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WATER PURIFICATION TECHNIQUES

by  Bob Fairchild

    Water can be contaminated by disease organisms, salts, metals, minerals, toxic chemicals and other materials.  These can be dangerous to human health as well as adversely affecting the color, odor, and/or taste.  Water may have to be purified to make it safe for drinking.  However, not all water has to be of the highest quality.  Lower quality water can be used for bathing, laundry, swimming and livestock and even lower quality water can be used for flushing toilets.

      Before choosing a treatment option, testing the water is recommended.  Testing should indicate whether treatment is needed and, if so, what contaminants need to be removed.  Once a problem is identified, appropriate treatment measures can be considered. The local health department is most helpful and may even test water samples for you.  Comprehensive water testing is becoming more accessible and more affordable.   Tests for all contaminants are usually available for less than $100

    1. Particle Removal Filters    

Solid particles are a common rural water system problem. Mechanical filters, which screen out particles larger than their pore size, will normally suffice.  If using rainwater or surface water, particles can be fairly large, so upstream filtering may be necessary to remove material before it enters the system.  For rainwater, it may be sufficient to divert the first few minutes of rainfall to wash the roof and then run the ensuing rain through a screen and gravel filter (see Cistern Supplement 3). If surface water from a pond or creek is used, relocating the intake may reduce the presence of such particles.  Screen filters can also be used in-line.  

  Settling, sand filters, and cartridge filters can be used to remove finer sediment particles.  In ponds, settling can be accomplished by adding powdered gypsum at a rate of 12 pounds per 7,000 gallons without adverse effects to human or marine life. Lime can be used to settle out particles in acidic water.  In a water system, settling can be accomplished by allowing the water sit quietly in a tank or by adding chemicals, such as alum, to assist the process.  Algae, which can make water cloudy and affect color and taste, can be eliminated using copper sulfate.  

    Two types of sand filters are commonly used:  rapid and slow.  Rapid sand filters remove sediment and cloudiness.  These consist of around 3 feet of medium sand over one foot of gravel and can be operated by gravity flow (with 3 to 6 feet of water over sand)  or in a closed vessel with pressure from a pump. Pressure sand filters for home use start at about $300.  Slow sand filters have the added benefit of reducing pathogen contamination in addition to removing sediment and cloudiness. Employing about 3 feet of fine sand over 6-8 feet of gravel, they are gravity-fed with water standing 1 to 3 feet over the surface of the sand.  In a slow sand filter, a biological mat builds as the water flows through it.  The slow sand filter is generally recognized as being the single best treatment process for improving the physical, chemical, and bacteriological quality of surface water.  It is ideal for small water systems serving 25 to 3,000 people, but can be custom made from concrete materials to accommodate larger numbers.  Size and cost depend upon water demand.  Some method of disinfection after sand filtering is generally recommended.

                 [Diagram 6  Sand Filter]    

Over time sand filters clog and so, require periodic maintenance.  Rapid sand filters are cleaned by back washing: pumping clean water back through the filter to wash out the fine particles.  Slow sand filters are cleaned by removing about one- half inch of sand from the top.  This sand is then roasted to remove contaminants and reused.  After about six inches of sand have been removed, clean sand should be added to the bottom of the filter by double digging

.                    [Diagram 7 Slow Sand Filter]    

Cartridge filters consist of a filter element enclosed in a housing.  Folded paper or wound string elements are used in disposable filters.  Elements typically cost $2-$5 each and housing, about $15.  Cartridge filters are best used to remove sediment from bacteriologically pure well or spring water.  If used with surface water or rainwater they can become a breeding ground for bacteria and algae.  For surface water, cleanable sediment filters are more appropriate.  These filters use a fine polyester mesh element that can be washed as needed and reused. The cost of a cleanable sediment filter with housing ranges from $35 to $70.

    The fossil skeletons of the marine algae diatoms, known as "diatomaceous earth" (DE), can be used as a filter medium to remove cloudiness from water.  A coating of this fine material is deposited on a filter fabric.  It is very effective at removing fine particles and cost around $300.  DE should be replaced regularly.  Mechanical filters all require regular maintenance, cleaning and replacement to ensure optimal system operation.

    2.  Disinfection    

Bacterial contamination is a common problem in private water systems, especially in rural areas.  Methods for disinfecting water include microfiltration, heat treatment, chlorination, distillation, reverse osmosis, iodine, ozone, and ultraviolet light.  Many of these methods are also effective at removing other contaminants from water (see Table 1).

Microfiltration    

Extremely fine ceramic filters are available that will remove bacteria from water.  Their pore size is 1 micron (0.00004") or less.  These are available as elements for standard or custom housings or as "filter candles" which can be attached through the bottom of a bucket to make a drip filter for drinking water.  To prevent clogging, they can be brushed clean and reused many times.  There is concern about bacteria growing through the filter medium.  Some manufacturers use silver compounds to prevent this.       Cost of a microfiltration unit with housing ranges from $50 - $150.  Elements cost from $30 to $100.  Hand pumped units are also available.  The more expensive models have finer and thicker ceramic.  Because of the flow rates involved, these are only feasible as single faucet units to treat drinking water. Microfiltration is the simplest and cheapest disinfection method.    

Heat Treatment    

Heat can be a very effective way to disinfect water. Boiling is the simplest method.  If water is brought to a rolling boil for two minutes, all bacterial, viruses and other microorganisms are killed.  Boiling is very energy intensive, expensive and inconvenient and is used mostly in emergencies.     Pasteurization is another effective method of disinfection. Water is heated to 140-160F for up to 15 minutes.  This is very effective but also energy intensive and expensive if done the usual way, with electric heat and circulating pumps.  It is possible to pasteurize water on a stove top in small batches. Solar pasteurization is also receiving greater attention.  Solar cookers or similar solar devices can be used to heat water.  In all cases, care must be taken to prevent recontamination.    

Chlorine    

Chlorination is the most popular water disinfection method used in this country.  Chlorine is very effective in killing microorganisms provided they are exposed to a high enough concentration for a long enough time.  Simple chlorination uses about one part per million (one gallon per 50,000 gallons of water).  Contact time of at least 30 minutes is needed.  With a cistern this is not a concern.  In a well system, chlorine is generally added at the pump to ensure adequate contact time.     Where 30 minutes of contact time is not possible, super chlorination can be used.  At levels of 5 ppm (one gallon chlorine bleach per 10,000 gallons water) contact time can be reduced to around 5 minutes.  At this level taste can be too strong and carbon filtration may be needed to remove it. Chlorination may be done manually or with automatic feed systems which range from $100 to $500.  Before using a water system, shock chlorination at 3 cups chlorine bleach per 100 gallons of water is recommended.  The entire system should be washed and pipes allowed to sit overnight filled with this concentration. Remember that at these concentrations, chlorine is not good for plants in wetlands.

    One advantage of chlorination is that residual chlorine in the water can prevent recontamination.  It will continue to kill microorganisms at low concentrations for a long time.  But there are problems with chlorination.  Reactions with iron, sulfur, ammonia, slime, organic materials, and other chemicals can reduce the effective level of chlorine.  In addition, some of the chemicals formed when chlorine reacts with organic chemicals are toxic or carcinogenic.  If these reactants are present, follow chlorination with slow carbon filtration or consider some other disinfection method.  All in all, the environmental community is reaching a consensus that chlorination should be avoided in favor of other methods if feasible.      

Distillation    

Distillation can be used to remove bacteria, salts, metals, and most other contaminants from water.  It is ineffective only against volatile organic chemicals.  Evaporation and recondensation separates the water from other contaminants.  Like boiling or pasteurization, it is very energy intensive and expensive.  Electric home units for drinking water production cost around $700.  Solar distillation is possible and has been used in sunny tropical areas with some success.  Costs can be high because of the amount of glazing required.    

Reverse Osmosis    

Reverse Osmosis uses high pressure to force water through an ultra-fine membrane.  This membrane separates at the molecular level, allowing small molecules like water to pass while blocking larger molecules like salts, metals, and organic chemicals.  This is a very effective but expensive filtration method.  Another disadvantage is that 4 to 9 gallons of water are required to make one gallon of drinking water.  The residual water has higher contaminant concentrations and must be disposed of or used in lower quality applications.  Electricity is generally necessary to achieve the high pressures required.  Small hand and battery powered units are available for emergency situations.  They can produce drinking water from seawater.  Cost can be in excess of $700.  Larger electric-powered units are available.  Cost is proportionally greater.  As with the elements in other filtration units, reverse osmosis membranes need periodic replacement.    

Iodine    

Iodine can also be used to disinfect water.  It is introduced in tablet form or as a solution to small batches of water or it can be automatically mixed with pumped water. Effective contact time is fifteen minutes under most circumstances.  Because it costs around twenty times more than chlorine, it is used primarily for emergencies and other special circumstances.    

Ozone    

Ozone is an activated form of oxygen.  It is created with electricity and mixed with water.  Ozone is such a powerful oxidizer that it kills microorganisms and breaks down organic chemicals.  Where organic chemicals are a concern, carbon filtration generally follows ozonation.  Drinking water units for home use can be hard to find.  Energy use and cost is high.        

Ultraviolet Light    

Ultraviolet light is another method of killing microorganisms in water.  Water is circulated in a thin layer past an ultraviolet bulb encased in a quartz sleeve.  The light energy kills microorganisms very quickly.  Clear water is needed for effective treatment.  Particles in the water can shade the bacteria from the light.  A mechanism for cleaning the quartz sleeve is generally incorporated into the unit, as is a mechanism for stopping water flow when light output is insufficient for disinfection.  Ultraviolet bulbs generally need replacement at least once a year.  Drinking water units for home use are hard to find.    

Carbon Filters      

Carbon, generally in the form of granular activated carbon, can be used to remove organic chemicals, including pesticides and chlorinated products as well as many tastes, odors and colors from water.  Carbon filters are available as cartridge filters for in-line use, or as loose granules for homemade drip filtering.  Fish tank carbon can be used in roughing filters, but its effectiveness is limited.  Carbon removes organic chemicals by attracting and holding the molecules.  For greatest effectiveness the water needs to flow slowly through the carbon. A drip filter is the most effective way to use carbon for treating drinking water.  Many cartridge type carbon filters are available for counter-top or under-sink installation.  Eventually carbon reaches its adsorption limit and must be replaced or it may begin releasing these chemicals back into the water.  Carbon filters can also serve as a growth site for bacteria.  They should be installed after bacterial treatment.  Carbon filters can also remove particulate.  Carbon block filters using granular carbon fused with a binder are especially effective as particulate filters.  Other ion exchange filters are available for iron, sulfur, and nitrates.

              [Diagram 8  Carbon Filter]    

Ion Exchange    

The best known example of ion exchange is a water softener where sodium ions replace ions of calcium and magnesium to reduce these hard-water minerals.  Softening of drinking water is not recommended because sodium can have negative health effects, especially for those with heart, kidney or circulatory problems. Salt water is used to recharge water softeners when their exchange capacity is exhausted.  This can be done manually or automatically or by water softener service companies that replace the tanks periodically.    

Other Treatment Systems    

Other treatment systems are available to remove specific contaminants from water, including lead, nitrates, sulfur, iron, and other compounds.  A variety of technologies can be used depending upon the contaminant.  Some are as simple as cartridges from special absorption resins, while others use aeration or chlorination in combination with special media filters.       The cost of producing drinking quality water from contaminated raw water can be high in small systems (thus, the rationale behind cisterns).  In many cases practicality may dictate treating only water  that is being used for human consumption to the highest level of quality.  Water for lower quality uses can be treated only to the required level of safety. Such a multi-quality system may require careful management, but can significantly lower costs for protecting health and is a sound ecological measure as well.

                ---------------------                  

 RESOURCES  
Carbon Filters and Disposable Sediment Cartridges:  Building and plumbing supply outlets.
Granular Activated Carbon (for do-it-yourself filter):  Calgon, P.O. Box 1346,  Pittsburgh, PA 15230.
Reverse Osmosis Filters:  Healthy Hardware, P.O. Box 3217, Prescott, AZ  86302;  
Real Goods, 966 Mazzoni St. Ukia CA  95482- 3471.      
Cleanable Sediment Filters & Ceramic Micro-filters:  EKAT  150 Gravel Lick Road, Dreyfus,  KY  40426;
 Lehman Hardware, P.O. Box 41, Kidron, OH  44636.                  

 REFERENCES
Troyan, Jerold T. and Sigurd P. Hansen, Treatment of Microbial Contaminants in Potable Water Systems.  Noyes Data Corp., 1989. Graham, N.J.D.  
Slow Sand Filtration.  Halsted Press, 1988. Campbell, Stu.  
The Home Water Supply -- How to Find, Filter, Store, and Conserve It.  Garden Way, 1983.
Planning for an Individual Water System.   American Association for Vocational Instructional Materials, 1983. Stone, Kenneth M.  How Safe is Your Water,   Garden Way, 1981. Dadd, Debra Lynn.  
Buyer's Guide to Water Purification Devices. Forest Knolle, CA: Earthwise ( P.O. Box 279,  94933). Coffel, Steve.
 But Not a Drop to Drink.  McMillan and Co., 1989.