“Some have heated together sulfur, realgar and saltpeter with honey; smoke and flame result, so that their hands and faces have been burnt, and even the whole house where they were working burned down”.
From a Taoist text tentatively dated to the mid 9th century. The first known use of saltpeter (potassium nitrate) was for the production of gunpowder – probably preceding its use as a fertilizer.
Occurrence: Nitrates, (NO3), are the end product of the aerobic stabilization of organic nitrogen, and as such they occur in polluted waters that have undergone self-purification or aerobic wastewater treatment processes. Nitrates also occur in percolating ground waters as a result of excessive application of fertilizer or leaching from septic tank systems and cesspools. In a few instances’ nitrates may be added to a stream or ground water by natural degradation, or directly by inorganic industrial wastes, but such sources are relatively insignificant. Wastes from chemical fertilizer-producing plants – apart from fertilizers in the field – are an increasingly important source of nitrate pollution, though they are certainly not the only source. The percolation of dairy farm or feedlot waste pond contents, or their excessive spreading on meadows and fields can contribute to nitrate contamination of groundwaters. Pollution of water sources due to nitrate existed long before chemical fertilizers were introduced. There also have been reported temporary nitrate spikes in river waters during the time of initial spring runoff of snow-melt.
In spite of their many sources, nitrates are seldom abundant in natural surface waters. Nitrates serve as an essential fertilizer for all types of plants from phytoplankton to trees. Photosynthesis is constantly utilizing nitrates and converting them to organic nitrogen in plant cells which, in turn, are ingested by animals and transformed into proteins, the eventual decay of which completes the cycle. (See “Nitrogen Cycle”, below). In deep ground waters, however, this action is not possible and, consequently it is in such waters that excessive and deleterious concentrations of nitrates are often found.
The Nitrogen Cycle
Exposure and Health Effects: Nitrate in water used to prepare baby formulas is of concern in infants whose immature stomach environment enables the conversion of nitrate to nitrite (NO2-) which is then absorbed into the baby’s blood stream where it occupies the sites on the hemoglobin designed for and normally occupied by oxygen. The reduction of the oxygen-carrying capacity of the child’s blood results in a blue cyanosis, “Blue Baby Syndrome” (Methemoglobinemia), a sometimes-acute condition in which health deteriorates rapidly over a period of days. Symptoms include shortness of breath and blueness of the skin. High nitrate levels may also affect the oxygen-carrying capacity of the blood of pregnant women.
Nitrates are rated among the poisonous ingredients of mineralized waters, with potassium nitrate being more poisonous than its sodium analog. In adults, excess nitrates cause irritation of the mucous linings of the gastrointestinal tract and bladder, with symptoms of diarrhea and diuresis. Drinking one liter of water containing 500 mg/L of nitrate can cause such symptoms.
Regulatory Background: Until 1962, the United States Public Health Service did not have a regulation for nitrates. At that time, however, a recommended limit of 45 mg/L as nitrate was established. Since United States Public Health Service (USPHS) standards were only non-enforceable guidelines, and not legally binding, the “Safe Drinking Water Act” (Public Law 93-523) was passed by Congress in 1974, which required the United States Environmental Protection Agency (USEPA) to establish enforceable standards for health-related drinking water contaminants to apply to all public water systems. Interim Primary Drinking Water Standards were set in 1975 and have since been amended and added to on several occasions. These standards, for the first time, set enforceable Maximum Contaminant Levels (MCLs), for a number of contaminants, among them nitrates. The nitrate MCL was set at 10 mg/L expressed as Nitrogen (N), or 45 mg/L if expressed as the ion (NO3_).
Since nitrates’ danger to infants is due to their conversion to nitrites in the infants’ intestinal tract, the USEPA wisely also set a limit for ingested nitrite. The nitrite (only) limit of 1 mg/L may, at first, seem rather low. It must be remembered, however, that nitrites are unstable intermediates in the nitrogen cycle. In aerobic environments, nitrites are rapidly oxidized to nitrates. Conversely, in anoxic systems, nitrites are reduced to harmless nitrogen gas. Even effluents from wastewater treatment plants rarely contain more than a fraction of a milligram per liter of nitrite. EPA has included the nitrite limit with the nitrate one. Thus, the MCL for the combination of nitrate and nitrite is 10 mg/L as N / 45 mg/L as NO3_.
Nitrate Removal Processes:
Several processes are available for the reduction of nitrates in, or the total removal of nitrates from, drinking water supplies. The processes, detailed below, are not listed in order of preference or applicability:
- Reverse Osmosis
- Ultra Filtration
- Strong Base Anion Resins
- Nitrate Selective Resins
- NSF Approved
- Bacteria reduce nitrate and nitrite to gaseous nitrogen. Limited almost entirely to wastewater treatment applications.
Reverse Osmosis, a membrane process, is primarily designed for the desalting of saline or brackish water by the application of hydrostatic pressure to overcome the water’s osmotic pressure, which drives the water to be treated through a semi-permeable membrane which allows passage of water molecules, but rejects those of dissolved contaminants. The process requires relatively expensive and fragile stacks of membranes, either cellulose acetate or thin film composite. Cellulose acetate membranes can be operated up to 400 psi, but are subject to biological attack and hydrolysis. They may also permit the salt passage to double after a service life of about three years. The more expensive thin film composite membranes are capable of the same or greater flux rate but at half the applied pressure, and they are reported to allow a less than 30% increase in salt passage after three years of service. Both membrane types require considerable pre-treatment of the water to prevent scaling, plugging, and colloidal or biological fouling.
Since the recovery of product water, as a percentage of feed, is a function of applied hydrostatic pressure, the process tends to be quite energy-intensive. Most reverse osmosis plants, depending on applied pressure, are designed for 75-80% recovery, i.e., up to 25% of the flow must be disposed of as a concentrated waste. While reverse osmosis is quite capable of removing nitrate, it is, of course not nitrate-only specific. Process operation, maintenance, pretreatment and membrane replacement costs tend to limit the process to reduction (rather than complete elimination) of nitrate, and then only in relatively small, low-volume, applications.
Nano Filtration, also known as “Membrane Softening” utilizes an ultra-low-pressure membrane designed to only permit passage of particles less than 1 nanometer (10 Angstroms) in size. Thus, it is very efficient (more efficient than Reverse Osmosis) in the removal of dissolved solids but is, of course, also not selective for nitrate only. As is the case for most other membrane processes, extensive pretreatment may be necessary to prevent particulate or bio-fouling of the membranes.
Micro-Filtration and Ultra-Filtration: Both of these low-pressure processes are considered to be promising technologies for dissolved solids removal. They can be applied over a wide range of water qualities, such as elevated turbidity, iron, manganese and, of course, nitrate. As with most membrane processes, provisions for adequate treatment of feed water must be provided to protect the membranes from fouling and to optimize their performance and service life expectancy. Disposal of reject water may become an issue if a sanitary sewer connection is not available.
Electrodialysis and Electrodialysis Reversal: Electrodialysis is an electrochemical membrane process initially developed for brackish or saline waters. Instead of hydrostatic pressure the process uses an applied direct current (DC) voltage to move anions and cations in water from alternate cells through semi-permeable membranes. This purifies a portion of the feed water while concentrating another. While capable of removing or reducing most ions, including nitrate, the process is equipment, energy and labor intensive. The process tends to be quite wasteful of water and produces a concentrated waste stream which must be legally disposed to a sewer line dedicated to brine waste waters.
Electrodialysis Reversal is a process in which the polarity of the electrodes is reversed on a controlled time cycle which reverses the direction of ion movement in a membrane stack. Reversing polarity provides for automatic flushing of scale forming minerals from the surface of the membrane. EDR typically requires little or no pretreatment to minimize fouling. ED/EDR systems are not considered economically viable for any but very small installations.
Anion-Exchange Nitrate Removal, the exchange of chloride ion for the nitrate ion, is currently the simplest and lowest-cost method for the removal or reduction of nitrate from contaminated groundwater to be used for drinking.
The anion exchange process for nitrate removal is similar to cation exchange softening except that (1) anions, rather than cations are being exchanged, (2) nitrate is a monovalent ion, whereas calcium and magnesium are divalent, and (3) nitrate, unlike calcium or magnesium, is not the most preferred common ion involved in the multi-component ion exchange process with a typical nitrate-contaminated groundwater. The latter two exceptions lead to some significant differences between softening and nitrate removal.
The resins used for nitrate removal are of the hydroxide or chloride-form Strong Base Anion (SBA) exchange resins, both of which contain the quaternary amine functional group [-N(CH3)3+, identified as “R” in the equations below] which is so strongly basic that it is ionized and is therefore useful as an anion exchanger over the entire pH range of 1 – 13. The exchange equations are:
(Hydroxide Form) R4N+OH- + NaNO3 → R4N+NO3 + NaOH
(Chloride Form) R4N+Cl- + NaNO3 → R4N+NO3- + NaCl
Fortunately, all SBA resins have a much higher affinity for nitrate than they do for chloride, and the chloride form equation proceeds at near neutral pH without any change in pH.
Weak Base Anion Exchange resins (WBA), by contrast, exchange chloride for nitrate only in the acidic region. If the solution is neutral or basic, no adsorption or exchange can take place. For these reasons the uses of WBA resins in water treatment are fairly uncommon, though some useful applications, (other than nitrate removal) are possible.
The technology of SBA resins is evolving so rapidly, and is so dynamic, that it is difficult to pin down any significant performance differences between resins and their manufacturers. Guter described several resins that were nitrate-selective with respect to sulfate, and their selectivities were in accord with the predictions of Clifford and Weber. In the regeneration of nitrate-laden resin it was found that sulfate is much easier to elute (off the resin) than is nitrate. To overcome this problem, studies have shown that partial regeneration, using dilute (0.5 Normal, rather than the more common 1.7 N) sodium chloride solution was more efficient. A further advantage of using a dilute regenerant is its greater amenability to biological denitrification of the spent brine. One obvious disadvantage of using dilute regenerants, of course, is that they produce greater volumes of wastewater and require longer regeneration times.
Because of its eutrophication potential, nitrate-contaminated brine cannot be discharged into surface waters or on land (other than lined evaporation ponds). In some municipal-scale applications the nitrate-laden brine is metered into sanitary sewers for subsequent biological treatment. This disposal must be closely coordinated with the local wastewater authorities to prevent ‘poisoning’ of the biological flora of the WW treatment plant.
Biological Denitrification is designed to bacterially reduce nitrite and nitrate to gaseous nitrogen through the application of facultative anaerobic heterotrophic microorganisms. A source of organic carbon, such as acetic acid, acetone, ethanol, methanol or sugar, is needed to act as a hydrogen-donor (oxygen acceptor) and to supply the carbon necessary for biological synthesis. Methanol is the preferred carbon source for this process, mainly because it is the least expensive synthetic compound available that can be applied without leaving a residual biological oxygen demand in the treated water. But this does not overcome the problem of adding organic carbon to a potential drinking water. That carbon constitutes a disinfection byproduct (DBP) precursor, which may very well render the water, after disinfection, violative of the total trihalomethanes (TTHM) and other DBP rules. Primarily for those reasons, the use of biological denitrification is limited almost entirely to wastewater treatment applications.
Pilot Testing: Since groundwater qualities can vary greatly, pilot testing is advisable. This is of particular importance for ion exchange applications, since the relative distribution of other, competing, anions will affect process performance.
Conclusion: Exceeding the nitrate plus nitrite MCL, 10 mg/L as N, 45 mg/L as NO3, has caused many water purveyors to be forced into Public Notification for violating the provisions of the SDWA. Fortunately, the reduction, or if necessary even complete elimination, of these contaminants, is readily achievable. Although the technology is advancing rapidly, at the time of this writing, Anion Exchange treatment may be considered as “Best Available Technology” for the removal or reduction of nitrates.