Nitrate Removal by Ion Exchange

Nitrates in concentrations above 10 ppm expressed as N*
(this can be expressed as 35.7 ppm as calcium carbonate or 44.3 ppm as nitrate)
are considered unsafe. Nitrates have no detectable color, taste or smell at the
concentrations involved in drinking water supplies, and they do not cause
discoloration of plumbing fixtures, so they remain undetectable to our senses.
Nitrates do not interfere in non-potable domestic uses such as laundering.
Therefore, nitrate removal processes must be either foolproof or include
extensive monitoring of the treated water to detect breakthrough or determine
the need for regeneration.

Infants are particularly susceptible to nitrates because
their digestive systems do not operate in the exact same manner as adults.
Nitrates are converted by bacteria in the stomach of infants to toxic nitrites.
At levels that would not cause harm to adults, nitrates can cause methemoglobinemia
in infants, a condition also known as "blue baby" syndrome.

Nitrates present in a water supply can be a symptom of other
contaminants in that source of water. Have a full public health analysis
performed on waters being considered for treatment. Disinfection may be
necessary to counter other health concerns.

Sources of Nitrates

There are both natural and man-made sources of nitrates in
groundwater. The main source of nitrate contamination appears to be from
agricultural operations, farm runoff and fertilizer usage. There also is some
nitrate formed in the atmosphere by oxidation of nitrogen oxides that are
emitted from power plants and internal combustion engines. One other man-made
source is industrial corrosion inhibitors that have leaked into the ecosystem.

Naturally occurring nitrate can result from a combination of
nitrogen and oxygen through electrical discharges (lightning). Also, nitrate is
formed by Nitrobacter bacteria by oxidation of nitrites.

Resin Treatment Choices

Standard anion resins.
The two types of standard resins commonly used for nitrate removal today are
Type 1 and Type 2 strongly basic anion exchange resins. The Type 1 resin
derives its ion exchange capabilities from the trimethylamine group. The Type 2
resin derives its functionality from the dimethylethanolamine group. The
relative order of affinity for the three most common ions in drinking water
compared to nitrates is

Sulfate > Nitrate > Chloride > Bicarbonate

Figure 1a illustrates a breakthrough curve for a type 2
anion resin treating a water containing nitrates.

Selective resins.
The term "nitrate selective" refers to resins that retain nitrates
more strongly than any other ions including sulfates. A variety of functional
groups can and have been placed into anion exchange resins that are nitrate
selective. Most of these resins are similar to the Type 1 resins, but they have
larger chemical groups on the nitrogen atom of the amine than the methyl groups
that comprise a Type 1 resin. The larger size of the amine groups makes it more
difficult for divalent ions such as sulfates to attach themselves to the resin.
This reorders the affinity relationships so that nitrate has a higher affinity
for the resin than sulfates even at drinking water concentrations. The affinity
relationship for nitrate selective resins in drinking water is

Nitrate > Sulfate > Chloride > Bicarbonate

A fair number of nitrate-selective resins have been
synthesized, but only two are available commercially--the tributylamine and
triethylamine types. Although both the tributylamine and triethylamine resins
are approved by most European countries for potable water applications, they
are not listed by FDA.

Generally speaking, nitrate selective resins have from 10 to
100 times higher relative affinities for nitrates against sulfates than the
standard resins. Because of this, it is the sulfate ion that would be
"dumped." The phenomenon known as "dumping" occurs when
nitrate concentration in the treated water exceeds the concentration in the raw
water. When a nitrate selective resin is run past the point of exhaustion, the
nitrate concentration of the treated water will not rise past the concentration
in the raw water.

Each of the two types of nitrate selective resin has its own
advantages, depending on the application. The triethlyamine structure, because
of its smaller size, yields a resin with a higher operating capacity than the
tributylamine type. However, the tributylamine may provide lower chemical
operating costs in large systems when regenerant use is minimized through brine
reclamation schemes.

Perchlorates & Technicium

The nitrate selective resins are designed to be sulfate
"deselective" and, therefore, favor the removal of nitrates. These
same sulfate de-selective resins are finding favor in other specialty
applications that need to minimize the impact of sulfate and other anions on
the removal capacity. Two applications that are receiving a lot of attention
today are the removal of perchlorate and technicium, both of which can pose
serious health threats by contaminating groundwaters. Perchlorate is a
byproduct of rocket propellant manufacturing and has been detected in the
groundwater in more than 30 states. Technicium is a radioactive isotope that
appears in the wastewater from some nuclear operations.

Nitrate Dumping

The Type 1 and Type 2 resins are considered non-selective
because of their greater affinity for sulfates. If a normal Type 1 or Type 2
resin is run past the end of the normal nitrate removal service cycle, sulfates
can continue to load onto the resin bed, pushing the nitrates off and causing
dumping. (See Figure 1a.)

When dumping occurs, the concentration of nitrates in the
treated water can approach the sum of the concentrations of both the sulfates
and nitrates in the raw water. In a water containing 80 ppm of nitrates as
calcium carbonate and 85 ppm of sulfates as calcium carbonate, overrunning the
unit will cause nitrate levels to rise until they approach 165 ppm as calcium
carbonate. Nitrate selective resins prevent this from occurring. (See Figures
1b and 1c.) In the event of a service overrun with the nitrate selective resin,
the highest nitrate level that can appear in the effluent is equal to the
nitrate level in the influent.

Resin Certification

The standard Type 1 and Type 2 resins are listed by the U.S.
Food and Drug Administration (FDA) for potable water applications in the USA.

Although both the tributylamine and triethylamine resins are
approved by the equivalent of the FDA organizations in essentially all European
countries for potable water applications, they are not currently listed by the
United States FDA. Only one or more brands of nitrate selective resin
(triethylamine functionality) have been certified by the Water Quality
Association's (WQA) Gold Seal Program. Ask your resin supplier to provide you
with copies of the certification.

Operation

The ion exchange process for the removal of nitrates is both
simple and effective. It operates in the same manner as a common water softener
and easily can remove much more than 90 percent of the nitrates. The process
uses a strong base anion exchange resin, which is regenerated with common salt.
The chloride (Cl) ion of the salt molecule is utilized by the anion exchange
site, the sodium (Na) ion passes right through the resin bed and does not
affect the process.

Operational Guidelines

The anion resins used in nitrate removal applications are
regenerated with ten percent brine at a dosage of about ten pounds per cubic
foot. The service flow rate can be between two and four gallons per minute per
cubic foot. A minimum resin bed depth of 30 inches is recommended, 36 inches is
preferred.

In many respects, the operation of a nitrate removal unit is
similar to an ordinary softener. The biggest difference during regeneration is
the backwash rate. The anion resins are less dense and require a backwash
flowrate about half that of softening resin.

In some household applications, nitrate removal is used solely
for drinking and cooking purposes. This reduces the volume demand
substantially, in some cases to the point that small POU throwaway filter
cartridges can be used instead of the larger regenerable systems. Cartridges
always should be filled with the nitrate selective resins to avoid nitrate
dumping.

Type 1 and Type 2 strongly basic anion resins have high
affinities for nitrates and are easily regenerated with common salt. All of the
anions found in potable water have varying affinities for the resin. Therefore,
they consume varying amounts of the resin's capacity. The amount of a
particular ion that the resin will hold varies directly with its affinity and
its relative concentration with respect to the other ions in the influent. At
the concentration levels involved in drinking water, sulfate has a higher
affinity for the Type 1 and Type 2 strong base resins than nitrate, while
nitrate is more strongly held than chloride and bicarbonate.

Safety Guidelines

It is essential that the unit not be overrun during service,
especially when standard anion resins are used, to prevent nitrate dumping.

* Use
automatic regeneration controls on the ion exchange unit.

- Meter
initiated (gallonage)

- Timeclock
controlled (time in service)

* Downgrade
the system to operate at 80 percent of the total capacity.

Also worth noting is the fact that nitrate-bearing water
should never be boiled. Boiling concentrates the water and the relative level
of nitrates would actually increase.

Waste Brine Concerns

The discharge of a salt regenerated nitrate unit is
typically sent to the on-site septic system. Some denitrification takes place
in the septic tank from anaerobic bacteria, where bacteria break down nitrate
to nitrogen and oxygen. Additional denitrification can take place during
percolation.

Pretreatment

Nitrate removal systems usually require only prefiltration
and dechlorination (if chlorine is present) as pretreatment. These two steps
are necessary to protect the anion bed from oxidation and physical fouling. Softening
ahead of the nitrate removal resin is not necessary except in cases of high pH
and high-hardness waters (four grains or higher) where the concentration of
carbonates and hydroxides in the resin bed could cause precipitation of calcium
or magnesium.

pH Effects

Anion resin in the chloride form removes not only nitrates,
but also sulfate and alkalinity. The removal of alkalinity can lead to a
reduction in pH of the product water in the beginning of the run. To minimize
this effect and add some buffering ability back to the water, soda ash (Na2
CO3) can be added to the brine tank. This will convert a portion of the resin
to the bicarbonate form during regeneration. A ratio of one lb./cu. ft. of soda
ash mixed with nine lbs./cu. ft. of salt can be used.

Comparing Resins

In standard resins, when sulfates are relatively low, the
nitrate takes up most of the resin's capacity, to about the same degree as in
the nitrate selective resins. Therefore, the higher total capacity of the standard
resins provides significantly higher operating capacities in all but those
cases where sulfates are present in large amounts.

Nitrate selective resins are best used in applications that
may not be monitored closely and an overrun may occur. The resins give
effective nitrate removal and prevent nitrate dumping. They can cost about 50
percent more than standard resins.

Operating Comparisons

Figures 1a, 1b and 1c show the performances of three resins (a
standard Type 2 resin, triethylamine-based nitrate selective resin and
tributylamine-based nitrate selective resin) on the same water supply after
being regenerated at 20 pounds of NaCl per cubic foot. These graphs show the
effluent concentrations of bicarbonates, sulfates and nitrates during the
service (exhaustion) cycle. The service cycles were allowed to run past the
nitrate breakthrough until the effluent and influent concentrations for each
ion were equal.

As an example in Figure 1a, the nitrate breaks through from
the bed of ResinTech SBG2 at about two-thirds of the run length before the
sulfates breakthrough. This is typical performance on this type of water for
either Type 1 or Type 2 resins. You also can see that the nitrate concentration
reaches a peak concentration of about twice the raw water concentration and
that the sulfate leakage occurs gradually, starting at about the time that the
nitrate begins to reach its peak level and that the sulfate never exceeds the
raw water value.

As another example, Figure 1b shows the sulfate breaks
through from the ResinTech SIR-100 bed 20 percent before the nitrate begins to
leak. This is typical performance for the triethylamine type resins. Notice how
gradual the breakthrough curve for nitrate is and that it never exceeds its
influent value. Also, you can see that the sulfate concentration reaches a
value of about 50 percent above its influent water concentration.

In Figure 1c, the sulfate begins leaking almost immediately
from the tributylamine type resin due to its very weak affinity for this type
of resin. You also can see in Figure 1c that the nitrate leakage also occurs
gradually, just like in Figure 1b.

Figures 2a, 2b and 2c show operating capacity curves for a
standard Type 2 anion resin and a triethylamine type nitrate selective anion
resin on waters containing 100 ppm each of bicarbonates, chlorides and nitrates
but with sulfate levels of 0, 100 and 300 ppm (0 percent, 25 percent and 50
percent, respectively). It can be seen that the standard resins have higher
operating capacities at sulfate levels up to 25 percent and that the selective
resin has a higher operating capacity when sulfate levels are above 50 percent.

Nitrate removal by ion exchange is the preferred technology
for whole house treatment. It is a low-cost method, operated in much the same
manner as a common water softener. Regeneration is simple and accomplished with
softener salt, the chloride ion from the salt being the reactive ion. Nitrate
selective resin is the logical choice to prevent any nitrate dumping. Cartridge
applications mandate the use of nitrate selective resins.

The future will find other applications for these selective
resins such as we are already are seeing with the removal of perchlorate and
technicium.

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