Water softeners and septic systems

Best for

Homes on a conventional septic tank and drainfield that is sized correctly and working, with a demand-initiated softener set for high salt efficiency.

Wrong fit

Undersized, aging or failing systems, sites where the drainfield is hydraulically limited, and advanced treatment units. Follow the system maker and local rules, and consider routing discharge elsewhere.

Tradeoff

Most of the research showing no harm was paid for by the softener industry. The findings are consistent, but independent extension advice leans more cautious.

The short answer

For a conventional septic tank and drainfield that is sized correctly and working, an efficient softener is generally fine. The research that exists points the same way: the problems show up when a softener uses far more salt than it needs, not from softening itself.

Two caveats matter. Most of the studies were funded by the water treatment industry. And some extension services and local codes still recommend or require sending softener discharge somewhere other than the septic system. Read both sides below, then check with your county.

What a softener sends down the drain

When a softener regenerates, it backwashes the resin, draws brine from the salt tank through it, and rinses. The wastewater carries the calcium and magnesium the softener removed, plus leftover sodium chloride. EPA’s 2002 onsite manual and Penn State Extension both put the volume at about 50 gallons per regeneration.

According to the industry’s own summary of the Virginia Tech work, the salt in that wastewater is diluted about sixfold by the rinse steps and then 160- to 400-fold once it mixes with the rest of the household’s wastewater.

The worries people raise come down to three questions, as Missouri Extension frames them:

1. Is the brine toxic to the bacteria that digest solids in the tank?
2. Does the regeneration flow stir up the tank and push solids into the drainfield?
3. Does sodium make clay soils in the drainfield swell and drain more slowly?

What the research found, and who paid for it

The Virginia Tech result is the most useful one for a homeowner. The authors found that calcium and magnesium in regeneration waste helped solids settle, while excess sodium from salt-heavy settings did the opposite. In their words, as quoted by WQA, the impact depends “on the level of hardness in the water, whether the regeneration waste is discharged to the septic tank, and the amount of excess sodium present in regeneration wastes.”

EPA’s view. EPA’s 2002 Onsite Wastewater Treatment Systems Manual has a fact sheet on softeners. It summarizes the Wisconsin and NSF studies and concludes that wastewater from properly operating and maintained softeners will not harm onsite systems that are designed, operated and maintained appropriately, and that the roughly 50 gallons per recharge does not cause hydraulic overload. The same fact sheet also says predicting impacts is “difficult and often inconclusive,” and elsewhere the manual says softener inputs should be eliminated where a site’s drainfield is hydraulically limited.

What independent extension guidance says

Extension advice is more mixed:

• University of Missouri Extension summarizes the Wisconsin and NSF results as a no to all three questions above, and says an undersized tank or heavy use of drain cleaners and antiseptic products is more likely to cause trouble.
• University of Minnesota Extension’s Septic System Owner’s Guide (1997) advises recharging the softener as infrequently as possible and rerouting softener recharge water outside the septic system, noting it “does not need to be treated.” Its tank-cleaning worksheet also counts a water softener or iron filter as a factor that shortens the time between pumpings.
• Penn State Extension says the roughly 50 gallons per regeneration “may or may not cause hydraulic overload.”
• University of Vermont Sea Grant says softener brine and backwash probably will not harm most septic systems, though they could call for a slightly larger tank and soil treatment area.

The fair reading: efficient softeners and healthy, properly sized septic systems coexist in many rural homes. The risk rises with inefficient settings, old timer-based softeners, small or failing systems and tight drainfields.

How to protect your septic system

WQA and the National Onsite Wastewater Recycling Association (NOWRA) published joint guidance in 2013 for conventional tank-and-drainfield systems. Combined with the research above, the practical steps are:

1. Use a demand-initiated (metered) softener. It regenerates only when capacity is used, saving salt and water. If you have an old timer unit, NOWRA and WQA suggest replacing it, or having it set to regenerate no more often than needed and switched off when the house is empty.
2. Set it for high salt efficiency. The Virginia Tech study saw benefits at 3,000 to 4,000 grains of hardness removed per pound of salt, which WQA describes as highly rated efficiencies. NOWRA and WQA say to make sure the salt setting matches the higher efficiency ratings. See water softener settings.
3. Size it and program it correctly. Enter your real hardness rather than a high “safe” number, which only adds regenerations and salt. See how to size a water softener.
4. Cut other water loads. Fix leaking toilet flappers, and keep sump pumps, floor drains and roof drains out of the system.
5. Go easy on chemicals. Avoid excessive bleach, strong disinfectants, “every-flush” toilet cleaners and caustic drain cleaners.
6. Keep up septic maintenance. Have the tank inspected and pumped on schedule.
7. If you suspect a problem, NOWRA and WQA suggest getting both a septic professional and a water treatment professional to look at it, since few people are expert in both.

When to route the discharge elsewhere

Consider sending softener discharge somewhere other than the septic system, where local code allows, if:

• Your system is undersized, old or already showing signs of failure.
• Your drainfield is hydraulically limited (EPA’s manual singles this out).
• You have an advanced treatment unit. The NOWRA/WQA guidance is written for conventional systems and says to follow the manufacturer’s instructions for advanced systems.
• Your county or state requires it. Rules vary, so ask your local health or environmental department before re-plumbing.

Don’t divert by default, though. In the Virginia Tech work, the poorer solids settling showed up both when softeners used excess salt and when the regeneration waste was diverted, because the calcium and magnesium in that waste were no longer reaching the tank. WQA’s summary describes the study as independent; it was funded by WQA’s research foundation, with an outside steering committee, so weigh it with that in mind.

A salt-free conditioner avoids the discharge entirely, but it does not remove hardness and usually needs iron-free water. See salt-free vs salt water softeners.

Bottom line

On a working conventional system, the evidence supports using a softener, provided it is efficient and set correctly. The research was largely industry-funded, but the mechanism it describes is consistent: calcium and magnesium in regeneration waste are not the problem; excess salt is. Spend your effort on efficient settings, and check the local code before you assume either way. For running costs of an efficient setup, see water softener cost.