Chloramine, like chlorine, is effective in controlling biofilm, which is a slime coating in the pipe caused by bacteria. Controlling biofilms also tends to reduce coliform concentrations and biofilm-induced corrosion of pipes. Because chloramine is not as reactive as chlorine with organic compounds, fewer taste and odor problems occur.

Ozonation

Ozonation is the process of feeding ozone into a water supply for the purpose of decolorization, deodorization, disinfection and oxidation. Ozone, a form of oxygen, is the most powerful disinfectant, but it is not effective in controlling biological contaminants in the distribution pipes because it does not have a long-lasting residual. Ozonation destroys bacteria and viruses and requires a shorter time period to treat water than most other water treatment methods. Ozone, a reactive gas, is made by subjecting oxygen to high electrical voltages. Ozone’s reactive nature allows it to readily react with and break up many organic compounds and kill bacteria and other organisms in the water supply. On-site production of ozone is energy-intensive. Ozone treatment is becoming more common in the United States as questions arise about disinfection byproducts. Ozone has been widely used in Europe for 100 years.

Table 6-3 Comparison of source water quality to federal standards. The figures are averages. Actual quality may vary at different times and places.

Membrane Filtration

Membrane filtration is a relatively recent development. Water is forced through membranes with small pores, and anything larger than the pore size is filtered out. Along with removing materials such as minerals, membrane filtration also can be used for disinfection.

Four major types of membrane filtration systems are in use: reverse osmosis (RO); nanofiltration (NF); ultrafiltration (UF); and microfiltration (MF). The main difference between the four types is the pore size of the membrane. This influences the amount of energy needed to force the water through the membrane. The membranes must be cleaned periodically. The smaller pore membranes require the most cleaning (backwashing) which is an energy-intensive process and wastes a significant amount of water. (See the section on desalinization below for further discussion of RO and NF.)

• RO has the smallest pore size and removes a great variety of materials from the water, from salts to organic materials and very small microbes. The water to be filtered must be pretreated to prevent pore clogging. In addition, the concentrate of materials removed by the filter is highly saline, causing loss of available water as well as disposal problems. This process is not primarily used for disinfection, although disinfection is achieved in the process of desalting water.
• NF has larger pores than RO and removes, pathogens, organics and some salts. Like RO the process requires pretreatment of water with chemicals or a sand-based system. NF has not been used commercially on a large scale for drinking water.
• UF has larger pores than NF and is highly effective in removing pathogens, including parasites such as giardia, but does not remove salts. Because it has large pores, UF does not leave a saline concentrate, although filters must be backwashed to keep the pores open. It is used primarily in the food and pharmaceutical industries, rather than large scale water treatment plants.
• MF has the largest pores of the membrane systems and removes particles, but not pathogens or organics. MF may be used as a pretreatment process for RO or NF, thus reducing some of the problems of those systems.

Several methods can be combined in the treatment process. Tucson Water initially chose ozone plus chloramine to treat CAP water because officials viewed this as the most effective treatment method with the least risk to human health. The ozone performs the initial disinfection while the chloramine provides the residual to control microbes throughout the distribution system. Tucson Water chose not to use chlorine in the CAP water treatment plant because of concerns about THMs. Tucson Water and some private water companies such as Metro Water, however, use chlorine to disinfect groundwater. THMs generally are not a concern with groundwater since little organic matter is found in groundwater to combine with chlorine.