(完整版)给排水毕业论文设计.docx

(完整版)给排水毕业论文设计 优秀论文审核通过 未经允许切勿外传 Disinfection Disinfection involves destruction or inactivation of organisms which may be objectionable from the standpoint of either to those responsible for supplying water, design of facilities for disinfection must necessarily be carefully executed. a. Chlorination. The application of chlorine to water is the preferred method of disinfecting water supplies at military installations. (1) Definitions. Terms frequently used in connection with chlorination practice are defined as follows: (a) Chlorine demand. The difference between the concentration of chlorine added to the water and the concentration of chlorine remaining at the end of a specified contact period. Chlorine demand varies with the concentration of chlorine applied, time of contact, temperature, and water quality. (b) Chlorine residual. The total concentration of chlorine remaining in the water at the end of a specified contact period, (c) Combined available residual chlorine. Any chlorine in water which . The most common source of nitrogen is ammonia, and compounds formed by the reactions between chlorine and ammonia are known as chloramines. The disinfecting power of combined available chlorine is about 25 to 100 times less than that of free available chlorine. (d) Free available residual chlorine. That part of the chlorine residual which . (2) Chlorination practice. (a) Combined residual chlorination, Combined residual chlorination entails the application of sufficient quantities of chlorine and ammonia, if ammonia is not present in the raw water, to produce the desired amount of combined available chlorine (chloramine) in a water. If enough ammonia is present in raw water to form a combined chlorine residual, only chlorine need be added to the water. Combined residual chlorination is generally used only when maintaining an adequate free chlorine residual in the distribution system is difficult or when objectionably . Due consideration of other TTHM control alternatives should be made before using chloramines,(see para 2-13). (b) Breakpoint chlorination. If a water contains - ammonia or certain nitrogenous organic matter which reacts with chlorine, the addition of chlorine causes the formation of chloramines until the ratio of elemental chlorine to ammonia compounds is about 5 to 1. Further addition of chlorine results in the oxidation of chloramines to gaseous nitrogen and nitrogen oxides, which decreases the quantity of chloramines present. After all of the chloramines oxidized, additional chlorine added to the water forms only free available chlorine. The point at which all of the chloramines oxidized and only free chlorine is formed is called the “breakpoint .“ If no ammonia is present in the water, there will be no breakpoint. The chlorine required to reach the breakpoint is usually about 10 times the ammonia nitrogen contentof the water. However, in certain waters, because of the presence of other chlorine consuming substances, as much as 25 times the ammonia nitrogen concentration may be required. Enough chlorine should be added past the breakpoint to ensure an adequate free chlorine residual. (c) Marginal chlorination. Marginal chlorination involves the application of chlorine to produce a desired level of total chlorine residual regardless of the relative concentrations of free or combined chlorine present. In marginal chlorination the initial chlorine demand satisfied but some oxidizable substances remain. (d) Chlorine dosages. Figure 2-4 provides minimum cysticidal and bactericidal free chlorine residuals and minimum bactericidal combined chlorine residuals for various pH and temperature levels. Since waterborne bacteria are the major concern at fixed installations, minimum bactericidal levels will be maintained in treated water in all parts of the distribution system under constant circulation. Even at lower pH levels, free chlorine residuals should not fall below 0.2 mgL and combined chlorine residuals should not fall below 2.0 mgL. If marginal chlorination is practiced, the total chlorine residual must not be less than 2.0 mgl. Whenever epidemological evidence indicates an outbreak of a nonbacterial waterborne disease such as amebiasis, infectious the area of a fixed military installation, cysticidal free chlorine residuals shall be maintained in the water supply. Further guidance on disinfection requirements may be obtained from the Surgeon Generals office. Air Force policy on minimum chlorine levels is established in AFR 161-44. (3) Other effects of chlorination. In addition to the disinfection achieved with chlorination, other beneficial effects should be noted. Since the oxidizing power of chlorine is the presence of free chlorine, sulfide is oxidized, nitrites are oxidized to nitrates, and soluble iron and manganese are oxidized to their insoluble oxides. Free chlorine also reacts with naturally occurring taste, odor and colorproducing organic substances to form chloro-organic compounds, e.g., trihalomethanes (see para 2- 13.b.). The US EPA, after much discussion over costs benefits, a maximum contaminant level for serving above 10,000 persons and disinfection practices will be required. (4) Application of chlorine. Chlorine may be applied to water of two forms: As gaseous elemental chlorine or as at all fixed installations. The cost of all plants larger than 0.5 mgd. For remote sites at fixed installations, some well sources require 5 gpm or less. These sources with small demands can use . (a) Point of application. Chlorine may be applied to water in a variety of locations in the water treatment plant, storage facilities, or distribution system. It is absolutely essential that the chlorine applied to the water be quickly and thoroughly mixed with the water undergoing treatment. If required, special chlorine mixing facilities should be provided. In conventional water treatment plants, chlorine may be applied . prior to any other treatment process (prechlorination), following one or more of the unit treatment process (postchlorination), and again in the more distant points of the distribution system (dechlorination). 1 Prechlorination., Prechlorination been used so the water would maintain a chlorine residual for the entire treatment period, thus lengthening the contact time. The coagulation, flocculation, and filtration processes were thought to be improved by prechlorination of the water, and nuisance algae growths in settling basins were reduced. In prechlorination, the chlorine was usually injected into the raw water at or near the raw water intake. Prechlorination was the most accepted practice of disinfection in the past. However, since many surface waters contain THM precursors that will combine with the free chlorine during prechlorination and form potentially carcinogenic THMs, such as chloroform, the point of application shifted further down the treatment process to take advantage of precursor removal during treatment. 2 Postchlorination. Postchlorination generally involves the application of chlorine immediately after filtration and ahead of the clear well. The design and construction of water treatment plants for military installations will include the necessary provisions for changing the locations of chlorine applications as may later be desirable for improving treatment or disinfection processes. 3 Dechlorination. Dechlorination is the practice of adding chlorine to water in the distribution system to maintain a minimum chlorine residual throughout the system. (b) Chlorination equipment. Hypochlorite salts must be applied to the water in solution form. Hypochlorite solutions are pumped by a diaphragm pump through an injection system into the water to be chlorinated. If elemental chlorine is used for disinfection, it shall be injected by solution-type chlorinators. Since chlorine solutions are acidic, many components of a chlorination system must be constructed of corrosion resistant materials such as glass, silver, rubber, or plastics. Maintaining the chlorination apparatus in a trouble-free state is essential, Key spare parts and repair kits for chlorination systems must be kept on system shall be installed in duplicate. (c) Automatic control. If automatic chlorination control is utilized, the chlorine feed rate should be controlled primarily by the rate of flow of water, with a signal from a downstream residual chlorine analyzer used to trim the feed rate. Provision for manual control during emergency situations must be included. (5) Superchlorination and dechlorination. Superchlorination may be necessary if there are large variations in chlorine demand or if available contact time is brief. Water which superchlorinated generally requires dechlorination before discharge to the distribution system. Dechlorination may be achieved through the application of sulfur dioxide, sodium bisulfite, or sodium sulfite, or by passing the water through granular activated carbon filters. The dechlorination process (and subsequent dechlorination, if necessary) shall be controlled so that the free residual chlorine remaining in the water is at least 0.2 mgL. Careful monitoring must be practiced to assure that potentially table 2-5 (6) Safety precautions for chlorination. The AWWA manual “Safety Practice for Water Utilities”contains safety recommendations regarding the use of chlorine. These recommendations shall be followed at all military water treatment facilities. Further discussion on safe operation of chlorination facilities for Army installations are contained in TB MED 576, appendix L. b. Alternate Disinfectants. If the use of chlorine as a disinfectant causes unacceptably large concentrations of chlorinated organic compounds, and if all other methods for reducing TTHMs exhausted, such as moving the point of chlorination, aeration, and special coagulant (as shown in table 2-3 for chloroform which is the main constituent of TTHMs in many cases) and if an alternate raw water source, such as a ground water source, is not available, an alternative disinfectant must be considered. Any alternate disinfectant system installed as the primary means of water disinfection shall facilities available and operative for stand-by use. Five alternative disinfectants are discussed below; ozone, chlorine dioxide, chloramines, ultraviolet (UV) radiation, and UV and Ozone combined. While chlorine is the least costly disinfectant, considering dosage and energy consumption basis. However alternate disinfectants are not significantly more expensive. (1) Ozone. Ozone is an extremely powerful disinfectant that used in Europe either as a sole disinfectant, or in conjunction with postchlorination to impart a persistent chlorine residual in the water distribution system. United States potable water plants the past used ozone to control taste and odor. Today ozonation is being increasingly used as a primary disinfectant prior to rapid mixing, floccula- . tion and filtration. Ozonation does not produce THMs. It is reduced to oxygen and does not leave any residual disinfectant. Hence, the need for postchlorination. Ozone is generated electrically, as needed using the electric discharge gap (corona) technique. Air or oxygen stream, a cooling water stream and alternating electric current are required. Efficient cooling is essential to reduce thermal decomposition of ozone. Bubble diffusers appear to be the most economic ozone contractors available. (2) Chlorine Dioxide, Chlorine dioxide is a the presence of their precursors. Chlorine dioxide uses in the United States limited to taste and odor control although it used elsewhere as a primary disinfectant and is presently receiving more attention in the United States. The common method of chlorine dioxide production is to react chlorine gas from a conventional chlorinator with a sodium chlorite solution. Following the mixing of the chlorine and sodium chlorite streams and prior to introduction into the main stream the mixed stream is passed through a packed column contactor to maximize chlorine dioxide production. A major disadvantage of chlorine dioxide is the formation of chlorate and chlorite which are potentially toxic. -. (3) Chloramines, The use of chloramines as a disinfectant fell into disuse after the introduction of breakpoint chlorination. To achieve the same disinfection ability of chlorine, 10 to 15 times the amount of chloramines are needed or longer contact time is required. More chloramines are needed if the influent water, Chloramines are easy to generate, feed, and produce a persistant residual that will remain through the water distribution system. Chloramines may be produced by introducing ammonia to the water stream prior to the addition of free chlorine. This process can be optimized for minimum THM production and maximum disinfection. Recently some concern over chloramine toxicity. (4) Ultraviolet Radiation. Ultraviolet (UV) radiation used on a large scale for drinking water supply disinfection. Most of its uses include product or process water disinfection where used to disinfect drinking water at remotely located cruise ships. Few large scale water processing plants use UV disinfection, although its application is feasible. UV disinfection does not leave a disinfectant residual and should be accompanied by postchlorination. Ultraviolet irradiation is also effective in oxidizing organic compounds in water, Water turbidity will inhibit the effectiveness of UV disinfection. (5) UV and Ozone, Recently there some experimentation in a combined UV and ozone contactor. Results from these tests show promise. However, there is no known water treatment plant operating with this method of disinfection. 消毒 消毒包括去除和毁灭一些影响人体健康和感官的有机物。消费者对健康水的供应是首要关心的，消毒设施的设计必须小心谨慎。 一、加氯消毒加氯消毒是军事装备供水首要的消毒方法 （1）定义 氯化的普遍应用实践介绍如下 a.氯需求量 加在水中氯的浓度与最后水中的氯的浓度是有一定的差异的，氯的需求量与氯气的浓度，接触时间，温度，水质有关。 b.余氯 经过整个的接触过程后留在水中的氯的浓度 c.混合态使用余氯 水中与氮气结合的氯气。水中常用的氮气来自氨水，将氯气与氨水混合可得到人们熟知的氯胺。混合氯的消毒能力大约为游离态余氯的25%100%。 d.游离态可使用余氯 没有与氨气结合的部分余氯。 （2）氯化消毒的使用 a. 氯化消毒剩余物的结合 氯化消毒剩余物的结合必须应用一些比较特别的氯和氨，如果天然水中不含氨，在水中可提取到量的可使用的氯的结合物。 b.氯化消毒断点 如果水中含有氨或某些可与氯反映的含氮有机物，加入的氯将会引起反映直到氯与氮元素的化合比大约为5：1。进一步添加氯气，导致氮和氮氧化物减少，降低了水中现存的氯化物的量。所有的氯化物被氧化后，额外加入水中的氯将只会形成不可利用的氯。所有氯化物被氧化形成不可用氯的这个点被称为断点。如果水中不存在氨也就不存在断点。 氯化消毒达到断点通常水中的氨氮比应为10倍，然而在海水中存在其他需要消耗氧的物质时，有多达25倍的氨氮必会被要求。为了确保足够的游离余氯，断点后应加入充足的氯。 c.边际氯化 边际氯化不考虑游离态余氯和混合态余氯总余氯应达到的水平，除了一些氧化物质的存在，最初的边际氯化应得到满足。 d.氯使用量 表2-4显示了最少的游离态杀菌余氯与最小的混合态杀菌余氯在不 同的温度，PH的情况下的效果。在固定装置中，水煤细菌是首先应考虑的因素。在指定的循环下，最小杀菌水平将会体现各部分水的处理体系。即使在较低的PH值情况下，游离态余氯不应低于0.2毫克升，混合态余氯不应低于2.0毫克升。边际氯化实现了，总余氯不得少于2.0毫克升。在一个具有固定的军事设施的地方也会有一些突发性的水疾病如传染性肝炎，血吸虫病。所以应保持尤里余氯供水，进一步的消毒措施