Water Softener Brine Recovery – A New Approach by Peter S. Cartwright, PE and Tom Cartwright

Introduction

As the world struggles with climate change and unprecedented weather events, our society is slowly
beginning to recognize the severity of the situation. Young people now understand and use terms like
sustainability and circular economy while demonstrating a commitment to addressing these issues.
Concerns such as water conservation and pollution control are gaining traction and the environment is
the better for it.

Pollution regulations have been in place for over 60 years, on both the state and national level; however,
we feel that the growing awareness of challenges to the health of our planet will result in even greater
regulatory activity. A significant source of pollution is chloride discharge from water softener
regeneration activities. The sodium ion exchange process for removing hardness (and to a limited extent
iron and manganese) from water supplies is used by close to 100% percent of the residential units in
place today. This process primarily utilizes sodium chloride, or occasionally, when cost is not an issue,
potassium chloride as the chemical required to replace hardness ions on the softening resin. To reverse
the chemical ' “preference'” of hardness attachment to the resin, a high concentration of sodium ion is
necessary, hence the discharge issue. The US EPA National Secondary Standard limit for chloride
concentration in drinking water is 250 mg/L.

Numerous states have chosen to address chloride contamination and have set their own discharge
standards. For example, the Minnesota Pollution Control Agency has set a Water Quality Standard
Value for chloride of 230 mg/L based on effects on aquatic life. Some municipalities have limited the
discharge of water softener regenerant into municipal sewer systems and California has instituted
onerous restrictions on the use of residential water softeners.

It is estimated that there are over 10 million residential and some 60,000 commercial/industrial water
softeners in the US, today. Since 2020, there has been exceptional growth of residential softeners in
North America (over 20 percent per year); however, a realistic sustainable annual growth rate appears to
be 8-10 percent. There are several no-salt softeners water “softeners” on the market, but for a
number of reasons, the industry has been slow to embrace them. IAPMO The International Association
of Plumbing & Mechanical Officials (IAPMO) has formed a Technical Subcommittee (Z601) to develop
a test/performance standard on scale reduction devices (Z601).

Background

Traditional water softening is a batch process in that when the resin has adsorbed its capacity of cations
and is “exhausted,” the unit must be taken off-line, and the resin regenerated with salt brine. This process
involves running a high concentration of sodium chloride (approximately 10-percent brine solution)
through the resin, which strips off the adsorbed hardness ions and replaces them with sodium ions. This
is accomplished automatically, initiated by a volume controller (self-regenerating). It is important to
remember that the chloride ion does not take part in this chemical reaction, but is part of the
regeneration chemical, sodium, or potassium chloride. On the other hand, because the chloride ion in
high concentrations is corrosive and otherwise detrimental to the environment, it is the pollutant of
concern. The excess brine goes down the drain with the removed hardness ions.

The issue

Both sodium and potassium chloride are highly soluble and, as a result, are difficult to remove from
water without applying sophisticated technologies such as reverse osmosis or evaporation. Although
reverse osmosis is widely used for desalting seawater (with the primary contaminant sodium chloride), it
is not practical for treating discharged water contaminated with these salts, which are normally directed
to the municipal wastewater treatment system. The very high energy costs associated with evaporation
also eliminates this technology from consideration. The most practical solution is to remove the chloride
salts from the regenerant waste before discharge and recycle them as part of the brine solution. To do
this, the hardness ions must be first separated from the salt solution.
The technology
Nanofiltration membranes (NF), are designed to distinguish between the valance of ionic components,
rejecting multivalent ions (e.g., calcium and magnesium), while allowing monovalent ions (e.g., sodium,
potassium and chloride) to pass through into the permeate stream. We have utilized this membrane
chemistry to create a A new, patented treatment approach has been developed to recover and reuse most
of the brine from the water softening regeneration process. It utilizes nanofiltration (NF) technology to
separate hardness ions from the brine solution, discharging the hardness to the drain and returning
recovered sodium/potassium chloride to the brine tank for reuse.

The treatment system utilizes a 20-30 gallon reclaim tank to directly collect the softener regenerant
during the regeneration event. A 1,000- psig pump directs this into the NF membrane with the
concentrate (mainly hardness ions) discharged to drain and the permeate (sodium/potassium chloride)
returned directly to the brine tank for reuse. Testing has shown that a minimum of 75 percent of the
sodium/potassium chloride that would normally be discharged can be recovered and reused. In addition
to the significant reduction in chloride pollution, this process provides:
? Significant savings in salt purchases by the end user.
? Less maintenance required for the end user – reducing the time spent adding salt to
the brine tank.
? Approximately 35-40 -percent reduction in water used during the regeneration
process.

Following is a detailed description of the treatment process:

1. Upon initial start-up, the brine tank, containing an initial charge of solid sodium/potassium
chloride is filled to the appropriate level with tap water (or softened water). After the first
regeneration, the system is configured to add the appropriate level of water back into the brine
tank – thus mostly eliminating the Fill Cycle of the water softener. (Some makeup may be
required to account for water lost by evaporation).
2. When the softener regeneration process initiates the brine draw/slow rinse cycle, the water passes
through a monitor where the conductivity of the brine draw/slow rinse water leaving the resin
tank is monitored and once the conductivity reaches a preset level, this regenerant water is
redirected into the reclaim tank. This same monitor/diverter valve system discharges to drain the
excess water from the final rinse portion of the regeneration process. This is required to prevent
overflow of the brine tank.
3. The reclaim tank contains two level switches. Once the regenerant water reaches the top level
control, a high pressure pump is activated. The pump directs this stream through the NF
membrane element at high pressure (600 – 1000 psi). The majority of the monovalent ions pass
into the permeate stream, which is returned to the brine tank, while most of the multivalent ions
(hardness ions, etc.) are discharged to the drain.
4. The system is configured so that the volume of treated water directed back into the brine tank is
equal to the amount of water introduced during the Filla normal fill cycle.
5. Once the water in the reclaim tank drops down to the lower float control level, the system shuts
down.
6. At this point, the electronics activate a set of solenoid valves that allow tap water (or softened
water), to flush through the pump and NF element thereby minimizing corrosion and fouling of
the pump, membrane and membrane housing from the high salinity water.
At the conclusion of the flushing cycle, the system shuts down until the next regeneration process.

Figure 1: Overall system configuration.
Conclusion
An IAPMO Technical Subcommittee has been formed to develop a test/performance standard (ASSE-
1088) to address performance requirements for water softener regeneration – brine reclaim. The water
conditioning industry thrives on innovation and new developments. This is no exception. The
completion of this standard should establish the credibility to provide incentive for state and local
regulators to address this technology as an innovative approach to mitigate the growing chloride
pollution problem, a black mark on our industry, known for its environmentally advanced technologies.

~Tom Cartwright, former Global Business Manager for GE Water, is owner and Chief Science Officer
of Envi H2O/Safeway Water Technologies, a Florida-based water treatment company. With degrees in
Chemistry, History and Religion, Mr. Cartwright has With over 40 38 years in water treatment. He,
he has lectured and provided training on water treatment equipment in 43 countries. Tom Cartwright has
15 patents to date and is a published author – with numerous technical articles, as well as one fictional
novel.

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