5 cm apart . A screen in a modern treatment plant removes might damage equipment or hinder further treatment . in some old
8、er treatment plants are cleaned by hand , but mechanical cleaning equipment is used in almost all new plants . The cleaning rakes are activated when screens get sufficiently clogged to raise the water level in front of the bars .In many plants , the second treatment step is a comminutor , a circular
9、 grinder designed to grind the solids coming through the screen into pieces about 0.3 cm or less in diameter .The third treatment step is the removal of grit or sand from the wastewater . Grit and sand can damage equipment like pumps and flow meters and must be removed . The most common grit chamber
10、 is a wide place in the channel where the flow is slowed enough to allow the dense grit to settle out . Sand is about 2.5 times dense than most organic solids and thus settles much faster . The objective of a grit chamber is to remove sang and grit without removing organic material .Organic material
11、 must be treated further in the plant , but the separated sand may be used as fill without additional treatment .Most wastewater treatment plants have a settling tank after the grit chamber , to settle out as much material as possible . Accordingly , the retention time is long and turbulence is kept
12、 to a minimum .The solids settle to the bottom of the tank and are removed though a pipe , while the clarified liquid escapes over a V-notch weir that distributes the liquid discharge equally all the way around a tank . Settling tanks are also called sedimentation tank or clarifiers . The settling t
13、ank that immediately follows screening and grit removal is called the primary clarifier . The solids that drop to the bottom of a primary clarifier are removed as raw sludge .Raw sludge generally has a powerfully unpleasant odor , is full of pathogenic , and is wet , three characteristics that make
14、its disposal difficult . It must be stabilized to retard further decomposition and dewatered for ease of disposal .The objective of primary treatment is the removal of solids , although some BOD is removed as a consequence of the removal of decomposable solids .A substantial fraction of the solids h
15、as been removed , as well as some BOD is a little P , as a consequence of the removal of raw sludge . After primary treatment the wastewater may move on to secondary treatment .Secondary TreatmentWater leaving the primary clarifier has not lose much of the solid organic matter but still contains hig
16、h-energy molecules that decompose by microbial action , creating BOD . The demand for oxygen must be reduce (energy wasted ) or else the discharge may create unacceptable condition in the receiving waters . The objective of secondary treatment is to remove BOD , where the objective of primary treatm
17、ent is to remove solids .The trickling filter consists of a filter bed of fist-sized rocks or corrugated plastic blocks over which the waste is trickled . The name is something of a misnomer since no filtration takes place . A very active biological growth forms on the rocks , and these organisms ob
18、tain their food from the waste stream dipping through the rock bed . Air either is forced through the rocks or circulates automatically because of the difference between the air temperature in the bed and ambient temperatures . Trickling filters use a rotating arm that moves under its own power , li
19、ke a lawn sprinkler , distributing the waste evenly over the entire bed . often the flow is recirculated and a higher degree of treatment attained .Trickling filtration was a well-established treatment system at the beginning of the twentieth century . in 1914 , a pilot plant was built for a differe
20、nt system that bubbled air through free-floating aerobic microorganisms , a process which became known as the activated sludge system . The activated sludge process differs from trickling filtration in that the microorganisms are suspended in the liquid .An activated sludge system includes a tank fu
21、ll of waste liquid from the primary clarifier and a mass of microorganisms . Air bubbled into this aeration tank provides the necessary oxygen for survival of the aerobic organisms . The microorganisms come in contact with dissolved organic matter in the wastewater . adsorb this material , and ultim
22、ately decompose the organic material to CO2 , HO2 ,some stable compounds , and more microorganisms .When most of the organic material , that is , food for the microorganisms , has been used up , the microorganisms are separated from the liquid in a settling tank , sometimes called a secondary or cla
23、rifier . The microorganisms remaining in the settling tank have no food available , become hungry , and are thus activated-hence the term activated sludge . The clarified liquid escapes over a weir and may be discharged into the receiving water . The settle microorganisms , now called return activat
24、ed sludge , are pumped back to the head of the aeration tank , where they find more food in the organic compounds in the liquid entering the aeration tank from the primary clarifier , and the process starts over again . Activated sludge treatment is a continuous process ,with continuous sludge pumpi
25、ng and clean-water discharge .Activated sludge treatment produces more microorganisms than necessary and if the microorganisms are not removed , their concentration will soon increase and clog the system with solids . Some of the microorganisms must therefore be wasted and the disposal of such waste
26、 activated sludge in one of the most difficult aspects of wastewater treatment .Activated sludge system are designed on the basis of loading , or the amount of organic matter , or food , added relative to the microorganisms available . The food-to-microorganisms (F/M) ratio is major design parameter
27、 . Both F and M are difficult to measure accurately , but may be approximated by influent BOD and SS in the aeration tank , respectively . The combination of liquid and microorganisms undergoing aeration is known as mixed liquor , and the SS in the aeration tank are mixed liquid suspended solids (ML
28、SS) . The ratio of influent BOD to MLSS , the F/M ratio , is the loading on the system , calculated as pounds (or kg ) of BOD per day per pound or kg of MLSS .Relatively small F/M ,or little food for many microorganisms , and a long aeration period (long retention time in the tank ) result in a high
29、 degree of treatment because the microorganisms can make maximum use of available food . Systems with these features are called extended aeration systems and are widely used to treat isolated wastewater source , like small developments or resort hotels . Extended aeration systems create little exces
30、s biomass and little excess activated sludge to dispose of .The success of the activated sludge system also depends on the separation of the microorganisms in the final clarifier . when the microorganisms do not settle out as anticipated , the sludge is said to be a bulking sludge , Bulking is often
31、 characterized by a biomass composed almost totally of filamentous organisms that from a king of lattice structure within the sludge floes which prevents settling . A trend toward poor settling may be the forerunner of a badly upset and ineffective system . The settle ability of activated sludge is
32、most often described by the sludge volume index (SVI) , which is reasoned by allowing the sludge to settle for minutes in a 1-Lcyliner . If the SVI is 100 or lower , the sludge solids settle rapidly and the sludge returned to the final clarifier can be expected at a high solids concentration . SVI i
33、s about 200 , however , indicate bulking sludge and can lead to poor treatment .Tertiary Treatment The effluent from secondary treatment meets the previously established effluent standards for BOD and SS . only phosphorus content remains high . The removal of inorganic compounds , including inorgani
34、c phosphorus and nitrogen compounds , requires advanced or tertiary wastewater treatment .Primary and secondary (biological) treatment are a part of conventional wastewater treatment plants . However , secondary treatment plant effluents are still significantly polluted . some BOD and suspended soli
35、ds remain , and neither primary nor secondary treatment is effective in removing phosphorus and other nutrients or toxic substances . A popular advanced treatment for BOD removal is the polishing pond , or oxidation pond , commonly a large lagoon into which the secondary effluent flows . such ponds
36、have a long retention time , often measured in weeks .BOD may also be removed by activated carbon adsorption , which has the added advantage of removing some is a completely enclosed tube , which dirty water is pumped into bottom and clear water exits at the top . microscopic crevices in the carbon
37、catch and hold colloidal and smaller particles . As the carbon column becomes saturated , the pollutants must be removed from the carbon and the carbon reactivated , usually by heating it in the absence of oxygen . Reactivated or regenerated carbon is somewhat less efficient than using virgin carbon
38、 , some of which must always be added to ensure effective performance .附录：污水的生物处理过程物水处理的目的是降解特定污染物的浓度使其达到污水排放不影响环境或形成健康威胁的程度，而且降低这些成分仅仅需要达到要求大的水平即可。应为任何给定的污水在来源，处理程度和类型方面是变化的，而这种变化是由工业决定的，而且处理程度往往依赖接纳水体的同化能力。氧垂曲线能显示多少BOD必须从污水中去除以致于使接纳水体的溶解氧不致降低太多。在大部分的污水处理要求的类型中，必须被去除的BOD数量是污水的一个标准和指标。为了便于讨论污水，假定一种“
39、典型的污水”而且假定这种污水通过处理必须达到以下的污水标准：BOD 15 mg/LSS 15 mg/LP 1 mg/L增加的污水标准应该被列出，但是为了说明目的我们仅仅考虑这三项。能达到这些污水标准的处理系统包括：1. 初级处理：固体物质非均匀化和污水排放均匀化的物理过程。2. 二级处理：依靠氧去除大部分生化物质的生物处理过程。3. 三级处理：物理，生物和化学过程的目的就是去除营养物质，像磷和有机物等，而且去除污水的臭味，色度和更多的氧化物。初级处理排放原污水到水域中最厌恶的一个方面是其中的漂浮物，因此，格栅被用在社区污水处理的首要位置，现在它们被作为处理方案的第一步，典型的格栅由一系列的钢条
40、组成，这些钢条之间大约相距2.5cm，在现代的处理方案中，格栅拦截了那些可能对设备造成破坏或者影响后续处理的物质，在过去的一些旧的处理方案中，格栅通过人工清洗；但是现在几乎所有的方案中都用机械清洗设备。当格栅被阻塞使水位上升到栅条以上时，清洗设备被启动。在许多方案中，第二步是粉碎，循环挤压固体物质通过格栅形成大约3.0cm或更小直径的小块。第三步处理是去处污水中的沙粒和石子，沙粒和石子会损坏设备，像水泵和流量计，因此必须被去除。大部分沉沙池是个很宽的空间，在这个渠道内水流慢到足够大密度的沙粒沉淀下来，沙粒的密度是大部分有机物密度的2.5倍，因此沉淀的比较快。沉沙池的目的就是去除沙粒和石子，而不
41、去除有机颗粒，在处理过程中有机物必须被进一步处理而分离除的出的沙粒可以被用作填充物而无需额外处理。大部分污水处理方案中在沉沙池后面都有一个沉淀池，目的就是尽可能的去除固体颗粒，因此，停留时间比较长，水流速度保持比较慢。固体沉淀到池子底部，然后通过管道被清除，而被澄清的液体则通过均匀布置在池子周围的V型溢流堰排放，沉淀池也被称为沉积池或澄清池，紧随在清渣和除沙之后的沉淀池也被叫作初次澄清池，沉淀在初次澄清池底部而被排放的固体称为原污泥。原污泥通常有很强的令人不快的气味，含有病原有机体，而且含水率高，这三个特点增加了它处理的困难，它必须被稳定为了防止它进一步腐化，也为了容易脱水处理。初级处理的目的
42、就是去除固体颗粒，然而一些BOD也在分离固体物质的一系列过程中被去除了。相当的一部分固体物质，还有一部分BOD和少量的P作为原污泥的产物而被去除了，被初级处理之后污水可能继续进行二级处理。二级处理水离开初次澄清池并没有去除多少固体有机物，而且仍含有能通过微生物作用分解的高能量分子，成为BOD。要求氧必须减少，否则，在接纳的水体中排放可能出现不能接受的情况。二级处理的目的就是去除BOD，而初级处理的目的则是去除固体颗粒。虑池由拳头大小的砾石或波纹塑料板的虑床组成，通过它污物被虑掉，命名可能有错误，因为根本没有过滤发生。活性微生物附着在砾石上，空气被强制通过砾石或自动循环，因为虑床空气温度和周围温
43、度的差异，虑池用一个循环调节系统降低它的能耗，像一个均匀布水装置，能均匀的在整个虑床上分布污水，而通常水流是循环的，因而更高程度的处理被实现。在20世纪初，滴虑是一种被证实了的处理系统。1914年，一个试验方案确立了一个不同系统，空气泡通过自由漂浮的好氧微生物，一种变的出名的活性污泥系统的处理方案。活性污泥处理不同于滴虑在于滴虑过程中微生物是悬浮在液体中的。一个活性污泥处理系统包括一个充满来自初次澄清池的污染液体的池子和一团微生物, 空气泡进入曝气池提供存活好氧有机物所必须的氧气，微生物进入与在污水中的溶解性有机物混合，吸附这些物质，最终将有机物分解成，等一些稳定的化合物和更多的微生物。当大部
44、分作为微生物食物的有机物被消耗掉，在一个沉淀池里微生物被从液体中分离，有时叫二次澄清池，在沉淀池里微生物仍旧没有食物可利用，变得饥饿，变成失去活性的活性污泥，澄清的液体从溢流堰流出而排入接纳水体。沉淀的微生物现在叫回流污泥，被抽送至曝气池前端，在那里它们从初次澄清池进入曝气池的液体的有化合物中找到更多的食物，处理再一次开始。活性污泥处理是一个连续的过程，连续的污泥回流和连续的清水排放。活性污泥处理产生的微生物多于需要的，如果微生物不被清除，它们的浓度将很快增加而形成固体物阻塞系统，因此，一部分微生物必须被去除，而处理这样多余的活性污泥也是污水处理中一个最困难的方面。活性污泥处理系统设计依据负荷
45、，有机物数量或营养物，而且联系到微生物的易变性，营养物与微生物（F/M）之比是一个主要的设计参数，F和M难于精确测定的，但是可以通过曝气池中的BOD和SS分别被估算，液体和微生物的混合物被称为混合液。在曝气池内SS是混合液悬浮固体（MLSS），BOD/MLSS，F/M是系统负荷，以计。相对小的F/M，或者少的营养物，而且一个长的曝气周期（长的停留时间在池中）产生一个高的处理效果，因为微生物能最大限度的利用营养物，有这些特点的系统被称为延时曝气系统，而且被用于单独的污水处理工艺，像小的工业部门或用于应急处理的旅馆。延时曝气系统产生少的剩余生物团，而且只有少的剩余活性污泥需要处理。活性污泥处理系统
46、的成功也依赖微生物最终在澄清池里的分离。当微生物没有像预期中的沉淀去除，则污泥被称为膨胀污泥，膨胀被描述为生物团有大量的丝状菌组成，形成了一个由污泥附着其上的空架结构而阻止了沉淀，低的沉淀率可能是一个低效率系统的先兆。活性污泥沉淀能力经常用污泥容积指数描述（SVI），SVI值的测定是通过在一个1L的圆筒内让污泥沉淀数分钟，如果SVI值是100或更低，则污泥固体沉淀快而且最终从澄清池流出的污泥可能被保持在一个高的固体浓度，然而，SVI值大约是200时，显示污泥膨胀而导致低的处理效果。三级处理来自二级处理的污水能达到前续要求的BOD和SS排放标准，仅仅磷含量仍旧是高的，去除无机化合物，包括无机磷和
47、氮化合物，就要求先进的或废水三级处理系统。初级和二级处理是传统废水处理方案的一部分，而且二级处理后污水被污染的程度仍然很高，一定程度的BOD和悬浮固体仍然存在，既不是初级也不是二级处理是有效的去除磷，氮或者碳的化合物的方法。一种流行的先进的BOD去除处理是人工塘或氧化塘，通常是二级污水流入一个大的环礁湖，像这样的塘有一个长的停留时间，经常保持在一周。BOD也可能被去除通过活性炭的吸附，它增加的去除的有点是被完全封闭在导管中，污水被从底部抽送进去，而清水则从上部流出，在活性炭中微小的入口能捕捉和吸附病毒和更小的颗粒，当炭柱变的饱和后，污物必须从炭中清除，使炭恢复活性，通常是通过在缺氧条件下加热。恢复活性或再生的炭效率仍较低，经常一些新的活性炭必须被添加进去以确保其工作效率。
