Nitrate

Nitrate

  Nitrate Reduction/Removal by lon Exchange

High nitrate (NO₃") levels in water supplies are prevalent in agricultural states in the United States, in areas of Canada, and in Europe.

High nitrate levels in water (over 10 ppm as nitrogen [N]) are considered a health hazard for infants less than six months old, as the nitrate is broken down in the gastrointestinal tract to the more dangerous nitrite (NO_2-). Infants affected by this condition are referred to as "blue babies," or having methemoglobinemia.

Methemoglobinemia is caused by nitrite (NO_2-), the chemically active form of nitrogen. Relatively low acidity (high pH) makes an infant's stomach an excellent environment for the bacteria that convert nitrate to nitrite. When an infant under six months of age or a newborn farm animal consumes nitrate (NO_3-) in water, the bacteria in its stomach convert the nitrate into potentially dangerous nitrite. This nitrite is then absorbed from the infant's intestine and enters into a complex chemical reaction with the hemoglobin in the infant's blood, changing it to methemoglobin.

Hemoglobin is the oxygen carrier in the blood; methemoglobin cannot carry oxygen. It is unable to reversibly bind the oxygen molecule.. As more and more of the blood hemoglobin is converted to methemoglobin, the capacity of the blood to carry essential axygen is reduced, and symptomns of oxygen starvation begin to occur.

Because oxygen starvation results in a bluish discoloration of the body, methemo-globinemia has been referred to as the "blue baby" syndrome.¹⁸

There is also medical concerm about the long-term effect of high-nitrate water ingestion by adults.19 A maximum level of 10.0 ppm of nitrate plus nitrite or 1 ppm of nitrite, expressed as nitrogen, is the current limit for safe drinking water set by the U.S. Environmental Protection Agency. Nitrates can be expressed in water analysis reports in three ways. The correct conversion of the 10.0 ppm-mg/L N is:

Nitrates have no detectable taste, odor, or smell at concentrations found in drinking- water sources. They do not cause any color, discoloration of plumbing fixtures, or other telltale signs. Therefore, they are undetectable by the human senses. Nitrates cause no prolem in nonpotable uses of domestic water, such as laundering, dishwashing, and bathing. They are only a concern where water is ingested.²⁰ For livestock, no water quality standards pertaining to nitrate levels have been established. However, a level of 100.0 ppm (mg/L) is considered the maximum for adult cattle, horses, pigs, and sheep by the agricultural community (Figure 6-5). Studies in Minnesota have shown that higher levels are not detrimental to adult livestock. Nitrates in the feed for livestock are more of a concern than nitrates in a water source.²¹

Nitrogen is a chemical element essential to all living matter. It occurs naturally in our environment-in soils, in air, and even in rainwater. Nitrogen can be found in several forms, including ammonia (NH₃), nitrate (NQ_3-), and as nitrite (NO_2-) Ammonia is present in the wastes of humans and animals. It can enter the soil (Figure 6-6) from inadequate or poorly managed septic or sewer systems, animal feedlots, or mature storage facilities. Microorganisms then convert the ammonia to nitrate.

To improve agricultural production, farms apply chemical fertilizers that consist nitrogen. This nitrogen is converted by the soil to nitrate (NO,). When more nitrogen nitrate accumulates than crops can use, rainwater, irrigation water, and snowmelt carry down the excess nitrogen through the soil and into the groundwater. This process is known as "leaching." How fast leaching occurs depends upon soil types. Water moves rapidly through sand or gravel, or where porous limestone bedrock underlies shallow soils.²⁴

Other sources for nitrates in water supplies are improper disposal of industrial waste water from metal finishing and from boiler blowdown of corrosion control compounds.

For limited quantities of drinking and cooking water in homes, two point-of-use treatment processes can reduce nitrate to the-10 ppm. Either counter-top or automatic home distillers can provide nitrate-safe water. The automatic distiller will produce on average 5-10 gallons (19-38L) every 24 hours. Likewise, hyperfiltration (reverse osmosis) under-the-sink-type systems, operating at 50 pounds pressure (psi) or more, will provide from 5-15 gallons (30-53L) per 24 hours. However, depending on pH temperature, and net pressure across the membrane, nitrate rejection may fall below 50 percent. The product water from either distillers or RO units, in addition to being nitrate-safe, will also be essentially mineral-free. (More details on both of these point-of-use processes will be covered in Chapter Nine.) Note: Nitrate-bearing water should ------ be boiled as a means of purification. Boiling would only concentrate and increase the nitrate content.

To treat nitrate-bearing water in all faucets in a home or office building, one low of method involves a system very much like a water softener. This process employs an anion ion exchange resin rather than a cation exchanger. The system is regenerated with sodium chloride (NaCI) salt in the same manner as a home water softener. In this application however, the chloride (CI) side of the salt is the reactive force (ion) regenerating the anion exchanger, and the sodium side passes through the resin bed with no chemical effect ----- the process. The following equation shows the chloride-nitrate exchange:

This "anionic softening," as it is often called, is basically the same process as described earlier in this chapter under "Strong Base Anion." The process that -------- standard Type II anion resin is not selective for nitrate only, Other anions, principally sulfates, are exchanged for chlorides as well. Bicarbonates are also exchanged during the initial portion of the service cycle and are later discharged by the resin. Since this ------ nitrate removal process also exchanges bicarbonates for chlorides, the pH value --------

Product water may also initially be reduced.²⁵ One way to buffer the product water pH is to add soda ash (Na₂CO₃) to the salt tank so that a more highly alkaline brine reaches the anion exchanger during regeneration. This, in turn, would convert part of the anion resin bed in the bicarbonate (HCO₃) form, rather the entire bed in the chloride form. The usual ratio of 1 pound (0.45kg) of soda ash to 9 pounds (4.1kg) of sodium chloride salt is used for this step.²⁶

Nitrate reduction by anion exchange is not as straightforward as water softening since anions have a pecking order of preference. This preferential ion exchange is referred to as anion selectivity. The usual order of the most common ions in potable waters when using a Type II anion (strong base) exchanger in the chloride (salt) cycle would be:

Sulfate (SO₄^2-) > Nitrate (NO_3-) > Chloride (CI-) > Bicarbonate (HCO_3-) ²⁷

Depending upon the levels of the above constituents in the raw water, the operation and capacity of the Type II anion exchanger will be influenced a good deal. Should the anion exchanger be run past the calculated exhaustion point for nitrate removal, sulfates will continue to be exchanged by the exchanger bed, in turn pushing off some of the nitrates already exchanged by the resin. Should this occur, the nitrate level in the product water can, for a short time, exceed the nitrate concentration in the raw water until sulfates break through. This phenomenon is known as "nitrate dumping."²⁸

Because of the variation in water composition from one region to another, a simplified method of determining anion exchange operating capacity is often used. In this method of calculation, the sulfate plus the nitrate content in the raw water are added together. This is known as the sulfate/nitrate capacity method. A Type II strong base anion exchanger (DVB/polystyrene) is the resin most often used for home, business, and municipal nitrate-reduction applications. To determine resin capacity, it is essential that a complete and accurate (all anions and cations) water analysis has been made of the raw water. Table 6-3 shows the average anion resin capacity based on the combined nitrate plus sulfate content versus total anion content. Since sulfates are more strongly held then nitrates, the sulfate/nitrate capacity rating method adds a degree of safety. High nitrate levels can be a symptom of other contaminants in a water supply. To ensure that a water is safe and potable, a prudent step with a nitrate-bearing water is to have a public health water analysis performed. Disinfection may also be necessary.

General operating conditions for the anion exchange nitrate/sulfate reduction process should include several parameters. First, a bed depth of at least 30 inches preferably 36 inches, should be used because of the slower kinetics of anion exchanges compared to cation resins. Service (intermittent) flow rate should be limited to 5.0 U.S. gpm (19 L/min.), with a flow controller installed downstream. Where the total hardness of the influent water is greater than 4.0 grains, presoftening the water will avoid potential calcium sulfate precipitation in the anion bed during regeneration.