Thursday, January 15, 2015

Water Purification, Part 3 - Technologies

Treating water to make it potable is done through a few basic processes (or a combination) to reach the desired quality of water.

Pasteurization is the process of heating water or food to a temperature high enough, and for long enough, to kill the pathogenic organisms present. Boiling is not needed; all that is needed is to get the water above 160° F (71° C) for a few seconds. There is a simple pasteurization indicator called a WAPI, but if you don't have one, just boil the water and let it cool. Overkill usually works.

Boiling only affects biological contaminants and may actually concentrate chemical contaminants (for an example, see my post concerning Microcystin and blue-green algae).

Distillation is boiling the water to a vapor state and then condensing it back to a liquid state. Distillation will remove all of the biological and most of the chemical contaminants, but chemicals that have a boiling point below that of water can carry over and actually be concentrated in the distillate; the larger the difference in boiling point there is between water and the chemical in question, the better chance there is that it will be removed. Anyone who has ever tried to distill alcohol will tell you how hard is is to get that last 5-10% of the water out of the alcohol (which is actually an azeotrope, and a subject for another time).

Distilling large quantities of raw water will require a lot of maintenance on the equipment to remove the sediment, known to boiler operators as "mud". Anyone who lives where the water is "hard" knows what a teapot looks like after a month's use, and the build up of lime reduces the efficiency of your boiler (and may cause failure its failure).

Cold means freezing. Freezing water will kill most of the larger and more complex organisms, but won't affect viruses or some of the more hardy forms of life. Chemical contaminants can be reduced or eliminated through freezing (depending on the specific chemical involved), but any chemical with a freezing point close to that of water has a good chance of being left in the ice.

Water is odd in the chemical world because it expands when it freezes, where most everything else contracts and gets denser. This is why ice floats, which is a good thing because it separates the water from the contaminates. Ice harvested from a lake or river starts off fairly clean and is usually potable untreated. Fresh snow and rain are also safe, unless there are abnormally high amounts of crap in the atmosphere (volcanic ash, nuclear fallout, ash from forest fires, etc).

Chemical Treatment
The three main chemicals used to treat water are Oxygen, Chlorine, and Iodine.

Oxygen works by burning (oxidizing) the organisms, and can also remove some of the metals found in raw water by converting them into forms that are easier to filter out. Potassium Permanganate is a strong oxidizer, and is a good source of oxygen for treating water (see this post for details). Ozone is a more energetic form of Oxygen that works well, but requires special handling and equipment.

Chlorine acts on the organisms by disrupting the cell walls. Pure Chlorine is a corrosive gas and is not to be played with unless you have the training and equipment to do so safely. Bleach contains both Cl and O, so it works both ways. I discussed bleach in an earlier post, specifically about the threat of blue-green algae.

Iodine tablets have a long shelf-life, small size, are solid instead of liquid, and work in the same manner as Chlorine. Iodine tablets are convenient because they are sized for a typical quart or liter canteen. Iodine works well on bacteria and common parasites, but does nothing for viruses. Both Iodine and Chlorine require time (typically 30 minutes) to kill the microbes present. Potable Aqua is one of the common brands of Iodine tablets.

Filtration is the process of passing water through a membrane or substance that allows the water molecules to pass through but traps impurities. Filter media is made up of a variety of things, with ceramic and plastic being the most common methods of removing biological impurities, and carbon being used to remove unwanted chemicals. Since the individual filters all have varying claims of efficiency, I'll try to list a few for you to compare. When doing your research, be careful to check the efficiency rating as well as the price and rated flow for each filter. Your mileage will vary, and I am not going to lie and tell you that there is one perfect filter that will fit everyone's needs. Most of the better filters on the market (or ones you can make) use more than one form of filtration. Simple is good, but when you're dealing with one of the most vital things you need to stay alive, it may help to add a level of complexity if it makes enough of an improvement in the quality of water.

Several filters also incorporate a coating or layer of silver as an additional antibacterial step. I'm not totally sure (meaning I haven't had it proven to me) of the effectiveness of silver as an antibacterial. There is plenty of anecdotal evidence of silver's effects on bacteria, but I haven't seen any honest, unbiased lab trials proving the claims.

A word of warning: if you live in an area where the ambient temperature drops below freezing, you must make provisions for keeping your filter warm after use. It is almost impossible to get all of the water out of a filter between uses, and because water expands when it freezes, the formation of ice crystals inside any filter will lead to rapid failure of the filter media. I carry a Sawyer Mini for many reasons, one of them being that I can cap the ends and hang it around my neck (inside my clothes) in between uses to keep it from freezing.

Coarse filtration
Also known as prefilters, anything from a clean piece of cloth to a bed of fine sand will remove large (> 10 micron) particulates and will extend the life of any further filtration method used. Sand filters are commonly used in the operation of swimming pools to remove algae and large debris, and would make a good prefilter for a large group. Just allowing cloudy water time to sit undisturbed will let the larger particles settle to the bottom of your container. If you carefully pour the clearer water off of the top (decanting) it will take some of the load off of your water filter and will help extend its life. Here's a repeat of a chart I posted before to give you an idea of the sizes of common contaminates.

"Cut-offs of different liquid filtration techniques" by Peter in s - Own work.
Licensed under Creative Commons Zero, Public Domain Dedication via Wikimedia Commons

Carbon filters
Carbon filters use activated carbon, a heat-treated form of charcoal, to trap chemical impurities. By heating charcoal in the absence of oxygen, the process opens up millions of micro-pores in the surface of the charcoal, making a filter media with a very high surface area and an affinity for impurities. Carbon filters are rarely seen alone; they are generally a part of a system that uses multiple methods of filtration to clean the raw water. Carbon is very effective at removing chemical contaminants but do almost nothing to remove biological contaminants (and may even serve as a breeding ground for some if left wet).

Ceramic filters
By using proprietary methods of making very small voids (empty spaces) or pores in a piece of ceramic, several companies have developed ceramic media that are capable of blocking anything larger than 0.1 micron. Large ceramic filters tend to use gravity to force the water through, while the smaller ones will have some sort of pump attached to allow the use of a smaller filter element and still get reasonable flow through it.
Just Water is one of the standard ceramic filters. Berkey, Katadyne, and Doulton are some other well-known names of ceramic filters. Ceramic filters generally have the ability to clean more water over their useful lifetime than plastic membrane filters, but are larger and more expensive. They are also more fragile since the filter media is brittle and heavy. Ceramic filters are easy to clean, usually requiring nothing more than wiping the media off with a clean sponge or scouring pad. Sometimes scrubbing with a toothbrush is needed to clean the surface and restore the flow rate. Several brands include a mesh or cloth sock over the filter itself as a prefilter. 
Variations in manufacturing will change the size of the pores, changing how effective they are at removing contaminants. Some of the simpler and cheaper ceramic filters are quite popular for NGO's (like Potters for Peace) to distribute or produce in third-world countries as a reliable and moderately effective method of cleaning water. Sometimes just getting the majority of the pollutants out can make a big difference.

Plastic filters
Plastic filters such as the Life Straw, the Sawyer Mini, and the Platypus use a series of tubes and the force of gravity or suction to force the water through them. The brands I listed are not the only ones on the market, but they are the ones with the best consumer and lab ratings. I tend to avoid buying life-support equipment made in factories in China and Mexico, since they don't seem to have a good grasp of the concept of quality control.

Micro-filtration/ Ultra-filtration
Micro-filtration uses very tightly controlled pores in a plastic membrane to allow very small things like water molecules to pass through, while rejecting anything bigger than the pores. There are two different modes of operation;
  • Laminar flow: the raw water is passed over the membrane and the filtered water (filtrate) is collected while the contaminants are carried away by the flow. This wastes some water, but doesn't require as much maintenance.
  • Radial flow: the raw water is forced through the membrane and the contaminants are trapped in or on the filter membrane. This method requires back-flushing to clear the membrane.

Reverse Osmosis (RO)
RO is a higher form of micro-filtration, capable of separating water from even dissolved solids like salt. RO requires high pressures to force the water through a semi-permeable membrane, and it is both slow and energy-consuming. Most RO units are laminar flow systems that force the raw water along the surface of the membrane: a portion of the water passes through the membrane and the impurities are carried away in the left-over water as a waste stream. Be aware that the waste stream can concentrate chemicals enough to be a hazardous waste, although that's normally only a problem with very large RO units. Maintaining RO systems consists of replacing the filter modules when they get fouled too badly.

UV light sterilization
Fairly new technology, UV light destroys microorganisms by disrupting the DNA within the individual cells. The main advantage to using UV light is the speed with which it kills microorganisms, usually measured in seconds. Industrial and municipal treatment plants use large UV light sources to disinfect water and there are a few rechargeable personal-use UV devices on the market. UV will not affect chemicals to any noticeable extent.

Ion exchange
The average water softener is a good example of ion exchange technology. Using resin beads of two types (anionic, or negatively charged, and cationic or positively charged) mixed together, they trap dissolved chemical contaminants based on the ionic charge of the chemical. The common pitcher-style home filters like Brita, Pur, and ZeroWater are ion exchange filters with a carbon stage to help remove bad tastes. The small household filters of this type are disposable, since they have no provision for regenerating the resin once it becomes saturated with impurities. Large ion exchange systems keep the anionic and cationic resins separate and are regenerated using strong acids and bases to knock the trapped impurities off of the resin, then rinsing them to remove the acid or base. Ion exchange methods will have little to no effect on biological contaminants by themselves.

Next week I will examine, in detail, some of the possible sources of raw water and what you may expect to find in them. This should help you decide what level of treatment you're going to need and what to look for in a filter.

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