misc.survivalism FAQ on water treatment V2.2 Last Update 7/19/98
Thanks to the following people for making additions, corrections, or
suggestions:
Richard DeCastro (decastro@netcom.com)
Henry Schaffer (hes@unity.ncsu.edu)
Alan T Hagan (athagan@sprintmail.com)
Logan Van Leigh (loganv@earthlink.net)
Carl Stiles (blitz1@airmail.net)
Alan also provided the wording for the disclaimer and copyright
notice.
Excluding contributions attributed to specific individuals all
material in this work is copyrighted to Patton Turner and all rights
are reserved. This work may be copied and distributed freely as long
as the
entire text, my and the contributor's names and this copyright notice
remains intact, unless my prior permission has been obtained. This FAQ
may not be distributed for financial gain, included in commercial
collections or compilations, or included as a part of the content of
any website without prior, express permission from the author.
======================================
DISCLAIMER: Safe and effective water treatment requires attention
to detail and proper equipment and ingredients. The author makes no
warranties and assumes no responsibility for errors or omissions in
the text, or damages resulting from the use or misuse of information
contained herein
Placement of or access to this work on this or any other site does
not mean the author espouses or adopts any political, philosophical or
meta-physical concepts that may also be expressed wherever this work
appears.
======================================
Water Storage:
Quantity. A water ration of as little as a pint per day has allowed
liferaft survivors to live for weeks, but a more realistic figure is 1
gallon per person per day for survival. 4 gallons per person/day will
allow personal hygiene, washing of dishes, counter tops, etc. 5 to 12
gallons per day would be needed for a conventional toilet, or 1/2 to
two gallons for a pour flush latrine. For short term emergencies, it
will probably be more practical to store paper plates and utensils,
and minimize food preparation, than to attempt to
store more water.
In addition to stored water, their is quite a bit of water trapped in
the piping of the average home. If the municipal water system was not
contaminated before you shut the water off to your house, this water
is still fit for consumption without treatment. To collect this water,
open the lowest faucet in the system, and allow air into the system
from a second faucet. Depending on the diameter of the piping, you
may want to open
every other faucet, to make sure all of the water is drained. This
procedure will usually only drain the cold water side, the hot water
side will have to be drained from the hot water heater. Again, open
all of the faucets to let air into the system, and be prepared to
collect any water that comes out when the first faucet is opened.
Toilet tanks (not the bowls) represent another source of water if a
toilet bowl cleaner is not used in the tank..
Some people have plumbed old hot water heaters or other tanks in line
with their cold water supply to add a always rotated source of water.
2 cautions are in order 1) make sure the tanks can handle the pressure
(50 psi min.), and 2) if the tanks are in series with the house
plumbing, this method is susceptible to contamination of the municipal
water system. The system can be fed off the water lines with a
shutoff valve ( and a second drain line), preventing the water from
being contaminated as long as the valve was closed at the time of
contamination.
Water can only be realistically stored for short term emergencies,
after that some emergency supply of water needs to be developed.
Water Collection:
Wells: Water can only be moved by suction for an equivalent head of
about 20' After this cavitation occurs, that is the water boils off
in tiny bubbles in the vacuum created by the pump rather than being
lifted by the pump. At best no water is pumped, at worst the pump is
destroyed. Well pumps in wells deeper than this work on one of three
principles:
1) The pump can be submerged in the well, this is usually the case for
deep well pumps. Submersible pumps are available for depths up 1000
feet.
2) The pump can be located at the surface of the well, and two pipes
go down the well: one carrying water down, and one returning it. A
jet fixture called an ejector on the bottom of the two hoses causes
well water to be lifted up the well with the returning pumped water.
These pumps must have an efficient foot valve as there is no way for
them to self prime. These are commonly used in shallow wells, bust
can go as deep as 350 feet . Some pumps for use the annular space
between one pipe and the well casing as the second pipe this requires
a packer (seal) at the ejector and at the top of the casing.
3) The pump cylinder can be located in the well, and the power source
located above the well. This is the method used by windmills and most
hand pumps. A few hand pumps pump the water from very shallow wells
using an aboveground pump and suction line. A variety of primitive,
but ingenious, pump designs also exist. One uses a chain with buckets
to lift the water up. Another design uses a continuous loop rope
dropping in the well and returning up a
small diameter pipe Sealing washers are located along the rope, such
that water is pulled up the pipe with the rope. An ancient Chinese
design used knots, but modern designs designed for village level
maintenance in Africa use rubber washers made from
tires, and will work to a much greater depth.
Obviously a bucket can be lowered down the well if the well is big
enough, but this won't work with a modern drilled well. A better idea
for a drilled well is to use a 2' length or so of galvanized pipe with
end caps of a diameter that will fit in the well casing. The upper
cap is drilled for a screw eye, and a small hole for ventilation. The
lower end is drilled with a hole about half the diameter of the pipe,
and on the inside a piece of rigid plastic or rubber is used as a
flapper valve. This will allow water to enter the pipe, but not exit
it. The whole assembly is lowered in the well casing , the weight of
the pipe will cause it to fill with water, and it can then be lifted
to the surface. The
top pipe cap is there mostly to prevent the pipe from catching as it
is lifted.
Springs: Springs or artesian wells are ideal sources of water. Like
a conventional well, the water should be tested for pathogens, VOCs
(Volatile Organic Compounds such as fuel oil or benzene), pesticides
and any other contaminants found in your area. If the source is a
spring it is very important to seal it in a spring box to prevent the
water from becoming contaminated as it reaches the surface. It is
also important to divert surface runoff around the spring box. As
with a well, you will want to periodically treat the spring box with
chlorine, particularly if the spring is slow moving. The spring my
also be used for keeping food cool if a spring house is build. If
this is the case, it is still recommended to build a spring box inside
the house to obtain potable water.
Surface water. Most US residents served by municipal water systems
supplied with surface water, and many residents of underdeveloped
countries rely on surface water. While surface water will almost
always need to be treated, a lot of the risk can be
reduced by properly collecting the water. Ideal sources of water are
fast flowing creeks and rivers which don't have large sources of
pollution in their watershed. With the small amounts of water need
by a family or small group, the most practical way to
collect the water is though a infiltration gallery or well. Either
method reduces the turbidity of the collected water making it easy for
later treatment.
Water Purification:
Contaminants:
Heavy Metals:
Heavy metals are only a problem is certain areas of the country. The
best way to identify their presence is by a lab test of the water or
by speaking with your county health department. Unless you are down
stream of mining trailings or a factory, the problem
will probably affect the whole county or region. Heavy metals are
unlikely to be present in sufficient levels to cause problems with
short term use.
Turbidity
Turbidity refers to suspended solids, i.e. muddy water is very turbid.
Turbidity is undesirable for 3 reasons 1)aesthetic considerations 2)
Solids may contain heavy metals pathogens or other contaminants, 3)
turbidity degreases the effectiveness of water
treatment techniques by shielding pathogens form chemical or thermal
damage, or in the case of UV treatment, absorbing the UV light itself.
Organic compounds
Water can be contaminated by a number or organic compound such as
chloroform, gasoline, pesticides, and herbicides. These contaminants
must be identified in a lab test. It is unlikely ground water will
suddenly become contaminated unless a quantity of
chemicals is allowed to enter a well or penetrating the acquifer. 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 swings in chemical
levels due to differences in rainfall, seasonal crop cultivation, and
industrial effluent levels
Pathogens
Protozoa: Protozoa cysts are the largest pathogens in drinking
water, and are responsible for many of the waterborne disease cases in
the US. Protozoa cysts range is size from 2 to 15 um (a micron is one
millionth of a meter), but can squeeze through smaller openings. In
order to insure cyst filtration, filters with a absolute pore size of
1 um or less should be used. The two most common protozoa pathogens
are Giardia Lamblia (Giardia) and Cryptosporidium (Crypto). Both
organisms have caused numerous deaths is recent years in the US, the
deaths occurring in the young and elderly, and the sick and immune
compromised. Many deaths were a result of more than one of these
conditions. Neither disease is likely to be fatal to a healthy adult,
even
if untreated. For example in Milwaukee in April of 1993, of 400,000
who were diagnosed with crypto, only 54 deaths were linked to the
outbreak, 84% of whom were AIDS patients. Outside of the US and other
developed countries, protozoa are responsible for many cases of Amebic
dysentery, but so far this has not been a problem in the US, due to
better wastewater treatment. This could change during a survival
situation. Tests have found Giardia and/or crypto in up to 5% of
vertical wells and 26% of springs in the US.
Bacteria: 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 .2 to
.6 um, and a .2 um filer 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.
Even in the US, E.coli is frequently found to contaminate water
supplies. Fortunately E. coli is relatively harmless as
pathogens go, and the problem isn't so much with E. coli found, but
the fear that other bacteria may have contaminated the water as well.
Never the less, dehydration from diarrhea caused by E. coli has
resulted in fatalities.
Viruses: 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 um, and are too small to be filtered out
completly by a mechanical filter (better filter designs, such as the
Katadyn may remove as much as 99% of viruses.) 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 US, 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 acase can be made for eliminating them now.
Physical Treatment
Heat Treatment
Boiling is one guaranteed 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 and extra minute for
every 1000 feet of elevation.. If one wishes to do this, a pressure
cooker would allow the water to be kept at boiling with out loosing
the heat to evaporation.
One reason for the long period of boiling my be to inactivate
bacterial spores (which can survive boiling), but these spore are
unlikely to be waterborne pathogens. Hepitatis A, a viral pathogen,
may not be inactivated with much of a safety margin unless water is
allowed to boil for 1 minute.
African aid agencies figure it takes 1 kg of wood to boil 1 liter of
water. Hardwoods and efficient stoves would improve on this.
Water can also be treated at below boiling temperatures, if contact
time is increased. A
commercial unit has been developed that treats 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
deg. F for 15 seconds.. Heat exchangers recover most of the
energy used to warm the water. Solar pasteurizers have also been
built that would heat three gallons of water to 65 deg. 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 most common use is aboard boats to produce
fresh water from salt water. 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 a imperfection in the
membrane that could allow molecules or whole pathogens to pass
through.
Using reverse to desalinate sea water osmosis requires considerable
pressure (1000 psi) to operate, and for a long time only electric
models were available. Competing for a contract to build a hand
powered model for the Navy, Recovery Engineering designed a model
that could operate by hand, using the waste water (90 percent of the
water is waste water, only 10% passes through the filter) to
pressurize the back side of the piston. The design was later acquired
by PUR. While there is little question that the devices
work well, the considerable effort required to operate one has been
questioned by some survival experts such as Michael Greenwald, himself
a survivor of a shipwreck. On the other hand the people who have
actually used them on a liferaft credit the availability of water from
their PUR watermaker for their survival.
PUR manual watermakers are available in two models: The Survivor 06
($500) produces 2 pints per hour, and the Survivor 35 ($1350)
produces 1.4 gal/hr. The latter model is also available as the Power
Survivor 35 ($1700), which produces the same
water volume from 4 Amps of 12 VDC, and can be disconnected and used
as a hand held unit A number of manufactures, including PUR, make DC
powered models for shipboard use.. PUR recommends replacing the O
rings every 600 hours on its handheld units, and a kit is available to
do this. Estimates for membrane life vary, but units designed for
production use may last a year or more. Every precaution should be
taken to prevent petroleum products from contacting the membrane as
they will damage or destroy the membrane. The prefilter must also be
regularly changed, and the membrane may need to be treated with a
biocide occasionally
Reverse osmosis filter are also available that will use normal
municipal or private water pressure to remove contaminates from water,
as long as they aren't present in the levels found in sea 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 and 2) If simple distillation is used, chemical contaminants
with boiling points below water (such as ethlyene gycol in automotive
antifreeze) will be condensed along with the water. Distillation is
most commonly used to remove dissolved minerals and
salts from water.
The simplest form of a distillation is a solar still. A solar still
uses solar radiation to evaporate water below the boiling point, and
the cooler ambient air to condense the vapor. The water can be
extracted from the soil, vegetation piled in the still, or
contaminated water (such as radiator fluid or salt water) can be added
to the still. While per still output is low, they are an important
technique if water is in short supply
Other forms of distillation require a concentrated heat source to boil
water which is then condensed. Simple stills use a coiling coil to
return this heat to the environment. These can be improvised with a
boiler and tight fitting lid and some copper tubing (Avoid
using lead soldered tubing if possible). FEMA suggests that, in an
emergency, a hand towel can be used to collect steam above a container
of boiling water. More efficient distillations plants use a vapor
compression cycle where the water is boiled off at
atmospheric pressure, the steam is compressed, and the condenser
condenses the steam above the boiling point of the water in the
boiler, returning the heat of fusion to the boiling water. The hot
condensed water is run through a second heat exchanger which
heats up the water feeding into the boiler. These plants normally use
an internal combustion engine to run the compressor. Waste heat from
the engine, including the exhaust, is used to start the process and
make up any heat loss. This is the method used
in most commercial and military desalinization plants
Inflatable solar stills are available from marine supply stores, but
avoid the WW2 surplus models, as those who have used them have had a
extremely high failure rate. Even new inflatable solar stills may only
produce from 30-16 oz under actual conditions,
compared to a rating of 48 oz/day under optimum conditions.
Desanilation kits are not the same as solar stills. These kits
contain a tablet that binds the salt up. One tablet will treat X
amount of water
Jade Mountain also offers the following portable models in travel
cases:
Traveler (WC106) 1 gpd,23 lb., 24x26x10 folded $695
Base Camp (WC107) 2 gpd,51 lb., 48x48x4 folded $895
Safari (WC108) Ruggedized version of the Base Camp, 48X48X5 $1095
Microfilters
Microfilters 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, which many experts do not consider to be a problem
in north America. Despite this the Katadyn microfilter has seen
considerable use around the world by NATO-member militaries, WHO,
UNHCR, 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. The Sweetwater Guardian suggests using
a freezer for short term storage.
Many microfilters may include silt pre filters, activated charcoal
stages, 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 with a coffee filter 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. If you are depending on the stage for
filtering the water you will have to keep up with how much water
passes through it.
New designs seem to be coming out every month. The best selling
brands seem to be the PUR, and Sweetwater Guardian. The Katadyn
doesn't sell as well to outdoor enthusiasts due to its high cost, but
for years it was state of the art for water purification and still has
a loyal following, especially among professionals in relief work.
Below is the data on a
few of the more common units, for a excellent field test of some
common units, see the December 96 issue of Backpacker magazine.
Note that the first price is for the filter, the second for the
replacement filter. The weight is from manufacturers literature if it
was not listed in the Backpacker article. Filter life is from
manufacturers literature and should be taken with a grain of salt.
Basic Designs Ceramic Filter Pump ($29/$15, 8 oz.) Cheap flimsy
filter, claimed to filter up to 500 gallons with a .9 um ceramic
filter. Not EPA rated, may not have passed independent lab tests,
prone to damage, filter element must be submerged in water.
General Ecology- First Need Deluxe ($70/$30, 20 oz) This filter uses
a structured matrix micro strainer, though General Ecology won't
reveal what the structure is. It has survived independent lab tests,
and filters particles to 4 um, while actually removing
viruses (the only filter capable of doing this) through electrostatic
attraction. The filter cartridges can't be cleaned (other than by
backflushing), but are good for 100 gallons. Pump design isn't the
best. Other models are available from the manufacturer.
Katadyn PF ($250/$145,22.7 oz). The original microfilter using a .2
um silver impregnated ceramic candle. An extremely thick filter
allows it to be cleaned many times for up to 14,000 gallons capacity.
While the Katadyn seems well made, one reader of this list reported
breaking the candle, and Backpacker Magazine broke the case during a
field test. The pump, while probably indestructible, is somewhat slow
and hard to use, requiring 20 lbs. of force on a small handle. The PF
also lacks a output hose as
the Katadyn engineers felt if would be a source of contamination.
Katadyn Combi ($185/$75 (ceramic)/$19 (carbon), 29 oz) A cheaper
version of the PF incorporating both ceramic and carbon stages. Much
faster filter than the PF. Katadyn Minifilter ($139/$59, 8.3 oz) A
smaller and cheaper version of the PF, easier to pump, but generally
not well received. Good for 200 gallons.
Katadyn Expedition ($680,$77 13 lb.) Similar filter to the PF (exact
same cartridge as the Drip Filter Below) but designed for much higher
production, stainless steel case with spade type D handle, produces
.75 gpm. Filter good for 26000 gallons.
Katadyn Drip Style Filter ($240, $77 12.5 lb.) Filter elements
similar to those in the PF are mounted vertically in top 3 gallon
plastic bucket, water drips through filters into second 3 gallon
bucket with faucet. 1 qt, per hour with the 2 filters included, a
third filter can be added to increase rate 50%. Some units are being
shipped with 3 filter. Each filter is good for 13000 gallons. The
mounting hardware for the filters is available for $10 to allow you to
make your own filter of what ever size is needed. Each mounting kit
requires a 1/2" hole in the bottom of the raw water container.
Katadyn Siphon Filter ($92, 2 lb.) Similar design to PF filter
element, but a siphon hose replaces the pump, filters 1-2 quarts per
hour (allow 1 hour for the filter to "prime" itself via capillary
action), but multiple filters can be used in the same container.
Collection vessel must be lower than raw water container. Good for
13000 gallons
MSR Miniworks ($59/$30, 14 oz) MSR's smaller filter, using a .3 um
ceramic element. Pump is well designed, and easy to use. Main
drawback is that the clean water discharge is from the bottom of the
filter, and no hose is provided. While the bottom is threaded for a
Nalagene bottle, it is a pain in the butt to fill a canteen or 2
litter bottle. Claimed to filter 100 gallons, Backpacker Magazine
feels this may be one of the few filters without a grossly inflated
rating
MSR Waterworks ($140/$30/$30, 17 oz) MSR's first filter with a .2
micron ceramic, membrane stage and a carbon stage. Other wise similar
to the Miniworks.
PUR Pioneer ($30/$4, 8 oz), newly introduced low end microfilter. .5
um, 1 lpm filter rate, 12 gallon capacity
PUR Hiker ($50/$20, 12 oz) PUR's microfilter only design, filters to
.5 um, 200 gallon capacity. Well liked, as are the other PUR filters.
Very compact. 200 gallon capacity
PUR Scout ($70/$35/$15, 12 oz) Combines a iodine resin stage , a 1.0
um filter, and a activated charcoal filter. 200 gallon capacity
PUR Explorer ($130/$45, 22 oz) PUR's top of the line model. bulky,
but well made, with a high output (1.4 lpm, faster than any of the
hand held models listed and one of the easiest to pump) Has a 1.0 um
filter plus a iodine resin stage, 300 gallon capacity
Sweetwater Walkabout($35/$13, 8.5 oz.) Sweetwater's low end filter,
0.2 um, .7 lpm, 100 gal capacity
Sweetwater Guardian ($60/$20, 11 oz) Uses a glass fiber and carbon
filter, filters to ..2 um, claimed to last for 200 gallons. A iodine
resin stage can be added that will kill viruses, and will last for 90
gallons. Pump is well designed, but it takes a few seconds to
pull a captive pin to fold for storage. Available in white or OD.
Timberline Eagle ($20/$13, 8 oz) At 1 um, this filter only does
protozoa, but is much easier to pump. lighter, and cheaper. Filter is
attached to pump, and must rest (but doesn't have to be submerged) in
water to be purified. Looks flimsy, but seems to hold
up. Claimed to last for 100 gallons.
It is also possible to build your own microfilter using diatomaceous
earth, sold for swimming pool filters (DE). Usually pressure is
required to achieve a reasonable flow rate. A DE filter will remove
turbidity as well as pathogens larger than 1 um.
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 24 inches 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 .2 m^3/m^2 of surface area). The
filter can be cleaned several times before the sand has to be
replaced.
Slow sand filter construction information: 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 form 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, at least 1.5 meters high. 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. The slots or the gravel
should be no closer than 20 cm to the walls. again to prevent the
water from bypassing the sand.
The sand for a SSF needs to be clean and uniform, and of the correct
size. The sand can be cleaned in clean running water, even if it is
in a creek. The ideal specs on sand are effective size (sieve size
through which 10% of the sand passes) between .15 and .35
mm, uniformity coefficient (ratio of sieve sizes through which 60%
pass and through which 10% pass) of less than 3, Maximum size of 3 mm,
and minimum size of 0.1 mm.
The sand is added to a SSF to a minimum depth of 0.6 meters.
Additional thickness will allow more cleanings before the sand must be
replaced. .3 to .5 meters of extra sand will allow the filter to work
for 3-4 years An improved design uses a geotextile
layer on top of the sand to reduce the frequency of cleaning. The
outlet of a SSF must be above the sand level, and below the water
level. The water must be maintained at a constant level to insure an
even flow rate throughout the filter. The flow rate can be
increased by lowering the outlet pipe, or increasing the water level..
One common idea for maintaining the water level is to use a elevated
raw water tank or pump, and a ball valve from a toilet .
While the SSF will begin to work at once, optimum treatment for
pathogens will take a week or. During this time the water should be
chlorinated if at all possible (iodine can be substituted). After the
filter has stabilized, the water should be safe to drink, but
chlorinating of the output is still a good idea, particularly to
prevent recontamination.
As the flow rate slows down the filter will have to be cleaned by
draining and removing the top few inches of sand. If a geotextile
filter is used, only the top 1/2" may have to be removed. As the
filter is refilled, it will take a few days for the biological
processes to
reestablish themselves.
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 a 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. The only filter I know of that uses only
activated charcoal, and doesn't required pressurized water is the
Water Washer ($59) Available from the
Survival Center.
Activated charcoal can be used in conjunction with chemical treatment.
The chemical (iodine or chlorine) will kill 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.
Activated charcoal can be made at home, though the product will be of
varying quality compared to commercial products. Either purchased or
homemade charcoal can be recycled by burning off the molecules
adsorbed by the carbon (The won't work with heavy metals of course.)
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.
Sol-Air Water Treatment: If sufficient desolved oxygen is available,
sunlight will cause the temporary formation of reactive forms of
oxygen such as hydrogen peroxide and oxygen free radicals.. This form
of water treatment is called solar photooxidative disinfection or
sol-air water treatment. Sol-Air water treatment has been shown to
dramatically reduce the level of fecal coliform bacteria. There is
some evidence that other bacteria and viruses may be affected also.
While not as reliable as other methods, it does offer a low tech
solution in emergencies. Sol-Air treatment requires bright sunlight,
and has been shown to be effective when ever the sun causes a distinct
shadow to be cast. Exposure to 4.5 hours of bright sunlight has been
shown to cause a thousand fold reduction in fecal coliforms in lab
tests
In order for Sol-Air to be effective, oxygen must be present.
Experiments have shown that shaking a bottle filled 3/4 with air will
restore oxygen levels to near saturation. As the treatment continues,
some of the oxygen will come out of solution, while other oxygen will
be consumed by the killed pathogens, so the shaking should be repeated
every few hours. Data shows that maximum activity occurs when the
water temperature is above 50 deg. C (122 deg. F) , so this method may
be unsuitable in colder climates unless special solar collectors are
used.
Either glass or plastic bottles may be used. Plastic bottles will
allow short wave ultraviolet radiation to pass, increasing the rate of
microbial inactivation, but may yellow with age, reducing light
transmission, and may leach plasticizers into the water at the
elevated temperatures that will occur. The leaching out of
plasticizers can be reduced by using bottles of PET (polyethlyene
terephtalate) rather than PVC. Glass bottles on the other hand are
more durable. Research has used bottles with 2 liters of capacity,
but if the water is free of turbidity, larger containers can be used.
Plastic bag, or some sort of flat glass container represent the ideal
container as this maximizes the solar energy received per ounce of
water.
Bottles should be filed 3/4 full in the early morning with water as
free of turbidity as possible. After capping the bottles the should
be shaken vigorously for a few minutes then placed upright in the sun,
where they will be not be shaded later in the day. The shaking should
be repeated at least three times during the day. At the end of the
day the water should be reasonably freed of bacteria, though it is
most practical to let the water cool for consumption the following
day. Each day a new batch should be treated due to the lack of a
residual disinfected.
After consumption of the water the bottle should be air dried to
prevent algae growth with continual use.
Improvised Mechanical Filter: If the materials aren’t available to
build a slow sand filter, or some other means of water treatment is
preferred, it may still be advantageous to mechanically filter the
water before treating it with chemicals or passing through a
microfilter. Generally the idea is to allow the water to flow as
slowly as possible through a bed of sand. In a municipal water
treatment plant this is called a rapid sand filter. The particular
design below is included, because the designer, a research engineer at
Oak Ridge National Laboratories, found it particularly effective at
removing fallout from water. The filter will do little or nothing to
remove pathogens, though removing suspended solids allow others water
treatment methods to work more effectively.
Expedient water filter, from Nuclear War Survival Skills, Cresson
Kearny, ORNL
1) Perforate the bottom of a 5 gallon bucket, or similar container
with a dozed nail holes even spread over a 4” diameter circle in the
center of the container.
2) Place a 1 and 1/2” layer of small stones or pebbles in the bottom
of the can. If pebbles aren’t available, marbles, clean bottle caps,
twisted coat hangers or clean twigs can be used.
3) Cover the pebbles with one thickness of terrycloth towel, burlap
sackcloth, or other porous cloth. Cur the cloth in a roughly circular
shape about three inches larger then the diameter of the can.
4) Take soil containing some clay (pure clay isn’t porous enough, pure
sand is to porous) from at least 4” below the surface of the ground.
(nearly all fallout particles remain near the surface except after
disposition ion sand or gravel.
5) Pulverize the soil, then gently press it in layers over the cloth
that covers the pebbles, so that the cloth is held snugly against the
walls of the can. The soil should be 6-7” thick.
6) Completely cover the surface of the soil layer with one thickness
of fabric as porous as a bath towel. This is to keep the soil from
being eroded as water is being poured into the filter. A dozen small
stones placed on the cloth near it’s edges will secure it adequately.
7) Support the filter on rocks or sticks placed across the top of a
container that is larger then the filter can (such as a dishpan)
The contaminated water should be poured into the filter can,
preferably after allowing it to settle as described below. The
filtered water should be disinfected by some method.
If the 6 or 7 inches of filtering soil is a sandy clay loam, the
filter will initially deliver about 6 quarter/hour. If the filter is
any faster than this then the fabric layer needs to be removed and the
soil compressed more. The filtering rate will drop over time as the
filter begins to clog up. When this happens the top 1/2” of soil can
be removed to increase the filtering rate. After 50 or so quarts, the
filter will need to be rebuild with fresh soil.
As with any filter, optimum performance will be achieved if sediment
in the water will be allowed to settle out before passing the water
through the filter
If the water is contaminated with fallout, clay can be added to help
the fallout particles to settle out. The procedure is as follows:
Fill a bucket or other deep container 3/4 full with contaminated
water. Dig pulverized clay or clayey soil from a depth of four or
more inches below ground surface and stir it into the water. Use
about 1 inch of dry clay or clayey soil for every 4” depth of water.
Stir until practically all of the clay particles are suspended in the
water. Let the clay settle for at least 6 hours. This will carry the
fallout particles to the bottom and cover them. Carefully dip out or
siphon the clear water and disinfect it.
Chemical Treatment
Chlorine: Chlorine is familiar to most Americans as it is used to
treat virtually all municipal water systems in the United States. For
a long time chlorine, in the form of Halazone tablets, was used to
purify small batches of water for campers and military troops. Later
questions emerged about the effectiveness of Halazone, and in 1989,
Abbot labs pulled it off the market. If Halazone Tablets are
encountered outside the US, the nominal shelf like is 6 months, and
the dosage is 2 tabs per liter. Until recently, there was no chlorine
product designed for wilderness/survival use available in the US.
Chlorine has a number of problem 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 superchlorination, 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
Chlorine, like iodine, will not kill Cryptosporidia.
Methods of chlorine treatment:
Bleach: Ordinary household bleach (such as Clorox) in the US
contains 5.25% sodium hypochlorite (NaOCL) and can be used to purify
water if it contains no other active ingredients, scents, or
colorings. Bleach is far from an ideal source due to its bulkiness
(only 5% active ingredient), and the instability over time of the
chlorine content in
bleach. Chlorine loss is farther increased by agitation or exposure
to air. One source claims chlorine loss from a 5% solution at 10%
over 6 months if stored at 70 deg. F. Nevertheless, this may be the
only chemical means available to purify water, and it is
far better than nothing. Normal dosage is 8 drops (.4 ml) per gallon.
Allow the treated water to sit for 30 min., and if there isn't a
slight chlorine smell, retreat. Note: USP standard medicine droppers
are designed to dispense .045-.055 ml per drop. Use of other
solvents or some chemicals can change this. The dropper can be
calibrated against a graduated cylinder for greater accuracy.
Some small treatment plants in Africa produce their own sodium
hypochlorite on site from the electrolysis of brine. Power demands
range from 1.7 to 4 kWh per lb. of NaOCL. 2 to 3.5 lbs. of salt are
needed for each pound of NaOCL. These units are fairly
simple and are made in both the US and the UK. Another system,
designed for China, where the suitable raw materials were mined or
manufactured locally, used a reaction between salt, manganese dioxide,
and sulfuric acid to produce chlorine gas. The gas was then allowed
to react with slaked lime to produce a bleaching powder that could
then be used to treat water. A heat source is required to speed the
reaction up.
AquaCure: Designed for the South African military, these tablets
contain chlorine and alum. The alum causes the suspended solids to
flocculate and the chlorine adds 8 PPM chlorine. This is a great way
to treat turbid water, though it will leave a lot of chlorine in
clear water (The one tablet/L could be halved for clear water.)
The US distributor for Aqua Cure is:
Safesport Manufacturing
Box 11811
Denver, CO 80211
1 800 433 6506
Bleaching Powder (Chlorinated Lime) can also be purchased and used as
a purification means if nothing else is available. Bleaching powder
is 33-37% chlorine when produced, but losses its chlorine rapidly,
particularly when exposed to air, light or
moisture.
Calcium Hypochlorite: Also known as High Test Hypochlorite. Supplied
in crystal form, it is nearly 70 % available chlorine. One product,
the Sanitizer (formally the Sierra Water Purifier) uses these crystals
to super chlorinate 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. The US
military
and most aid agencies also use HTH to treat their water, though a test
kit, rather than superchlorination, is used to insure enough chlorine
is added. This is preferable for large scale systems as the residual
chlorine will prevent recontamination
Usually bulk water treatment plants first dilute to HTH to make a 1%
working solution at the rate of 14g HTH per liter of water. While
testing to determine exact chlorine needs are preferable, the solution
can be used at the dose rate of 8 drops/gallon, or for
larger quantities, 1 part of 1% solution to 10,000 parts clear water.
Either of these doses will result in 1 PPM chlorine and may need to be
increased if the water wasn't already filtered by other means.
When test kits are available, the WHO standard is a residual chlorine
level of 0.2 to 0.5 mg/l after a 30 min. contact time. The may
require as much as 5 mg/l of chlorine to be added to the raw water.
Iodine:
Iodine's use as a water purification method emerged after WW2, when
the US 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. However 4 individuals with
preexisting
cases of hyperthyroidism became more symptomatic while consuming the
water.
Nevertheless. experts are reluctant to recommend iodine for long term
use. 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.
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 deg. 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 um), 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
Sources of Iodine:
Tincture of Iodine:
USP tincture of iodine contains 2% iodine and 2.4% sodium iodide
dissolved in 50% ethyl alcohol. For water purification use, the
sodium iodide has no purification effect, but contributes to the total
iodine dose. Thus it is not a preferred source of iodine, but
can be used if other sources are not available. 0.4 cc's (or 8 drops)
of USP tincture (2% iodine) added to a liter of water will give the 8
mg/l (same as 8 PPM). If the iodine tincture isn't compounded to USP
specs, then you will have to calculate an equal
dose based on the iodine concentration.
Lugol's solution:
Contains 5% iodine and 10% potassium iodide. 0.15 cc (3 drops) can be
added per liter of water, but 3 times more iodine is consumed compared
to sources without iodide.
Betadyne (povidone iodine) Some have recommended 8 drops of 10%
povidone iodine
per liter of water as a water treatment method, claiming that at low
concentrations povidone iodine can be regarded as a solution of
iodine. One study indicated that at 1:10,000 dilution (2
drops/liter), there was 2 PPM iodine, while another study resulted
in conflicting results. However, at 8 drops/liter, there is little
doubt that there is antimicrobial effect. The manufacturer hasn't
spent the money on testing this product against EPA standard tests,
but in other countries it has been sold for use in field water
treatment.
Kahn-Vassher solution. By adding a sufficient amount of iodine
crystals to a small bottle, an almost unlimited supply of saturated
iodine solution can be produced. As long as crystals remain in the
bottle, the solution is saturated. Concentration of the iodine is
dependent of temperature, either condition at ambient temperature can
be assumed, or commercial models such as Polar Pure incorporate a
liquid crystal thermometer to determine dose
One criticism of this method is the chance of decanting iodine
crystals into the water being treated. This isn't that much of a
problem as iodine is very weakly toxic, but the polar pure
incorporates a collar into the neck of the bottle to help prevent
this. Another disadvantage to this method is that the saturated
iodine solution must be kept in glass
bottles, and is subject to freezing, but this is hardly an
insurmountable problem. Freezing, of course, doesn't affect the
crystals.
This is the method I use, but I do use the commercial polar pure
bottle, and refill it as necessary with USP crystals. During a
crisis, or extended camping trips I would microfilter the water first,
so a much lower dose of iodine is needed.
With the Polar Pure bottle, dosage information is provided. Otherwise
a 1 oz bottle can be used to carry the solution. The bottle is filled
with water after use. At the next use, 1/2 of the supernate (15 cc)
is poured off into a liter of water. at 68 deg. F, this will yield
a dose of 9 mg/l. To use this method with a microfilter to get a 0.5
PPM concentration,
either large batches of water need to be treated (1/2 oz to 4.5
gallons would be .5 PPM), or a TB syringe or medicine dropper can be
used to measure doses. A USP medicine dropper should give 20 drops
per ml.
Iodine can also be dissolved in alcohol to make a solution of known
concentration . I am not aware of any commercial products, but a
pharmacy could compound one for you, or you could do it your self.
One suggested formula is 8g iodine/100 cc ethyl alcohol which yields
enough solution to disinfect 250 gallons of water. at the rate of .1
cc (2 drops)/liter to give a concentration of 8 mg/l
Tetraglycine hydroperiodide (e.g. Potable Aqua) This is the form of
iodine used by the US military for field treatment of water in canteen
sized batches. Usual dose in one tablet per quart of water to give a
concentration of 8 mg/l. Two tablets are used in
cloudy or cold water or contact time is doubled. The major downside
of this product is that the product will loose its iodine rapidly when
exposed to the air. According to the manufacturer, they have a near
indefinite life when sealed in the original bottle, but
probably should be discarded within a few months of opening. The
tablets will change color from gun metal gray to brown as they lose
the iodine, and you should see a brown tint to the water after
treating.
Iodine Resin Filter: Some commercial microfilter incorporate a iodine
resin stage to kill viruses and bacteria, with out putting as much
iodine in the water as if it had been added to the raw water. A few
products rely exclusively on an iodine resin stage. Downside of these
filters are their fragile nature, dependency of effectiveness on flow
rate and the
inability to identify when they need to be discarded. If you are
going to use one where the water is known to be contaminated with
viruses, then one of the better known brands such as the PUR or
Sweetwater Viraguard is recommended. More than one pass
through the filter may be necessary in cold weather.
Resins do have the advantage of producing less iodine in the water for
the same antimicrobal effect as for the most part, they only release
iodine when contacted by a microbe. The downside is that physical
contact between the microbe and the resin is
needed.
Silver: Silver has been suggested by some for water treatment and may
still be available outside the US. Is 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 accumulation 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.
Katadyne markets a silver based water treatment product called
Micropur. The manufacturer recommends a 2 hr contact time at a dose
of 1 tab per liter and states the product is "For the Disinfection and
storage of clear water. Reliably kill bacterial agents of enteric
diseases, but not worm eggs, ameba, viruses. Neutral to
taste...insure protection against reinfection for 1-6 months." The
following forms are available:
Micropur Tablets MT1 1 tablets/qt 25 gal
MT2 1 tablet/5qts 62.5 gal
Micropur Fluid MF 75 10 drops/gal 75 gals
MF250 “ “250 gals
Micropur Crystal MC250 1 packet/gal 250 gal
MC 2500 1 spoon/25 gal 2500gal
MC12500 1 spoon/250 gal 12500 gal
Potassium Permanganate:
Potassium Permanganate is no longer commonly used in the developed
world to kill pathogens. It is much weaker than the alternatives,
more expensive, and leaves a objectionable pink or brown color. 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:
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%
inactivation of rhinovirus with a 1.5% solution in 24 minutes.
Hydrogen Peroxide is more effective against bacteria, though Fe+2 or
Cu+2 needs to be present as a catalyst to get a reasonable
concentration-time product.
Coagulation/Flocculation agents:
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
form 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 the 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 know 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 need 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.
Treatments requiring electricity:
Ozone: Ozone is used extensively in Europe to purify water. Ozone, a
molecule composed of 3 atoms of oxygen rather than the 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 O2. (one source quotes a half life of only 120 minutes
in distilled water at 20 deg C). Ozone is expected to see increased
use in the US as a way to avoid the production of
Trihallomethanes. 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 and 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
colloidally suspended particles. This causes 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 efficacy of UV 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. Any
purchased UV filter should be checked to insure it at least complies
with the 1966 HEW standard of 16 mW.s/cm^2 with a maximum water depth
of 7.5 cm ANSI/NSF require 38 mWs/cm^2 for primary water treatment
systems. This level was chosen to give better than 3 log (99.9%)
inactivation of Bacillus subtillis. This level is of little use
against Gairdia, and of no use against Crypto.
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, and 3) Lack of a Residual Disinfectant.
Questionable or Dangerous methods of water treatment:
1) Aerobic 07. Also sold as Aerobic Oxygen. The company refuses to
release the disinfectant. It maybe chlorine dioxide, a well known, if
somewhat unstable, disinfectant. The company has shown company
sponsored tests showing effectiveness against viruses and bacteria
(but not against Giardia). No independent testing has been performed,
nor has anybody provided concentration-time data for the product.
2) Survival Straw: This product claims to destroy and eliminate
impurities including bacteria, protozoa. fungi, chemicals and heavy
metals using a matrix of metal alloy. The manufacturer claims the
product’s media meets EPA and FDA specs, which is no indication of
the filter’s effectiveness. The filter violates a number of laws of
physics since it claims that it destroys heavy metals and pathogens
without filtering them.
(
geocities.com/mark_l_anderson)