Water Softener for Septic Systems: Safe Picks and Limits

We have not tested these products. The guidance below comes from published sources, and the product details come from each maker’s own page.

What the research says

The strongest study is a 2013 Virginia Tech report by Novak and Hogan. The Water Quality Research Foundation, part of the water-treatment industry, paid for it. The researchers filled lab columns with solids from working septic tanks, fed them brine, and also sampled real tanks in New York and North Carolina. Their finding: what matters is how much extra sodium the brine carries compared with calcium and magnesium. A softener that removes about 4,000 grains of hardness per pound of salt improved the tank effluent compared with tanks where the brine was sent elsewhere. One that removes about 1,000 grains per pound increased solids and oxygen demand in the effluent. The authors say a lab cannot fully copy a field tank and that more long-term field data is needed.

University of Missouri Extension, summarizing University of Wisconsin and NSF studies, says brine is not toxic to septic bacteria, does not carry solids into the drain field, and did not hurt a normal absorption field. A 1994 Virginia Department of Health memo draws on the same Wisconsin work.

Other agencies are more cautious. Rhode Island’s environmental agency says brine disrupts settling of solids in the tank and does not recommend sending it to a septic system. It reports that Massachusetts and Connecticut prohibit it. The Virginia Tech report adds that Delaware barred new installations after March 2009 from discharging regeneration waste to a septic system. The EPA’s SepticSmart guide says some water treatment systems, softeners included, can send hundreds of gallons of water to a septic tank and suggests asking a licensed plumber about alternative routing.

In plain words: the evidence is mixed. None of the studies cited here shows that brine from an efficient softener damages a healthy system, and the main industry-funded study found a benefit. But the newest controlled study came from the industry, some states and local agencies still advise against the practice, and real tanks differ. The problems that keep coming up are inefficient softeners, extra water, clay soil and small or neglected tanks.

Regeneration water: how much goes to the tank

Penn State and Nebraska put the water used at about 50 gallons per regeneration. Rhode Island says a properly used softener regenerates one to three times a week and produces 40 to 150 gallons of brine a week, and that a badly set one can produce much more. A 1994 Virginia memo allowed softeners without any change to the septic design when they sent 60 gallons or less per cycle and 180 gallons or less per week. Treat that as one old state’s yardstick, not a current rule for your county.

The volume matters because it adds to the daily load on the tank and drain field. Missouri Extension says a washing machine is a bigger load than a softener. Ask the installer to set the regeneration time for when the washer and dishwasher are idle.

Published figures for the products below: the Fleck 5600SXT 48,000-grain lists about 25 to 50 gallons per regeneration. The Matrixx 1 cu ft model lists 33, 59 or 67 gallons depending on its salt setting, so its highest-salt setting runs above the Virginia 60-gallon figure.

Salt efficiency and demand-initiated regeneration

A timer-based softener regenerates on a schedule whether or not the resin needs it. Penn State says timed models waste water and salt. A demand-initiated (metered) unit counts the gallons used and regenerates only when the resin is nearly full. NDSU adds that recharging too early wastes salt and water. The Water Quality Association says most softeners sold today are demand-initiated, and that a unit running at 3,000 to 4,000 grains of hardness per pound of salt will not harm a septic system.

Salt efficiency is the second half of the story, and the Virginia Tech results show why:

Salt efficiency (grains removed per pound of salt)	Sodium compared with calcium and magnesium in tank effluent	What the study found
About 1,000	11 to 1	Higher solids and oxygen demand in the effluent
About 2,000	5 to 1	The negative effect was greatly lessened
About 4,000	3 to 1	Effluent better than when the brine was diverted

You can estimate your own unit’s number by dividing its grain capacity by the pounds of salt it uses. The Matrixx 1 cu ft model lists 15,000 grains at 3 lb of salt, 24,000 at 6 lb and 32,000 at 15 lb. That works out to about 5,000, 4,000 and 2,100 grains per pound. The same unit can sit in the good zone or the poor zone depending on a setting. The Fleck 48,000-grain lists about 45,000 grains at 15 lb and 36,000 at 9 lb, which is about 3,000 and 4,000 grains per pound.

The practical lesson: buy for the capacity you need at a low-salt setting, not the nameplate number at the maximum setting. The softener size calculator shows how hardness and household size set that capacity.

Potassium chloride swaps sodium for potassium. It costs more, and NDSU says about 10 percent more of it is used per backflush. The studies above looked at sodium brine, so potassium is not proven to be better for a drain field. See the potassium chloride comparison.

Drain-field soil

Sodium can change how some soils behave. The Gallatin Local Water Quality District in Montana notes a concern that in clay that shrinks and swells, a lot of sodium over many years could cut the soil’s ability to take water. Rhode Island mentions reports of leachfields clogged by soil minerals reacting with brine. The Wisconsin work cited by Missouri found the opposite: the calcium in the backwash acted like gypsum, which is used to open up clay soils.

So the soil question is open. If your drain field sits in clay, or it is already slow or soggy, ask a septic professional or your county health department before adding a softener. The system’s design or permit records usually name the soil type.

Sodium and chloride also pass through the drain field toward the water table. Gallatin warns they can reach your own well. That is one reason Rhode Island wants any dry well 100 feet or more from wells.

Tank size and condition

Missouri Extension says an undersized septic tank and heavy use of drain cleaners and household antiseptic products are more likely to disrupt a system than softener brine. The WQA and NOWRA guidance lists fixing leaking toilet flappers and valves, because they overload a system over time, and avoiding excess bleach and caustic drain cleaners.

The EPA says household septic tanks are typically pumped every three to five years. A tank that is overdue has less room for solids to settle, which is the thing the brine studies worry about. Know your tank size in gallons and the date of the last pumping before you shop. For a new system, Montana’s fact sheet advises sizing the tank and drain field for the added softener flow.

Should the brine go somewhere else?

Rhode Island recommends a below-ground dry well instead of the septic system. Some states go further and prohibit brine in the septic system. The Virginia Tech study cuts the other way: if an inefficient softener sends brine away from the tank, the calcium and magnesium no longer reach it, and the effluent can get worse.

Two more cases need a check. Rhode Island says brine can disrupt an advanced treatment system and voids the maker’s warranty on the common ones there, so ask the maker of an aerobic or advanced unit first. Montana does not allow backflush to the septic system when the softener also feeds outdoor irrigation lines. Local rules decide, so call the county health department before any plumbing change.

Softeners with published features that fit

Three published numbers are what separate a fit from a guess: water per regeneration, salt per regeneration with the capacity at that salt dose, and whether the valve regenerates on demand.

Product	Price	Regeneration	Published water and salt
Matrixx MXX-100 (1 cu ft)	From $1,466.95	Metered, Bluetooth app	33, 59 or 67 gal; 3, 6 or 15 lb
Fleck 5600SXT 48,000-grain	$1,995	Metered valve	25 to 50 gal; about 7 lb
Fleck 5600SXT 32,000-grain	$1,495	Metered valve	About 5 lb; no water figure
American Valve ScaleRX	$215	None, no salt	Not applicable

Matrixx. The 15 GPM model is $1,635 and the 20 GPM model is $1,775. The maker claims up to 70 percent less salt and water than standard settings. No NSF/ANSI 44 softener listing is shown; the certifications named cover the valve and components. Its setting choices make it the easiest of these to tune to a septic system, as long as you leave it off the maximum-salt setting.

Fleck 5600SXT 48,000-grain. From Mid Atlantic Water, with a 5-year valve and 10-year tank warranty. It is sized for one to four people on water up to about 15 grains per gallon. The compact 32,000-grain model lists about 5 lb of salt per regeneration but no water figure, and its seller usually starts homes at the 48,000-grain size because a smaller unit regenerates more often. See the Fleck 5600SXT review.

SoftPro Elite HE. From $1,327 to $1,477, depending on size. Its page lists precision variable brining and clear-water iron handling up to 3 ppm, but it gives no salt-per-regeneration figure, so ask for one in writing before you count it as a fit. It also suits wells, as covered in the well water softener guide.

Salt-free. The ScaleRX uses no salt, power or filters and carries a 76 percent scale-reduction rating from IAPMO R&T testing. SpringWell lists its FutureSoft with no salt, drain line or backwash, from $2,184 as a bundle with a whole-house filter, and it needs iron, manganese and hydrogen sulfide at 0 ppm. Neither softens: they reduce scale, but hard water still dries on glass and soap still lathers poorly. Our salt-free softener guide and the softener versus conditioner comparison cover the trade-off.

Who should skip a standard salt softener

• Anyone where the local rule bars brine in a septic system. Rhode Island’s agency reports that Massachusetts and Connecticut prohibit it, the Virginia Tech report says Delaware restricted it, and a county can be stricter than its state.
• Anyone with an aerobic or advanced treatment system until the system’s maker says in writing that softener discharge is allowed.
• Anyone whose tank is overdue for pumping, or whose drain field is already slow, soggy or in clay.
• Anyone who needs real softening from a salt-free device. It will not give you soft water.
• Anyone with a lot of iron in the water. Installers count each 1 ppm of dissolved iron as about 5 grains of hardness when they size a unit, so iron makes it regenerate more often. See the best water softener guide.

Buying mistakes to avoid

• Judging by the nameplate grains. A 32,000-grain unit gives that number only at a high salt dose, which is the least efficient setting.
• Buying a timer-based unit. It regenerates on a schedule whether or not the resin is full.
• Skipping the written numbers. If a page gives no water or salt per regeneration, ask the seller before you buy.
• Forgetting the plumbing. A separate brine line or dry well is extra work and may need approval. See the softener cost guide.
• Leaving it on factory settings. Set the hardness to your lab result, not a guess.

Questions to ask before you buy

• The seller: How many gallons of water and pounds of salt per regeneration at the setting I will use? How many grains per pound is that? Does the valve regenerate on demand?
• The county health department: Is brine from a softener allowed in my septic system? Are there setbacks to wells?
• A septic professional: What size is my tank, when was it last pumped, what is the soil, and should the brine go elsewhere?