عدد المساهمات : 3443
تاريخ التسجيل : 15/09/2009
العمر : 49
الموقع : مصر
|موضوع: water treatment methodes1 الإثنين يناير 04, 2010 4:23 pm|| |
In an emergency, boiling is the best way to purify water that is unsafe because of the presence of protozoan parasites or bacteria.
If the water is cloudy, it should be filtered before boiling. Filters designed for use when camping, coffee filters, towels (paper or cotton), cheesecloth, or a cotton plug in a funnel are effective ways to filter cloudy water.
Place the water in a clean container and bring it to a full boil and continue boiling for at least 3 minutes (covering the container will help reduce evaporation). If you are more than 5,000 feet above sea level, you must increase the boiling time to at least 5 minutes (plus about a minute for every additional 1,000 feet). Boiled water should be kept covered while cooling. From Drinking Water for Emergency Use (pdf file). You can also look at recommendations of the EPA.
The advantages of Boiling Water include:
Pathogens that might be lurking in your water will be killed if the water is boiled
Boiling will also drive out some of the Volatile Organic Compounds (VOCs) that
might also be in the water. This method works well to make water that is
contaminated with living organisms safe to drink, but because of the
inconvenience, boiling is not routinely used to purify drinking water except
The disadvantages of Boiling Water include:
Boiling should not be used when toxic metals, chemicals (lead, mercury, asbestos,
pesticides, solvents, etc.), or nitrates have contaminated the water.
Boiling may concentrate any harmful contaminants that do not vaporize as the
relatively pure water vapor boils off.
Energy is needed to boil the water
In many ways, distillation is the reverse of boiling. To remove impurities from water by distillation, the water is usually boiled in a chamber causing water to vaporize, and the pure (or mostly pure) steam leaves the non volatile contaminants behind. The steam moves to a different part of the unit and is cooled until it condenses back into liquid water. The resulting distillate drips into a storage container.
Salts, sediment, metals - anything that won't boil or evaporate - remain in the distiller and must be removed. Volatile organic compounds (VOCs) are a good example of a contaminant that will evaporate and condense with the water vapor. A vapor trap, carbon filter, or other device must be used along with a distiller to ensure the more complete removal of contaminants.
The advantages of Distillation include:
A good distillation unit
produces very pure water. This is one of
the few practical ways to remove nitrates,
chloride, and other salts that carbon
filtration can not remove.
Distillation also removes pathogens in the
water, mostly by killing and leaving them
behind when the water vapor evaporates.
If the water is boiled, or heated just short
of boiling, pathogens would also be killed.
As long as the distiller is kept clean and is
working properly the high quality of treated
water will be very consistent regardless of
the incoming water - no drop in quality over
No filter cartridges to replace, unless a carbon filter is used to remove volatile
The disadvantages of Distillation include:
Distillation takes time to purify the water, It can take two to five hours to make a
gallon of distilled water.
Distillers uses electricity all the time the unit is operating
Distillers requires periodic cleaning of the boiler, condensation compartment, and
Countertop Distillation is one of the more expensive home water treatment
methods, using $0.25 to $0.35 of electrical energy per gallon of distilled water
produced - depending on local electricity costs. The cost of ownership is high
because you not only have the initial cost of the distillation unit to consider, but you
also must pay for the electrical energy for each gallon of water produced. If it cost
you $0.25 to distill each gallon, and you purified 10 gallons per week, you would
pay $130 for your 520 gallons of distilled water each year.
Most home distillation units require electricity, and will not function in an
emergency situation when electrical power is not available.
Reverse Osmosis (RO):
Water pressure is used to force water molecules through a membrane that has extremely tiny pores, leaving the larger contaminants behind. Purified water is collected from the "clean" side of the membrane, and water containing the concentrated contaminants is flushed down the drain from the "contaminated" side. The average RO system is a unit consisting of a sediment/chlorine pre filter, the reverse-osmosis membrane, a water storage tank, and an activated-carbon post filter. They cost from about $150 to over $1,500 for point of use systems.
The advantages of Reverse Osmosis include:
Reverse osmosis significantly reduces salt, most other inorganic material present
in the water, and some organic compounds. With a quality carbon filter to remove
any organic materials that get through the filter, the purity of the treated water
approaches that produced by distillation.
Microscopic parasites (including viruses) are usually removed by properly
functioning RO units, but any defect in the membrane would allow these
organisms to flow undetected into the "filtered" water - they are not
recommended for use on biologically unsafe water.
Though slower than a carbon or sediment water filter, RO systems can typically
purify more water per day than distillers and are less expensive to operate and
Reverse Osmosis systems also do not use electricity, although because they
require relatively high water pressure to operate, they may not work well in some
The disadvantages of Reverse Osmosis include:
Point of Use RO units make only a few gallons of treated water a day for drinking
RO systems waste water. Two to four gallons of "waste" water are flushed down
the drain for each gallon of filtered water produced.
Some pesticides, solvents and other volatile organic chemicals (VOCs) are not
completely removed by RO. A good activated carbon post filter is recommended
to reduce these contaminants.
Many conditions affect the RO membrane's efficiency in reducing the amount of
contaminant in the water. These include the contaminant concentration, chemical
properties of the contaminants, the membrane type and condition, and
operating conditions (like pH, water temperature, and water pressure).
Although RO filters do not use electricity, they depend on a relatively high water
pressure to force the water molecules through the membrane. In an emergency
situation where water pressure has been lost, these systems will not function.
* However, if a high quality activated carbon filter is used for the post filter, it could
probably be disconnected and used to siphon water through in an emergency to
reduce many contaminants.
RO systems require maintenance. The pre and post filters and the reverse
osmosis membranes must be changed according to the manufacturer's
recommendation, and the storage tank must be cleaned periodically.
Damaged membranes are not easily detected, so it is hard to tell if the system is
functioning normally and safely.
You might want to check NSF International to see if the Reverse Osmosis system you are interested in purchasing is certified.
A reverse-osmosis system is a good treatment option for people who have
unacceptably high levels of dissolved inorganic contaminants in their drinking
water which can not be removed effectively or economically by other methods.
Water from shallow wells in agricultural areas that contains high nitrate levels is
a good example of a situation where RO would make sense. Most people,
however, who are using municipal water water that is subject to EPA regulations
usually have acceptably low levels of nitrates. Maximum nitrate levels should be
able to be determined by requesting a water quality report from your local water
The topic of water filters is complicated because there are so many models available (over 2,500 different models manufactured by more than 500 companies), and because there are so many types of filtration strategies and combinations of strategies used. The basic concept behind nearly all filters, however, is fairly simple. The contaminants are physically prevented from moving through the filter either by screening them out with very small pores and/or, in the case of carbon filters, by trapping them within the filter matrix by attracting them to the surface of carbon particles (the process of adsorption).
There are two main types of filters (sediment and activated carbon), and
sometimes they are combined into a single unit. A third type, which will be
considered as a separate topic, is reverse osmosis.
You will read about micron or sub micron filtration. This is a measure of how good
the filter is at removing particles from the water - smaller is better. A micron is a
unit of measure - one micron is about 1/100 the diameter of a human hair. A
filter that removes particles down to 5 microns will produce fairly clean-looking
water, but most of the water parasites, bacteria, cryptosporidia, giardia, etc. will
pass through the pores. A filter must trap particles one micron or smaller to be
effective at removing cryptosporidia or giardia cysts. Viruses can not be effectively
removed by any filtration method. In theory, reverse osmosis will remove viruses,
but a small flaw in the membranes would allow viruses to pass undetected
into the 'filtered' water.
Click here too view a figure that compares the relative size of several biological
contaminants with the pore size of some common filters.
A benefit of all home filtration systems is that they are passive. That is, they
require no electricity to filter the water, and normal home water pressure is used
to force the water though the filter. The only routine maintenance required is
periodic replacement of the filtration element. As long as the cost of the
replacement filter elements is reasonable, owning a even a high-end water filter
can be very inexpensive if you look at the long term costs and compare it
with other solutions.
Sediment Filters - Solid Particles are Strained Out of the Water:
Fiber Filters: These filters contain cellulose, rayon or some
other material spun into a mesh with small pores. If you take
a piece of cloth and pour water containing sand through it
you will get the picture. Suspended sediment (or turbidity) is
removed as water pressure forces water through tightly
wrapped fibers. Some small organic particles that cause
disagreeable odors and taste may also be removed. These
filters come in a variety of sizes and meshes from fine to
coarse, with the lower micron rating being the finer. The finer
the filter, the more particles are trapped and the more often
the filter must be changed.
Fiber filters are often used as pre-filters to reduce the
suspended contaminants that could clog carbon or RO filters.
Fiber filters will not remove contaminants that are dissolved
in the water, like chlorine, lead, mercury, trihalomethanes or
other organic compounds.
Ceramic Filters: Ceramic filters are much like fiber filters
and use a process where water is forced through the pores
of a ceramic filtration media. This provides mechanical
filtration only. This type of filter can reduce asbestos
fibers,cysts (if the pores are one micron or smaller), some
bacteria (with pore sizes in the 0.2 - 0.8 micron range**) and
other particulate matter.
Ceramic filters will not remove contaminants that are
dissolved in the water, like chlorine, lead, mercury,
trihalomethanes or other organic compounds, nor will they
remove viruses. These filters may be used as a back-end to
an activated carbon filter to provide a more thorough removal