We may organize
water treatment technologies into three general areas: Physical
Methods,
Chemical Methods, and Energy Intensive Methods. Physical methods of
Wastewater treatment
represents a body of technologies that we refer largely to as
Solid-liquid
separations techniques, of which filtration plays a dominant role.
Filtration
technology can be broken into two general categories - conventional
and
Non-conventional.
This technology is an integral component of drinking water and
Wastewater
treatment applications. It is, however, but one unit process within a
Modern water
treatment plant scheme, whereby there are a multitude of equipment
And technology
options to select from depending upon the ultimate goals of
Treatment. To
understand the role of filtration, it is important to make distinctions
Not only with
the other technologies employed in the cleaning and purification of
Industrial and
municipal waters, but also with the objectives of different unit
Processes.
Chemical methods
of treatment rely upon the chemical interactions of the
Contaminants we
wish to remove from water, and the application of chemicals that
Either aid in
the separation of contaminants from water, or assist in the destruction
Or
neutralization of harmful effects associated with contaminants. Chemical
Treatment
methods are applied both as stand-alone technologies, and as an integral
Part of the treatment
process with physical methods.
Among the energy
intensive technologies, thermal methods have a dual role in
Water treatment
applications. They can be applied as a means of
sterilization, thus
Providing high quality
drinking water, and/or
these
technologies
can be applied to the processing of the solid wastes or sludge, generated from
water treatment
Applications. In
the latter cases, thermal methods can be applied in essentially the
Same manner as
they are applied to conditioning water, namely to sterilize
sludge
Contaminated
with organic contaminants, and/or these technologies can be applied
To volume
reduction. Volume reduction is a key step in water treatment operations,
because ultimately there is a tradeoff between polluted water and hazardous
solid waste.
Energy intensive
technologies include electrochemical techniques, which by and
Large are
applied to drinking water applications. They represent both sterilization
And conditioning
of water to achieve a palatable quality.
All three of
these technology groups can be combined in water treatment, or they
May be used in
select combinations depending upon the objectives of water
Treatment. Among
each of the general technology classes, there is a range of both
Hardware and
individual technologies that one may select from. The selection of not only the
proper unit process and hardware from within each technology group, but the
optimum combinations of hardware and unit processes from the four groups depends
upon such factors as:
- How
clean the final water effluent from our plant must be.
- The
quantities and nature of the influent water we need to treat.
- The
physical and chemical properties of the pollutants we need to remove Or render
neutral in the effluent water.
- The
physical, chemical and thermodynamic properties of the solid wastes Generated from
treating water.
- The
cost of treating water, including the cost of treating, processing and Finding a home
for the solid wastes.
If we
start with the first technology group, then filtration should be thought of as
Both a unit
process and a unit operation within a water treatment facility. As a
Separate unit
process, its objective is quite clear: namely, to remove suspended
Solids. When we
combine this technology with chemical methods and apply
Sedimentation
and clarification (other physical separation methods), we can extend
The technology
to removing dissolved particulate matter as well. The particulate
Matter may be
biological, microbial or chemical in nature, as such, the operation
Stands alone
within its own block within the overall manufacturing train of the
Plant. Examples
of this would be the roughening and polishing stages of water
Treatment. In
turn, we may select or specify specific pieces of filtration equipment
For these unit
processes.
The following
are some of the major contaminants that are of concern in water purification applications,
as applied to drinking water sources, derived from groundwater:
Heavy
Metals - Heavy metals represent
problems in terms of groundwater
Pollution. The
best way to identify their presence is by a lab test of the water. There are
concerns of chronic exposure to low levels of heavy metals in drinking water.
Turbidity
-
Turbidity
refers to suspended solids, i.e. muddy water, is very turbid.
Turbidity is
undesirable for three reasons:
- aesthetic
considerations,
- solids may
contain heavy metals, pathogens or other contaminants
- turbidity
decreases the effectiveness of water treatment techniques by Shielding
pathogens from chemical or thermal damage, or in the case of UV (ultra
violet) treatment, absorbing the UV lights itself.
Organic
Compounds - Water can be
contaminated by a number of organic
Compounds, such
as chloroform, gasoline, pesticides, and herbicides from a variety of
industrial and agricultural operations or applications. These contaminants must
be identified in a lab test. It is unlikely groundwater will suddenly become contaminated,
unless a quantity of chemicals is allowed to enter a well or penetrating the
aquifer. One exception is when the aquifer is located in limestone.
Not only will
water flow faster through limestone, but the rock is prone to forming
Vertical
channels or sinkholes that will rapidly allow contamination from surface
Water. Surface
water may show great variations in chemical contamination levels
Due to
differences in rainfall, seasonal crop cultivation, and industrial
effluent
Levels. Also,
some hydrocarbons (the chlorinated hydrocarbons in particular) form
A type of
contaminant that is especially troublesome. These are a group of
Chemicals known
as dense nonaqueous phase liquids, or DNAPLs. These include
Chemicals used
in dry cleaning, wood preservation, asphalt operations, machining,
And in the
production and repair of automobiles, aviation equipment, munitions, and electrical
equipment. These substances are heavier than water and they sink quickly into
the ground.
This makes
spills of DNAPLs more difficult to handle than spills of petroleum products. As
with petroleum products, the problems are caused by groundwater dissolving some
of the compounds in these volatile substances. These compounds can then move
with the groundwater flow. Except in large cities, drinking water is rarely
tested for these contaminants.
Disposal of chemicals that have low water
solubility and a density greater than water result in the formation of distinct
areas of pure residual contamination in soils and groundwater. These chemicals
are typically solvents and are collectively referred to as Dense Non-Aqueous
Phase Liquids (DNAPLs). Because of their relatively high density, they tend to move
downward through soils and groundwater, leaving small amounts along the
migratory pathway, until they reach an impermeable layer where they collect in
discrete pools.
Once the DNAPLs have reached an aquitard they
tend to move laterally under the influence of gravity and to slowly dissolve
into the groundwater, providing a long-term source for low level contamination
of groundwater.
Because of their movement patterns DNAPL
contamination is difficult to detect, characterize and remediate.
Pathogens
-
These
include protozoa, bacteria, and viruses. Protozoa cysts are the
largest
pathogens in drinking water, and are responsible for many of the waterborne disease
cases in the U.S. Protozoa cysts range is size from 2 to 15 µm, but
can squeeze through smaller openings. In order to insure cyst
filtration, filters with an absolute pore size of l µm or less should be used.
The two most
common protozoa pathogens are Giardia Zamblia (Giardia) and Cryptosporidium
(Crypto). Both organisms have caused numerous deaths in recent years in
the U.S. and Canada, the deaths occurring in the young and elderly, and
the sick and immune compromised.
Bacteria are
smaller than protozoa and are responsible for many diseases, such as
Typhoid fever,'
cholera, diarrhea, and dysentery. Pathogenic bacteria range in size
From 0.2 to 0.6
µm, and a 0.2 µm filter is necessary to prevent transmission.
Contamination of
water supplies by bacteria is blamed for the cholera epidemics,
Which devastate
undeveloped countries from time to time.
Viruses are the
2nd most problematic pathogen, behind protozoa. As with protozoa, most
waterborne viral diseases don't present a lethal hazard to a healthy adult.
Waterborne
pathogenic viruses range in size from 0.020-0.030 µm, and are too
Small to be
filtered out by a mechanical filter. All waterborne enteric viruses
Affecting humans
occur solely in humans, thus animal waste doesn't present much
Of a viral
threat. At the present viruses don't present a major hazard to people
Drinking surface
water in the U.S., but this could change in a survival situation as
The level of
human sanitation is reduced. Viruses do tend to show up even in remote areas,
so a case can be made for eliminating them now.
INTRODUCING THE
PHYSICAL TREATMENT METHODS
The following
technologies are among the most commonly used physical methods
Of purifying
water:
Heat
Treatment - Boiling is one way to
purify water of all pathogens. Most experts feel that if the water reaches a
rolling boil it is safe. A few still hold out for
Maintaining the
boiling for some length of time, commonly 5 or
10 minutes, plus
An extra minute
for every l000 feet of elevation. One reason for the long period of
Boiling is to
inactivate bacterial spores (which can survive boiling), but these spore are
unlikely to be waterborne pathogens. Water can also be treated at below boiling
temperatures, if contact time is increased. Commercial units are available for
Residential use,
which treat 500 gals of water per day at an estimated cost of
$1/1000 gallons
for the energy. The process is similar to milk pasteurization, and
Holds the water
at 161˚ F for 15 seconds. Heat exchangers recover most of the
Energy used to
warm the water. Solar pasteurizers have also been built that can heat three
gallons of water to 65 ˚C and hold the
temperature for an hour. A higher temperature could be reached, if the
device was rotated east to west during the day to follow the sunlight.
Regardless of the method, heat treatment does not leave any form of residual to
keep the water free of pathogens in storage.
Reverse
Osmosis - Reverse osmosis forces
water, under pressure, through a
Membrane that is
impermeable to most contaminants. The membrane is somewhat
Better at
rejecting salts than it is at rejecting non-ionized weak acids and bases and
Smaller organic
molecules (molecular weight below 200). In the latter category are
Undissociated
weak organic acids, amines, phenols, chlorinated hydrocarbons, some pesticides
and low molecular weight alcohols. Larger organic molecules and all pathogens
are rejected.
Of course, it is
possible to have an imperfection in the membrane that could allow molecules or
whole pathogens to pass through. Using reverse osmosis to desalinate seawater
requires considerable pressure (1000 psi) to operate.
Reverse osmosis
filters are available that will use normal municipal or private water pressure
to remove contaminate from water. The water produced by
Reverse osmosis,
like distilled water, will be close to pure H2O.
Therefore
mineral intake may need to be increased to compensate for the normal mineral
content of water in much of the world.
Distillation
-
Distillation
is the evaporation and condensation of water to purify
Water.
Distillation has two disadvantages:
1)
A large
energy input is required.
2)
If simple
distillation is used, chemical contaminants with boiling points below
Water will be
condensed along with the water. Distillation is most commonly used
To remove
dissolved minerals and salts from water.
Micro
filters - Micro filters are
small-scale filters designed to remove cysts,
Suspended
solids, protozoa, and, in some cases, bacteria from water. Most filters
Use a ceramic or
fiber element that can be cleaned to restore performance as the
Units are used.
Most units and almost all made for camping use a hand pump to
Force the water
through the filter. Others use gravity, either by placing the water
To be filtered
above the filter (e.g. the Katadyn drip filter), or by placing the filter
In the water,
and running a siphon hose to a collection vessel located below the
Filter (e.g.
Katadyn siphon filter).
Microfilters are
the only method, other than boiling, to remove Cryptosporidia. Microfilters do
not remove viruses. Despite this, the Katadyn microfilter has seen considerable
use around the world by NATO-member militaries, and other aid organizations.
Microfilters
share a problem with charcoal filter in having bacteria grow on the filter
medium. Some handle this by impregnating the filter element with silver, such
as the Katadyn,
Others advise
against storage of a filter element after it has been used. Many
Microfilters may
include silt prefilters, activated charcoal stages, or an iodine resin.
Most filters
come with a stainless steel prefilter, but other purchased or improvised
Filters can be
added to reduce the loading on the main filter element. Allowing time for
solids to settle, and/or prefiltering will also extend filter life.
Iodine matrix filters will kill viruses that
will pass through the filter, and if a charcoal stage is used it will remove
much, of the iodine from the water. Charcoal filters will also remove other dissolved
natural or manmade contaminates. Both the iodine and the charcoal stages do not
indicate when they reach their useful life, which is much shorter than the
filter element.
Slow
Sand Filter - Slow sand filters pass
water slowly through a bed of sand.
Pathogens and
turbidity are removed by natural die-off, biological action, and
Filtering.
Typically the filter will consist of a layer of sand, then a gravel layer in
Which the drain
pipe is embedded. The gravel doesn't touch the walls of the filter,
So that water
can't run quickly down the wall of the filter and into the gravel.
Building the
walls with a rough surface also helps. A typical loading rate for the
Filter is 0.2
meters /hour day (the same as 0.2 m3/m2 of
surface area). The filter can be cleaned several times before the sand has to
be replaced. Slow sand filters
Should only be
used for continuous water treatment. If a continuous supply of raw
Water can't be
insured (say, using a holding tank), then another
method should be chosen. It is also important for the water to have as low
turbidity (suspended solids) as possible.
Turbidity can be reduced by changing the method of collection (for example,
building an infiltration gallery, rather than taking water directly from a creek),
allowing time for the material to settle out (using a raw water tank),
Prefiltering or
flocculation (adding a chemical, such as alum to cause
the suspended material to floc together.) The SSF filter itself is a
large box. The walls should be as rough as possible
to reduce the tendency for water to run down the walls of the filter, bypassing
the sand. The bottom layer of the filter is a gravel bed, in which a slotted
pipe is placed to drain off the filtered water.
Activated
Charcoal Filter - Activated
charcoal filters water through adsorption;
Chemicals and
some heavy metals are attracted to the surface of the charcoal, and
Are attached to
it. Charcoal filters will filter some pathogens, though they will
Quickly use up
the filter adsorptive ability, and can even contribute to
Contamination,
as the charcoal provides an excellent breeding ground for bacteria
And algae. Some
charcoal filters are available impregnated with silver to prevent
This, though
current research concludes that the bacteria growing on the filter are
Harmless, even
if the water wasn't disinfected before contacting the filter. Activated charcoal
can be used in conjunction with chemical treatment. The chemical (iodine or
chlorine) will lull the pathogens, while the carbon filter will remove the treatment
chemicals. In this case, as the filter reaches its capacity, a distinctive chlorine
or iodine taste will be noted. The more activated charcoal in a filter, the longer
it will last. The bed of carbon must be deep enough for adequate contact with
the water.
Production
designs use granulated activated charcoal (effective size or 0.6 to 0.9 mm for
maximum flow rate). Home or field models can also use a compressed carbon block
or powered activated charcoal (effective size 0.01) to increase contact area.
Powered charcoal
can also be mixed with water and filtered
out later. As far as life of the filter is concerned, carbon block filters will
last the longest for a given size, simply due to their greater mass of carbon.
A source of pressure is usually needed with carbon block filters to achieve a
reasonable flow rate.
CHEMICAL TREATMENT
CHLORINE
Chlorine has a
number of problems when used for field treatment of water. When chlorine reacts
with organic material, it attaches itself to nitrogen containing compounds
(ammonium ions and amino acids), leaving less free chlorine to continue
disinfection. Carcinogenic trihalomethanes are also produced, though this is
only a problem with long-term exposure. Trihalomethanes can also be filtered
out with a charcoal filter, though it is more efficient to use the same filter
to remove organics before the water is chlorinated. Unless free chlorine is
measured, disinfection can not be guaranteed with moderate doses of chlorine.
One solution is super
chlorination, the addition of far more chlorine than is needed.
This must again
be filtered through activated charcoal to remove the large amounts
Of chlorine, or
hydrogen peroxide can be added to drive the chlorine off. Either
Way there is no
residual chlorine left to prevent recontamination. This isn't a problem, if the
water is to be used at once.
Chlorine is
sensitive to both the pH and temperature of the treated water.
Temperature
slows the reaction for any chemical treatment, but chlorine treatment
Is particularly
susceptible to variations in the pH as at lower pHs, hypochlorous
Acid is formed,
while at higher pHs, it will tend to dissociate into hydrogen and
Chlorite ions,
which are less effective as a disinfectant. As a result, chlorine
Effectiveness
drops off when the pH is greater than 8.
Calcium
Hypochlorite, also known as High Test Hypochlorite (HTH) is supplied
In crystal form;
it is nearly 70% available chlorine. One product, the Sanitizer
(Formally the
Sierra Water Purifier) uses these crystals to superchlorinate the water
To insure
pathogens were killed off, then hydrogen peroxide is added to drive off
The residual
chlorine. This is the most effective method of field chlorine treatment.
IODINE
Iodine's use as
a water purification method emerged after World War 2, when the
U.S. military
was looking for a replacement for Halazone tablets. Iodine was found
To be in many
ways superior to chlorine for use in treating small batches of water.
Iodine is less
sensitive to the pH and organic content of water, and is effective in
Lower doses.
Some individuals are allergic to iodine, and there is some question
About long term
use of iodine. The safety of long-term exposure to low levels of
Iodine was
proven when inmates of three Florida prisons were given water
Disinfected with
0.5 to 1.0 ppm iodine for 15 years. No effects on the health or
Thyroid function
of previously healthy inmates was observed. Of 101 infants born
To prisoners
drinking the water for 122- 270 days, none showed detectable thyroid
Enlargement.
Iodine is
normally used in doses of 8 PPM to
treat clear water for a 10 minute contact time. The effectiveness of this dose
has been shown in numerous studies. Cloudy water needs twice as much
iodine or twice as much contact time. In cold water (Below 41˚ F
or 5˚ C) the dose or time must also be doubled. In any case doubling the
treatment time will allow the use of half as much iodine. These doses
are calculated to remove all pathogens (other than cryptosporida) from
the water. Of these, giardia cysts are the hardest to kill, and are what
requires the high level of iodine. If the cysts are filtered out with a microfilter
(any model will do since the cysts are 6 µm), only 0.5 ppm is
needed to treat the resulting water.
Water treated
with iodine can have any objectionable taste removed by treating the
Water with
vitamin C (ascorbic acid), but it must be added after the water has stood for
the correct treatment time. Flavored beverages containing vitamin C will accomplish
the same thing. Sodium thiosulfate can also be used to combine with
Free iodine and
either of these chemicals will also help remove the taste of chlorine as well.
Usually elemental iodine can't be tasted below 1 ppm, and below 2 ppm
the taste isn't objectionable. Iodine ions have an even higher taste threshold
of 5 ppm. Note that removing the iodine
taste does not reduce the dose of iodine
Ingested by the
body.
Average American
iodine intake is estimated at 0.24 to 0.74 mg/day, higher than the RDA of
0.4 mg/day. Due to a recent National Academy of Science recommendation that
iodine consumption be reduced to the RDA, the EPA discourages the
use of iodized salt in areas where iodine is used to treat drinking water.
SILVER
Its use is
currently out of favor due to the EPA's establishment of a 50 ppb MCL (Maximum
Contaminate Level) limit on silver in drinking water.
This limit is
set to avoid argyrosis, a cosmetic blue/gray staining of the
skin, eyes,
And mucous
membranes. As the disease requires a net accumulation of 1 g of silver in the
body, one expert calculated that you could drink water treated at 50 ppb for 27
years before accumulating 1 g. Silver has only be proven to be effective
against bacteria and protozoan cysts, though it is quite likely also effective against
viruses. Silver can be used in the form of a silver salt, commonly silver
nitrate, a colloidal suspension, or a bed of metallic silver.
Electrolysis can
also be used to add metallic silver to a solution. Some evidence has suggested
that silver deposited on carbon block filters can kill pathogens without adding
as much silver to the water.
POTASSIUM
PERMANGANATE
Potassium
permanganate is no longer commonly used in the developed world to kill
pathogens.
It is much
weaker than the other alternatives cited, more expensive, and leaves a
Objectionable
pink or brown color. Still, some underdeveloped countries rely on it,
Especially in
home-use applications. If it must be used, 1 gram per liter would
Probably be
sufficient against bacteria and viruses (no data is available on it
Effectiveness
against protozoan cysts). Hydrogen Peroxide can be used to purify
Water if nothing
else is available. Studies have shown of 99 percent inactivation of
Poliovirus in 6
hr with 0.3 percent hydrogen peroxide and a 99% in-activation of
Rhinovirus with
a 1.5% solution in 24 minutes. Hydrogen
Peroxide is more
Effective
against bacteria.
COAGULATION/FLOCCULATIONA
GENTS
While
flocculation doesn't kill pathogens, it will reduce their levels along with
Removing
particles that could shield the pathogens from chemical or thermal
Destruction and
organic matter that could tie up chlorine added for purification. 60-98% of
coliform bacteria, 65-99% of viruses, and 60-90% of giardia will be
Removed from the
water, along with organic matter and heavy metals.
Some of the
advantages of coagulation/flocculation can be obtained by allowing the particles
to settle out of the water with time (sedimentation), but it will take a while for
them to do so. Adding coagulation chemicals, such as alum, will increase the rate
at which the suspended particles settle out by combining many smaller particles
into larger floc, which will settle out faster. The usual dose for alum is
10-30 mg/liter of water. This dose must be rapidly mixed with the water, then
the water must be agitated for 5 minutes
to encourage the particles to form flocs. After this at least 30 minutes
of settling time is need for the flocs to fall to the bottom, and them the
clear water above the flocs may be poured off.
Most of the
flocculation agent is removed with the floc, nevertheless, some question the
safety of using alum due to the toxicity of the aluminum in it. There is little
to no scientific evidence to back this up. Virtually all municipal plants in
the US dose the water with alum. In bulk water treatment, the alum dose
can be varied until the idea dose is found. The needed dose varies with the pH
of the water and the size of the particles. Increase turbidity makes the flocs
easier to produce not harder, due to the increased number of collisions between
particles.
ENERGY
INTENSIVE TREATMENT TECHNOLOGIES
OZONE
Ozone is used
extensively in Europe to purify water. Ozone, a molecule composed
Of 3 atoms
of oxygen rather than two, is formed by exposing air or oxygen to a
High voltage
electric arc. Ozone is much more effective as a disinfectant than chlorine, but
no residual levels of disinfectant exist after ozone turns back
into 0,.
(One source
quotes a half life of only 120 minutes in distilled water at 20 ˚C).
Ozone is expected
to see increased use in the U.S. as a way to avoid the production
And formation of
trihalomethanes, and while ozone does break down organic
Molecules,
sometimes this can be a disadvantage as ozone treatment can produce
Higher levels of
smaller molecules that provide energy source for
Microorganisms.
If no residual disinfectant is present (as would happen if ozone
Were used as the
only treatment method), these microorganisms will cause the water quality to
deteriorate in storage. Ozone also changes the surface charges of
Dissolved
organics and colloidially suspended particles. This cause
Microflocculation
of the dissolved organics and coagulation of the colloidal
particles.
UV LIGHT
Ultraviolet
light has been known to kill pathogens for a long time. A low pressure
Mercury bulb
emits between 30 to 90 % of its energy at a wave
length of 253.7 nm, right in the middle of the UV band. If water
is exposed to enough light, pathogens will be killed. The problem is that some
pathogens are hundreds of times less sensitive to UV light than others.
The least sensitive pathogens to UV are protozoan cysts. Several studies show
that Giardia will not be destroyed by many commercial UV treatment units.
Fortunately, these are the easiest pathogens to filter out with a mechanical
filter. The efficiency of treatment is
very dependent on the turbidity of the water. The more opaque the water is, the
less light that will be
Transmitted
through it.
The treatment
units must be run at the designed flow rate to insure
sufficient
Exposure, as
well as insure turbulent flow rather than plug
flow. Another problem
With UV treatment
is that the damage done to the pathogens with UV light can be
Reversed if the
water is exposed to visible light (specifically 330-500 nm) through
A process known
as photoreactivation. UV treatment, like ozone or mechanical
Filtering,
leaves no residual component in the water to insure its continued
Disinfection.
The US EPA explored
UV light for small scale water treatment plants and found
It compared
unfavorably with chlorine due to
1) Higher costs.
2) Lower reliability.
3) Lack of a residual disinfectant.
