the ultrafilteration methodes one of final treatment of water/waste water treatment methodes
Activated Carbon Filters:
Activated carbon (AC)
is particles of carbon that have been treated to increase their surface area and increase their ability to adsorb a wide range of contaminants - activated carbon is particularly good at adsorbing organic compounds.
You will find two basic kinds of carbon filters
Granular Activated Carbon (GAC)
and
Solid Block Activated Carbon (SBAC).
Contaminant reduction in AC filters takes place by two processes, physical removal of contaminant particles, blocking any that are too large to pass through the pores (obviously, filters with smaller pores are more effective), and a process called adsorption by which a variety of dissolved contaminants are attracted to and held (adsorbed) on the surface of the carbon particles.
The characteristics of the carbon material (particle and pore size, surface area, surface chemistry, density, and hardness) influence the efficiency of adsorption.
AC is a highly porous material; therefore, it has an extremely high surface area for contaminant adsorption.
One reference mentions " The equivalent surface area of 1 pound of AC ranges from 60 to 150 acres (over 3 football fields)".
Another article states, "Under a scanning electron microscope the activated carbon looks like a porous bath sponge.
This high concentration of pores within a relatively small volume produces a material with a phenomenal surface area: one tea spoon of activated carbon would exhibit a surface area equivalent to that of a football field."
( estimates of surface area vary from different sources)
AC is made of tiny clusters of carbon atoms stacked upon one another.
The carbon source is a variety of materials, such as peanut shells, coconut husks, or coal. The raw carbon source is slowly heated in the absence of air to produce a high carbon material.
The carbon is activated by passing oxidizing gases through the material at extremely high temperatures.
The activation process produces the pores that result in such high adsorptive properties.
An article about Activated Carbon states that "Activated carbon is one of the best tools which can be used to reduce risks to human health and provide an aesthetically pleasing product at reasonable cost."
The article also describes how AC works and has some of the best scanning electron microscope pictures of actual AC granules I have seen.
The adsorption process depends on the following factors:
1) physical properties of the AC, such as pore size distribution and surface area;
2) the chemical nature of the carbon source, or the amount of oxygen and hydrogen associated with it;
3) chemical composition and concentration of the contaminant;
4) the temperature and pH of the water; and
5) the flow rate or time exposure of water to AC. Interesting article: What is Activated Carbon, Adsorption, History of AC.
The effectiveness of carbon filters to reduce contaminants is affected
by the factors affecting adsorption listed above and three additional
characteristics of the filter, contact time between the water and the carbon
material, the amount of carbon in the filter, and pore size.
The length of contact time between the water and the carbon material,
governed by the rate of water flow and the amount/volume of activated
carbon, has a significant effect on adsorption of contaminants. More contact
time results in greater adsorption.
The amount of carbon present in a cartridge or filter affects the amount
and type of contaminant removed. Less carbon is required to remove taste-
and odor-producing chemicals than to remove trihalomethanes.
Pore size characteristics will be discussed in greater detail on the GAC
and SBAC pages, but GAC filters contain loose granules of activated
carbon while in SBAC filters, the activated carbon is in the form of very
small particles bound into a solid, matrix with very small pores.
Because of the filter characteristics discussed above, the most effective
Point of Use activated carbon filters are large SBAC filtration
systems, and the least effective are the small, pour-through pitcher filters.
Activated carbon filter cartridges will, over time, become less effective at
reducing contaminants as the pores clog with particles (slowing water flow)
and the adsorptive surfaces in the pores become filled with contaminants
(typically not affecting flow rate).
There is often no noticeable indication
that a carbon filter is no longer removing contaminants, so it is
important to replace the cartridge according to the manufacturer's
instructions.
The overall water quality (turbidity or presence of other
contaminants) also affects the capacity of activated carbon to adsorb a
specific contaminant.
It is important to note, particularly when using counter-top and faucet-mount
carbon filtration systems, that hot water should NEVER be run through
a carbon filter.
I have seen warnings about possible damage to the filter
from hot water.
Perhaps more importantly, hot water will tend to release
trapped contaminants into the water flow potentially making the water
coming out of the filter more contaminated than the water going in.
------------------------------
Granular Activated Carbon (GAC):
In this type of filter, water flows through a bed of loose activated carbon
granules which trap some particulate matter and remove some chlorine,
organic contaminants, and undesirable tastes and odors.
The three main
problems associated with GAC filters are: channeling, dumping, and
an inherently large pore size.
Most of the disadvantages discussed
below are not the fault of the activated carbon filtration media, rather, the
problem is the design of the filters and the use of loose granules of
activated carbon.
The advantages of GAC filters include:
Simple GAC filters are primarily used for aesthetic water treatment,
since they can reduce chlorine and particulate matter as well as improve
the taste and odor of the water.
Loose granules of carbon do not restrict the water flow to the extent of
Solid Block Activated Carbon (SBAC) filters.
This enables them to
be used in situations, like whole house filters, where maintaining a good
water flow rate and pressure is important.
Simple, economical maintenance. Typically an inexpensivefilter
cartridge needs to be changed every few months to a year, depending
on water use and the manufacturer's recommendation.
GAC filters do not require electricity, nor do they waste water.
Many dissolved minerals are not removed by activated carbon.
In the
case of calcium, magnesium, potassium, and other beneficial minerals,
the taste of the water can be improved and some (usually small) nutrient
value can be gained from the water.
The bottom line is that GAC filters are effective and valuable water treatment devices, but their limitations always need to be considered.
A uniform flow rate, not to exceed the manufacture's specifications, needs to be maintained for optimal performance, and the filter cartridge must be changed after treating the number of gallons the filter is rated for.
The disadvantages of GAC filters include:
Water flowing through the filter is able to "channel" around the carbon
granules and avoid filtration.
Water seeks the path of least resistance.
When it flows through a bed of loose carbon granules, it can carve a
channel where it can flow freely with little resistance.
Water flowing
through the channel does not come in contact with the filtration
medium.
The water continues to flow, however, so you do not realize
that your filter has failed - you get water, but it is not completely filtered.
Pockets of contaminated water can form in a loose bed of carbon
granules.
With changes in water pressure and flow rates, these
pockets can collapse, "dumping" the contaminated water through the
filter into the "filtered" flow.
Since the carbon granules are fairly large (0.1mm to 1mm in one
popular pitcher filter), the effective pore size of the filter is relatively
large (20 - 30 microns or larger).
GAC filters, by themselves, can not
bacteria.
As described above, hot water should NEVER be run through a
carbon filter
Also, if you think of a bed of charcoal that traps an occasional
bacterium, picks up a bit of organic material, and removes the chlorine
from the water, you can see how these filters might become breeding
grounds for the bacteria they trap.
You will see warnings about GAC
filters suggesting you run water through them for a few minutes each
morning to flush out any bacteria.
If you are considering purchasing a
GAC filter be sure to ask the representative about what they
recommend you do about this potential problem (I was told by one
sales person, that if the filter was not going to be used for a few days,
I should remove the filtration element, place it in a plastic bag, and
store it in the refrigerator until I returned).
Unless the filter plugs up or you notice an odor in the "filtered water",
it may be difficult to know when the filter has become saturated with
contaminants and ineffective. That is why it is necessary to change
filter cartridges according to the manufacturer's recommendation.
----------------
Solid Block Activated Carbon (SBAC):
Activated carbon is the primary raw material in solid carbon block filters;
but instead of carbon granules comprising the filtration medium, the
carbon has been specially treated, compressed, and bonded to form a
uniform matrix.
The effective pore size can be very small (0.5 - 1
micron). SBAC, like all filter cartridges, eventually become plugged or
saturated by contaminants and must be changed according to
manufacturer's specifications.
Depending on the manufacturer, the filters
can be designed to better reduce specific contaminants like arsenic,
MTBE, etc. (an example)
The advantages of SBAC filters include:
Provide a larger surface area for adsorption to take place than Granular
Activated Carbon (GAC) filters for better contaminant reduction.
Provide a longer contact time with the activated carbon for more
complete contaminant reduction.
Provide a small pore size to physically trap particulates. If the pore size
is small enough, around 0.5 micron or smaller, bacteria that become
trapped in the pores do not have enough room to multiply, eliminating a
problem common to GAC filters.
Completely eliminate the channeling and dumping problems
associated with GAC filters.
SBAC filters are useful in emergency situations where water pressure
and electricity might be lost.
They do not require electricity to be
completely effective, and water can even be siphoned through them.
SBAC filters do not waste water like reverse osmosis.
Many dissolved minerals are not removed by activated carbon.
In the
case of calcium, magnesium, potassium, and other beneficial minerals,
the taste of the water can be improved and some (usually small) nutrient
value can be gained from the water.
Simple, economical maintenance. Typically an inexpensive filter
cartridge needs to be changed every few months to a year, depending
on water use and the manufacturer's recommendation.
This combination of features provides the potential for greater adsorption
of many different chemicals (pesticides, herbicides, chlorine, chlorine
byproducts, etc.) and greater particulate filtration of parasitic cysts,
asbestos, etc. than many other purification process available. By using
other specialized materials along with specially prepared activated carbon,
customized SBAC filters can be produced for specific applications or to
achieve greater capacity ratings for certain contaminants like lead,
mercury, arsenic, etc.
The disadvantages of SBAC filters include:
SBAC filters, like all activated carbon filters, do not naturally reduce the
levels of soluble salts (including nitrates), fluoride, and some other
potentially harmful minerals like arsenic (unless specially designed) and
cadmium.
If these contaminants are present in your water, reverse
osmosis would usually be the most economical alternative followed by
distillation.
As described above, hot water should NEVER be run through a
carbon filter
As SBAC filters remove contaminants from the water they gradually
lose effectiveness until they are no longer able to adsorb the
contaminants.
There is no easy way to determine when a filter is
nearing the end of its effective life except that the 'filtered' water
eventually begins to taste and smell like the unfiltered water.
The
manufacturer's guidelines for changing filter cartridges should always
be followed.
You might want to check NSF International to see if the SBAC filtration system you are interested in purchasing is certified.
An example of a certified list of contaminants significantly reduced by a high-end SBAC filtration system (** at 200 percent of capacity - that's after filtering twice the rated volume of the filter**):
For Standard 042 - aesthetic
Chlorine Reduction, Class I
(actual chlorine reduction 99.9%)
Particulate Reduction, Class I
(actual particulate reduction down to 0.5 micron)
Chloramine Reduction
(actual chloramine reduction greater than 96%)
Taste and Odor Reduction
For Standard 053 - contaminants of health concern and measured percent reduction
Asbestos Reduction >99%
Chlordane Reduction >99.8%
Cyst Reduction 99.9%
Lead Reduction >98%
Mercury Reduction >99.3% (pH 8.5) >91.4 (pH 6.5)
MTBE Reduction 96.6%
PCB Reduction >98%
Toxaphene Reduction >93%
TTHM Reduction (Trihalomethanes) >99.8%
Turbidity Reduction >99%
VOC Reduction (volatile organic chemical) Reduction - you will see a
long list of specific VOCs (individual percent reduction for the various
VOCs can be found on the product certification sheet below, but
most are 98-99% or more).
A few SBAC filtration systems have been certified for arsenic reduction:
----------------
What about the inexpensive GAC pitcher filters and faucet mount filters? What
will they do for your water purification needs?.
The answer to that question depends on what your water treatment goals are and the
amount of filtered water you plan to use.
The most popular brands are GAC filters
that will sometimes also contain granules of an ion exchange medium (you can see
and hear the granules rattling around).
They do reduce the level of some contaminants of health concern and thus are
better than nothing.
They are very limited in the type and number of contaminants they remove,
typically chlorine and perhaps lead, copper, and/or cysts are reduced.
At about $5 - $10 for a filter cartridge capable of treating about 35 - 40 gallons,
your filtered water will cost about $0.14 - $0.25 per gallon ( $58 - $90 per year if
you use 35 gallons per month).
If you are using (or would like to use) much more
than a gallon per day, and/or would like to remove more than a minimum number of
contaminants, you should probably examine other options.
Since they contain a very small amount of very loose GAC granules, they CAN
NOT be considered effective treatments for most biological contaminants
or chemical contaminants of health concern.
The pitcher filters which contain
a microfilter are typically certified to reduce cysts. All of these filters are mostly
designed to improve the aesthetics of drinking water (taste & odor improvement)
and reduce levels of a limited number of harmful contaminants.
They are subject to
possible bacterial growth channeling, and all the other problems of larger GAC
filters.
How about the Solid Carbon Block faucet mount filters?
The solid carbon block faucet mount filters are more effective than GAC filters in
reducing contaminants.
These filters, by nature, are quite small, though, and because filter effectiveness is
dependent on contact time of the water with the filter media, a larger, high-quality
solid carbon block filter will be more effective at reducing contaminants at the
same flow rate.
The difference is size can be striking - 4 ounces of activated
carbon for a faucet mount filter vs. 32 ounces for a high-end filter, over 7 times
more filter media.
Like the pitcher filters, the replacement cartridges for faucet mount filters tend to
be relatively more expensive (typically $0.14 - $0.25 per gallon) compared to a
high-quality solid block activated carbon replacement filter which will filter water for
about $0.07 per gallon.
Using 2 gallons of filtered water per day would cost
between $100 and $180 to replace 4-5 ounce faucet mount cartridges and about
$50 to replace a 32 ounce high-end filter.
محطات معالجة الصرف الصناعى والصحى بالطرق البيولوجية والكيميائية
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