Pure Water Occasional, April, 2023
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Greetings from Pure Water Products, the Pure Water Gazette, and the Pure Water Occasional.
In this Occasional you'll hear about the basics of desalination of sea water, lead pipe replacement, increasing ocean water temperatures, PFAS, TDS, VOCs and weight gain caused by drinking water. Another chapter in the shameful story of Camp Lejeune water contamination and the truth, the real truth about National Garden Hose Day. And, as always, there is much, much more.
Thank you for reading, and sincere thanks from Pure Water Products for your continuing support. |
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Does the Permeate Pump Save Water?
by Gene Franks
The permeate pump has been a popular accessory for domestic undersink reverse osmosis (RO) units for a number of years. It is a non-electric, water-powered device that works by isolating the RO membrane from back pressure from the storage tank.
Standard undersink RO units produce water into a small pressurized storage tank. As the tank fills, the RO unit has to push against increasing back pressure from the storage tank. The greater the back pressure, the more inefficient the RO unit becomes. Standard undersink RO units that store water in a pressure tank never perform as well as the membrane rating. A 4:1 (four gallons to drain for each gallon of product water) membrane, when used on a standard undersink RO unit, actually performs much less efficiently because of back pressure from the storage tank. In informal tests that we did at Pure Water Products, actual efficiency performance of a standard RO in simulated home usage was around 7:1. The unit performed at 4:1 when the tank was empty but the efficiency got progressively worse as pressure built in the storage tank.
The purpose of the permeate pump is to allow the RO unit to produce water into an almost pressure-free chamber. The pump then pushes the stored water into the the pressurized storage tank. Water leaves the RO membrane housing in two streams: the purified water (permeate) and the reject or drain water (brine). The pump uses the energy from the brine to push the permeate into the storage tank.
The pump manufacturer claims that the pump will save water and also improve the TDS performance of the RO unit. This is discussed in detail, with manufacturer’s charts, in a 2011 issue of the Pure Water Occasional.
An earlier Pure Water Gazette piece reported an informal test done with a single home RO unit that looked at the question of whether the pump improved TDS rejection performance as claimed by the manufacturer. The test showed no improvement in TDS performance with the pump as compared with the same unit without the pump.
The TDS test was done with a standard Black & White undersink RO unit with the GRO 50/50 water saving membrane. The membrane performance rating is 1:1–one gallon of brine for one gallon of permeate. The same unit, now a year older, was used to test the permeate-to-brine efficiency with and without the permeate pump. We ran six trials: three with a standard unit without permeate pump and using the regular Payne shutoff valve, and three with an Aquatec ERP 500 permeate pump installed and the same Payne shutoff.
Discussion
Standard RO membranes are set up with a ratio of about 4 (drain) to 1 (permeate). With no tank to push back against, a 24 gpd membrane installed with a 250 ml/m drain line flow restrictor will run about 95 gallons of water to drain while producing 24 gallons of drinking water. This works out to around a 4:1 ratio. The GRO 50/50 membrane used in this test is a 50 gpd membrane, paired with a 150 ml/m brine restrictor, so the theoretical production is 50 gallons of permeate while sending 57 gallons to drain. Essentially a 1:1 ratio. (To convert milliliters per minute to gallons per day, multiply by 0.38.) Significantly, a smaller-than-usual storage tank was used to move things along faster. The tank is about a 7.5″ X 9″ petite version, which the manufacturer calls a “2 gallon” size, but which for practical purposes holds about one gallon.
The trial consisted simply of starting with a full tank, draining the tank to catch and measure the content, then allowing the RO unit to refill the tank while catching the brine for measurement. Three trials each were done with and without the pump. Results were surprisingly uniform. The table below reports the averages in percent recovery, which means the percentage of the water that goes into the RO unit that comes out as permeate. Keep in mind that a standard 4:1 membrane setup without tank has a percent recovery ratio of about 20%; with a tank in a standard residential setting the percent recovery might be 12%.
The Test with GRO Membrane
| Setup |
Total Permeate in fluid ounces |
Total Brine in fluid ounces |
Recovery Percentage |
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| GRO Membrane, without pump |
114 (0.89 gal.) |
160 (1.25 gal.) |
41.6% |
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| GRO Membrane, with pump |
120 (1.07 gal.) |
118 (0.92 gal.) |
50.4 % |
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The conclusion is, then, that the permeate pump does indeed save water, even when running with the stingy GRO membrane. With a conventional 4:1 membrane, the saving is more pronounced. Permeate pump manufacturer Aquatec’s performance chart shows conventional membrane performance with the pump at about 12:1 when the storage tank is 60% full. Our tests show overall performance of conventional units averages about 7:1 under normal stop and go residential use, while the permeate pump unit with the same membrane maintains a consistent 4:1 throughout the fill cycle.
One final note about the GRO 50/50 membrane. Although it would appear from the 41% recovery that it is performing at less than the advertised 50% recovery rate, its performance is really quite remarkable. The 50% setup is the performance you get without a tank (as in a tankless “countertop” RO unit). You should get the essentially same performance with a permeate pump, whose job it is to nullify the back pressure from the storage tank. That is what we got in our test.
Here is a summary of simulated performance figures to put all this in perspective:
| Setup |
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Approximate Recovery Percentage |
| Standard membrane in undersink standard RO with tank |
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12% |
| Standard membrane in tankless countertop RO |
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20% |
| Standard membrane in undersink RO with permeate pump |
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20% |
| GRO membrane in tankless countertop RO |
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50% |
| GRO membrane in undersink RO without permeate pump |
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40% |
| GRO membrane in undersink RO with permeate pump |
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50% |
Adding a permeate pump to a standard RO unit improves its recovery percentage by around 10%. Replacing the standard membrane with a 50/50 GRO membrane increases recovery by about 30%.
Does this mean, then, that you should always buy an RO unit with a permeate pump? Not at all. There is more to be considered than water saving. The permeate pump makes the RO unit a more complicated device that can be a bit harder to install, harder to maintain and more difficult to troubleshoot if there is a problem. It also makes some noise, although the current version ERP-500 model is very quiet. Saving 8% or 10% in water consumption may not be worth it.
If saving water is the main objective, I suggest adding the GRO membrane first, and then the permeate pump.
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Getting Drinking Water from the Sea
by B. Sharper
Pure Water Gazette numerical wizard Bee Sharper rolls out some numbers on production of fresh water by sea water desalination.
Editor's Note: This article first appeared in the Pure Water Gazette in 2014, so some of the information is out of date. A lot has happened in sea water desalination in ten years, but the basics remain the same.
Approximate number of desalination plants in the world as of 2013 — 17,200.
Daily production capacity of these plants in gallons — 23 billion.
Percentage of these plants that make potable water from sea water — 59%.
Percentage that make potable water from brackish water — 22%.
Percentage that make potable water from river water and wastewater — 9%.
Total dissolved solids (TDS) count of the saltiest of sea waters (e. g,.the Arabian Gulf) — 50,000 mg/L.
Total dissolved solids (TDS) count of most ocean water — 35,000 mg/L.
Typical chloride content of sea water — 19,000 mg/L.
Typical calcium content of sea water — 400 mg/L.
Typical sodium content of sea water — 10,500 mg/L.
Typical number of viruses present in one drop of sea water — 1,000,000.
Nominal pore size of a reverse osmosis membrane used for desalination — 0.0001 to 0.001 microns.
Nominal pore size of a nanofiltration membrane — 0.001 microns.
Psi equivalent of one bar of pressure — 14.5.
Pressure required to treat sea water by reverse osmosis desalination — 55 to 70 bars (800 psi to 1000 psi).
Pressure required to treat brackish water by reverse osmosis desalination — 15 to 35 bars (220 psi to 500 psi).
Percentage of salts, organics, and microbes that are removed from sea water by high pressure reverse osmosis — 99%+.
Estimated per gallon cost of producing fresh water by desalination with reverse osmosis –1/2 cent.
Desalination plant at Al Khaluf in Oman
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Note: Below is a straightforward explanation of the highly problematic group of water contaminants known as VOCs. The article is from the Calgon Carbon Corporation, a leading supplier of water treatment media. We’ve edited a bit for clarity.
Volatile organic compounds (VOCs) are a large class of substances that may be found in various water sources. What follows is a straightforward guide to understanding VOCs, specifically, what qualifies a compound as a VOC, how to detect them, and how to treat and remove them from the water system.
As the name implies, a VOC is defined primarily by two things. The first is that the chemical is volatile, which means that it easily changes state from liquid to gas with a relatively small amount of energy. Most VOCs have a comparatively low molecular weight, which is one of the reasons for this volatility. The other key defining factor is that it is organic, meaning the molecule is composed primarily of carbon atoms.
Most VOCs are manmade products, although a few, such as acetone, are also naturally occurring. Nearly all VOCs end up in the water supply via industrial processes, chemical spills, or other human activity. Half come from industrial processes, 45% from motor vehicles, and 5% from consumer solvents.
Federally regulated VOCs are listed under the Safe Drinking Water Act (SDWA). Of course, each individual state may have its own list of regulated VOCs beyond those in the SDWA.
For regulated VOCs, the EPA and state agencies set two levels for each chemical: a maximum contaminant level (MCL) and a maximum contaminant level goal (MCLG). The MCL is the largest permissible concentration of the chemical in water, while the MCLG is the concentration at which there is no known or expected health risk.
VOC Types And Properties
Not all VOCs behave the same. There are three sub-classifications of VOCs based on their boiling points:
- VVOCs (very volatile organic compounds). With boiling points of <0°C to 50-100°C, many of these exist solely in a gaseous state. Examples include butane, propane, and trichloromethane.
- VOCs (volatile organic compounds). Although the term VOC is often used to describe chemicals from all three subcategories, technically it applies only to those with boiling points in the 50-100°C to 240-260°C range. Some examples are ethanol, acetone, and vinyl chloride.
- SVOCs (semi-volatile organic compounds). The least volatile subclass is defined by boiling points from 240-260°C to 380-400°C. Phthalates, many pesticides (including DDT), and nitrobenzene are some such examples.
VOCs can be further categorized as either hydrophobic (repel water) or hydrophilic (attract water). Hydrophobic VOCs (e.g., benzene) usually have smaller molecular weights and do not dissolve easily in water, which makes them relatively easier to move to a gaseous state. By contrast, hydrophilic VOCs (e.g., acetone) tend to have higher molecular weights and are more easily dissolved in water, which makes them relatively harder to move into a gaseous state.
Removing VOCs From Water
There are two primary methods for removing VOCs from source water.
Air Stripping. The process of forcing air through water works well on VOCs with lower boiling points (especially VVOCs) and/or those that are hydrophobic. This includes chemicals such as vinyl chloride, methyl chloride, chlorofluorocarbons, and methane.
Activated Carbon. Higher-molecular-weight VOCs won’t be as responsive to air stripping. For these chemicals, a granular activated carbon (GAC) filter is a more effective solution. The activated carbon can adsorb most VOCs, including those that are more difficult to remove via air stripping. Because VOCs diffuse quickly through the carbon bed, however, it is important to ensure the carbon has a high iodine number. Usually, about 1,000 to 1,100 is ideal to reduce the number of changeouts.
Establishing A Buffer For VOCs
It’s rare for water treatment plants to discover new VOCs in their source water. Most water systems are well established, and the challenge is less about tackling a previously unencountered chemistry but rather struggling to meet established and new MCLs.
That said, chemical spills can happen anywhere. For example, a city that pulls from a riverway with heavy boat traffic is always susceptible to some type of spill, as anything that is on a boat can end up in the water. Even chemical spills on land, such as a tipped gas tanker, can result in VOCs in groundwater. A GAC system that is in place for everyday VOCs will act as a buffer, ready to adsorb new contaminants should they enter the source water.
Pure Water Gazette Afternote
A much more complete listing of commonly encountered VOCs can be found on the Pure Water Occasional website. Calgon Corporation’s article is intended mainly for water treatment plant operators. Treatment options for residential users do not include air stripping. For residential use, activated carbon is by far the best protection. Coconut shell carbon is generally regarded as the treatment of choice for VOCs and this can be in the form of carbon beds (backwashing whole house filters), cartridge style whole house or drinking water filters filters, or the carbon filters that are part of undersink reverse osmosis units. The main thing to know about VOC protection with carbon filters is that VOCs are much more difficult to remove than chlorine. For VOC removal filters need a slower flow rate to allow more residence time and more frequent replacement. The need for frequent replacement and slower flow rates is evident in the following from the spec sheet of the CTO Plus coconut shell carbon block.
Capacity in gallons:
- Chlorine: 240,000 @ 7 gpm,
- Chloramine: 12,000 @ 3 gpm,
- PFAS: 21,000 @ 3 gpm,
- VOC: 4,500 @ 2 gpm
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As a general rule of thumb for residential users, VOCS are very difficult to remove, so a carbon filter that removes VOCs can be expected to remove most chemical water contaminants whether they have been tested for specifically or not.
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Water News Briefs for April 2023
Lead Pipe Replacement Moves at a Snail’s Pace
In 2018, almost 30 cities across New York state received federal money to carry out a specific, urgent task: removing lead service lines that poison drinking water. The city of Troy, NY – which sits across the Hudson River and just north of Albany – was among them, receiving $500,000. But five years later, city leaders have failed to spend a single dollar of that money, and have yet to remove a single lead pipe. The revelation emerged at a city council meeting this winter, raising all sorts of questions. Chief among them is why the city hasn’t spent the money. Troy’s failure illustrates the challenges small cities face when trying to address environmental injustices like lead pipes. Full article from The Guardian.
World’s Ocean Temperature Hits All-Time High
The temperature of the world’s ocean surface has hit an all-time high since satellite records began, leading to marine heatwaves around the globe, according to US government data. Climate scientists said preliminary data from the National Oceanic and Atmospheric Administration showed the average temperature at the ocean’s surface has been at 21.1C since the start of April – beating the previous high of 21C set in 2016. “The current trajectory looks like it’s headed off the charts, smashing previous records,” said climate scientist Prof Matthew England. Hotter oceans provide more energy for storms, as well as putting ice sheets at risk and pushing up global sea levels, caused by salt water expanding as it warms. Marine heatwaves can also have devastating effects on marine wildlife and cause coral bleaching on tropical reefs. The Guardian.
Railroads Follow Rivers
Decisions made more than 150 years ago about where to run railroad tracks have significant consequences today when trains derail. That’s because rail lines usually follow rivers. At a time when climate change is altering rainfall and flooding patterns increasing the risk of washouts and mudslides on tracks, it is increasingly risky and irresponsible to run large trainloads of petroleum and hazardous chemicals along the banks of rivers. The Colorado River, for example, furnishes water for 40 million people. A significant derailment along the Colorado could be devastating. USA Today.
Industry Knew about the Dangers of PFAS Decades Ago, but Kept It Secret
Industry sponsored studies documented PFAS toxicity in 1978 but this information was not shared with the US EPA until 2000. PFAS have been linked to a range of health problems, including testicular and kidney cancers, decreased birth weight, and thyroid disease. While most companies have stopped producing two forms of PFAS— perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS)—the chemicals persist in drinking water systems, and new forms of PFAS are raising concerns. This revelation underlines the basic truth that allowing industry to voluntarily regulate itself does not work. Strong governmental oversight is essential.
Ice Sheets Are Shrinking Much Faster Than Predicted
New research shows that the massive ice sheets at the top and bottom of our planet are shrinking much faster than previously thought. The international study compiled satellite measurements over time and depicts what one researcher described as a “devastating trajectory.” PBS News.
Garden Hose Art Like the Decorative Basket Above Was Once Featured in National Garden Hose Day Events
National Garden Hose Day Is On the Ropes Because of the Political Climate
National Garden Hose Day, a lusty national holiday celebrated in June, reached peak popularity as an early summer good time event in the years preceding the pandemic. Garden Hose Day festivities were curtailed beginning in 2020 due to public health concerns, and, unfortunately, efforts to bring the holiday back have faltered because of the divisive political climate.
Many US cities have already cancelled Garden Hose celebrations this year because of fears of transgender participation in the Garden Hose Tug, a tug of war event that is often the center of Garden Hose festivities. Critics also point out that the garden hose is not mentioned either in the Bible or the US Constitution and that the shape of the garden hose itself has sexual implications and should not be seen by children under 18. In Texas and Mississippi there are now bills before the state legislature to remove books from libraries that have pictures of garden hoses.
(Since fake news has become so common that it’s hard to recognize these days, the Pure Water Gazette wishes to advise that the article above is completely phony. Please do not sue or write abusive letters. While we’re at it we’ll confess to having invented and shamelessly promoted a non-existent holiday for a number of years. To our credit, we did not try to sell you a tee shirt or a NFT depiction of Garden Hose Superman blasting the enemies of the Second Amendment with a high powered fire hose. Here’s a Garden Hose Day article from 2013, back in the good old days before politics got so nasty that you couldn’t enjoy a good old summer Garden Hose Tug or Hose Blast competition.)
Can Water Cause Weight Gain?
Important research published in the journal Obesity establishes a definite link between weight gain and PFAS in drinking water. “Our study adds new evidence that being overweight isn’t just about a lack of physical activity and unhealthy eating habits – PFAS are increasingly suspected to be a contributing factor.” The PFAS exposures in the European participants are quite comparable to levels in America, so my concern is that our exposures to PFAS are making it difficult for us to avoid getting overweight.” Full article in Pure Water Gazette.
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TDS
TDS stands for “Total Dissolved Solids.” Solids might also be called dissolved minerals, ionic species, or salts. TDS is usually measured in ppm (parts per million) or mg/L (milligrams per liter), which are essentially the same. TDS is, in short, a measurement of all the dissolved mineral content of the water. A test for TDS does not measure chemicals or pathogens; a low TDS count does not mean that water is safe to drink.
TDS is measured often by laboratories with a conductivity meter, which quantifies the water’s ability to conduct electricity. The higher the mineral content, the better it conducts electricity. For more practical purposes, a TDS meter, which works on the same principle, is used. Conductivity is read in micromhos per centimeter. The familiar TDS meter, an inexpensive and very handy tool, converts conductivity to ppm TDS for convenience.
The TDS Tester is an effective tool that gives an instant reading of any water. Just turn it on and insert the bottom part of the tester into the water. More information.
There is often confusion about TDS meters and what the readings mean. TDS meters measure the performance of reverse osmosis units, distillers, and deionizers, but except for limited use by professionals, they do not measure the performance of filters or water softeners. Softeners and filters do not affect TDS readings significantly. A softener, to illustrate, removes calcium and magnesium ions but the TDS reading will not be affected significantly because the softener adds a more-or-less equal amount of sodium in exchange. The TDS reading of softened water is usually slightly higher than the TDS of the untreated water. You need a hardness test to judge softener performance, not a TDS meter. Filters, especially when they are new, usually add TDS (the phenomenon is called “TDS throw”). Likewise, the performance of softener alternatives, either tank-style or electronic, cannot be measured by a TDS meter.
Classifying Water by TDS
Although other TDS classifications may differ slightly, here is a good basic TDS breakdown from a publication of the Water Quality Association of America:
Water Type
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TDS, in mg/L
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Fresh Water
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<1,000
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Brackish Water
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1,000-5,000
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Highly Brackish Water
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5,000-15,000
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Saline Water
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15,000-30,000
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Sea Water
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30,000-40,000
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Brine
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40,000-300,000+
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Note that in standard usage these classifications are applied loosely. “Brine,” is used in water treatment for the salty water used to regenerate a softener or for the reject water from a reverse osmosis unit. In the case of the RO unit, the “brine” (a.k.a. “concentrate”) could be less than 50 mg/L in TDS. And although the song says that the moon was bright and shiny out on the briney, even sea water doesn’t technically qualify as brine. Similarly, saline often means any salty solution, and brackish is often used just to mean really bad water without specific reference to its TDS.
The EPA suggests an upper TDS limit for drinking water of 500, although many cities exceed this limit without dire consequences. For residential water use, when water gets above 1,000 TDS it is starting to border on being unusable, although some well owners grit their teeth and put up with problems like badly stained fixtures, stopped up plumbing, or water so high in sodium that it isn’t good for plants. Actually fairly high TDS water can be usable but it isn’t pleasant to deal with.
Hardness does not always result from high TDS. In our area in Texas, for example, much of the well water is high in sodium but naturally soft. If water has a TDS of 600 and a hardness reading of 2 grains (about 35 ppm), you can be virtually certain that it has a lot of sodium in it. If the high TDS consists mainly of calcium and magnesium (the hardness minerals), it can be softened, but the resulting water will be high in sodium.
Treating High and Low TDS
Treating low TDS is not common, but it can be done by using filters with a sacrificial medium like calcite. As water passes through the filter, it dissolves some mineral content and the TDS goes up. This can be done for point of entry (whole house) or point of use (drinking water only) applications. Small filters are now often used to “remineralize” reverse osmosis water. Minerals are dissolved by the low TDS water passing through the filter, raising the TDS count.
Lowering TDS is done by reverse osmosis, the most common method used in residential settings, distillation, or deionization. Reverse osmosis reduces TDS 90%+ (99% for larger, high pressure units), while distillation and DI (deionization) units can reduce TDS to a zero meter showing. Filters do not reduce TDS, not even the extremely tight ones.
Practical TDS Tips for Residential RO Users
The main use for a handheld TDS tester is to verify the performance of your RO unit’s membrane. While TDS is not in itself a targeted “contaminant” like lead, arsenic, or nitrates, the TDS meter verifies the health of the RO membrane. If the RO unit is reducing dissolved solids by 90%, you can be sure it’s also doing a good job on aluminum and fluoride.
The purpose of the TDS test is to tell you when to change your membrane. If you have an excellent TDS reading, that does not mean you don’t need to change your filter cartridges. The membrane should be changed on need—as indicated by the TDS test—but cartridges are changed on time. In fact, keeping the cartridges fresh is the best way to protect your membrane.
The worst time to do a TDS test is immediately after changing your filter cartridges. The new carbon postfilter will produce a “TDS throw” that will make your TDS reading high. Take a TDS test before you change your cartridges. The same principle applies to new RO units. You won’t get a reliable TDS test until the unit is a couple of weeks old. If you want to test shortly after installation, take loose the tube going into the storage tank and take your sample there—before the water goes through the post filter.
An acceptable TDS reading is a matter of personal preference. On our residential RO units, we usually change membranes when the unit consistently fails to reduce TDS by 85% or so.
To determine this, test first the tap water from the faucet, then compare it with the water coming out of the RO unit. This is called “% rejection,” and the formula is TDS of tap water minus TDS of RO water divided by TDS of tap water times 100. To illustrate, our local tap water usually runs around 180 TDS. So, if we test an RO system that shows a TDS reading of 15, the arithmetic would be 180 minus 15 = 165 divided by 180 = around 9.16 X 100 = about 92% rejection. That’s fine.
The best advice is don’t obsess over TDS readings from a home unit. TDS is somewhat fickle and can be changed by variables like water pressure and the amount of water being used. Don’t be too quick to change a membrane if you get one bad test.
(This article as well as hundreds of others is available any time you want them on the Pure Water Gazette website. It's an easy site to use and we don't sell advertising or beg you for information while you're there.)
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Places to visit for additional information:
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Thanks for reading. The next Occasional will be out eventually--when you least expect it.
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