By C.F. “Chubb” Michaud, MWS
Can a Diet High in Nitrates Be Dangerous?
The presence of nitrates and nitrites in our food and water is associated with an increased risk of gastrointestinal cancer in adults and methemoglobinemia in infants. High nitrates in groundwater should be a red flag to proceed with caution. The usual sources of nitrates are fertilizer runoff, septic leakage, mine drainage, refuse dumps, animal feedlot seepage, and decaying plant debris. None of these are very appetizing.
Elevated nitrate levels may indicate other contaminants are present and warrant thorough testing of the water. Although nitrates occur naturally, they are rarely detected at levels above 1 part per million NO₃-N[1]. Any values above 3 parts per million (ppm) generally indicate that its presence is a result of human activity.
While dietary guidelines for humans encourage the consumption of greens and other vegetables, the majority of nitrate intake comes from foods such as leafy greens, beets, celery, citrus, and nuts. Only about 20 percent of nitrate consumption comes from beverages, including drinking water.
Ingested nitrates support the production of nitric oxide in the body, which improves blood flow and reduces blood pressure. However, nitric oxide is oxidized to nitrites, which are the real troublemakers. Nitrites react with hemoglobin to form methemoglobin, which is a relatively poor transporter of oxygen in the blood. This can result in oxygen deprivation (methemoglobinemia, also known as blue baby syndrome), which can be fatal to infants and immunocompromised individuals. For adults, long-term exposure to high nitrate levels can be carcinogenic, increasing the risk of colon, kidney, and stomach cancers. Nitrates can also react with amines and amides to form nitroso compounds, also known to be carcinogenic.
According to the European Food Safety 2017 Guidelines, the acceptable amount of nitrite per day is about 0.06 milligrams (mg) per kilogram (kg) of adult body weight. For the average 70 kg (155-pound) person, that equals 4.2 mg of nitrite per day. Cured bacon contains about 120 ppm of nitrites, or about 4.5 mg per piece! Other foods may contain nitrites to preserve color. The average consumer in the U.S. has a diet containing 75-100 mg of nitrate per day, mostly from processed meats.[2]
In 2015, the World Health Organization classified processed meat as carcinogenic if it contained added nitrates. That put processed meats in the same risk category as smoking tobacco and asbestos exposure.[3]
What Is the Safe Level for Nitrates in Drinking Water?
The U.S. Environmental Protection Agency (EPA) maximum contaminant level (MCL) for nitrate in drinking water is currently set at 10 ppm NO₃-N and 1 ppm for NO₂-N.[4] This reporting level measures the nitrogen content only and does not reflect the total ionic level. 10 ppm NO₃-N equals 44.3 ppm of nitrate ion and 35.7 ppm of nitrate as CaCO₃. 1 ppm NO₂-N equals 3.3 ppm as NO₂ ion and 3.6 ppm as CaCO₃.
The EPA maximum contaminant level goal (MCLG) is the same as the MCL. National and international independent studies indicate that nitrate levels above 0.14 ppm can lead to an increased risk of colorectal cancer. However, the EPA should review the “safe” limits in drinking water, since consuming water with nitrates above the MCL is unhealthy and can be dangerous for adults and fatal to infants. Consuming water with nitrates below the MCL may still be unhealthy and dangerous.
Municipalities will blend an offending well with one low in nitrates to produce a legally acceptable water product. This may be legal, but it is not healthy. In addition, the EPA MCL does not apply to private wells. A 2007 U.S. Census Bureau report showed that more than 15 million households were served by private wells, and 10 percent of those (more than 2 million homes) were delivering water greater than 10 ppm NO₃-N.[5] Private wells are exempt from the MCL limits, but those who consume water from them are not exempt from the consequences.
Why Doesn’t the EPA Lower the MCL if It Is Not Protective?
The simple answer is cost. There are currently 9,000 regional utilities in the U.S. supplying water with nitrate levels in the 5 ppm NO3-N range. Even a medium-size municipality can incur costs of $10 million to $15 million to set up a nitrate-reduction plant. Perhaps this explains why the EPA has chosen to look the other way.
Nitrate reduction presents great opportunities for qualified vendors of water-treatment equipment. It is estimated that there is an untapped market of over $2 billion for new systems that can reduce the nitrate problem—even though the U.S. consumer is not aware that there is one.
What Is the Best Available Technology for Treating Nitrates in a Residence?
Reverse osmosis is 83 percent to 92 percent effective in reducing nitrates. However, it produces far too much waste to be practical for centralized municipal treatment. The most effective and most applied technology for nitrate remediation is ion exchange.
Ion exchange can reduce nitrates to undetectable levels, which allows for blending down on out-of-compliance wells. It is also the preferred technology for treating residential wells. Ion exchange is compact and wastes very little water (2 percent to 3 percent). It has a wide flow range for any system size. It can be started, stopped, and ramped up at will and can regenerate automatically. Ion exchange also has a very successful 50-year history of reliability. In addition, by using the proper resin, other trace anionic contaminants, such as uranium, chrome, selenium, and arsenic, can be removed in the process.
There are two generalized classes of nitrate-removal resins, nitrate selective and nonselective, and there are applications for both.
What Are the Ion Exchange Resin Choices?
Standard strong base anion gel resins are not nitrate selective. They will exchange a chloride for both nitrate and sulfate. However, as they approach exhaustion, they will release the nitrates first (this is called dumping). This can create an effluent product water that is higher in nitrates than the original feed water. The resins will, however, produce lower leakages and higher capacities when the feed water is high in nitrates and low in sulfates (above a 2:1 ratio).
In addition, it is always a good idea to underrate the capacity of nitrate systems by up to 50 percent if feed water is variable. If the nitrates are several times the MCL, installing a lead/lag setup using two units may be best. This is also good insurance to avoid the consequences of dumping.
Nitrate-selective resins also pick up both nitrates and sulfates, but, approaching exhaustion, they release the sulfates first (“sulfate deselective”). This type of resin is the best choice when the feed water is lower in nitrates and higher in sulfates. Sulfate content impacts the capacity and the leakage, and in very high total dissolved solids feeds, the leakage may exceed the MCL, making a lead/lag setup necessary. Downgrade the capacity by about 30 percent for insurance.
The difference between putting in nitrate-reduction equipment and a softener is in what could go wrong. Anion resins can pick up trace contaminants, such as heavy metals and arsenic, and may be subject to dumping those elements at dangerous levels. When working with treatment systems with health-effect qualities, it is imperative to secure a quality water analysis to avoid unintended consequences. Strong base anion exchangers have a much higher selectivity for nitrates than for nitrites. Use an adequate safety margin, and consult an expert if there are doubts.
Anion Chemistry
The majority of anion exchangers are made from styrene cross-linked with divinylbenzene. The resins are then treated with intermediates plus an amine compound to provide functionality. They most commonly end up with an exchangeable chloride. The resins have a positive charge and pick up negatively charged ions. Type I gel anions are functionalized with trimethylamine. Type II gels have a dimethylethanolamine group and possess a weaker charge. The weaker charge makes them easier to regenerate and gives them a higher capacity for a given level of regenerant. The nitrate-select resins are either triethylamine (TEA) or tripropylamine (TPA). The larger amine group offers a hindrance for the larger sulfate molecule and makes a weaker connection with it. This is what makes this resin sulfate deselective. There is also a super-selective version for nitrate resin made using a tributylamine. This is so highly selective for nitrate that it is difficult to regenerate using sodium chloride (NaCl) brine.
Type II gel is a workhorse for nitrate removal and is often the choice by municipalities that have on-site control over the system operation. Type II is the best choice for high-nitrate feed water with relatively low sulfate levels. Care must be taken to make sure the system feed is free of iron to avoid resin fouling. In addition, when Type II resins are regenerated with salt, some of the bicarbonates come off as carbonates, which can cause issues if the feed water is higher than about 15 grains hard. In that case, citric acid is often fed with the brine during regeneration to reduce hardness fouling. It is recommended that gel resins be regenerated with soft water. Type II resins are a bit more expensive than Type I resins. On the plus side, Type I resin tends to dump nitrate less as it exhausts.[6]
For water that is higher in sulfates or has unknown levels of sulfate, nitrate-selective resins are the safe bet. The TEA and TPA versions will demonstrate a higher capacity and avoid dumping. Costs are a little higher than for Type II resins. Super-selective resins demonstrate the highest selectivity for nitrates without dumping. However, the selectivity is so high that regeneration may take three to four times as much salt.
References
1. USGS.gov website
2. Web MD, Foods High in Nitrate, Nov 2022
3. “Cancer: Carcinogenicity of the consumption of red meat and processed meat,” World Health Organization, October 26, 2015, https://www.who.int/news-room/questions-and-answers/item/cancer-carcinogenicity-of-the-consumption-of-red-meat-and-processed-meat.
4. US EPA web site Current MCLs
5. US Census Bureau, Current Housing Reports, Series H150/07, American Housing Survey for the United States: 2007. Washington, DC: US Government Printing Office, 2008.
6. Guter, Gerald A, US Patent #4,479,877 July 28, 1983
About the author
C.F. “Chubb” Michaud, MWS, is the technical director and CEO of Systematix Company of Buena Park, CA, which he founded in 1982. He has served as chair of several sections, committees, and task forces within the Water Quality Association (WQA), as well as served as a past director and governor. He served on the Pacific Water Quality Association (PWQA) board, chairing the Technical and Education Committees for 12 years. Michaud is a proud member of both the WQA and PWQA Halls of Fame, has been honored with the WQA Award of Merit, and is a two-time recipient of the PWQA Robert Gans Award. A frequent and well-published author and speaker, Michaud has contributed over 100 original papers on water treatment techniques and holds four U.S. patents on ion exchange technologies. He holds a BS and an MS degree from the University of Maine.
About the Company
Systematix Company, founded in 1982, is an innovative media supply company with a focus on ion exchange media, processes, and systems design. Expert advice is offered for the asking. The company can be reached at (714) 522-5453, or email [email protected].