Home Water Purification
©1999 Loren Amelang
Most Americans have grown up thinking pure, fresh water comes out of the wall at the turn of a tap. Many of us who live with home power systems are also providing and maintaining our own water supply. We become our own electric company, and water treatment company as well!
The simplest conventional private water system starts with an electric pump large enough to directly supply the maximum water flow rate required. Conventional pumps draw a lot of power, so you must have a large battery bank and inverter, even if the pump is only on for short periods of time. This system also needs a small pressure tank with a "drawdown" of about twenty gallons (76 liters) to prevent the pump from cycling on and off excessively. The water in the pressure tank is typically replaced several times a day so there is effectively no storage of water.
Off-Grid Water Systems
My first off-grid water system did not store much water. The old-fashioned windmill style pump was fitted with a small electric motor driven by PV panels. It filled up a fifty gallon (189 liter) barrel next to the well. A small DC pump lifted this water up to another fifty gallon barrel wedged into a huge oak tree. All of the water was used and replaced almost every day. I could use the fifty gallons by gravity flow as rapidly as I wanted to, but then I had to wait for it to be refilled.
When I became impatient enough to add more storage capacity to the water system, I began learning about water treatment. If the same water sits in a tank for days, biological contaminants that were insignificant in the incoming water have time to multiply and become a problem. The sediment filter cartridges required to protect the DC pressure pump grew algae until they were blocking the flow and causing the pump to cavitate noisily. Enough people reported vague "digestive upsets" that I began filtering all drinking water through my Katadyn filter.
My real water treatment education was forced on me by a new property with a real water problem — iron. The plumbing fixtures were all bright orange, and the water looked like tea and smelled foul, right out of the well. If this water was allowed to stand, it grew iron bacteria. One slowly oozing outdoor faucet had a two inch (5 cm) layer of what looked like butterscotch pudding coating its pipe!
Water Treatment Categories
Some systems must try to make "good" water from whatever "bad" water is available. The water may be unacceptable because of sediment (mechanically removable particles), dissolved minerals (hardness), chemical contaminants (from off flavors to toxic poisons), or biological hazards (giardia, non-fecal coliform, fecal coliform, or specific infectious agents). Some of these problems require their own unique solutions. You might also have several problems combined in your water source. If you suspect you are starting with "bad" water, be sure to have it tested for a broad spectrum of various contaminants. A local government agency may be able to provide some kinds of testing, but you will probably need to have additional tests done by a commercial laboratory.
If you have good water, just keep it clean and fresh through storage and distribution. If the water in all of your tanks, pipes, and water heaters is not used up and replaced each day, you probably need to provide some type of preventive water treatment. The simplest, cheapest, and most common is chlorination, either by adding solid tablets which slowly dissolve in a storage tank, or by injecting liquid chlorine into the plumbing through a specialized metering pump. Unfortunately, chlorinated water is one of the things many of us left the cities to avoid.
I'm not a water treatment expert, just a wary consumer who has searched diligently for reasonable solutions to several water problems. My experience is a bit different from what you may have read in articles geared towards people served by public water systems. That's because all of my water systems have been individual and private. I've come up with three principles which any proposed water treatment solution must deal with.
1. The Medium is Messy
Most common water filters use some kind of mechanical barrier. The common pleated paper "sediment" cartridge has 20 micron openings and will keep your faucet screens from clogging up with debris, but does nothing about smaller contaminants. Likewise, large system "sand" filters trap only large, visible particles. The "backpacking filters," designed to make a biologically safe drink from whatever water you can find, have openings of 2 microns or less, and can catch bacteria and other infectious particles.
Reverse osmosis filters use a mechanical barrier which prevents biological contaminants and even dissolved minerals from passing through. Carbon granule or block filters rely on chemical attraction (adsorption) of contaminants to the filter medium rather than physically blocking their passage.
All of these devices provide a marvelous home for living things. Carbon filters attract biological contaminants along with their food supply. The sediment cartridges required by my close tolerance Slowpump clog with living algae long before they ever fill with mineral crud. If you have iron bacteria, they will not only live in the sediment cartridge, they will eat it. When you replace it, the pleated paper will simply be gone.
Many households use barrier or adsorption filters without obvious problems. This is because most shared water systems use chlorination to kill anything living in the water.
The filters with barriers under 2 microns, designed for unchlorinated water, are almost always used intermittently for treating small batches. They are allowed to dry out between uses. If they were to be used in a permanent installation where they were continuously wet, bacteria which could not pass through in their full grown size would eventually grow through the barrier during cell division. Some brands of this type of filter have chemical additives that inhibit bacterial growth.
Carbon filter cartridges work well when they are new, attracting a high percentage of all types of contaminants and hanging on tight. Unfortunately, an infestation of living things can quickly cover the whole surface, rendering the filter totally useless. Even without a biological infestation, there is no way for you to know when the cartridge has reached its limit for attracting contaminants, and has become a source of contamination rather than a solution.
Reverse osmosis filters trap practically all biological, chemical, and mineral contaminants, even the large quantities of dissolved minerals which make water "hard." Unfortunately, these filters must be flushed of their refuse with pressurized water. There is a lot of waste in this process — up to ten gallons for every gallon of purified water. This brings us to the second principle...
2. The Refuse Must Go Somewhere
In most installations, this slightly contaminated waste stream from typical reverse osmosis filters goes right down the drain. Since few people can afford a reverse osmosis system big enough to filter more than their personal drinking water, this waste of 10 to 40 gallons (38 to 151 liters) every day is generally tolerated. Small barrier style batch filters must occasionally be cleaned, generally by scrubbing the contaminants loose by hand and flushing them down the drain with potable water. Again, their limited size makes the impact of this problem tolerable.
Disposable filter cartridges, such as pleated paper sediment cartridges and carbon granule or block cartridges, are thrown in the domestic trash and probably eventually into a landfill. Along with the contaminants goes an inseparable maze of paper, plastic, carbon, and various other materials. Obviously none of these systems are attractive when scaled up large enough to treat all of the water used in a home or homestead. When you talk to a water treatment company about handling more than your personal drinking water, you will probably hear about one of two different systems — backwashable media or ozone.
Backwashable media systems use common sand, or some other granular mineral, inside a tank. The tank is plumbed into the water line much like a cartridge filter. Conventional water softeners are backwashable media systems, using a special ion-exchange medium which can be regenerated using common salt. Different water problems require different media, and sometimes require a series of different tanks with different media. Backwashable media systems share the biological problems of the smaller barrier filters. Because of this, you will probably be sold a continuous chlorination system to go in front of your filter setup.
The advantage of backwashable media is that you do not have to physically remove and clean a cartridge to get rid of the refuse. The system generally includes programmable timers which control the backwashing process while you sleep. This convenience is costly, requiring a high volume pump and a lot of potable water to backwash the refuse out of the tank and medium. You will also need to put this large volume of contaminated refuse somewhere. The control systems typically assume full time AC power is available to run the timers and high volume pumps. If you do not want the chlorine residual in your kitchen and shower, you will need a third system to remove it at the point of use.
Backwashable media systems rely on a combination of chemical reactions between their media and your contaminants, and mechanical trapping of particulates and precipitates in the water. Ozone systems could be described with the same words, except the granular "media" is replaced by ozone gas.
Ozone is a highly unstable molecule consisting of three oxygen atoms, formed by electrostatic discharge (such as lightning) or by exposure of air to strong ultraviolet light. It desperately wants to come apart into normal oxygen (two atoms of oxygen in a relatively stable molecule) and free oxygen ions which are ready to combine with dissolved minerals or biological contaminants. If it can't find anything else to combine with, most of the free dissolved oxygen will combine with itself into stable molecular oxygen within minutes of the ozone being released into the water.
Many dissolved mineral contaminants will react with the free oxygen to form solid oxides, which can then be trapped by a mechanical filter. Ozone is particularly effective at oxidizing iron (turning it to rust particles). In addition to reacting with minerals, ozone kills and destroys bacteria and viruses. Chlorine, which must be added to backwashable media filters, is considered a strong oxidizing agent, but pure free oxygen atoms are far more effective against simple biological molecules. Free oxygen atoms also leave no chlorine residue. In fact, the dissolved oxygen left after ozone treatment is beneficial to plants and animals.
In case you are worried, the ecological nightmare connected to ozone does not apply to ozone water treatment systems. Ozone is destroyed far up in the stratosphere by the breakdown of chlorofluorocarbon compounds, creating "ozone holes." Ozone which escapes into the air at ground level is oxidized to pure oxygen within hours, and cannot migrate nearly high enough to affect ozone holes one way or another.
Ozone water treatment systems come in many types and sizes, from those that fit in the plastic housing of a common sediment cartridge filter, to giant systems for whole cities. The ones I have experience with are designed to treat one domestic water system, cost under $2,000 (not including the required storage tank), and use a pleated paper cartridge filter which is similar to (but not interchangeable with) a swimming pool filter cartridge. Ozone water treatment systems are often sold by direct mail. Unfortunately, they have a bit of an aura of "snake oil" about them, even though the big plumbing wholesalers now carry essentially the same units.
If your water system already uses an unpressurized storage tank of 1,500 gallons (5,678 liters) or more, just hang the filter cartridge in your tank, plumb the injector unit into the pipe from your water source to the tank, and hook up the wiring and ozone tubing. If you don't have the storage tank, you will need to add one and an additional pressure pump and controls.
The glaring disadvantage of these ozone systems, which the salespeople will not describe as vividly as I do here, is that the refuse must go somewhere. Depending on what is in your water and how much you use, you will need to remove and hand wash the filter cartridge between once a week and once a month.
To do this, climb up on top of your storage tank, remove the lid, and pull this ten inch diameter by twenty inch high (25 by 50 cm) filter cartridge up by its chain. It will be full of water and coated all over with up to one eighth inch (3 mm) of slime, which will fall back into your drinking water supply unless you lift the filter smoothly and at just the right speed.
Once you have it out and on the ground, you will probably spend half an hour spraying the goop out of the pleats in the filter. A gas or electric powered pressure washer (like the ones sold for washing cars) would make short work of this job and use much less water. Clean the ozone "diffusing stone" by soaking it in concentrated Muriatic Acid for a couple of minutes, and place the cartridge back into the tank.
The advantages of ozone make this ritual quite tolerable to me. For the new property, a comparable granular media iron removal system would have been much more expensive and more complex to install. It would have required a larger, freeze-protected pump house, and would have involved complex automation controls that the residents of the property might not have understood. We also would have needed a new drainage system for the high flow rate of the backwashing process, and a chlorine injection system to prevent the filter from becoming a bacterial colony.
For my own off-grid home, the issues were non-fecal coliform, algae, and preserving freshness in my new 3,000 gallon (11,356 liter) storage tank. While I could have taken care of these problems cheaply with chlorine injection, I was not willing to ruin my delicious water and fresh air with chlorine and its byproducts. While the ozone system is not particularly effective against the calcium hardness of my water, it has eliminated all of the black manganese stains from the plumbing fixtures.
I make double use of the ozone generator by diverting the ozone to my outdoor hot tub when its filter is running. This has eliminated the tissue algae problem that nothing else would touch. I have also cut the chlorine feed to the tub to one fourth of the previous amount. And the ozone does seem to make the chlorine residual less offensive.
The ozone filter unit is cleverly designed. The ozone gas, released inside the filter cartridge, causes the water in the storage tank to circulate around the tank and through the filter, trapping particles on the outside of the cartridge as it enters. No chlorine is required, because the ozone gas is continually circulating through the tank and the filter medium. This destroys all biological contaminants in the water and prevents any problem of biological colonization of the filter itself. Ozone also causes the majority of mineral contaminants to precipitate out of solution and become trapped in the filter cartridge. While the end product is not as "soft" as water from a dedicated water softener, we noticed a dramatic reduction in the amount of soap required for bathing and laundry after the ozone unit went in.
The ozone treatment system uses a small compressor, forcing air past the ultraviolet lamp ozone generator. From there, the compressor pumps the ozone into the filter unit. The compressor and lamp use about 50 watts of power. They are intended to run continuously. The lucky thing for those of us using home power systems is that the amount of ozone treatment required depends upon how much water you are using — really upon how much untreated water you add to the storage tank. Definitely let the ozone run while you are pumping untreated water into the tank. In fact, most of these systems automatically divert all of the ozone into the incoming water line while your pump is running. Run the ozone long enough to mechanically filter out all of the particulates that will precipitate out of the incoming water.
Since ozone has a residual life measured in minutes, be sure that the ozone runs at least daily and preferably several times a day to prevent biological regrowth. This is true even if you are not adding untreated water to the tank. Keep in mind that the unit does not have to run continuously if you don't have the power available. In fact, it's a natural diversion load for solar electric systems, since most of us use and pump more water when there is more sun.
3. You Will Never Catch Them All
If your water has a biological problem, or if your usage is slow or intermittent so that water sits stagnant in your pipes and filter systems, face the fact that you will never kill every last contaminant. You must consider how your system deals with regrowth or recontamination past the point of purification. The vast majority of water systems deal with this by creating a chlorine residual in the filtered water.
Residual chlorine is free chlorine that remains after all of the oxidizable biological material in the water has been "burned out." Residual chlorine is what you are measuring when you use a pool test kit with the yellow "OTO" indicator. As in a swimming pool, you want that part or so of active chlorine remaining per million parts of water, even in the most remote parts of the plumbing, at all times. When this active chlorine finds some new biological material to "burn," you get that characteristic chlorine odor.
I have no experience with chlorinating already purified water. The reports say that the unpleasant smell and the supposedly carcinogenic THMs do not occur when there is nothing left in the water to be oxidized. Chlorine advocates claim that it's possible to create a chlorine residual sufficient to prevent any biological regrowth problems, no matter how long the water sits stagnant, with none of the undesired effects. To try this, you will need an expensive metering pump, and you will have to feed it power and concentrated chlorine.
With four people living 500 feet (152 m) of pipe away from our ozonated tank, and someone here all the time, we have never noticed significant biological regrowth even without any chlorine residual. But another 100 feet (30 m) beyond, there is another home served by the water system. This house is occasionally unoccupied for up to a month at a time, and there we definitely see problems.
The old filter housings grow algae, and the water heater tank (which was thoroughly contaminated with iron bacteria before the ozone system went in) grows the "butterscotch pudding" iron bacteria. This system has been flushed and shock treated with chlorine many times, but you will never catch them all, and given the chance, the biological problems will come back. We have learned to run freshly ozonated water through all the pipes occasionally when the second house is vacant.
Things That Work
If your water comes from a shared system providing a chlorine residual, and you don't suspect toxic chemical contamination, you can choose a treatment system based on aesthetics. The chlorine should take care of enough of the incoming biological contaminants. If you like the taste of the water when you're using a particular filter, go for it. But if you are using a filter which removes the chlorine, do not store the filtered water! If your under-counter unit removes chlorine and then stores a gallon (3.8 liters) of filtered water, use it regularly to keep it fresh. Flush it completely before drinking any if it has gone unused for a few days.
If you provide your own water, and especially if you know it has or occasionally might have a biological contamination problem, your choices are much more limited. The simplest source of biologically safe drinking water is the "backpacking filter" used on small batches of water. Drink the water before the inevitable missed contaminants multiply, and clean and dry the filter element between uses. I prefer the Katadyn brand which has been proven in use for over fifty years. Although it's impregnated with silver to retard bacterial growth, the filter does nothing about mineral or chemical contaminants.
If you have the required water pressure and can live with the continuous waste, a small reverse osmosis unit will provide biologically safe drinking water, and will remove mineral and chemical hazards too. I don't have firsthand experience with this, but I suspect that it would have problems treating the iron bacteria-laden water which is now handled by our ozone system. But it would make an ideal "finishing filter" after an ozone system, because there is continuous flow through the unit and only the microbiologically pure water is stored.
If you have lots of power to spare, a distillation unit will handle bacterial and mineral problems in your drinking water. It will boil the water into steam and recondense the vapor. There are reports that chemical contaminants with boiling points similar to water may sneak through the distillation process. If your water is hard or has lots of particulates, you will definitely have a cleaning problem.
For treating your whole water supply, especially on a low power budget, ozone is probably the best compromise. If you suspect a chemical contamination problem, or if your usage is too low to keep a dependable ozone residual in your household plumbing, you may also want to use one of the other filters.
Things That May Not Work
Assuming you might have a biological problem and that you are not providing a chlorine residual, all of the common carbon-based countertop, under-counter, and carafe filters are suspect. They should not be used even as "finishing filters" after an ozone system because you will never catch them all. They provide a reservoir of stagnant water which is not protected by the short residual of ozone.
There are some cute "ultraviolet" or "ozone" systems which fit in the space format of the common sediment filter cartridge, and are often sold in combination with matching sediment and carbon cartridges. They even come in 12 volt DC versions. Unfortunately, they miss the mark on all three of my principles.
The ultraviolet or ozone treatment happens only as the water flows through the tiny cartridge chamber, and the lamp is far too small to kill all the contaminants in one quick pass. There is no continuous circulation of ozone through the sediment or carbon cartridges, leaving them unprotected from biological "messiness." And if the treatment causes biological or mineral precipitation, there is nowhere for it to go but into the carbon filter until it's saturated. From there, the precipitation will contaminate your supposedly filtered water.
Your Mileage May Vary
For a single system, ozone seems to handle many water treatment needs quite well, but water problems are as individual as home power users, and ozone might not solve yours. It can't remove sodium, chlorides, fluoride, nitrates (fertilizer contamination), or sulfates (but it does remove hydrogen sulfide).
The goal of my research was to find a generally recognized and relatively ready-made water treatment solution, able to handle all of the water used on a rural property with a minimum of user intervention. I already have plenty of homemade energy systems to maintain, and I was not anxious to risk the health of people who had not chosen to be part of an experiment.
Your balance of water problems versus creativity, time, money, and personal energy is undoubtedly different than mine. When you consider buying an existing solution or creating your own unique system, ask yourself how it deals with the three major issues I have pointed out.
As an example, you could power a distillation unit with sunshine, but the medium is messy — how will you keep your distiller from becoming an algae farm as it cools down in the evening? The refuse must go somewhere — how will you clean the accumulated minerals and crud out of the heating area? And you will never catch them all — will you provide a chlorine residual so you can store the water for use when there is no sun?
I realize I have not provided you with ready-made answers to your water treatment problems. Only you know what you want fixed about your water, and what resources are available to accomplish that task. The marketplace is full of dealers who will try to convince you that the particular technology they happen to sell is best. I believe I have armed you with three critical questions that will help you to zero in on the system that will work best for you.
Loren Amelang lives and works in a solar powered ridgetop home, with solar DHW backed up by wood, and a solar greenhouse which provides year-round space heat if the sun is out. But he has blown his "wireless" purity by adding an ISDN connection to the internet.
Loren Amelang, Box 24, Philo, CA 95466-0024 loren (symbol) pacific (punctuation) net
For a reprint of Should You Use A Water Filter? Consumer Reports, July 1997, send $3 and request report #RO162. CU/Reprints, 101 Truman Ave., Yonkers, NY 10703-1057
Continuous circulation ozonation/filtration systems for use in storage tanks: Triple O Systems, Inc., 1550 Dell Ave., Unit E, Campbell, CA 95008 * 408-378-3002 * Fax: 408-378-7155 * email@example.com * www.tripleo.com
Katadyn Water Filters, Black Mountain Stores, 1721 N. Texas, Odessa, TX 79761-1226 * 800-760-7942 * 915-580-7175 * Fax: 915-332-1053 * firstname.lastname@example.org * www.katadyn.net
Reverse osmosis filters starting at $525 list are available from Authorized Distributors of Nature's Sunshine Products * email@example.com * www.naturessunshine.com
Water Quality Association, 4151 Naperville Road, Lisle, IL 60532-1088 * 800-749-0234 * 630-505-0160 * Fax: 630-505-9637 * firstname.lastname@example.org * www.wqa.org
Water Technology Magazine and WaterNet, 13 Century Hill Drive, Latham, NY 12110 * 518-783-1281 * Fax: 518-783-1386 * email@example.com * www.waternet.com
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