Chapter 6. Ensuring Safe Drinking Water - Water in the Tucson Area: Seeking Sustainability

Water in the Tucson Area: Seeking Sustainabliity

ch. 6, pp. 71 - 73

Chapter 6: ENSURING SAFE DRINKING WATER

CORROSION

Corrosion of a metal pipe is a process that involves primarily oxygen gas and the spontaneous flow of electricity. Salts found in water have a secondary role in this process. Oxygen gas (O2) is found in small concentrations in water (4-9 mg/l), as well as in the atmosphere (20 percent by volume). Both Tucson groundwater and CAP are well oxygenated waters, though CAP has higher levels of dissolved oxygen. We can explain metal corrosion by the fact that most metals are very good conductors of electricity and that they have a natural tendency to want to react with oxygen. During the corrosion process oxygen molecules combine with electrons from the metal pipe and acid from the water to form water molecules. The process results in the formation of metal oxides (rust in the iron pipes), which is soft, reddish-brown and has a metallic taste. In essence then the metal pipe dissolves during the corrosion process.

Metal pipe corrosion can start on both sides of a pipe and often does. Poorly protected iron pipes that are buried in wet or waterlogged areas can corrode faster from the outside than from the inside.

Salts and alkalinity can either accelerate or inhibit pipe corrosion by forming scale (calcium carbonates) that limits the contact of oxygen with the metal surface or by dissolving this protective scale, as previously described.

Salt constituents such as chloride and sulfate ions also seem to have an indirect role in pipe corrosion. High concentrations of these two ions reduce calcium carbonate scale formation which inhibits corrosion. The concentrations of chloride and sulfate ions are about four to five times higher in CAP water. These two ions may be significant contributors to the corrosive effects attributed to CAP water.

Water treatment, which is necessary to prevent diseases such as cholera and dysentery, may cause health problems, mainly because of certain byproducts. EPA has become concerned about some of the byproducts of various disinfection processes and is currently funding research to determine the possible extent that byproducts may cause health problems.

Water treatment can serve five very different functions:
Remove disease-causing microbes (disinfection);
Remove toxic materials such as TCE;
Reduce corrosivity;
Reduce hardness or TDS;
Improve taste, odor or appearance.

Each water quality problem is very different and must be solved by different treatment methods. When determining appropriate treatment for CAP water, each problem must be examined separately. In some cases a single treatment method will deal with more than one problem, but in most cases each type of problem requires a different solution.

Filtering Out Particles

Filtration/flocculation, which removes suspended particles from the water, is basic to most forms of treatment. Filtered particles include clays and silts, natural organic matter, precipitants from other treatment processes, iron and manganese and microorganisms. Filtration clarifies water and enhances the effectiveness of disinfection. Filters may be made of a variety of materials, including sand, anthracite, aggregate or activated charcoal. Before filtration, a flocculation agent is added to cause minute particles in the water to coagulate into larger particles to enhance filtration.

REMOVING DISEASE-CAUSING MICROBES

Chemical Disinfection

Water is usually disinfected before it enters the distribution system to ensure that dangerous microbes are killed. Further, if the water travels a distance to the customer or is stored for a period of time in system reservoirs, the disinfectant must remain effective long enough to prevent disease. Chlorine, ozone, and chloramines are most often used for initial disinfection because they are very effective disinfectants. A small amount of chlorine is often added at the end of the process to retain disinfection.

Chlorine has been used throughout this century for disinfection of drinking water to protect public health from diseases caused by bacteria, viruses and other disease causing organisms. Chlorine is highly effective and has become the most widely used disinfectant throughout the world since its introduction in the late nineteenth century. The decline of typhoid and cholera in the twentieth century is a direct result of chlorination of drinking water. While chlorine is effective, there are drawbacks to its use. These include odor, which customers can sometimes detect. Chlorine tends to dissipate in air (which is why swimming pools must be repeatedly chlorinated) and does not remain stable in the distribution system for long periods of time. Also, when used to treat water, chlorine can react with organic substances to form trihalomethanes (THMs) which are toxic disinfection byproducts. Further, when stored or transported as a gas (the usual procedure for large treatment systems) chlor ine can be highly toxic if accidentally released. Some water companies have switched to other forms of disinfectants because of these problems.

Chloramines, the monochloramine form in particular, have been used as disinfectants since the 1930s. Chloramines are produced by combining chlorine and ammonia. Chloramine is a weaker disinfectant than chlorine, but is more stable, thereby extending disinfectant benefits throughout a water utility’s distribution system. In fact, the primary use of chloramine is as a secondary disinfectant for maintaining a disinfectant residual in the distribution system. Chloramine is not as reactive as chlorine with organic material in water, and therefore produces substantially lower concentrations of disinfection byproducts in the distribution system. Because the chloramine residual is more stable and longer lasting than free chlorine, it provides better protection against bacterial regrowth in systems with large storage tanks and dead-end water mains.

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