Onsite Wastewater Treatment Systems
The following is the U.S. Environmental Protection
Agency?s Onsite Wastewater Treatment Systems Special Issues Fact Sheet on
Water Softeners. This document supports the WQA's position on softener
wastes and the fact that they are not harmful to septic systems. Home water
softeners, which periodically generate a backwash that is high in sodium,
magnesium and calcium concentrations, can affect wastewater treatment
processes, the composition and structure of the infiltration field biomat and
the underlying soil. However, attempts to predict whether impacts will occur and
to estimate their severity are difficult and often inconclusive.
Water softeners remove 'hardness' (dissolved
calcium and magnesium) through ion exchange processes. Incoming hard water
passes through a tank of high-capacity ion exchange resin beads supersaturated
with sodium. The calcium and magnesium ions in the water attach to the resin
beads, replacing the sodium, which is released into the water. The softened
water then is distributed for use throughout the house.
Over time, the ion exchange resin beads become saturated
with calcium and magnesium ions. When this occurs, the tank must be recharged
by flushing with a salt brine solution. Sodium ions reclaim their position on
the resin beads, and the calcium and magnesium ions are released into the backwash
water. The backwash water then exits the tank and is discharged to the
wastewater treatment system. The number of times the tank is recharged and the
amount of wastewater generated depends on a number of factors including the
hardness of the water, amount of water used, size of the water softener and
capacity of the resins to remove calcium and magnesium.
The wastewater generated during the recharge phase of the
water softening process mixes with other household wastewaters, enters the
septic tank and eventually moves to the soil adsorption field. Studies
conducted by soil scientists at the University of Wisconsin and the National
Sanitation Foundation conclude that the wastewater effluent generated from
properly operating and well-maintained water softeners will not harm onsite
systems that are designed, operated and maintained appropriately. Specifically,
the studies conclude the following.
concentrations of calcium and manganese in the softener backwash water have no
deleterious effect on the biological functions occurring in the septic tank and
may, in some cases, be helpful.
additional volume of wastewater generated (typically about 50 gallons per
recharge cycle) is added slowly to the wastewater stream and does not cause any
hydraulic overload problems.
structure in the soil absorption field is positively affected by the calcium
and magnesium ions in water softener effluent.2
Regarding the last conclusion, some people have the
misconception that the salt brine that enters the ion exchange tank also exits
the tank as wastewater. In fact, the influent with its high concentration of
sodium ions is very different than the effluent, which has a high concentration
of calcium and magnesium ions. Consequently, the potential for chemical
clogging of clayey soil by sodium ions is reduced. The calcium and magnesium
input may even help improve soil percolation.
Risk Management Issues
The human health impacts of ingesting softened water
increasingly are discussed in addition to the traditional benefits of reduced
use of surfactants and plumbing repair requirements. The choice of the
homeowner to soften or not to soften will factor into all arguments. Also, the
preceding descriptions are predicated on whole-house-supply softening. Today point-of-use
devices designed for use with specific features in the house make the
traditional advantages and disadvantages less clear.