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Distillation

 

Introduction
Principles of Distillation
Distillation Units
Operation and Maintenance
Costs

Introduction
Distillers can effectively remove most or all contaminants, including minerals, metals, organic chemicals, and microorganisms from water. Since distilled water has no minerals, some people claim distilled water tastes flat or slightly sweet. Distillation also kills or removes microorganisms, including most pathogens. Distillation can also remove organic contaminants, but its efficiency depends on the chemical characteristics of the contaminant. Distillation efficiency also decreases as the TDS of the water increases. Volatile organic chemicals (VOCs) like benzene and TCE vaporize along with the water and recontaminate the distilled water if not removed prior to distillation. Some distillation units may initially purge some steam and volatile chemicals. These units should be properly exhausted to prevent indoor air contamination. Some home distillation units have activated carbon filters to remove VOCs during distillation.

Distillation process

click to enlarge

Principles of Distillation
The principle for operation of a distiller is simple. Water is heated to boiling in an enclosed container. As the water evaporates, inorganic chemicals, large non-volatile organic chemicals, and microorganisms are left behind or killed off in the boiling chamber. The steam then enters condensing coils or a chamber where the steam is cooled by air or water and condenses back to a liquid. The distilled water then goes into a storage container, usually 1.5 to 3 gallons in capacity.

Distillation Units
Also called stills, distillation units generally consist of a boiling chamber (where the water enters, is heated, and vaporized), condensing coils or chamber (where the water is cooled and converted back to liquid), and a storage tank for treated water. Distillation units are usually installed as point-of-use (POU) systems that are placed near the kitchen faucet and used to purify water for drinking and cooking only. Home stills can be located on the counter or floor, or attached to the wall, depending on size. Models can be manual, partially automated, or fully automated.

Distillers vary from small, round units that distill less than one quart of water per hour to rectangular carts that distill about one-half gallon of water per hour.

Operation and Maintenance
As with all home water treatment systems, distillation units require some level of regular maintenance to keep the unit operating properly. Contaminants left in the boiling chamber need to be regularly flushed out. Even with regular removal of the brackish (saline) residues, calcium and magnesium scale will quickly collect at the bottom of the boiling chamber. Over time, this scale reduces heat transfer and should regularly be removed either by hand scrubbing or by soaking with acetic acid. Vinegar is commonly used to clean home distillers.

Although minerals that can cause corrosion and scaling are removed during distillation, distilled (and RO) water is very corrosive (aggressive). It should not be stored or transferred in metal pipes.

Costs
Small still units (capacity: 1.5 gallons, 6 liters) cost $250 or more. Large units (capacity: 15 gallons, 57 liters) vary from $450 to $1,450 in purchase price.

Distillation is the most effective, but also the most expensive (energy-intensive) form of water purification. The power rating of the still and the local electricity rates determine the cost of operation of these units. To calculate the cost to produce one gallon of water, multiply the price of a kilowatt hour times the rated kilowatt hour use of your model, times the number of hours it takes to produce one gallon of water. For example, if local electricity costs 0.10 cents per kilo watt hour and your unit is rated at 800 watts (or 0.8 kilowatt hour) and it takes 4 hours to produce one gallon of water, your operating cost is 0.32 cents per gallon, excluding purchase and maintenance costs.

(Portions of this text has been adapted from Plowman, F.T., "Distillation," Durham: University of New Hampshire Cooperative Extension, Water Quality Fact Sheet 21, 1989.)

Go to other treatment methods:
Particle and Microfiltration Filters
Activated Carbon Filters
Reverse Osmosis
Ion Exchange Water Softening
Disinfection of Drinking Water
Other Treatment Methods


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