给排水毕业论文

兰州交通大学毕业 设计 领导形象设计圆作业设计ao工艺污水处理厂设计附属工程施工组织设计清扫机器人结构设计 (论文) Disinfection Disinfection involves destruction or inactivation of organisms which may be objectionable from the standpoint of either health or esthetics. Inasmuch as the health of water consumers is of principal concern 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 has combined with nitrogen. 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 has not combined with nitrogen. (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 high levels of TTHMs would be formed as a result of free residual chlorination. 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 have been oxidized, additional chlorine added to the water forms only free available chlorine. The point at which all of the chloramines have been 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 has been 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 mg/L and combined chlorine residuals should not fall below 2.0 mg/L. If marginal chlorination is practiced, the total chlorine residual must not be less than 2.0 mg/l. Whenever epidemological evidence indicates an outbreak of a nonbacterial waterborne disease such as amebiasis, infectious hepatitis, or schistosomiasis in 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 General’s 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 high, in the presence of free chlorine, hydrogen 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, has chosen a maximum contaminant level for serving above 10,000 persons and has indicated a water treatment industry to avoid costly modifications to existing plants. To reach the US EPA’s future maximum contaminant level for TTHM’s, more significant changes in disinfection practices will be required. (4) Application of chlorine. Chlorine may be applied to water of two forms: As gaseous elemental chlorine or as hypochlorite salts. Gaseous elemental chlorine shall be used for water disinfection at all fixed installations. The cost of hypochlorite salts is prohibitive in 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 hypochlorite for disinfection. (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 has often 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 has been 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 hand. Critical components of the chlorination 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 has been 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 mg/L. Careful monitoring must be practiced to assure that potentially harmful levels of TTHMs are not exceeded. A summary of TTHM regulations are presented in 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 TTHM’s have been 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 have chlorination 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 has been 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 have in 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 highly effective disinfectant producing minimal THMs in the presence of their precursors. Chlorine dioxide uses in the United States have been limited to taste and odor control although it has been 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