Water Softener Use Raises Questions for Septic System Owners
From Pipeline, a newsletter of the National Small Flows Clearinghouse, Winter, 2001
Parts of the U.S. have what is commonly referred to as “hard
water,” and people who live in these areas battle the problems that hard
water creates. The most common defense against hard water, which is a
nuisance but not a health hazard, is to install a home water softener.
Water softening involves exchanging calcium and magnesium
minerals present in the water-which cause the hardness-with sodium. As
the water softener processes gallon after gallon of hard water on a
daily basis, the treatment capability of the softener becomes depleted
and must be recharged or regenerated.
Regenerating the unit uses a large quantity of sodium-rich
water, called “brine,” that must be disposed of. In homes with onsite
septic systems, this brine flows into the septic tank and eventually
makes its way to the system’s drainfield. People using
home water softeners often wonder whether these units might cause
problems for their onsite septic systems. Some common questions
consumers ask are:
- Do water softeners hurt the bacteria that work in a septic tank?
- Does the additional amount of water from water softener regeneration affect a septic system’s performance?
- Does the concentrated salt water used in regeneration decrease the drainfield’s ability to absorb wastewater?
Unfortunately, experts don’t all agree on the answers to
these questions. Research has been done that resulted in acceptable
conclusions to many people in the industry, but some authorities believe
that more studies are needed to determine what impact, if any, brine
has in a septic system.
Because of these differences of opinion, this Pipeline issue
is unlike most others. Ordinarily we offer information that gives
readers concrete answers. But because water softener brine disposal
remains somewhat controversial, Pipeline will provide opinions on the
research that has been done to date. We know people want
information about this subject. At this point, our mission is to help
readers become better-educated consumers so that they can make informed
decisions when trying to resolve their hard water problems. Some steps
that homeowners can take to minimize their concerns about the safe
operation of their onsite sewage systems are included.
Readers are encouraged to reprint this issue or any Pipeline
articles in flyers, newspapers, newsletters, or educational
presentations. Please include the name and phone number of the National
Small Flows Clearinghouse (NSFC) on the reprinted information and send
us a copy for our files. If you have questions about
reprinting articles or about the topics discussed in the newsletter,
please contact the NSFC at (800) 624-8301 or (304) 293-4191.
Home septic systems
Knowing how a septic system works helps homeowners understand why adding something like water softener regeneration brine may cause problems. A
conventional septic system consists of a septic tank, a distribution
box, and a drainfield, all connected by pipes. When wastewater flows
from the house, it is temporarily held in the septic tank where heavy
solids (sludge) settle to the bottom. Lighter materials float on the
surface of the water in the tank and are called the scum layer. This
separation is known as primary treatment.
The solids that collect in the bottom of the tank and the
materials that float in the scum layer are partially decomposed with the
help of bacteria that occur naturally in human waste. The liquid
between the solids and the scum flows out of the tank through a baffle
(or a tee) and into a distribution.
As sludge accumulates in the bottom of the tank and its level
rises, new wastewater coming from the house has less time for suspended
particles to settle into the sludge layer.These suspended particles can
flow into the absorption field. The septic tank must be
pumped out periodically to remove the accumulated sludge and scum and to
prevent clogging the drainfield box. The distribution box evenly
separates the flow into a network of drainfield pipes. Each pipe has
holes in its underside that allow the water to drain into gravel-filled
trenches. The water slowly seeps into the soil beneath the trenches
where it is further treated. This process is called secondary treatment.
What makes water hard? And how does it become soft?
As water flows through layers of rock underground, it picks
up loose particles and dissolves minerals from its surroundings. Because
of this characteristic and the kind of rock common in many aquifers,
calcium and magnesium minerals are frequently found in household water.
Water with substantial amounts of calcium and magnesium is referred to
as “hard water.”
Hard water minerals reduce water’s ability to function
effectively in our homes. For instance, bath soap combines with the
minerals and forms a pasty scum that accumulates on bathtubs and sinks.
Homeowners must use more soap and detergent in washing, so expense for
these products increases. These minerals also combine with
soap in the laundry, and the residue doesn’t rinse well from fabric,
leaving clothes dull. Hard water spots appear on everything that is
washed in and around the home — from dishes and silverware to the family
Hard water not only affects household cleaning, but the
minerals also can build up on the inside of pipes in the plumbing
system. And in water heaters, the minerals settle on the heating
element, the walls of the tank, in the hot water pipes, and in faucets
where they produce a scale (similar to the original rock) that reduces
the efficiency and life of the hot water system.
Water Softeners Make Water Work Better
Water softeners combat this nuisance by eliminating the
minerals that cause hard water. The most common kind of water softener
is a mechanical appliance plumbed directly into the home’s water supply
intake. (See figure 1) The water softener exchanges calcium and
magnesium with sodium in a process called ion exchange.
The water softening system consists of a mineral tank and a
brine tank. The water supply pipe is connected to the mineral tank so
that water coming into the house must pass through the tank before it
can be used.
mineral tank holds small beads (also known as resin) that carry a
negative electrical charge. The positively charged calcium and magnesium
(called ions) are attracted to the negatively charged beads. This
attraction makes the minerals stick to the beads as the hard water
passes through the mineral tank. (See figure 3.)
Eventually the surfaces of the beads in the mineral tank
become coated with the calcium and magnesium minerals. To clean the
beads, a strong sodium (salt) solution held in the brine tank is flushed
through the mineral tank. Sodium ions also have a positive electrical
charge, just not quite as strong as that of calcium and magnesium. This
large volume of sodium ions overpowers the calcium and magnesium ions
and drives them off of the beads and into the solution. The sodium
solution carrying the minerals is then drained out of the unit. Some
sodium ions remain in the tank attached to the surfaces of the beads.
The normal water softening cycle operates like this: Hard
water enters the mineral tank. Inside the tank, the calcium and
magnesium ions carried in the water attach themselves to the beads. The
surfaces of the beads eventually hold their limit of calcium and
magnesium and can’t remove any more from the water. At this point the
water softener must be "regenerated". (See figure 2.) The three-step
regeneration cycle can be scheduled according to a timer or by a flow
The first step, called the backwash phase, reverses the
water’s flow and flushes any accumulated dirt particles out of the tank
and down the drain. Next, in the regeneration or recharge phase, the
sodium rich brine solution flows from the brine tank into and through
the mineral tank. The brine washes the calcium and magnesium off the
beads. In the final phase, the mineral tank is flushed of the excess
brine, which now also holds the calcium and magnesium, and the solution
is disposed of down the drain.
Sodium ions from the previous regeneration cycle cling to the
beads. Now when the hard water flows into the mineral tank, the calcium
and magnesium ions change places with the sodium ions on the resin. The
displaced sodium ions remain dissolved in the water.
Reducing regeneration brine flow
- Install a water softener whose
backwash/regeneration cycle is based on need, not on a timer. A water
softener operated by a time clock regenerates the mineral tank on a
regular schedule, regardless of how much water has been used. A
softening unit that is regulated by a flow detection meter measures the
amount of water that has been used and regenerates the water softener
accordingly. These units can cost $100 to $450 more than timer-regulated
- Buy a water softener with a large mineral tank. The larger
tank may cost more initially, but it will not have to be recharged as
frequently as a smaller tank.
- Be more conservative with household water use. Less water used
in the home means that less water will be measured going through the
softener process; therefore, a unit with a flow detection meter won’t
have to regenerate as often.
Researchers Look for Answers
The Water Quality Research Council and the Water Quality
Association (WQA) supported two studies in the late 1970s: one by the
National Sanitation Foundation (NSF International) in Ann Arbor,
Michigan, and the other conducted by the Small Scale Waste Management
Project (SSWMP) at the University of Wisconsin in Madison. Both studies
compared the performance of home sewage treatment systems with and
without added water softener brine.
The two studies were designed to help answer questions
consumers ask about their water softeners. The SSWMP research sought to
determine if a water softener’s brine affects a drainfield’s ability to
absorb wastewater. NSF investigated whether the influx of brine from a
water softener’s regeneration phase affects the processes that occur in a
sewage treatment tank. Researchers also wanted to find out if the
additional water discharged during backwash and regeneration (up to an
extra 50 gallons) plus that water’s flow rate into the septic tank
interfere with the settling and floatation processes.
NSF’s researchers used individual aerobic wastewater
treatment units to study possible effects the brine might have on
treatment processes in the tank. (See the Winter 1996 Pipeline for
details about aerobic treatment units.) The normal performance of both
septic tanks and aerobic tanks depends on the presence of active
bacteria living in the system. These bacteria help break down the solids
in the wastewater. An aerobic treatment system uses bacteria that
require oxygen to live; whereas, an anaerobic system, such as a
conventional septic tank, treats wastewater by using bacteria that
thrive in conditions lacking air. If high doses of sodium from water
softener regeneration and other household products flow into the tank,
the bacteria could be affected. If the bacteria are negatively affected,
the system might not operate at its full potential, and some of the
solids might not fully decompose.
Researchers from NSF found that brine wastes had no negative
effects on the bacterial population living in the aerobic treatment
tank, even when the system was loaded with twice the normal amount of
brine. The tests determined that water softener wastes actually help
with treatment processes. WQA’s final report states that the wastewater
has “a beneficial influence on a septic tank system by stimulating
biological action in the septic tank and caused no operational problems
in the typical anaerobic or the new aerobic septic tanks.” In other
words, the researchers in this study found that microorganisms living
and working in a home aerobic treatment system are not harmed by water
Researchers also found that the additional amount of water
discharged to a treatment tank during the regeneration process had no
negative impact. The question concerned whether the volume and flow rate
of the regeneration brine might overload the system and cause carry
over of solids into the drainfield. The study found that the volume of
water discharged was comparable to or less than that from many automatic
washing machines and other household appliances. Researchers also found
that the wastewater flowed into the treatment tank slowly enough so
that it caused minimal disturbance.
The study at the University of Wisconsin-Madison examined
whether regeneration brine affected the soil in a septic system’s drain
field. This research was prompted by the common knowledge that sodium
causes some soil particles to swell, thereby reducing water's ability to
seep readily through the soil. Researchers found that the water
softener regeneration brine did not reduce the percolation rate of water
in the absorption field of a normally operating septic system. This
conclusion was reached because the brine contains sodium, but it also
includes significant amounts of calcium and magnesium. The calcium in
the brine acts similarly to gypsum, a calcium rich substance routinely
used to increase the porosity of clay soils in agriculture. The research
report stated that calcium, therefore, helps counteract any negative
effects of the sodium.
Most manufacturers and many industry experts agree with the WQA ’s position.
As with most scientific research, these two studies answered
each of the proposed questions under the specific conditions of the
research project. Because other variables exist that weren’t part of the
study’s setup (e.g., problems that might occur because of a poorly
functioning home water softening unit) some people feel that more
research needs to be done to completely resolve the disputed issues.
The NSF study, for example, used an aerobic treatment tank
rather than an anaerobic tank (a conventional septic tank). Conventional
septic systems are much more common than aerobic treatment units. An
aerobic system often has a pretreatment tank to settle out much of the
solids. Aerobic systems require air to be injected into the tank to
support the growth of the suspended aerobic bacteria that digest solids
in the wastewater.The wastewater in the tank is constantly stirred to
mix in the air.
On the other hand, a conventional septic tank separates
solids from wastewater by settling. In a properly functioning
conventional system, most of the solids sink to the bottom of the tank
leaving the liquid portion relatively clear. The anaerobic bacteria do
their work without the wastewater in the tank being agitated. Would
the same results have been found if a conventional septic tank had been
used? National Small Flows Clearinghouse Senior Engineering Scientist
David Pask remains skeptical. He tells of residential drainfield
failures that he has seen where the distribution pipes were plugged with
"a noxious fibrous mass" that, under microscopic inspection, appeared
to be made up of grease and cellulose fibers identical to fibers from
Pask said the homes all had ion exchange water softeners
connected to their water supplies, and softener brine discharged into
each home’s septic system. His discussion of the situation with
colleagues led him to question if the sodium in the "plug flow" of brine
might cause metabolic shock to the bacteria in the septic tank. This
shock could cause the bacteria to be less able to digest the cellulose
fibers, which then might be carried over into a septic system’s
"I still believe that the case for discharge of softener wash
to the septic tank is unproven and that some research is justified,"
Pask said. "As for research needs ...I would add to the list the effects
of plug flow of brine on the digestion of cellulose and scum components
in a standard septic tank."
Terry Bounds, an engineer and respected expert in the
wastewater industry, also would like to see more research done before
any conclusive statements are made about the effects of water softener
regeneration brines in septic systems. In the summer 1994 issue of Small
Flows (the precursor of the Small Flows Quarterly magazine) Bounds
stated that in his work he has seen noticeable differences between
septic tanks with and without water softener brine discharges in septic
tank effluent pump (STEP) systems (an alternative wastewater collection
and treatment method) and in conventional systems. Bounds said that in
the tanks with added water softener discharge, he saw reduced scum layer
development, carryover of solids and grease to the pressure sewer
collection system, and a less distinguishable "clear zone" that might
mean solids remain suspended instead of settling in the tank.
Today, Bounds contends that research has focused on water
softener discharge of regeneration brine under ideal conditions. Water
softeners that malfunction or are not used correctly (i.e., timed to
regenerate too frequently) may cause septic system problems. "Our
experience with regard to the operation and maintenance of systems that
discharge water softener backwash to septic tanks is that it has a
detrimental effect on the effluent that is discharged," Bounds said. "As
a researcher, I believe that when "all" the variables and processes are
evaluated and monitored, measured performance and science will share
close results. So far, I have seen no research that compares to typical
environmental engineering sciences in anaerobic digesters. Most of the
reports that I’ve seen suggest that this research still needs to be
National Small Flows Clearinghouse (NSFC)
The NSFC offers a variety of technical assistance and free
and low-cost information and materials about wastewater technologies for
small communities. Just a few of the NSFC’s many resources and services
are mentioned in this newsletter. Call the NSFC at (800) 624-8301 or
(304) 293-4191 or visit our Web site at http://www.nsfc.wvu.edu/ for more information.
Water Quality Association
The Water Quality Association (WQA) is the
international trade association representing the household, commercial,
industrial, and small community water treatment industry. WQA is a
resource of information, product testing, and professional certification
for all water users. Contact the WQA at (630) 505-0160 or visit their
Web site at http://www.wqa.org/.
(formerly called the National Sanitation Foundation)
International is a private, nonprofit organization devoted to research,
education, service, and training. NSF tests and publishes standards for
products related to public health and the environment.Contact them at
(800) 673- 6275 or visit their Web site at http://www.nsf.org/.