Illinois Water Supply Planning



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There are a wide range of methods available for the treatment of potable (drinking) water. Primary reasons for treatment are to address taste and odor concerns, to reduce operation and maintenance costs and consumer complaints about excessive mineral scale formation and corrosion, and to comply with state and federally mandated regulations for controlling contaminants that pose public health risks. The treatment option chosen will depend upon the problem type and severity. Some of the more common water treatment methods are aeration, coagulation and flocculation, sedimentation, filtration, chlorination/chloramination, ozonation, water softening (lime soda), water softening (ion exchange), reverse osmosis, and corrosion control.

Aeration

Aeration Tank
 Aeration Tank
Aeration uses a system of trays, cascades, or sprays to break up water into small particles and expose it to the atmosphere. The same effect can be accomplished with diffusion aerators that bubble compressed air into the water. Aeration has many benefits. Concentrations of substances that cause taste and odor problems and health concerns, such as volatile organic compounds and hydrogen sulfide, are substantially reduced. Levels of free carbon dioxide, a gas that can increase the corrosiveness of the water, are reduced significantly, and oxygen levels in the water can increase. While this may increase the potential for corrosion, it also facilitates the oxidation of iron and manganese when they are present, which is a necessary first step to their removal through filtration and/or sedimentation.

Coagulation and Flocculation
Coagulation and flocculation is a process where chemicals are added, with mixing, to a stream of water to increase the removal efficiency of suspended particulate matter or colloidal material. These materials tend to have a negative charge that causes them to repel each other. Coagulants are compounds that negate this repelling action. Common coagulants are aluminum sulfate (alum), ferric chloride or sulfate, and various polymers that sometimes incorporate those salts. Flocculation refers to the process of agglomeration of the particulate matter into larger particles (floc) that are then removed through sedimentation and/or filtration. Formation of a floc heavy enough to settle is promoted by some type of mixing. Common particulate contaminants removed by these processes are silt, organic material, minerals, bacteria, and viruses. Removal efficiency varies widely depending upon the specific contaminant and other water quality parameters.

Sedimentation

Sedimentation Tank
 Sedimentation Tank
Sedimentation can be used to clarify water with or without coagulation and flocculation. Because flocculation greatly increases efficiency and reduces time required for removal of particulate matter, sedimentation is seldom used as a stand-alone process. While there are a number of different designs, sedimentation normally uses a circular, square, or rectangular basin or tank through which the water flows slowly to allow completion of flocculation and settling. The water is usually then routed through a filter to complete the clarification process.

Filtration

Filtration Tank
 Filtration Tank
Filtration is the passage of water through some type of porous medium to remove undesirable suspended solids. This filtration can be accomplished by gravity flow or pressure. The media used for filtration are selected for the specific contaminants that are to be removed. The most common media used in filters are natural silica sand, anthracite coal, and granular activated carbon. Naturally occurring manganese greensand sometimes is used in combination with potassium permanganate to remove iron and manganese from groundwater supplies. The shape, size, and depth of the media bed in the filter are adjusted to obtain the desired flow rate and effective porosity. Materials collected by the filter must be removed regularly by backwashing (reversing the flow through the filter to lift out the accumulated contaminants and suspended solids). Because this backwash may contain very high concentrations of potentially dangerous contaminants and biological material, new federal regulations require careful monitoring and control when filtration is used.

Chlorination/Chloramination

Chlorine Tank
 Chlorine Tank
State and federal regulations require the addition of chlorine, or a combination of chlorine and ammonia that forms various chloramine compounds, to all community water systems in Illinois. Community water supplies must retain a small residual concentration of free chlorine or chloramine throughout their distribution systems. Chlorine, a strong oxidizer, is very effective at either killing or deactivating most microorganisms. Solid, gaseous, and liquid forms are readily available and economical to use. However, chlorine has some significant drawbacks under certain conditions. It can react with naturally occurring organic materials in surface waters to form suspected carcinogens such as trihalomethanes (THMs) and haloacetic acids (HAAs). Chlorine also begins to lose effectiveness at higher pH levels (above 8.0) and has a limited life span in a distribution system. Chloramine, the disinfectant of choice for almost all surface water systems, is a less effective disinfectant, has a much lower potential for forming THMs or HAAs, is more effective at higher pH levels, and has substantially longer life in a distribution system. Chlorine is still the better choice for most groundwater supplies because they tend to have lower pH levels and very low concentrations of naturally occurring organic matter that is a precursor for forming THMs and HAAs.

Ozonation

Sewage Tank
 Sewage Tank
Ozone is very reactive gas formed with the use of electrical discharges in the presence of oxygen. The resulting compound is an extremely effective disinfectant. It will destroy or inactivate virtually all pathogens commonly found in natural waters, and it provides excellent control of taste and odor problems. Ozone also destroys many organic precursors that can form THMs or HAAs with the addition of chlorine or chloramines. Some European countries have used ozone extensively for decades, but it has only found popularity in the United States within the last 10–15 years. Ozone must be generated on site and requires fairly expensive equipment and substantial power requirements that make its use somewhat costly. Because ozone is so reactive, and has a very short effective life span in a system, another disinfectant must be used that will provide a disinfectant residual throughout the distribution system to protect consumers. Either chlorine or chloramine is suitable for this purpose; the best selection depends upon other system and water quality parameters.

Water Softening (Lime Soda)
Use of lime and soda ash in hard waters to reduce the concentration of dissolved minerals such as calcium and magnesium has been widespread for many years, but the expense of operation and the cost of sludge disposal has resulted in few new plants in the last decade. The addition of lime and soda ash raises the pH and causes the precipitation of calcium and magnesium hardness salts, which produces large quantities of sludge that must be dried and disposed of regularly. In addition to reducing hardness, the lime-soda softening process has some efficacy as a bactericide and partially removes other inorganic contaminants. Lime-soda softening has been proven to reduce the concentration of heavy metals such as arsenic, mercury, barium, and lead. Arsenic, in particular, is a problem for many Illinois groundwater supplies. While lime-soda softening will remove some arsenic, additional treatment may be required to lower arsenic below the established federal guidelines. Lime-soda softening can also reduce radium concentrations by as much as 70 percent. The presence of substantial concentrations of these contaminants can limit the use of the sludge produced and greatly increase the cost of sludge disposal.

Water Softening (Ion Exchange)

Water Softener/Heater
 Water Softener/Heater
Ion exchange is a process whereby an undesirable ion (such as calcium) is replaced by another ion (such as sodium) in the water stream. This is accomplished by passing the water through a bed of media with a high concentration of the more desirable ion. This process was first used in the 1800s with clay minerals known as zeolite earths. The resin used for ion exchange today is a synthetic material (divinyl benzene cross-linked with polystyrene) similar to plastic that has a much higher exchange capacity, and it is still often referred to as zeolite resin when used for softening purposes. There are many different types of ion exchange units, but the only one commonly used in the production of drinking water is cation exchange units operated in the sodium cycle. These are better known as softeners. These units will remove calcium and magnesium ions and replace them with sodium ions. They also will remove most other positively charged ions such as iron, manganese, lead, and even radium. Frequent regeneration of softeners with salt brine is necessary to recover the ion exchange capacity lost as sodium ions are replaced with other cations. Similar to the problems with disposal of lime-softened sludge when high concentrations of contaminants are present, there can be problems with disposal of the used salt regenerate solution. Some states severely limit the use of softeners on the basis of the high concentrations of salt that are regularly dumped into the sanitary system.

Reverse Osmosis
Reverse osmosis is a process where water is passed over a semipermeable membrane under high pressure. Commercial units typically operate at over 250 pounds pressure to produce dissolved solids removal rates of 95–98 percent. Reverse osmosis also removes many organic materials, bacteria, and viruses. A drawback of this process is that it discharges between 25 and 50 percent of the influent water (incoming) as wastewater that may require treatment. This, along with power requirements and replacement of the membranes when necessary, are the principal operating expenses associated these units. High initial equipment costs and operational costs are offset by the lack of physical oversight required on the part of a water plant operator. These devices are becoming more popular in drinking water treatment because they are so effective in removing contaminants. Utilities often blend the effluent (treated water) from a reverse osmosis train with untreated water to lower contaminants below regulatory limits without absorbing excessive capital costs.

Corrosion Control

A Valve
 A Valve
Some naturally occurring waters are corrosive to distribution mains and household plumbing. This can dramatically reduce the service life of piping and plumbing materials, and corrosion of iron in distribution mains results in consumer complaints due to “red water” that stains clothing and fixtures. Corrosive water can also be responsible for unsafe levels of lead and copper from household plumbing. Community water systems are required to monitor lead and copper levels in household plumbing, and to institute corrosion control treatment if either exceeds a prescribed action limit.

The types of treatment that can be used for corrosion control in drinking water systems are very limited. The most common methods are the adjustment of pH and alkalinity levels and the addition of phosphates. There are two classes of phosphates used in drinking water, orthophosphates and polyphosphates. The orthophosphates are excellent corrosion inhibitors, but are relatively insoluble in higher pH waters. Polyphosphates are more soluble and also provide the added benefit of complexing suspended particles of iron and keeping them in solution. This eliminates the “red water” problem. However, in most cases, they are less effective than orthophosphates as corrosion inhibitors. For these reasons, many commercial phosphate products blend both ortho and polyphosphate to obtain the maximum benefit. One possible drawback to these blended products is that some utilities have indicated that phosphates seem to promote bacterial regrowth in low flow areas or at the ends of long runs of distribution mains under certain conditions.

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