原文链接：

https://awwa.onlinelibrary.wiley.com/doi/epdf/10.1002/opfl.1673

原文作者：

Hunter Adams, Steve Ash, Keisuke Ikehata, Laith Furatian, Mark Southard

原文出处：J Opflow

翻译：阮辰旼

Operators Need to Know Basics of Distribution System Water Quality

运营人员需要了解的输配系统水质基础

Once source water (surface water or groundwater) passes through a treatment plant for purification, the treated water has to be moved to customers for daily use. When the treated water moves from the plant to the distribution system entry point, the highly controlled conventional and advanced water treatment processes discussed in earlier articles in the “Operators Need to Know” series can become more difficult to manage.

当原水（地表水或地下水）通过水厂的净化处理后，就必须要被输送到用户来进行日常的使用。当处理过的水从水厂到居民室内分配系统的入口过程中，在“运营人员需要知道”系列的前两篇文章中讨论的常规处理和深度处理过程就难以对之进行控制。

There are a wide variety of distribution system concerns that operators must deal with daily, including many that focus their attention on achieving adequate pressure and flow. Some of these areas may be within the water system operator's control and some may not, but it's important for operators to understand these concerns to supply each customer with a sufficient volume of water at adequate pressure.

运营人员每天都必须处理各种各样的输配系统的问题，其中有许多问题的关注点是保证足够的压力和流量。其中有些问题涉及的领域可能在供水系统运营人员的控制范围内，有些可能在控制范围外，但运营人员必须了解这些问题，以便以足够的压力向每个用户提供足够的水量。

In addition, it's critical for operators to deliver safe water that satisfies the quality expectations of customers and regulators. The distribution and storage system is a vital component of a multiple-barrier approach to preventing contamination, and each component must be optimized to provide the maximum level of protection from contamination as water is delivered to consumers. This article covers some of the most common factors that can affect water quality in distribution systems.

此外，供水企业提供安全的水，使满足用户和监管机构对水质的期望，这一点至关重要。输配过程和储水系统是防止水质污染的多重屏障中重要的组成部分，每个组成部分都必须进行优化，以便在将水输送给用户过程中防止污染，提供最大程度的保护。本文涵盖了一些可能影响输配系统水质的最常见因素。

WATER AGE

水龄

One of the water quality challenges that operators may face is minimizing water age, or residence time, in the distribution system. Residence time refers to the amount of time that water spends in the distribution system between the entry point and the point of use. It's best to minimize the time the water spends in the distribution system.

运营人员可能面临的水质挑战之一是尽量减少输配系统中的水龄或称为停留时间。停留时间是指水从进入点到使用点之间在输配系统中停留的时间，水龄越短越好。

To help reduce the residence time, an operator should have a plan to fill and drain the distribution system storage tanks on a regular basis when possible. Overhead storage tanks typically fill overnight when water demand is lowest and then are allowed to drain during the day when water demand is highest. This “cycling” ensures that the water in the tanks doesn't stay in residence longer than necessary. For example, an operator may choose to cycle an overhead storage tank by filling it to within 90% of its capacity and then letting it drain to 30% capacity, then repeating that cycle. When this is done on a regular basis, it helps to ensure the water doesn't stay in the tank long enough to become “stale.” Occasionally, the operator may even choose to “deep cycle” the tank to a lower percent capacity to ensure the stored water gets used and doesn't remain in the tank longer than necessary. The levels to which the operator fills and drains are an operational decision typically based on the operator's experience and the water system's requirements.

为了帮助减少停留时间，运营人员需要在可能的情况下对输配系统储水池的进水和排水周期保持一个基本的规律。高位水池一般在夜间用水量需求低的时候装满，在白天用水量高的的时候逐渐排空。这种“循环”可以保证水在水池中的停留时间不会长于必要的水平。例如，某个运营人员对某个高位水箱选择采取进水到90%容量并放空到30%容量的运行周期，并反复循环执行该策略。当确定了这样一个运行规则并执行，就能帮助确保该水池中的水不会停留时间过长而“陈旧”。偶尔的，运行人员也会选择实行“审读循环”的策略，让水池中的水存留的更少，以贮水背充分使用，而不是比平时停留更长的时间。具体如何确定水池进水和放空的策略，就取决于运营人员的经验，以及供水系统的需求了。

Other means of reducing residence time is through flushing dead-end mains, which are sections of the distribution system where the piping isn't looped and water can sit for extended periods of time and become stale. Many systems may already have a dead-end main flushing program in place, whereby they flush these areas on a regular basis (e.g., weekly or monthly). If your system doesn't have a flushing program in place, flushing dead-end mains can improve water quality and may reduce customer complaints and possible water quality violations. Flushing can be accomplished by installing automatic flushing devices or by using a fire hydrant (Photo 1).

缩短停留时间的其他方法是通过冲洗管网的死水区，死水区是输配系统中管道不循环的部分，水可能长时间停留在那里而变得陈旧。许多供水系统可能已经有了冲洗管网死水区的计划，他们定期冲洗这些区域（如每周或每月）。如果目前供水系统还没有死水区的冲洗计划，那么进行冲洗可以改善水质，并可能减少用户投诉和可能的水质违规。冲洗可以通过安装自动冲洗装置或使用消火栓来完成（图片1）。

Reducing residence time in the distribution system can prevent an array of potential water quality problems such as biofilm formation, nitrification, corrosion, and disinfection byproduct (DBP) formation. In systems that use chloramines as a terminal disinfectant, having a high residence time can promote nitrification and can create bacterial regrowth in the system, leading to biofilm production. Biofilm can begin to grow in areas of the system where the water has become stale and where disinfectant residuals have diminished. Biofilm provides an environment where pathogenic bacteria, such as Escherichia coli and Legionella, can grow because disinfectant can't effectively reach them. Moreover, biofilm creates a disinfectant demand and can rapidly diminish disinfectant residuals. Because biofilm is organic material (bacteria), it can react with chlorine products to create DBPs such as trihalomethanes (THMs).

减少输配系统中的停留时间可以防止一系列潜在的水质问题，比如生物膜的形成、硝化反应、腐蚀和消毒副产物（DBPs）的生成。在使用氯胺作为终端消毒的系统中，较长的停留时间会促进硝化作用，并在系统中发生细菌再生现象，导致生物膜的产生。生物膜一般可以在系统中水已经变质或残留消毒剂减少的位置开始生长。生物膜为大肠杆菌和军团菌等致病菌的生长提供了环境，因为消毒剂无法有效地到接触到它们。此外，生物膜会对消毒剂造成额外的消耗，可以迅速减少消毒剂的残留量。而且，因为生物膜是有机物质（细菌），它可以与氯化物反应，生成消毒副产物，如三卤甲烷（THMs）。

Extended residence time can also increase the chances for corrosion. Carbon dioxide can dissolve into the water in the distribution system, creating carbonic acid. This lowers the water's pH and, given enough residence time in the system, can lead to corrosion inside the distribution system's pipes and storage tanks. In addition, corrosion can oxidize iron in the system to create rust, which increases the chance of “red water” calls from customers. Stale water may also give rise to various taste and odor (T&O) issues that can lead to customer complaints. These topics will be discussed in greater detail in subsequent sections.

过长的停留时间同样可能增加腐蚀的机会。二氧化碳会被溶解在水中进入输配系统，形成碳酸。这会降低水的pH值，如果还有足够长的停留时间，就会导致输配系统中管道和水池内部的腐蚀。此外，腐蚀的过程会氧化系统中的铁形成铁锈，增加被用户称之为“红水”的机会。“陈旧”的水也会增加各种嗅和味的可能，导致用户的投诉。这些问题在随后的章节里会更详细地讨论。

BIOFILMS

生物膜

Water that is safe to drink is neither sterile nor chemically pure. It contains microorganisms and organic and inorganic solutes. These solutes, particularly naturally occurring organic matter, provide nutrients and energy to heterotrophic bacteria that initially enter the distribution system as individual cells or clumps of cells suspended in the finished water and are referred to as planktonic. In their planktonic state, heterotrophic bacteria don't find the distribution system to be a hospitable place when nutrient levels are sufficiently low and/or an adequate disinfectant residual is present. Under conditions that inhibit microbial growth, the water may be referred to as biologically stable, a desirable state for consumers that essentially extends the “shelf life” of the product.

可以安全饮用的水并不是完全无菌的，也不是化学上绝对纯净的。它可能含有微生物以及有机和无机溶质。这些溶质，特别是自然的有机物，为异养细菌提供了营养和能量，这些细菌最初以单个细胞或悬浮在出厂水中的细胞团的形式进入输配系统，被称为浮游生物。在浮游状态下，当营养水平足够低和/或有足够的消毒剂残留时，输配系统不会被异养细菌认为是个好地方。具有这种抑制微生物生长的条件，水可以被称为生物稳定，这是用户希望得到的状态，基本上可以延长水的“保质期”。

Planktonic cells in the distribution system may adhere to surfaces such as pipe walls. Cells that have adhered to surfaces will encounter a new environment that is generally more favorable than the one they endured in the bulk water. A biofilm may eventually develop on surfaces in a complex microbial community involving multiple types of bacteria and a significant amount of material excreted by cells known as extracellular polymeric substance. Biofilm structure can take on complex forms that offer protection to cells against disinfectants, retain nutrients, and promote the growth of colonies that ultimately may release planktonic cells to colonize downstream surfaces (Figure 1). Biofilm is the natural home of bacteria, with the planktonic state serving as a means of dispersal. If significant amounts of particulate matter are allowed to settle at the bottoms of pipes and storage facilities, then the available surface area for biofilm formation increases significantly.

输配系统中的浮游细胞可能粘附在管壁等表面。粘附在表面上的细胞将进入一个新的环境，这个环境通常比它们在水中生存的环境更有利。在，生物膜最终可能会在拥有一个复杂的微生物群落的表面中形成，包括涉及多种类型的细菌和细胞排泄的大量物质，这些物质被称为胞外聚合物质。生物膜的结构可能是复杂的形式，以便为细胞提供保护，抵御消毒剂，保留营养物质，并促进菌落的生长，最终可能释放出浮游细胞，在输配系统的更下游表面增殖（图1）。生物膜是细菌的天然家园，浮游状态则是细菌对外散布的一种手段。如果允许大量的颗粒物质在管道和储存设施的底部沉淀，那么可供形成生物膜的表面积就会大大增加。

NITRIFICATION

硝化作用

Free ammonia (NH₃) naturally occurs in surface waters at low levels. It's used in water treatment with the addition of chlorine (Cl₂) to produce chloramines. Chloramines are longer lasting than Cl₂ and don't have the same risks of DBP formation potential as Cl₂, but they have their own set of potential issues that must be monitored. Systems that use chloramines as a disinfectant can become problematic as water ages and nitrification occurs.

游离氨（NH₃）在地表水中天然地存在，但浓度很低。它在水处理中与氯（Cl₂）一起使用会生成氯胺。氯胺比Cl₂更持久，也不像Cl₂那样有生成消毒副产物的风险，但它也有自己的一些潜在的问题，必须加以监测。使用氯胺作为消毒剂的供水系统会随着水龄的延长和硝化作用的发生而变得有问题。

Nitrification is the process by which NH₃ is oxidized. Chloramines degrade over time, and NH₃ levels can rise as degradation occurs. NH₃ can be used as a food source for certain microorganisms, leading to an overall degradation in water quality as nitrification progresses. As microorganisms metabolize NH₃ (Figure 2), nitrite (NO₂^–) is produced and is then further metabolized and converted to nitrate (NO₃^–). Nitrate doesn't readily degrade and can build up in distribution systems to create water quality problems and harmful health effects (e.g., blue baby syndrome). The maximum contaminant level (MCL) for nitrate-N at the entry point is 10 mg/L, but it isn't currently regulated in the distribution system.

硝化作用是NH₃被氧化的过程。氯胺随着时间的推移而降解，NH₃水平会随着降解的发生而上升。NH₃可作为某些微生物的食物来源，随着硝化作用的进行，导致水质的整体恶化。当微生物代谢NH₃时（图2），会产生亚硝酸盐（NO₂^-），然后进一步代谢并转化为硝酸盐（NO₃^-）。硝酸盐不容易降解，可以在输配系统中堆积，造成水质问题和有害的健康影响（如蓝婴综合症）。用户的接入处的硝酸盐-N的最大污染物水平（MCL）是10毫克/升，但目前在输配系统中没有进行限制。

The distribution system should be regularly monitored for early warning signs of a nitrification event. Water quality parameters, such as pH, temperature, alkalinity, chloramines, NH₃, NO₂^–, and NO₃^–, should be monitored so action can be taken if a nitrification event is detected. Common indicators of the occurrence of some degree of nitrification can be seen in the following field test parameters: decreasing pH; warmer temperatures; decreasing chlorine and chloramine concentrations; and an increase in NH3, NO₂^–, and/or NO₃^– (Table 1). An increase in any one of these nitrogen species can indicate nitrification in that particular area of the system.

输配系统应定期监测硝化事件的早期预警表现。应监测的水质参数如pH值、温度、碱度、氯胺、NH₃、NO₂^-和NO₃^-，以便在发生硝化反应时采取行动。发生某种程度的硝化反应可以从以下常见指标的即时数据变化趋势中看出：如pH值下降；温度升高；氯和氯胺的浓度下降；NH₃、NO₂^-和/或NO₃^-增加（表1）。各种价态的氮中的任何一种浓度增加都可以表明系统中该特定区域中发生了硝化反应。

Nitrification can be controlled through regular monitoring and flushing at low-usage/high-water-age areas, such as areas with low customer occupancy and/or dead-end mains. Nitrification also can be problematic in areas of aging infrastructure, as older pipes with tuberculation provide increased surface area and increase the potential for biofilm formation. For this reason, it's also beneficial to monitor heterotrophic bacteria by performing heterotrophic plate counts (HPCs), which provide an estimate of the bacterial population in a water sample. It's generally accepted that when the concentration of HPC bacteria is lower, water quality is higher.

硝化反应可以通过定期的监测，以及对低用水量/高水龄的地区进行冲洗来控制，例如用户入住率低的地区和/或管网的死水区。在基础设施老化的地区，硝化反应也可能成为问题，因为有沉积物结垢的旧管道提供了更大的表面积，增加了形成生物膜的可能性。因为异养菌平板计数（HPCs）能够帮助估计水样中的细菌数量因此，因此通过异养菌平板计数来监测异养菌也是有帮助的。人们普遍认为，当异样菌的浓度较低时，水质较好。

In Texas, the Texas Commission on Environmental Quality began enforcing the use of nitrification action plans (NAPs) in 2015 to help control nitrification issues in public water systems (PWSs) using chloramines. Each PWS is required to monitor entry points and sample sites that are representative of the distribution system; many PWSs use Revised Total Coliform Rule sites as their NAP sites. Yellow “alert” and red “alarm” triggers are used to indicate when and what action must be taken based on sample results (e.g., supervisor notification, corrective actions, etc.). NAPs act as an early warning system to detect and mitigate water quality degradation from nitrification in distribution systems.

在德克萨斯州，德克萨斯州环境质量委员会于2015年开始强制施行硝化反应行动计划（NAPs），以帮助控制使用氯胺的公共供水系统（PWS）中的硝化问题。每个公共供水系统被要求监测用户接入点和输配系统中有代表性的采样点；许多供水系统使用修订的总大肠菌群规则点作为其NAPs监测点，触发黄色“警报”和红色“报警”用于表明何时以及根据样本结果必须采取何种行动（例如，主管通知、纠正行动等）。NAPs作为一个早期预警系统，可用于监测和缓解输配系统中硝化反应造成的水质恶化。

CORROSION

腐蚀

Corrosion is a natural process that degrades materials such as metals, metal alloys, and concrete through electrochemical reactions (rusting). Because corrosion typically produces chemical compounds that have lower physical-chemical properties, such as tensile strength, compressive strength, elasticity, and chemical resistance, than the parent materials, corrosion is one of the leading causes of structural and mechanical failure in infrastructure, including water distribution systems. Unlike metals and other inorganic materials, plastics are made of organics and usually don't undergo electrochemical degradation. Instead, plastics often are degraded via direct contact with solvents and other aggressive chemicals and/or photochemical (ultraviolet light) processes.

腐蚀是一个自然发生的过程，通过电化学反应（生锈）使金属、金属合金和混凝土等材料钝化。由于腐蚀通常产生的化合物的物理化学性质较差，如抗拉强度、抗压强度、弹性和耐化学性等，因此，腐蚀是基础设施（包括输配系统）发生结构和机械故障的主要原因之一。与金属和其他无机材料不同，塑料是由有机物制成的，通常不会发生电化学反应。相反的，塑料经常会通过直接接触溶剂和其他腐蚀性化学品和/或光化学（紫外线）过程而被降解。

Corrosion can be caused by many factors, including oxygen, moisture (water), acidity, alkalinity, and electric current. Water distribution systems convey water and usually are buried underground where soil moisture is present. In addition, the quality of water conveyed affects corrosion in the distribution system. All of these factors make corrosion a perpetual challenge for water distribution system operators. Because corrosion is a relatively slow process and often starts where it's usually invisible or inaccessible, regular monitoring and routine preventive maintenance are crucial.

腐蚀可由许多因素引起，包括氧气、水分（水）、酸度、碱度和电流。输配系统输送水，通常被埋在地下，那里也有土壤水分存在。此外，输送的水的水质也会影响到输配系统的腐蚀。所有这些因素使腐蚀成为输配系统运营人员的一个长期挑战。由于腐蚀是一个相对缓慢的过程，而且往往开始于平常看不见或无法触及的地方，所以定期监测和日常预防性维护是至关重要的。

In addition to structural and mechanical failures, such as leaks, main breaks, and valve and pump failures, corrosion inside the water distribution pipes (i.e., internal corrosion) can cause several health, aesthetic, and operational issues, including the following:

除了结构和机械性故障，如泄漏、主管道断裂、阀门和水泵故障外，输配管道内部的腐蚀（即内部腐蚀）也会造成一些健康、感官和运营中的问题，包括以下内容。

● Leaching of metals, such as copper, iron, lead, and zinc
          金属物质的泄露，比如铜、铁、铅和锌

● Lower chlorine residuals
          较低的余氯浓度

● Microbial regrowth, biofilm formation, and potential positive coliform samples
          微生物的再生，生物膜的形成，以及潜在的大肠杆菌阳性

● Potential nitrification
          潜在的硝化反应

● Discolored water
          有颜色的水

● Turbidity
          浑浊度

● Metallic taste
          金属味

● Sediment formation and pressure problems
          导致发生管道沉积以及压力问题

There are several types of corrosion, including uniform corrosion, pitting corrosion, galvanic corrosion, and microbiologically influenced corrosion. Direct contact of two types of metals or alloys should be avoided to prevent galvanic corrosion. Microbial activity, such as nitrification and sulfate reduction, can influence corrosion. Controlling biofilm formation in the pipes is critical to prevent microbiologically influenced corrosion.

腐蚀有几种类型，包括均匀腐蚀、点状腐蚀、电化腐蚀和微生物影响的腐蚀。应避免两种类型的金属或合金直接接触，以防止电化腐蚀。微生物活动，如硝化作用和硫酸盐还原，也会影响腐蚀的程度。控制管道内生物膜的形成对于防止微生物造成的腐蚀至关重要。

1. From left, anodizing/reducing action causes pitting in the pipe as elemental iron (Fe) dissolves into corrosive water. Ferrous (dissolved iron [Fe²⁺]) ions are released into the water column.

1. 从左起，阳极氧化/还原作用导致管道中的点蚀，因为管道中的铁元素（Fe）溶解到腐蚀性的水中，铁离子（溶解的铁[Fe²⁺]）被释放到水中。

2. Electrons (e^-) freed from the reduction of elemental iron react with dissolved oxygen (O₂) and water (H₂O), producing hydroxide ions (OH^-) in a cathodic/oxidizing reaction.

2. 铁元素还原后释放的电子（e^-）与溶解的氧气（O₂）和水（H₂O）反应，在阴极/氧化反应过程中产生氢氧根离子（OH^-）。

3. Hydroxide ions (OH^-) are released into the water column and react with dissolved ferrous ions (Fe²⁺), forming iron(II) hydroxide (Fe(OH)₂).

3. 氢氧根离子（OH^-）被释放到水体中，与溶解的亚铁离子（Fe₂₊）反应，形成氢氧化铁（II）（Fe（OH）₂）。

4. Fe(OH)₂ undergoes dehydration, producing a variable amount of H₂O, and rust (iron oxide [Fe₂O₃]) precipitates out of the water column and attaches to the water main. As stated previously, rust increases surface area and increases biofilm formation potential.

4. Fe(OH)₂发生脱水，产生不同数量的H₂O，铁锈（氧化铁[Fe₂O₃]）从水体中沉淀出来，附着在水管上。如前文所述，铁锈增加了管道的比表面积，增加了生物膜的形成潜力。

Water corrosivity can be assessed using several index values, such as Langelier saturation index (LSI), Ryznar stability index, aggressive index, and calcium carbonate precipitation potential. LSI is one of the most widely used index values and can be calculated by the following equations:

水的腐蚀性可以用几个参数来评估，如朗格莱尔饱和度指数（LSI）、雷兹纳稳定性指数、侵蚀性指数和碳酸钙沉淀潜力。LSI是最广泛使用的指数值之一，可以通过以下公式计算。

LSI=pH–pH_s     [1]

where pH_s is the saturation pH and can be calculated by:

其中pH_s是饱和pH值，可通过以下方式计算：

pHs=(9.3+A+B)–(C+D)     [2]

其中，

A=（log10溶解性总固体-1）/10，

B=-13.12×log₁₀（摄氏温度+273）+34.55，

C=log₁₀[碳酸钙（以Ca²⁺计）]-0.4，

D=log₁₀[碱度（以CaCO₃计）]。

A negative LSI value indicates that the water is corrosive, whereas a positive value indicates that the water is scale-forming. Seriously corrosive (LSI < –0.5) and scale-forming (LSI > 0.5) conditions should be avoided. Hard water is often oversaturated with calcium carbonate (CaCO3) and causes excessive scaling, which can reduce the internal diameter of pipes and fixtures, increasing the pressure and causing them to fail. Slightly scaling water (LSI > 0 and < 0.5) is desirable because it produces a protective CaCO₃ coating in the water distribution system. If the finished water is corrosive, pH adjustment and/or the use of corrosion inhibitors, such as zinc phosphate and silicates, can be used to control corrosion. Table 2 shows parameters that affect corrosion and how water quality degrades as corrosion potential increases.

LSI如为负值表示水有腐蚀性，如为正值表示水有成垢性。应避免严重的腐蚀倾向（LSI<-0.5）和结垢倾向（LSI>0.5）的情况。硬水通常是碳酸钙（CaCO3）的过度饱和，并导致过度结垢，这会减少管道和设备的内径，增加管网压力甚至导致它们失效。轻微结垢的水（LSI>0且<0.5）是可取的，因为它在输配系统中会形成一层保护性的CaCO₃涂层。如果出厂水具有腐蚀性，可以使用pH值调整和/或使用腐蚀抑制剂，如磷酸锌和硅酸盐来控制腐蚀性。表2显示了影响腐蚀的参数以及水质如何随着腐蚀潜力的增加而恶化。

CHLORINE BOOSTING AND DISINFECTION BYPRODUCTS

补氯以及消毒副产物

To prevent microbial regrowth and positive coliform samples in water distribution systems, a proper level of disinfectant residual—either as free chlorine, chloramine, or chlorine dioxide—needs to be maintained. Disinfectant residual decays as the water ages in pipes, reservoirs, and tanks. Maintaining a proper residual is sometimes challenging, especially in warmer months. Where chloramine is used as a secondary disinfectant, ammonia released from the chloramine decay can lead to nitrification, T&O issues, and positive coliform samples. Several physical-chemical parameters influence the stability of disinfectant residuals, including pH, temperature, alkalinity, hydrogen sulfide (H₂S), nitrite, bromide, and dissolved organic carbon (DOC). If a stable disinfectant residual can't be achieved, booster chlorination needs to be added.

为了防止输配系统中微生物的重新生长，以及防止大肠菌群样品检测的阳性，需要保持适当的消毒剂残留，无论是游离氯、氯胺，还是二氧化氯。消毒剂余量会随着水在管道、中间水库和水池水箱中的老化而衰减。保持适当的消毒剂余量有时是个挑战，特别是在气候温暖的月份。在使用氯胺作为二级消毒的地方，氯胺衰变释放的氨可能导致硝化作用、嗅和味的问题，并导致大肠菌群样品检测阳性。有几个物理化学参数会影响消毒剂残留物的稳定性，包括pH值、温度、碱度、硫化氢（H₂S）、亚硝酸盐、溴化物和溶解性有机碳（DOC）。如果不能保持稳定的消毒剂残留量，就需要补氯。

Boosting free chlorine residual is relatively straightforward compared to boosting chloramine. However, additional chlorinated DBPs, such as THMs and haloacetic acids, would likely form due to the additional free chlorine; careful consideration needs to be made to prevent the exceedance of their MCLs within the distribution system, especially if the DOC concentration (i.e., DBP formation potential) in the finished water is high.

与提高氯胺相比，提高游离氯的余量是相对简单的。然而，额外的氯化消毒副产物，如三卤甲烷和卤乙酸，可能会因为增加了额外的游离氯而生成。因此需要仔细考虑，以防止它们在输配系统中超过最大污染物限值（MCL），特别是如果出水中的溶解性有机碳浓度很高的情况（即消毒副产物的生成势会很高）。

Chloramine residual boosting is more challenging because operators need to consider several different factors, including the concentrations of free ammonia, (mono)chloramine, and total chlorine, in the water to be boosted. The feed ratio of free chlorine to ammonia-N is usually set to 5:1. It's important to maintain a low level of free ammonia after the boost to prevent the formation of dichloramine and trichloramine, which may cause customer complaints due to T&O. At the same time, breakpoint chlorination should be avoided. If excessive free ammonia is present, the chances of nitrification will increase. The actual concentrations of free ammonia, monochloramine, and total chlorine should be monitored continuously using an analyzer after the boosting because several factors may affect actual chloramine residual, including water temperature, chlorine strength degradation (if sodium hypochlorite is used), and chemical feed pump variations.

提高氯胺残留量更具挑战性，因为运营人员需要考虑几个不同的因素，包括待提高的水中的游离氨、一氯胺和总氯的浓度。游离氯与氨氮的进料比例通常设定为5:1。重要的是要在补氯后保持较低的游离氨水平，以防止形成二氯胺和三氯胺，否则可能会导致用户因嗅和味而投诉。同时，应避免折点加氯。如果存在过量的游离氨，硝化的机会就会增加。补氯后应使用分析仪连续监测游离氨、一氯胺和总氯的实际浓度，因为有几个因素可能会影响实际的氯胺余量，包括水温、氯的下降（如果使用次氯酸钠）以及化学进料泵的变化。

It's critical for operators to properly monitor residuals and adjust chemical feed in storage tanks where chloramine boosting is practiced because chemical equilibrium along the breakpoint curve can shift when waters with different levels of chloramine residuals are mixed. This is particularly important when boosting is used in systems that blend two sources of finished water using different disinfectants. The chlorine to ammonia-N ratio can increase after blending as the concentration of free ammonia decreases, eventually driving unintentional breakpoint chlorination and a decrease in desired monochloramine.

对于运营人员来说，正确监测消毒剂余量并调整使用补氯用的储罐中的氯胺化学品进料是至关重要的，因为当不同水平的氯胺残余物混合在一起时，折点曲线的化学平衡会发生变化。当补氯用于混合两种使用不同消毒剂的成品水的系统时，这一点尤其重要。混合后，随着游离氨浓度的降低，氯与氨氮的比例会增加，最终导致无意的折点和所需的一氯胺减少。

CROSS-CONNECTIONS AND BACKFLOW

交叉连接和倒流

It's now well-established that the greatest risk to water in the distribution system is from the entry of contaminants due to cross-connections and backflow events. Although the distribution system must deliver finished water at the required pressure and flow for use by customers, water once supplied must never be allowed to return to the system. Unfortunately, a long list of known contamination events has been documented in water supplies of all sizes, and an even greater number of unknown events is suspected to have occurred.

现在已经公认的是，输配系统中最大的风险是由于交叉连接和倒流事件导致的污染物进入。尽管输配系统必须以规定的压力和流量输送出厂水供用户使用，一旦供应到用户的水决不允许再回流到输配系统中去。不幸的是，在各种规模的供水系统中，已经记录了一长串已知的类似的污染事件，甚至更多的未知的污染事件也可能已经悄然发生。

Cross-connection and backflow can affect water quality as well as expose the public to microbial and chemical hazards. Immediate corrective actions may include isolation and flushing, followed by a review of the testing and inspection records and procedures of the utility's cross-connection control program. Investigating backflow events may benefit from mapping customer complaints and using easily measured water quality parameters to locate the contamination's source.

交叉连接和回流会影响水质，并使公众暴露在微生物和化学污染的危险中。可立即采取的纠正措施包括审查公用事业部门对管线交叉连接的控制计划中的测试和检查记录及程序，然后隔离和冲洗。调查倒流事件可能会受益于定位用户投诉来源，并使用容易测量的水质参数来定位污染源。

UNDERSTANDING WATER QUALITY IN THE DISTRIBUTION SYSTEM

理解输配系统中的水质

By understanding how water quality can be affected by factors in the distribution system, operators can produce treated water that can lessen the impact of water age, biofilm formation, nitrification, corrosion, and DBP formation potential. This overview article should make operators aware of the critical nature of their role in water treatment and the delivery of a safe, aesthetically pleasing product to consumers. For a more comprehensive look at this topic, see AWWA's Manual of Water Supply Practices M68, Water Quality in Distribution Systems, along with other excellent resources in the association's online store (www.awwa.org/store).

通过了解水质如何受到输配系统中各种因素的影响，运营人员可以最终向用户提供降低水龄、减少生物膜形成、减少硝化、腐蚀和消毒副产物生成可能性的水。这篇概述文章应使运营人员意识到他们在水处理和向用户提供安全、感官好的水中的关键作用。