Water treatment field guide: softeners, filtration, and RO for plumbers

Water treatment, and why the test comes first

Water treatment is the equipment you put on a building's incoming water to change what is in it: a softener for hardness, filters for sediment and chlorine, an oxidizing filter for iron, a neutralizer for low pH, reverse osmosis for dissolved solids, and UV for bacteria. Each piece fixes one problem. None of them fixes all of them, and putting in the wrong one fixes nothing while costing the same money.

The single decision that drives everything else is the water test. You do not size a softener, pick a filter micron, or specify a UV lamp until you know what is actually in the water. Hardness, iron, manganese, pH, total dissolved solids, chlorine, and any contaminant flagged for the source: those numbers are the design inputs, the same way load and length are the inputs to a feeder. The water service guide covers how the water gets into the building. This guide is about conditioning it once it is there.

Treat the problem you have, not the problem the brochure sells. A softener will not touch a chlorine taste. A carbon filter will not touch hardness. A salt-free conditioner will not remove a single grain. Match the device to the test result, in the right order, sized for the flow, and most of the callbacks on this work never happen.

Why test the water before you treat it?

Test the water first because every piece of treatment downstream is sized and selected from the numbers, and guessing wrong wastes the equipment budget on the wrong problem. A certified lab test, or a utility consumer confidence report on municipal water, gives you the baseline. On a well you order the test; on a city service you can start with the published report and confirm the parts that matter at the tap.

The numbers that drive the design are a short list. Hardness, reported in grains per gallon or ppm, sizes the softener. Iron and manganese, in mg/L, decide whether you need an oxidizing filter ahead of the softener. pH tells you whether the water is eating the copper. Total dissolved solids points at reverse osmosis. Chlorine or chloramine sets the carbon stage and warns you the disinfectant residual is there to protect against bacteria. On a private well you also test for coliform bacteria and nitrate, because nobody else is.

Retest after a problem changes. Well chemistry shifts with the season and the water table, and a municipal supply can change source or switch from chlorine to chloramine without telling the customer in a way they notice. A treatment train sized to last year's test can be wrong this year.

What does hard water do?

Hard water is water carrying dissolved calcium and magnesium, and what it does is leave those minerals behind as scale everywhere it gets heated or evaporates. Inside a water heater the scale bakes onto the elements and the tank bottom, insulating the heat from the water, so the unit works harder, runs longer, and dies early. The water heater guide covers what that scale does to each heater type. Hard water is the reason it happens.

Past the heater, hardness shows up as the white crust on faucet aerators and showerheads, the film on glass and tile, the soap that will not rinse and the curd in the laundry. Soap reacts with calcium and magnesium to form scum instead of lather, so you use more of everything and clean less. Restriction builds in pipes and valves over years as scale narrows the bore.

The cost is not the water bill. It is the shortened life of every appliance that heats or moves the water, the higher energy to push heat through scale, and the fixtures and valves replaced ahead of their time. On a commercial dish machine, an ice maker, or a boiler, the scale math gets expensive fast, which is why hardness is usually the first thing treated.

Grain per gallon (gpg)

The common US hardness unit; 1 gpg is about 17.1 ppm as calcium carbonate

Scale

Calcium and magnesium deposited as a hard crust where water is heated or evaporates

How does a water softener work?

A water softener removes hardness by ion exchange. The tank is full of resin beads that hold sodium ions, and as hard water passes through the bed, the resin grabs the calcium and magnesium and releases sodium in their place. The water that leaves is soft. This is the only common treatment that actually takes the hardness minerals out of the water rather than changing how they behave.

The resin fills up. Once the beads are loaded with calcium and magnesium they stop exchanging, and that is what regeneration fixes. The control valve runs a cycle: a backwash that lifts and rinses the bed, a brine draw that pulls concentrated salt solution from the brine tank across the resin so the flood of sodium drives the hardness back off the beads, a slow rinse, and a fast rinse that flushes the spent brine and loosened hardness to the drain. Then it refills the brine tank for next time. The hardness and the salt go down the drain together.

Salt is the consumable. A sodium-cycle softener regenerates on sodium chloride; potassium chloride is the swap for customers cutting sodium, at higher cost. The customer keeps the brine tank full of salt, and that is most of the ongoing maintenance. Run it dry and the resin never recharges, so hard water breaks through and nobody notices until the scale comes back.

Sizing the softener, and metered vs timer regeneration

Size a softener so it regenerates on a sane schedule, which means matching the grain capacity to the daily hardness load. The working number is hardness in grains per gallon times the daily water use in gallons. Figure roughly 75 gallons per person per day on a residence, so four people at 15 gpg run about 4 people times 75 gallons times 15 gpg, near 4,500 grains a day. Add about 5 grains of capacity for each 1 mg/L of iron the softener is asked to carry, and add reserve so it is not regenerating daily. Manufacturer capacity ratings and the salt dose set the real number, so size to their tables.

Metered regeneration beats timer regeneration on almost every job. A metered (demand-initiated) valve counts the gallons actually used and regenerates when the measured capacity is spent, so it uses salt and water in proportion to demand and rides out a slow week or a full house. A timer just regenerates every so many days no matter what, which either wastes salt on a light week or runs out of soft water when a houseful of guests shows up. Spec metered unless cost rules it out.

Undersizing is the common failure. A softener sized too small for the hardness regenerates constantly, burns salt, and still lets hard water break through near the end of each cycle. When the customer complains of spotting that comes and goes, the unit is usually undersized for the load, not broken.

What is the difference between salt-based and salt-free?

The honest answer is that salt-based softeners remove hardness and salt-free conditioners do not. A salt-based ion-exchange softener pulls the calcium and magnesium out of the water. A salt-free unit, usually template-assisted crystallization (TAC), leaves every grain of hardness in the water and only changes its form so it is less likely to stick as scale.

TAC media gives the dissolved calcium and magnesium a surface to crystallize on, forming tiny stable crystals that tend to stay suspended and wash through instead of plating onto pipe and heater surfaces. It uses no salt, no electricity, and no drain, and it makes no brine to dispose of. For scale control on a building that cannot take a brine discharge, or where the customer wants no sodium added, it has a real place.

What it does not do matters. The water is still hard. Soap still will not lather, glass still spots, and a hardness test still reads the same grains, because the minerals are still in there. Sell TAC as scale control, never as softening. The instant a customer is told salt-free water will feel soft or stop the spotting, the callback is booked, because the test will prove it never removed anything.

Sediment filtration and the prefilter

A sediment filter pulls suspended solids out of the water: sand, silt, rust flakes, and grit. It is a mechanical strainer rated in microns, and it goes first in the train because dirty water fouls everything downstream. A clogged sediment cartridge is cheap. A softener resin bed or a fouled RO membrane is not.

Micron rating sets what it catches. A coarse spin-down screen at 50 to 100 micron knocks out sand and the big stuff and backflushes with a valve. A cartridge or bag filter at 5 to 20 micron takes the finer silt, and the tighter you go, the more often it loads up and needs changing. Watch the nominal versus absolute rating: a nominal 5 micron catches most of that size, an absolute 5 micron catches essentially all of it, and the difference matters when something downstream needs clean water to work.

On a well with real sediment, a two-stage prefilter earns its keep: a coarse spin-down to handle the volume, then a finer cartridge to polish. Size the housing for the flow so the filter is not the choke point, and put a pressure gauge before and after so the customer can see the cartridge loading instead of waiting for the pressure to fall off at the fixtures.

Carbon filtration for chlorine, taste, and odor

Activated carbon takes out chlorine, chloramine, taste, odor, and many organic compounds by adsorption: the contaminants stick to the huge internal surface of the carbon as the water passes through. It is the stage that makes city water taste like water instead of a pool, and it removes the chlorine that would otherwise shorten softener resin and degrade an RO membrane. Carbon does not remove hardness, dissolved minerals, or nitrate, so it is a partner to the softener, not a replacement.

Carbon comes two ways. A backwashing carbon tank holds a deep bed of loose granular carbon, gives long contact time, and periodically backwashes to declump and reclassify the bed; it is the choice for whole-building flow and chlorine removal. A carbon block cartridge is a fixed element with tight pore structure that filters and adsorbs at once, common at point of use and for finer taste-and-odor polishing, and it is changed rather than backwashed.

Carbon has a service life that is invisible. It does not clog like sediment; it just quietly fills up and stops adsorbing, and the chlorine starts passing through. Chloramine in particular needs more contact time and the right catalytic carbon, so confirm what the utility uses before you spec the bed. Change or rebed on the manufacturer's gallon or time rating, not on how the water looks, because exhausted carbon looks identical to fresh.

Iron and manganese, and the well-water stain

Iron and manganese are a well-water problem, and they announce themselves with stains: orange-brown from iron, black from manganese, on every fixture, in the toilet tank, and in the laundry. The water can run clear from the tap and then stain minutes later as the dissolved metal hits air and oxidizes. A metallic taste comes with it.

The treatment is an oxidizing filter. A manganese greensand or catalytic media oxidizes the dissolved iron and manganese into solid particles, then filters those particles out and backwashes them to the drain. Greensand is regenerated with potassium permanganate as the oxidizing coating is consumed, and the bed needs regular backwashing to clear the accumulated metal. These filters work over a moderate range, commonly cited around 3 to 10 mg/L of combined iron and manganese, with the exact range, media, and regeneration set by the manufacturer and the water chemistry.

Order matters here. Oxidizing iron filtration goes ahead of the softener, because heavy iron will foul softener resin and a low dose of iron the softener might handle still eats into its hardness capacity. A softener can remove a small amount of dissolved iron, but it is not an iron filter, and counting on it for that is how the resin gets fouled and the spotting comes back.

Acidic water, low pH, and copper corrosion

Acidic water, pH below 7, corrodes the plumbing from the inside, and on copper it is unmistakable: blue-green stains in sinks and tubs, a metallic taste, and pinhole leaks in pipe that should have lasted decades. It is a well-water problem most often, but soft and low-mineral water can be aggressive too. Ignore it and the customer reroofs the copper system over the next few years one leak at a time.

Two treatments raise pH. A calcite neutralizer is a tank of crushed calcium carbonate that the acidic water slowly dissolves on contact, raising pH and adding a little hardness; it suits slightly acidic water, commonly in the pH 6.0 to 6.5 range, and the media is simply topped off as it dissolves. For very low pH, generally below 6.0, a soda ash feeder injects sodium carbonate solution with a metering pump to bring the pH up, which adds no hardness but needs a pump and solution maintenance.

Test after the neutralizer, not just before. Calcite adds hardness as it dissolves, so a building that neutralizes acidic well water may then need softening it did not need at the wellhead. The two pieces interact, and the train has to be designed for the water as it actually leaves each stage.

Where does reverse osmosis fit?

Reverse osmosis fits at the tap, not the whole building, on most jobs. RO pushes water under pressure through a semipermeable membrane that rejects dissolved solids, commonly cutting total dissolved solids by around 96 to 98 percent, along with many contaminants a softener and carbon cannot touch. It is the answer for high TDS, nitrate, arsenic, and a long list of dissolved problems, and for drinking water that has to taste and test clean.

The catch is the reject water. RO sends a meaningful share of the feed water to the drain as concentrate, often on the order of a couple gallons rejected per gallon produced depending on the membrane and pressure. A point-of-use unit under the kitchen sink makes the few gallons a day the household actually drinks and cooks with, so the waste is small and the membrane is cheap to change. A whole-building RO treats every gallon that flushes a toilet or waters a lawn, which wastes water at scale and needs pumps, tanks, and remineralization to be usable.

So the usual right answer is point-of-use RO for drinking and cooking, paired with a whole-house softener and carbon for the rest. Whole-building RO is reserved for the specific case where the incoming water is bad enough across the board, very high TDS, brackish, or contaminated, that nothing smaller solves it. Spec that deliberately, knowing the water and energy cost, not as a default.

UV disinfection for untreated water

UV disinfection kills bacteria, viruses, and cysts by passing the water through a chamber lit by an ultraviolet lamp, with no chemical added and no taste or residual left behind. It is the microbiological treatment for water that is not chlorinated, which mostly means private wells and other non-municipal sources where a coliform test came back positive or the source is at risk.

UV only works on water it can shine through. Sediment, iron, and high hardness shade the bugs from the light and coat the quartz sleeve, so UV goes last in the train, after the water is clear. That is the whole reason for the order: filter and soften first, disinfect the polished water. A UV unit on dirty water gives a false sense of safety while passing live organisms in the shadow of every particle.

It leaves no residual, which is the trade-off. UV disinfects at the point it sits, but it does nothing for the piping downstream, so a contaminated or stagnant branch past the lamp is back to untreated. Size the unit to the flow so every gallon gets the rated dose, change the lamp on schedule even though it still glows after it has lost output, and keep the sleeve clean.

The treatment train order

The order of the train is not a preference, it is a sequence each stage depends on. The common arrangement is sediment first, then any iron or pH correction, then the softener, then carbon, then point-of-use RO or whole-house UV last. Each stage protects the ones behind it, and getting the order wrong fouls equipment that was working fine.

Sediment leads so grit never reaches the resin or the membrane. Iron oxidation and pH correction come early because iron fouls resin and low pH eats everything. The softener goes ahead of carbon and RO so the membrane and the carbon are not fighting hardness scale. Carbon strips the chlorine before RO, because chlorine attacks many RO membranes, and before UV it removes nothing the UV needs but it is fine there. UV goes dead last so it disinfects water already clear of the particles that would shadow the lamp.

There is one judgment call worth naming. Chlorine protects against bacteria, and carbon removes that chlorine, so on a system that then sits with long stagnant runs you have taken away the residual that was holding bacteria down. That is the link between the carbon stage and the stagnation problem later in this guide, and it is why dechlorinated water on a building with dead legs deserves a second thought.

Point-of-entry vs point-of-use

Point-of-entry treats the whole building at the service entrance; point-of-use treats one fixture, usually under a sink. The split decides what you treat where, and matching the technology to the right point saves both money and water. Softening, sediment, carbon, iron, and pH are whole-building problems, so they go point-of-entry where they protect every fixture and the water heater. RO and sometimes UV are point-of-use, because treating only the water you drink is far cheaper than treating the water you flush.

The mistake runs both directions. Putting RO whole-house when point-of-use was right wastes water and money on toilet and irrigation water that never needed it. Putting a drinking-water cartridge only under the kitchen sink when the whole house has hardness leaves every shower, heater, and appliance scaling. Read the test, decide which problems are building-wide and which are drinking-water-only, and place each device accordingly.

A clean layout is often both: point-of-entry softener and carbon for the whole building, with a point-of-use RO at the kitchen for drinking and the ice maker. That covers the scale and chlorine everywhere and the dissolved-solids problem where it counts, without the cost and waste of treating every gallon to drinking standard.

Sizing the train for flow without a pressure drop

Every device in the train has a service flow rating, and the whole train has to pass the building's peak flow without dropping the pressure at the fixtures. A softener, a carbon tank, and a sediment housing each have a rated gallons-per-minute through which they treat properly and below a pressure loss; gang several undersized devices in series and the cumulative drop shows up as weak showers when the dishwasher and a hose run at once.

Size to the peak demand, not the average. The fixture-unit method that sizes the water service also sizes the treatment that the service feeds: figure the simultaneous demand the building can actually pull, then pick each treatment device so its rated flow covers that peak with margin. The water service guide covers the fixture-unit demand calculation; the treatment train has to be sized to the same peak, or it becomes the restriction the whole system was sized to avoid.

Pressure drop stacks. A clean sediment cartridge might cost a couple psi, a softener a few, a carbon bed a few more, and as the cartridge loads the drop grows. Add the stages at peak flow, compare to the available pressure, and leave headroom for the filters loading up between changes. The reading at the gauge under load is the truth; the rating on the box is the starting point.

The brine drain and the air gap

A softener regenerates by flushing brine and hardness to a drain, and that drain connection is a code item, not a convenience. The discharge must be indirect, through an air gap, into an approved receptor: a floor drain, a standpipe, a laundry tub, or a similar fixture. It cannot tie directly into the waste piping, because a direct connection could siphon sewage back into the softener and from there into the potable water.

The air gap is a physical vertical separation between the end of the drain line and the flood rim of the receptor, with no possible back-siphon path. Plumbing codes commonly call for an air gap of at least twice the diameter of the drain line and in no case less than the code minimum above the receptor, so confirm the dimension in the adopted code (IPC or UPC) and any local amendment. A backwashing filter, an RO drain, and a softener brine line all follow the same indirect-waste rule.

Two more drain realities. The receptor has to handle the regeneration flow rate, which is more than a trickle, or it overflows during a cycle. And brine is salty: discharging softener brine where it reaches a septic system or a sensitive sewer can be restricted by local rule, and some jurisdictions limit or ban softener discharge for that reason. Check where the brine is allowed to go before you set the unit, not after.

The bypass, service, and what wears out

Put a bypass on the treatment so the building keeps water while the equipment is serviced or swapped. A softener bypass valve, or a three-valve bypass loop around any treatment device, lets you isolate the unit, change a cartridge, rebuild a control valve, or troubleshoot without shutting the whole building down. A train with no bypass means every service call is a water shutoff, and that is the first thing that gets cursed during a cartridge change.

The maintenance is predictable, so set the customer's expectation up front. Salt gets topped off in the brine tank, the most frequent task. Sediment and carbon block cartridges are changed on a gallon or pressure-drop schedule. Backwashing media beds backwash on the controller and get rebedded on the manufacturer's life. RO membranes and prefilters are changed on schedule. UV lamps are replaced annually even though they still light, because output falls before the lamp dies.

Resin and media wear out on a longer clock. Softener resin commonly lasts well over a decade on clean water, less where chlorine or iron is attacking it, which is another reason carbon and iron filtration ahead of it pay off. When soft water quality falls off and the unit is sized and salted right, suspect fouled or aged resin before you condemn the valve.

Treating to protect the water heater, boiler, and fixtures

The strongest case for treatment is equipment protection, because scale and corrosion cost far more than the treatment does. Softening hard water before it reaches a water heater keeps scale off the elements and the tank, holds the efficiency where the rating says it should be, and adds years to the unit. The water heater guide walks through what scale does to tank, tankless, and heat pump units; treating the hardness is how you stop it.

It scales up from the residence. A tankless heater is especially scale-sensitive because it heats a thin passage fast, so hard water descales it on a short clock and many makers void the warranty without treatment. A commercial boiler, an ice maker, a steam table, a dish machine, a humidifier, and a cooling tower all live or die on water quality, and on those the scale and corrosion math justifies the treatment on first cost alone.

Corrosion is the other half. Low pH eats copper and steel, high TDS and chlorides drive pitting, and oxygen and microbiological growth attack closed systems. Matching the treatment to what the equipment needs, soft water for the heaters and boilers, low-chloride and pH-corrected water where corrosion is the threat, protects the expensive iron downstream. The treatment is the cheap insurance on the costly equipment.

Stagnation, biofilm, and Legionella

Treatment is half the water-quality picture; movement is the other half. Legionella and other bacteria grow in biofilm on the inside of premise piping, and they grow worst where water sits warm and still: dead legs, oversized mains running slow, capped-off branches, and fixtures that rarely run. Stagnant water loses its disinfectant residual, warms into the growth range, and lets biofilm establish, and once biofilm is set it shelters the organisms from later treatment.

This intersects with treatment in one way worth flagging. Carbon removes the chlorine residual that was suppressing bacteria, so a dechlorinated building with stagnant runs has lost a control it used to have. That is not a reason to skip carbon; it is a reason to design out the stagnation: avoid dead legs, size piping to keep velocity up, and flush low-use runs on a schedule, commonly at least weekly, in line with CDC and ASHRAE guidance for premise plumbing.

On larger and higher-risk buildings this becomes a managed water-safety program, and the standards for it are a topic of their own. The point for the treatment installer is narrow: do not create a stagnant, dechlorinated, warm dead leg and walk away, because that is exactly where the problem grows.

Commercial and process water: boilers, cooling towers, kitchens

Commercial and process water is treated to a tighter spec than domestic water, because the equipment it feeds is less forgiving and the failure is more expensive. Boiler feedwater is softened and often further treated to strip hardness and dissolved oxygen, because scale on the tubes wastes fuel and a thin scale film can overheat and fail the metal. The treatment a boiler needs is set by its pressure and the manufacturer, and it is a specialty beyond a domestic softener.

Cooling tower makeup is its own problem. A tower concentrates whatever is in the makeup water as it evaporates, so hardness, TDS, and silica climb and scale and fouling follow, managed with treatment and controlled blowdown. Kitchens have specific needs too: hardness scales dish machines and ice makers and spots glassware, while steam and combination ovens often call for a defined water quality the equipment maker specifies. The right water for a dish machine is not the same as the right water for a steam oven.

Match the treatment to the process, and pull in the specialist where the process demands it. A general softener and carbon train serves the kitchen sinks and the domestic side; the boiler, the tower, and certain kitchen equipment carry their own water-treatment requirements that the equipment documents and the relevant trade specs control. Treat the building water to a sane baseline, then meet each process spec on top of it.

Does municipal water need treatment, or just wells?

Both can need treatment, but for different reasons, and a well almost always needs more. Municipal water arrives already treated and disinfected to the federal Safe Drinking Water Act standard, chlorinated or chloramined and tested by the utility, so on city water the usual jobs are hardness, the chlorine taste, and drinking-water polishing. The bacteria question is mostly handled before the water reaches the property.

A private well is the owner's responsibility and nobody else's. It is not routinely tested, not disinfected, and exposed to whatever the geology and the surroundings put into it: hardness, iron and manganese, low pH, sediment, nitrate from agriculture, and bacteria from surface intrusion. So a well typically needs the fuller train, sediment and iron and pH correction and softening, with UV for disinfection because there is no residual protecting it.

The design rule follows from the source. On municipal water, start from the consumer confidence report and confirm hardness and chlorine at the tap. On well water, start from a full lab panel including coliform bacteria and nitrate, and design for everything the test flags. Same equipment catalog, different starting assumptions, and the test settles which devices the building actually needs.

Field checklist

What to document

Write down the test result, the treatment it drove, and the maintenance it needs, so the next person can see why each device is there and keep it running. A treatment train nobody documented becomes a row of tanks nobody understands, serviced wrong or not at all.

Record the water test numbers and date, each device with its capacity or rating and where it sits in the train, the softener salt type and regeneration setting, the cartridge and media change intervals, the drain connection and air gap, and the retest after startup. If a neutralizer or a salt-free unit is in the train, note what it does and does not change, because that is the line that gets misremembered later.

Common mistakes

• Selling and installing treatment without a water test, then fixing the wrong problem.
• Undersizing the softener for the hardness so hard water breaks through at the end of each cycle.
• Selling salt-free TAC as softening when it removes no hardness and only conditions against scale.
• Leaving out the sediment prefilter and letting grit foul the softener resin and the RO membrane.
• Putting the softener ahead of an iron problem and fouling the resin with iron the unit cannot handle.
• Ignoring low pH and watching copper pinhole over the next few years.
• Specifying whole-house RO where point-of-use was right, wasting water on every flushed gallon.
• Connecting the brine drain directly to waste with no air gap, or to a drain too small for the regeneration flow.

Standards and references

The product side runs on NSF/ANSI certifications, and naming the right one tells you what a device is actually verified to do. NSF/ANSI 44 covers cation-exchange water softeners. NSF/ANSI 42 covers aesthetic reduction, chlorine, taste, and odor. NSF/ANSI 53 covers contaminants with a health effect. NSF/ANSI 58 covers reverse osmosis systems, and NSF/ANSI 55 covers UV systems, with Class A rated for disinfection of microbiologically unsafe water and Class B for supplemental treatment of disinfected water. Match the certification to the claim, and read what the unit is listed for rather than the marketing.

The water test itself should come from a certified laboratory, or from the utility's consumer confidence report on municipal supply, with private wells tested for coliform bacteria and nitrate as a baseline. The plumbing code, the International Plumbing Code or the Uniform Plumbing Code as adopted, governs the indirect-waste and air-gap requirements for the brine and backwash drains, with local amendments that can also restrict where softener brine may discharge. Confirm the adopted edition and amendments with the AHJ before you rely on a specific dimension.

On the microbiological and process side, CDC and ASHRAE guidance address Legionella control in building water systems, and boiler, cooling tower, and specialty kitchen equipment carry their own water-quality requirements set by the equipment manufacturer and the relevant trade specifications. Sizing numbers, capacities, doses, and ranges in this guide are typical figures to start from; the water test, the manufacturer's data, and the adopted code control the final design.

Units, terms, and conversions

Water-quality numbers show up in a few unit systems across a lab report, a utility report, and an equipment sheet, so the same value can read differently depending on the page.

Hardness is reported in grains per gallon (gpg) or in ppm, also written mg/L as calcium carbonate, where 1 gpg is about 17.1 ppm. Iron, manganese, and other contaminants are in mg/L, which equals ppm for dilute water. Total dissolved solids is in ppm or mg/L. Filter ratings are in microns, nominal or absolute. Flow is gallons per minute, and pressure drop across a device is in psi. UV dose is in millijoules per square centimeter.

Hardness (gpg / ppm)

Dissolved calcium and magnesium; 1 grain per gallon is about 17.1 ppm as calcium carbonate

TDS

Total dissolved solids, all dissolved matter in the water, in ppm or mg/L

Ion exchange

Swapping calcium and magnesium on a resin for sodium, the way a softener removes hardness

TAC

Template-assisted crystallization, a salt-free conditioner that alters scale but removes no hardness

Micron

Particle size a sediment filter catches; nominal catches most, absolute catches nearly all

Point-of-entry / point-of-use

Whole-building treatment at the service versus single-fixture treatment, usually under a sink

Related tools

Calculators and readiness checks for this work

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