Water Treatment in Remote and Austere Environments

 21st October 2022

Introduction

Working in remote areas of the world and some under-developed regions increases the likelihood of exposure to untreated or contaminated water which must be treated before it can be safely consumed or used for wound cleaning or irrigation.  Natural surface water may be contaminated with organic or inorganic material from land and vegetation or industrial chemical pollutants (1).  In developed regions, natural disasters and major incidents can affect domestic water supplies, rendering available water undrinkable unless treated.  Even in seemingly ‘pristine’ areas of unspoiled beauty, the presence of humans, natural wildlife and cattle pose a risk almost everywhere (with the possible exception of snowmelt and glacial water) where faecal matter may contain Escherichia coli (E Coli) and Salmonellae (2,3).

Enteric (relating to the intestines – so, typically ingested) pathogens are the leading cause of diarrhoea and vomiting among travellers.

Microbiologic aetiology

Waterborne pathogens that may cause gastrointestinal illness fall into four major categories:

• Bacteria -      free-living cells that can live inside or outside a body

• Viruses -       non-living collection of molecules that need a host to survive.

• Protozoa -    single-celled organisms, like bacteria, but contain a nucleus and other cell structures, making them similar to plant and animal cells.

• Helminths - worm-like parasites that survive by feeding on a living host

Definitions

• Screening - the process of removing coarse material from heavily contaminated water, prior to secondary or tertiary treatment methods.

• Disinfection - the process of inactivation or removal of microorganisms.

• Sterilisation -  the process of killing all microorganisms.

• Safe drinking water – (also referred to as ‘potable’) does not mean that water is sterile, rather, the concentration of microorganisms is too low to expect any risk to human health.

• Conservation - describes procedures which prevent microbiological recontamination of previously “safe” water.

• Turbidity – the cloudiness or haziness of a fluid due to suspended particles and matter.

Water treatment techniques

There are several techniques that can be utilised to treat water, usually requiring a two-step process to ensure completely safe drinking water and the techniques used will be determined by a number of factors including, but not limited to;

• Available resources

• The volume of water required

• The speed of the process(es)

• The duration of required drinking water (hours, days or weeks?)

• The condition of the water, prior to treatment

How likely is illness due to contaminated water?  

There are a number of factors at play, including, how contaminated the water is (almost always a function of the presence, proximity and concertation of humans and animals) as well as the efficacy of the treatment (4, 5) and without treatment, waterborne diseases can spread rapidly, resulting in large-scale disease and, in some cases, death.  (6, 7).  Microorganisms with a small infectious dose (eg, giardia, cryptosporidium, shigella spp, hepatitis A, E coli, and norovirus - the leading viral disease risk in water contaminated with human waste) may cause illness even from inadvertent drinking during water-based activities such as swimming (8).

Unfortunately, appearance, smell, and taste are not reliable indicators to estimate water safety.

Pre-treatment - Screening

Some water, especially heavily turbid water with sediment and other particulate matter will need to be pre-treated first.

• Particulate matter can clog filtration systems, rendering them ineffective or, in the worst case, inoperable.

• Particulate matter can reduce the effectiveness of chemical treatments (9-15).

Sedimentation

Sedimentation is the separation of suspended particles such as sand and silt that are large enough to settle rapidly by gravity.  Most microorganisms, especially protozoan cysts, also settle eventually, but this takes much longer (16).  Simply allowing the water to sit undisturbed for about one hour or until sediment has formed on the bottom of the container and then decanting or filtering the clear water from the top through a coffee filter or finely woven cloth will remove many larger particles from the water. 

Flocculation

Flocculation can remove smaller suspended particles and chemical complexes too small to settle by gravity – including microorganisms, heavy metals and some chemicals and minerals (15, 17).  Flocculation is a process that promotes the formation of larger particles by gentle mixing.

Alum (aluminium sulphate) is commonly used as a flocculant and is available on-line.  In an emergency, bleaching powder, baking powder, or even the fine white ash from a campfire can be used (18).

Add approximately 1 large pinch (1ml or 1/8 tsp) of flocculant per 4 litres (approximately 1 gallon) of water and stir or shake for about one minute to mix, and then agitate gently and frequently for at least 5 min to assist flocculation.  If the water is still cloudy, add more flocculent and repeat mixing.  After at least 30 min for settling, pour the water through a fine-woven cloth or paper filter. 

Although most microorganisms are removed with the flocculant, a second process of filtration and chemical disinfection (below) will need to be completed to ensure disinfection. 

Millbank bags

Millbank bags are densely woven canvas bags that can effectively screen even heavily turbid water although the process is painfully slow and the instructions have to be followed closely for success.

1. Soak the Millbank bag thoroughly prior to use.   If the material is not entirely saturated the capillary action will not work and the bags will simply retain the water.

2. Suspend the bag and fill the bag completely with water to be treated.

3. Allow the water to drain onto the floor (not into a container) until the water level has dropped down to the black line printed on the Millbank bag.   This ensures that the first volume of water to pass through the material has ‘rinsed’ the bag of the ‘dirty’ water which was used to initially soak it.   Collecting this water will contain any untreated water as well as surface contaminants.

4. Begin to collect the water from below the bag.  As this water is still untreated, only coarsely filtered, do not collect the water directly into a water bottle where cross-contamination can occur.  Collect into a clean container before treating with chemical disinfection and finally decanting into your water bottles.

5. It is widely stated that the Millbank bag should fill a one-litre container within 5 minutes although it is widely experienced that this can often take up to 20 minutes.   If the process takes longer than this the most common issue is not completely saturating the material prior to use.

6. Once the water has been treated the Millbank bag should be dried and stored away

7. After repeated use, the inside of the bag will need to be cleaned to remove the build-up of sediment and algae.

Advantages:

• Simple process.

• Low cost

• Low maintenance requirement.

• Suitable for large volumes.

Disadvantages:

• Time-consuming process

• Only removes fine-to-course particulates – a secondary or tertiary treatment is required.

• Instructions must be followed closely to ensure effective treatment.

Filtration

Mechanical or gravity filers can remove fine particulate matter including bacteria, protozoa, helminths – and depending on the pore-size – viruses. 

All filters eventually clog from suspended particulate matter (present even in clear streams), requiring cleaning or replacement of the filter.

Viruses often clump together and to other larger particles or organisms, resulting in an aggregate large enough to be trapped by the filter; in addition, electrochemical attraction may cause viruses to adhere to the filter surface (19-21).  Bacteria can also grow on filter media and potentially result in some bacteria in filtered water, but pathogenic bacteria and illness have not been demonstrated (22).

As a filter clogs, it requires increasing pressure to drive the water through it, which can force microorganisms through the filter or damage the filter.  A crack or eroded channel in a filter will also allow passage of unfiltered water.

Most portable filters are microfilters (0.2nm) that can readily remove protozoan cysts and bacteria but may not remove all viruses, which are much smaller than the pore size of most field filters. (23, 24)

Filters using ceramic elements with a pore size of 0.2 nm (such as the Katadyn Pocket Filter and the MSR MicroWorks) can reduce viral loads by more than 90%, but they are not adequate for complete removal of viruses (25).   Modern hollow-fibre designs with a pore size of 0.02 microns (such as the MSR Guardian) are required for complete microbial removal, including viruses; they can also remove colloids and some dissolved solids (26, 27) with the added benefit of increased flow rates of around 2.5L/min versus around 1L/min for ceramic cartridges. 

A disadvantage of hollow-fibre technology is that they cannot be exposed to freezing temperatures as moisture in the fibres will expand and damage the fibres.  Ceramic cartridges can be field-maintained more easily, cleaned and dried out.

Advantages:

• Relatively simple procedure

• Scalable procedure with products available for individual and group use.

• Can improve optical quality of the water by reducing turbidity.

• No bad taste or smell associated with chemical treatments.

• Some materials (adsorbing substances like activated carbon or nanocomposites) can remove bad tastes and smells as well as some toxins and chemicals.

• Hollow fibre filters or nanocomposite filters are lighter than classical ceramic filters. 

Disadvantages:

• Only hollow fibre filters with a pore size of 0.02 nm or nanocomposite filters can remove viruses so a second, chemical, treatment is recommended.   

• Clogging is a frequent problem.  The smaller the pores the safer the water, but also the more frequent the problem of clogging.   If possible, use clear water.  Do not increase the pressure of filtration as this can force microbes through the system and contaminate your water.

• Depending on the material, filters are breakable (especially ceramic), so handle the equipment with care.

• Most filters are damaged when freezing containing water remnants, resulting in microscopic cracks compromising disinfection.

• With some filters, the need for replacement is only indicated by gradually requiring increasing filtration pressure.  However, if the filter is damaged filtration pressure remains low in spite of an urgent need for replacement.

• Some filters (e.g. some nanocomposite filters) give no sign of being “used up”, so the user has to keep track of how many litres have been treated until a defined volume has been reached.  In reality, this is sometimes several thousand litres, which would be rare.

• Water is not conserved, so recontamination of treated water is a risk.  The filter itself may become contaminated or contamination can come via the mouthpiece in systems designed to increase pressure by sucking.  For these reasons some filters are impregnated with silver ions.

Improvised filtration techniques

Sand filters can be highly effective at removing turbidity and improving microbiologic quality (99% efficacy), depending on their design and operation (28, 29).

An emergency sand filter can be made in a 20 L (5.3 gal) bucket, composed of a 10 cm (3.9 in) layer of gravel beneath a 23 cm (9.1 in) layer of sand; a layer of cotton cloth, sandwiched between two layers of wire mesh, separates the sand and gravel layers. (9)

The optimum depth of a community or household sand filter is 2m, with diameter determined by the volume of water needed although a sand filter also can be improvised with stacked buckets of successive filter layers with holes in the bottom to allow water passage.

This simple filter method can reduce the number of larger germs like giardia cysts and eggs or larvae of parasites (helminths) (30), it should be (relatively) effective against vibrio cholerae because this germ tends to agglomerate with organic material (31) and other bacteria and viruses can be reduced significantly (32).

Disinfection

Whilst filtration can remove almost all particulates and many pathogens, a second stage treatment should be used to remove all remaining pathogens.

Heat

All common enteric pathogens are readily inactivated by heat at pasteurization temperatures (60-70°C), although microorganisms vary in heat sensitivity, with protozoan cysts being the most sensitive to heat, bacteria intermediate, and viruses less sensitive (33-45).  Only bacterial spores are more resistant, but they are not generally enteric pathogens (35).

As enteric pathogens are killed within seconds by boiling water rapidly at temperatures >60°C (140°F), the traditional advice to boil water for 10 min to ensure safe drinking water is excessive.  The time required to heat water from 55°C (131°F) to a boil works toward disinfection; therefore, any water brought to a rapid boil should be adequately disinfected (46).

Boiling for 1 min is recommended by the US CDC to account for user variability in identifying boiling points and adds a margin of safety.  The boiling point decreases with increasing altitude (only 82.8°C at 5,000 meters, for example), but this is not significant compared with the time required for thermal death at these temperatures.

Advantages

• Simple method, nearly no failure.

Disadvantages

• Slow process

• Must have available fuel

Chemical treatment

Chlorine and iodine are the most commonly used chemicals for water treatment in an outdoors context but iodine has fallen out of favour due to the concerns of toxicity, especially long-term use and vulnerability of certain groups including pregnant women, those with known hypersensitivity to iodine; persons with a history of thyroid disease, even if controlled on medication; persons with a strong family history of thyroid disease (thyroiditis); and persons from countries with chronic iodine deficiency. (47)

Iodine can be used but only for short periods of time in low volumes.  Chlorine is a preferred option for long-term or repetitive use.

Given adequate concentrations and contact times, both iodine and chlorine are effective disinfectants with similar biocidal activity under most conditions (48) but chlorine is still advocated by the WHO and the CDC as a mainstay of large-scale community, individual household, and emergency use (48-50).

The most commonly available form of chlorine is hypochlorite (household bleach at 6% to 8.25% or concentrated swimming pool granules or tablets at 70%).  There is extensive data on the effectiveness of hypochlorite in remote settings (51-54).

As a general rule, use 3 to 4 mg/L as a target concentration for clear surface water.  Lower concentrations (eg, 2mg/L) can be used for back-up treatment of questionable tap water or high-quality well water.

The efficacy of chlorine, as with all chemical treatments is relative to the turbidity of the water prior to treatment (12).  The presence of organic and inorganic contaminants (mainly nitrogen compounds from the decomposition of organisms and their wastes, faecal matter, and urea) can react with chlorine to form compounds with little or no disinfecting ability, effectively decreasing the concentration of available chlorine (10,56) as such, screening is essential for very dirty water and filtration is recommended.

If screening or filtration is not possible, typical recommendations for field treatment are to double the amount of chlorine in cloudy water (57,58).

Chlorine has a distinctive ‘swimming pool’ taste which can be reduced with the addition of ascorbic acid (vitamin C – which converts chlorine into chloride).  Ascorbic acid is available as a powder but is more commonly found in flavoured drink mixes (hence the “screech” orange and lemon drink sachets found in UK ration packs to be added after the use of chlorine ‘puritabs’.).   It is important to add the ascorbic acid or drink sachet after the required contact time as chloride is not a disinfectant and adding it too soon will reduce or even negate any disinfectant properties of the chlorine.

Advantages:

• Easy process

• Cheap and easy to obtain.

• Where chlorine tablets are not available, alternatives (e.g. household bleach) can be used in an emergency.

• Effective against most waterborne pathogens.

• No fuel or equipment required.

Disadvantages:

• Efficacy of treatment is subject to many variables, e.g. water temperature, pH, organic contamination, turbidity.

• Can be time consuming - up to 2hrs for turbid water.

• Susceptible to dosing errors if tablets are not used.

• Chlorine compounds have a limited effectiveness against protozoa like giardia lamblia and cyclospora and require higher dosages or longer contact times.

• There is no effectivity against Cryptosporidium parvum at practical dosages and contact times.  Eggs and larvae of several helminths show an increased resistance against hypochlorous acid.

• Chlorine and other halogens have a distinctive, off-putting taste.   The taste can be neutralised with ascorbic acid but once added, the disinfection effect ceases.

• Chlorine products lose their effectiveness when exposed to sunlight and oxygen, so they – and the treated water - must be stored appropriately.  Chlorine treatment is ideal for camp settings where treated water can be sat, in the shade, unagitated.  There may be potential loss of efficacy when chlorine-treated water is being carried in a rucksack or vehicle as oxygen can cause the chlorites to disassociate.

• Chlorine products have limited conservation capability.

Chlorine dioxide

Chlorine dioxide (ClO2), has been used for many years to disinfect domestic water supplies but until recently, has not been used in tablet form for low volume water disinfection.

After a tablet is added to water, a series of complex chemical reactions occurs, generating chlorine dioxide, but it does not react with water molecules to form hypochlorous acid (59).   Some of the intermediary chemical compounds may also have antimicrobial activity.

Due to the absence of hypochlorous acid, ClO2 has no taste or odour in water.  It is capable of inactivating most waterborne pathogens, including cryptosporidium parvum oocysts (60-62) and is at least as effective a bactericide as chlorine and far superior for virus and parasite inactivation (63).

Add one tablet per litre and wait 10 mins. For heavily contaminated water, use two tablets and wait 30mins.

A only real disadvantage for field use of tablets is the long reaction or contact time required. ClO2 does not produce a lasting residual conservation, and water undergoing chlorine dioxide disinfection must be protected from sunlight.

Advantages

• Chlorine dioxide is a potent water disinfectant requiring less concentration and contact times than hypochlorites.

• It is effective against all relevant waterborne pathogens, even cryptosporidium parvum.

• In contrast to hypochlorites, chlorine dioxide is also effective in alkaline water (pH 8-9).

• After disinfection, chlorine dioxide leaves less chlorine taste or odour.

Disadvantages

• Time-consuming

• Since no residuals are formed, recontamination is possible.

• Chlorine dioxide tablets are relatively expensive at £10-£15 ($12-$17) for 30 tablets, up to 30 litres of water.

• Chlorine Dioxide tablets can degrade in their packaging making dosing inaccurate/

• Chlorine Drops are more stable in long-term storage but it is a more complicated and time-consuming process.

• Disinfected water should be used up quickly, the storage of water disinfected by chlorine dioxide is not recommended since the substance is relatively volatile (keep bottles closed whenever possible).

Potassium Permanganate

Potassium permanganate has known antibacterial and disinfection effects in domestic and industrial water treatment (64) but there is not much evidence to support its use as an emergency water disinfectant despite its ubiquitous inclusion in survival kits.

Potassium permanganate can be used in an emergency at a 1:10,000 / 0.01% solution (65, 66) to reduce bacterial and viral contamination or as an adjunct in combination with another technique, but cannot be strongly recommended as a primary means of water disinfection (67).

Ultraviolet Light

At sufficient doses, all waterborne enteric pathogens are inactivated by ultraviolet radiation (UVR).  UV light in the range of 250 to 270 nm is the most effective (68). 

UV-C rays disrupt the DNA of the microorganism primarily by causing the formation of dimers between bases. As a consequence, the DNA strands cannot be copied and replicated anymore.  This way the microorganism is unable to multiply and cause an infection.

The germicidal effect of UV light is the result of action on the nucleic acids of microorganisms and depends on light intensity and exposure time.  In sufficient light intensities and duration, all waterborne enteric pathogens are inactivated by UVR (51) with vegetative cells of bacteria being significantly more susceptible to UVR than bacterial spores or viruses.   Giardia and cryptosporidium are particularly susceptible to UVR, possibly because of their relatively large size (69-71)  however bacterial spores and some strains of viruses show a higher resistance against UV light than vital bacteria and protozoa (72).

The UV waves must strike the organism, so the water must be free of particles that could act as a shield (73, 74).

The SteriPEN® is a handheld device emitting mainly UV-C radiation with a wavelength of 254 nm.

The effectiveness of this method depends on the characteristics of the water (e.g., turbidity, germ concentration) and handling of the device.  In general, all microorganisms are susceptible to UV-C radiation.  

Advantages:

• Water disinfection with the SteriPEN® is an easy and fast method to achieve safe drinking water. At about 180 g, the SteriPEN® is lighter than a ceramic filter (> 400 g) and disinfects water in less time than chemical treatment (90 seconds vs. 0.5-2 hours).

• UV light does not change the water’s aspect, smell, or taste in contrast to chemical by-products.

Disadvantages:

• Fragility of the lamp and limited lifetime of batteries - four AA lithium batteries are necessary for 100 disinfection cycles.

• Rechargeable SteriPEN® models require an external power source after 20-50 litres.

• Water needs to be absolutely clear to guarantee an adequate disinfection, because particles in water scatter the UV radiation. 

• Droplets in the cap and neck of the water bottle are not disinfected and pose a risk of recontamination making water storage inadvisable.

• Disinfection with the SteriPEN® does not remove toxins or heavy metals from the water.

• As UV light is beyond the visible spectrum it is impossible to determine whether the UV light source is actually on and working, especially in daylight.

Solar Disinfection – “UV – SODIS”

The combined UV and thermal effects of prolonged sunlight exposure have been shown to safely disinfect and substantially improve the microbiologic quality of water in a resource-limited environment (75-88).  Temperatures above 50 to 60°C can be sufficient to obtain potable water within 1 hour, independently of UV radiation. (32, 89)

Unlike UV-C, the cell damage occurs mainly indirectly via the formation of reactive oxygen species in water and thermal disinfection (90).  

While most waterborne pathogenic bacteria are inactivated within 5 to 6 hours of sun exposure (mid-latitude midday summer sunshine (91)), some viruses and protozoa are less amenable to SODIS (90).

Contaminated water may be filled into transparent bottles, preferably lying on a dark surface, and exposed to sunlight for a minimum of 4 hours with intermittent agitation (92).  There may be additional benefit from shaking the bottle for 30 seconds prior to exposure to encourage the formation of oxygen reactive species (75).

Advantages:

• If applied correctly viable pathogenic germs are reduced significantly to non-detectable levels after exposure time. 

• SODIS requires only glass or clear plastic bottles.

• The use of sunlight is probably the cheapest and easiest method to disinfect large quantities of water.

Disadvantages:

• There is a plurality of influencing factors like temperature, water turbidity, and intensity of UV radiation on disinfection time and efficacy.

• People make use of SODIS without any instruction on disinfection time according to the local circumstances.  This makes SODIS an uncontrolled, not reliable method.

• Water needs to be clear for SODIS to be effective and bottles have to be in a good condition (no scratches which scatter the UV radiation). 

• Only where high temperatures can be achieved these preconditions are not mandatory.

Summary

The methods of water treatment employed are dependent upon the situation and the means available.

• For a lone worker or traveller, on a short excursion, without expectation of prolonged dependence upon treating potentially contaminated water, chlorine dioxide tables would be ideal.  This could be enhanced with the addition of a disposable coffee filter for screening turbid water.

• For a multi-day activity, a personal hollow-fibre filter or a combination of a personal ceramic filter with cholinre would be more appropriate.

• For a longer dependence or a small group, a larger, more robust filter such as the Katadyn Pocket Filter or MSR Guardian would be recommended.

• For larger groups and timeframes a large-volume gravity filter such as the MSR Auto Flow or Platupus Gravity Works should be considered.   This may require larger volume screening with Millbank bags for heavily contaminated or turbid water, with the addition of chlorine tablets or bleach, if a ceramic filter is used. This water would need to be stored in the shade and unagitated for maximum confidence.

• In extremis, small volumes of water for individuals can safely be obtained – albeit slowly with a combination of screening through a rudimentary filter, if required, and improvised methods of treatment such as SODIS or hypochlorite bleach.

• For long term, large scale management, sand filters and hypochlorite bleach can be used.   Given the time requirements for effective treatment as well as the increased demand for potable water, resource management will need to be strictly employed as well as diligent sanitation techniques to prevent cross or re-contamination of treated water.

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