Home Water Treatment Equipment: An Overview

Introduction

Reports of contamination have raised concern about the safety of our public and private drinking water. One response has been an increasing number of products claiming to make water safe or "pure". Often the consumer is not sure if treatment equipment is needed or would help and has to evaluate many claims when selecting treatment methods and products.

When considering the purchase of water treatment equipment, first determine the quality of the water and what constituents may need to be removed or treated. This information will help you choose the equipment best suited for the specific problem. To determine quality, have the water tested. In most cases, it is best to use an independent (preferably state certified) laboratory to help answer this question. It is not economically feasible to test for all possible contaminants.

Identification of the type and level of contaminants is important in identifying potential problems and selecting appropriate treatment equipment. No single water treatment method treats all water problems and all methods have limits. In many cases, a combination of treatment methods may be required to address a specific problem.

Filters

Filters can be classified as mechanical (microfiltration) or adsorptive. Mechanical filters are used to remove small amounts of suspended material such as sediment, sand, rust and precipitated iron particles from the water. Mechanical filters can include small mesh screens, but commonly are cartridges constructed of natural or synthetic fibers. A filter media, such as sand, also may provide filtration. The water line pressure forces water through the filter material, trapping foreign particles. Some mechanical filters can be backwashed to remove trapped particles, but many require cartridge replacement when water flow slows appreciably.

Activated carbon, a form of charcoal, is the primary adsorptive filter material. The carbon adsorbs (the contaminants adhere to the surface of the carbon particles) some inorganics that cause odors and disagreeable tastes, including chlorine. Carbon filters also will remove many specific organic chemicals, including some pesticides. They also can be used to remove radon. Typically, they are used to remove tastes, odors and small amounts of organic contaminants from drinking and cooking water. The longer the water contacts the filter, the more time the carbon has to remove impurities. In general, the larger the filter the longer the water flow path will be and the longer the contact time.

Carbon filters are commonly installed at the point of use. They come in a variety of sizes and designs. Small units can fit on kitchen faucets, while larger units, often called in-line filters, are installed under the kitchen sink. Large units suitable for treating all household water are available for treating contamination such as radon.

Carbon filters must be replaced regularly to maintain their absorptive capacity. Cartridge replacement frequency depends on the quantity of water used, the amount of carbon in the filter and the amount of contamination in the water. A regular filter replacement schedule is necessary since there is no easy method for detecting when a filter is no longer working effectively. Closely follow the manufacturer's recommendations for filter replacement.

Carbon filters do not remove nitrate, bacteria or dissolved minerals. In fact, carbon filters should only be used with bacterially safe water. There has been some concern about the filter media serving as a location for bacteria growth. However, Environmental Protection Agency studies have shown that bacteria growth in the filters studied was nonpathogenic, meaning they did not cause water-borne disease. If a filter is not used for several days, it is wise to allow the water to run a few minutes to purge the filter of possible nonpathogenic bacteria. Some manufacturers treat the carbon with silver to prevent bacterial growth. This silver only prevents bacterial growth in the filter; it does not disinfect bacterially unsafe water.

Water Softeners

Dissolved calcium and magnesium cause hard water. Hard water interferes with the cleaning action of soaps and detergents used for laundering, dishwashing and bathing. It also affects appliances and plumbing. For example, scale from the calcium and magnesium precipitates can build up in hot water pipes, water heaters, plumbing fixtures and appliances. This scale increases the cost of heating water and reduces the life of appliances.

The common water softening methodology uses ion exchange. The hard water passes through a tank containing high capacity ion exchange resin beads. The beads are saturated with sodium to cover both their exterior and interior surfaces. As water passes through the bed of softening materials, calcium and magnesium ions attach to the resin beads while the sodium on the resin is released into the water.

Eventually, the beads become saturated with calcium and magnesium ions and no sodium ions remain. When this occurs, the softener must be regenerated or recharged by flushing the ion exchange resin with a salt brine solution. This will replenish sodium ions on the resin. Frequency of regeneration depends on the hardness of the water, the amount of water used, the size of the unit and the capacity of resins to remove hardness.

A water softener will remove small amounts of dissolved iron (5 to 10 ppm). However, if there is oxidized iron or iron bacteria in the water, the ion-exchange resin will become coated or clogged and lose its softening ability. In this case, another method of iron removal such as an iron filter or a chlorination-filter combination is needed before the water passes through the softener.

Distillers

Distillation removes virtually all contaminants including nitrate, sodium and sulfate, as well as most organic compounds. The heat used for the distillation process also kills bacteria. Removal of minerals produces water that can have a bland taste.

When the distiller is operating, tap water is heated to boiling (usually in a stainless steel tank). Steam is produced, rises, and leaves most contaminants behind. The steam enters condensing coils, where it is cooled and condensed back to water. The distilled water then goes into a storage container. Water is removed directly from the storage container or piped to a special faucet.

Free-standing distillers can be placed on a counter or on the floor near an electric outlet. Some distillers are installed permanently and connected to a special faucet, usually at the kitchen sink.

Large distillers can distill about one-half gallon of water per hour. Smaller units produce less than one quart of water per hour. The cost of producing distilled water depends on the efficiency of the appliance and the local electrical rate. Although distillers have no parts that need to be regularly replaced, they are not maintenance-free. Scale must be removed from the boiling tank. Frequency of cleaning the distiller varies with the quantity of contaminants in the water and the amount of water distilled. White vinegar or a manufacturer's cleaner is used for cleaning. Some units may have automatic flushing cycles to assist with boiling tank cleaning.

Before buying a distiller, evaluate the quantity and quality of treated water desired; the contaminants to be removed and the concentration of those contaminants; total costs; and the available alternatives, such as bottled water.

Reverse Osmosis Units

A reverse osmosis unit can be up to 95 percent effective in removing a wide variety of inorganic contaminants such as nitrates, calcium and magnesium. It also will remove some organic chemicals. A reverse osmosis membrane is most efficient when new with the removal efficiency dropping as it ages. Typically, a reverse osmosis unit is used to treat only drinking and cooking water.

A reverse osmosis unit often includes: a pre-filter to remove sediment; an activated carbon filter to remove organics and contaminants causing odors and taste; a semi-permeable membrane through which water flows under pressure; a tank to hold the treated water; and a drain connection for discharging concentrated contaminants. Reverse osmosis units may be located under the sink or in a remote location, depending on the size of the water holding tank. Reverse osmosis units are available in different production capacities. The capacity of the unit, in gallons per day, needs to be matched to household water needs. Most households find five gallons per day adequate for drinking and cooking needs.

Contaminants are removed by forcing water through a membrane having microscopic holes that allow water molecules, but not larger compounds, to pass through. Water flushes away the contaminants held by the membrane. Most units discharge 40 percent, or more, of the total water into the waste water system.

Disinfection

Most private wells do not need continuous disinfection. A well contaminated with bacteria often can be made safe by locating and controlling the source of the bacteria and then shock chlorinating the well and entire water distribution system. With shock chlorination, a strong solution of chlorine is introduced into the well and then circulated throughout the entire plumbing system. The chlorinated water is allowed to stand in the system for a period of time and then discharged.

In cases where continuous chlorination is needed, a chemical metering device is used to inject liquid chlorine into the system or dry chlorine pellets are dropped into the well. Adequate contact time must be allowed for the chlorine to function. A certain amount of chlorine must remain in the system (called chlorine residual) in order to maintain the disinfecting capability. A carbon filter can be used to remove the chlorine before using the water for drinking and cooking. Chlorination does not remove nitrates or other chemicals, but may oxidize organics and some minerals such as iron. Mechanical filters are used to remove oxidized precipitates.

Disinfection units using ultraviolet light and ozone are also available. These units are not used as widely as chlorine. Ultraviolet and ozone disinfection provide instantaneous disinfection and do not provide disinfection beyond the treatment point like chlorine.

Iron Removal

Iron and manganese in the water will stain clothes and plumbing fixtures. When the iron and manganese are dissolved they cannot be seen in water. When water containing dissolved iron and manganese is exposed to air, the chemical form is changed by oxidation and solid particles (precipitates) are formed. The iron compounds will usually appear as reddish, rust colored particles and the manganese compounds will be black particles floating and settling in the water.

Water with a high iron or manganese content is not considered a health problem, but it can be objectionable in taste, odor or appearance if iron is present in amounts greater than 0.3 mg/l or manganese is present in amounts greater than 0.05 mg/l.

Iron bacteria are nuisance organisms often associated with soluble iron in water. Because they cause a slime buildup, they can be quite objectionable with iron concentrations as low as 0.1 mg/l soluble iron.

The presence of iron bacteria is indicated by a gelatinous slime on the inside wall of the toilet flush tank and gelatinous "rusty slugs" being discharged at the tap. Periodic high doses of chlorine are required to control iron bacteria (200 to 500 mg/l chlorine). This shock chlorination requires that the chlorine contact all parts of the water system along with the well.

Four types of iron removal equipment are available:

1. Iron filters. Iron filters are only useful for removing soluble iron and manganese; precipitated iron particles will quickly plug them. They appear similar to water softeners but contain a bed of natural or synthetic manganese green sand. Manganese dioxide oxidizes iron and manganese and the oxidized particles are then filtered out in the lower part of the bed.
   
   The filter bed must be backwashed frequently to remove the accumulation of iron particles. For backwashing, a flow rate more than double the normal service flow rate is usually required. The exhausted manganese must be recharged by adding potassium permanganate.
   
   Since the slime produced by iron bacteria will clog the filter, the iron bacteria must be controlled before using an iron filter.
2. Water softeners. Water softeners contain an exchange resin that will remove soluble iron on an ion-exchange basis (the same way calcium and magnesium are removed in water softening). All water softeners will remove some iron. Depending on the kind of resin used and the regeneration process, up to 5 mg/l of soluble iron can be removed. Iron bacteria must be controlled since the slime produced by iron bacteria will clog the resin and reduce its effectiveness.
3. Polyphosphate feeders. These units can handle up to 3 mg/l of iron in solution. They contain a phosphate compound which coats the soluble iron and prevents its oxidation when the water is exposed to air. The compound is not effective against iron that has already oxidized.
   
   When some waters are heated, the raised temperature will reduce the effectiveness of the polyphosphate so that oxidized iron will accumulate in the water heater. The heated water will be rusty and unsatisfactory for many home uses. Polyphosphate is most effective in cold water.
4. Chlorination and filter. Chlorination followed by filtration through a sand filter can remove any quantity of iron in any form. The chlorine oxidizes and precipitates the iron and the filter removes the particles. Carbon filtration may be required to remove excess chlorine.

1. Table I. Quick Reference to Water Treatment Devices.

Table II. Quick Reference for Correcting Household Water Quality Problems.

Many types of water treatment equipment are available to address water quality problems. Tables I and II can provide quick references to assist with treatment equipment selection. In addition, key steps to use in selecting water treatment equipment are:

1. Use appropriate tests to correctly identify the problem or problems which need to be addressed.
2. Identify options for correcting the problem.
3. Decide if whole-house or point-of-use treatment is most appropriate.
4. Select reputable dealers; ask for references.
5. Obtain second opinions.
6. Check to see if the proposed equipment has been tested or validated by independent organizations such as the National Sanitation Foundation or Water Quality Association.
7. Talk with others who have the same equipment you are considering.
8. Identify all costs including purchase price, and installation, operating, and routine required maintenance costs.
9. Understand what maintenance will be required.
10. Understand how to determine if the equipment is operating satisfactorily.
11. Determine if the system has adequate capacity for your needs.
12. Determine the expected life of the equipment and components.
13. Understand any warranty provided with the equipment.