Drinking Water Treatment - Distillation

Drinking Water and Human Health December 06, 2010|Print

EFFECTIVE AGAINST: Some heavy metals, dissolved solids, some bacteria and viruses, inorganic materials such as nitrate, sodium, fluoride, and sulfate, and some toxic organic chemicals.

NOT EFFECTIVE AGAINST: Most volatile and semi-volatile organic compounds (VOCs), and some bacteria.

Contents

Distillation

Distillation is one of the oldest water treatment processes. Water is boiled and the resulting steam is collected and cooled backed to water in a separate chamber. The treated water thus produced is called distilled water that is relatively free of many contaminants. Generally, distillation is used to supply water only for drinking or special uses.

Distillation units are point-of-use devices that disinfect water, reduce the concentration of toxic metals, and remove some organic contaminants. Home distillation units are usually electrical devices, but oil, wood, or any other heat source can also be used. They can either be counter top units that are manually filled as water is needed, or they can be automatic and be connected to the household plumbing for continuous operation.

Principles

A supply of feed water is diverted from the cold waterline of the household distribution system, which allows continuous operation. The water is heated to boiling. The resulting steam flows into a condenser – a tube or coil cooled with cold water or air. The steam condenses in the cold tubing, and treated water is collected below the condenser. The distilled water thus produced then goes into a storage tank. With very little other treatment, distillers produce nearly pure water.

One way to save water and energy is to purchase a distiller that uses diverted water as the coolant before sending it to the distillation chamber. During the cooling process, the temperature of the coolant water increases, thus reducing the energy necessary for evaporation once the water reaches the distillation chamber. A maximum water temperature of 70 degrees Fahrenheit is suggested for such a design. Air is another coolant option that avoids waste.

Distillation normally removes over 99.9 percent of the dissolved materials, regardless of their effects on water quality. Dissolved minerals impart flavor to water. Because distillation removes minerals, distilled water is often flat and tasteless. Removing dissolved solids can make the water corrosive, so the storage tank and other distiller components must be made of corrosion-resistant materials. Since distillation is generally a point-of-use treatment, the household water distribution system need not be corrosion-resistant.

There are certain volatile and semi-volatile organic compounds that may not be removed by distillation. If these contaminants are present in the water, they should be removed prior to distillation. Preheating the water in a vented part of the distiller before it enters the boiling chamber will allow the VOCs to escape. The vapors must be vented to the outside so they do not accumulate in the home. If undesirable organic chemicals remain in the distilled water, post-treatment with an activated carbon device may remove them.

Manufacturers usually state minimum water quality parameters for the untreated water. Very hard water or water with high dissolved solids concentrations reduce the efficiency of the distiller. For more efficient distillation, pre-treat excessively hard water with a water softener.

A good distiller includes a water level sensor that shuts off the heat source if the boiling vessel runs dry. Others may have a temperature sensor to prevent overheating.

Types of Units

Distillers vary from small, round units that distill less than one quart of water per hour to larger, rectangular carts, which distill about one-half gallon of water per hour. Because distillation units produce a small amount of treated water, they are typically installed as point-of-use units at the faucet and not used to treat all the water entering the house.

Before purchasing a system, verify that the treatment system you are purchasing has been tested and certified by a third party (for example National Sanitation Foundation) to ensure manufacturer’s claims. Distillers can be filled with water either manually or by a connection to a water supply line. Permanently installed water distillers should have a drain opening to remove contaminated water. Faucets facilitate the draining of countertop units.

Storage containers store the distilled water. The containers hold from one and a half to 15 gallons of water. All types of storage containers are suitable when properly maintained as directed by the manufacturer. Automatic features on units include reset switches and timers that make automatic operation possible on some installed models. These features might be desirable when distilled water is used continuously.

Production rates vary with the type of condensing system. Air-cooled devices typically produce 1 gallon of untreated water for each gallon of treated water. Water-cooled units may require 5 to 15 gallons of untreated water for each gallon of treated water. About four hours are required to produce 1 gallon of water.

Electricity requirements also vary. Portable units, which generally produce 3 to 4 gallons per day, typically have 500- to 700-watt heating elements. Larger units have heating elements of 1,000 to 1,500 watts and produce 8 to 12 gallons per day. Home units generally operate on 115 volts (normal household power). It takes about 800 watts of electricity to distill 1 quart of water in one hour. Costs will vary based on the temperature of the incoming water and system design, but the electrical costs for distilled water usually range from $0.25 to $0.33 per gallon.

A continuously operating, off-line distiller for home use should have a storage tank to hold treated water until it is needed. The storage tank capacity may provide an estimate of a distillation unit’s capacity. Most tanks drain by gravity, but some may have a pump to deliver treated water to other locations.

Maintenance

Regardless of the quality of the equipment purchased, it will not perform satisfactorily unless managed in accordance with the manufacturer’s recommendations for maintenance, cleaning, and part replacement. Keep a log book to record water test results, equipment maintenance and repairs. Any water that remains in the distillation unit is concentrated with minerals and metals, so dispose of it carefully. Cleaning frequency will depend on the level of minerals in the water and the amount of water being used. In some cases the mineral built-up can be dissolved with pure water. In other cases, the mineral built-up needs to be dissolved by dilute acid cleaners, such as lemon juice or vinegar in a heated condition. It is important to keep mineral build-up on the heating element to a minimum, as it reduces heat transfer and results in higher energy costs. Mineral build-up may also necessitate periodically replacing the heating element, although an individual element should last approximately three years.

How often to clean a distiller depends on the water quality – especially the hardness level. After installation, check the boiler chamber and heating element in continuously operating units on a weekly basis for scale accumulation. Check countertop units after each distillation cycle. Develop a regular maintenance schedule based on these initial observations. For continuously operating distillation units, periodically draining or sending to waste part of the water from the boiling chamber may prevent solids from precipitating and may eliminate some solids that have already precipitated. Always follow the manufacturer’s cleaning recommendations.

Other Considerations

Ensure the system you choose is installed and operated according to the manufacturer’s instructions. After installation, retest both the raw water (prior to treatment) and the treated water at a state certified laboratory to ensure it is working properly and removing the contaminants. You should continue to test the quality of both the raw and treated water annually or more frequently (quarterly or semi-annually) if high levels of contaminants are present in the raw water. Frequent testing will also help you determine how well your treatment system is working and whether maintenance or replacement of components may be necessary. Distillation units require electricity, although power usage can be decreased in units with automatic shut-off devices.

Questions to Ask Before You Buy

Before purchasing a water treatment device, have your water tested at a [state certified laboratory] to determine the contaminants present. This will help you determine if distillation is an effective treatment method for your situation. See Questions to Ask Before You Buy A Water Treatment System for more information.


Adapted from: Wagenet,L., K. Mancl, and M. Sailus. (1995). Home Water Treatment. Northeast Regional Agricultural Engineering Service, Cooperative Extension. NRAES-48. Ithaca, NY.