Nitrates and Nitrites

High nitrate (NO₃^-) concentrations in drinking water resources present a potential risk to the health of the public.  Background nitrate concentration in surface water is usually below 5 mg/L. In Pakistan, the standard set by NIH and PCRWR is 10 mg N0₃^- - N or 45mg/L NO₃, which is consistent with the USEPA standards. 

Nitrogen is a nutrient that plants cannot live without and nitrate (NO₃) is the primary source.  Due to this fact, nitrogen fertilizers are applied to crops in order to encourage healthier plant growth.  However, nitrate contamination occurs when there are more nitrates present in the soil than the plants are able to consume.  This excess nitrate can move easily through soil and rocks when carried by snowmelt, irrigation and rain, ultimately ending up in the groundwater.  Besides fertilizers, other possible sources of nitrate in groundwater include waste dumps, animal feedlots, landfills, lack of sewage disposal and defective septic tanks. Groundwater contamination is enhanced when the soil is sandy or gravely due to a high hydraulic conductivity.  In addition, contamination is more likely in areas where the water table is close to the surface, or results from seepage. 

There are two major concerns with elevated nitrate levels including Lake Eutrophication and human health.  There are ways to prevent high concentrations, but since it will take many years to see the results of improved fertilizer management, nitrate removal may be the only option for many communities.

Nitrogen Chemistry

Nitrogen chemistry is complicated due to its numerous oxidation states the element assumes in its compounds such as, NO₃^- (+5), ammonium NH₄⁺ (-3) and nitrite NO₂^- (+3).  Bacterial action on organic matter, such as animal wastes, releases ammonia, which may be oxidized to nitrite by bacteria called Nitrosomonas.  Further oxidation then occurs by another bacterium known as Nitrobacteria increasing the oxygen demand, which can have negative effects on the environment by lowering the dissolved oxygen content in the water. High nitrates levels are therefore always accompanied by a high coli form count. In addition to the above reactions, gentrification is competing simultaneously, which converts nitrite and nitrate ions into nitrogen gas and nitrogen monoxide when denitrifying bacteria is present.

The nitrate ion is the most oxidized form of nitrogen and is chemically non reactive in aqueous solutions. Excess nitrate concentrations in water can cause some unintended consequences on the environment and human health.

Unintended Consequences of Nitrates

Eutrophication

Eutrophication of lakes occurs due to the denitrification of nitrate, where nitrates are reduced, and produce N₂ gas as the end product.  The nitrogen produced in this process is available for biological nitrogen fixation.  A few genera of blue-green algae (such as Anabaena, Nostoc and Gloeotrichia) are able to fix nitrogen and do so at the surface of the lake.  The oxygen produced by algae is prevented from diffusing into the blue-green algae by structures called heterocyst.  The heterocyst consume the oxygen and prevent nitrogen from entering the cells.  This fixation process can speed up eutrophication.  Eutrophication results in low dissolved oxygen content, high BOD (biochemical oxygen demand) and reduced sunlight penetration reaching the aquatic macrophytes.  Entropic lakes lack diversity and generally contain only a few thriving species many of which are bacteria and aquatic worms.  The ultimate end result is the destruction of fish habitat.  The most distinct characteristic is usually a thick blanket of blue-green algae covering the surface of the pond or lake.

Human Health

The major concern affecting human health pertains to infants less than six months of age.  In sufficient quantities, at nitrate concentrations exceeding 10 mg/L, the possibility of a health hazard is significant towards infants.  This health hazard is due to a bacterium that exists in their gastrointestinal tract that converts nitrate to nitrite (NO₂).  The nitrite produced then reacts with haemoglobin to form methemoglobin, which does not carry oxygen.  As more and more hemoglobin is converted, the infant receives less oxygen to the brain resulting in slate blue skin, vomiting, diarrhoea, mental retardation and/or suffocation leading to death.  This is a syndrome known as methemoglobinemia or “blue baby” syndrome.  After six months of age, nitrate is absorbed and secreted without conversion to toxic nitrite.  There is evidence that other health problems are associated with nitrate including stomach cancer, birth defects, hypertension, enlarged thyroid and lymphoma, but studies are conflicting and inconclusive.

Nitrite had been found to react with amines and amides to form nitrosamines and nitrosamides, which have been found to induce cancer in rodents.  There is no other group of carcinogens known that have the ability to induce such a wide variety of tumours in organs, ranging from lung, oral, brain, skin, leukaemia, bladder, to name a few.

Nitrate Contamination Prevention

There are several ways to prevent nitrate from entering drinking water reserves.  For example, wells need to be isolated from possible sources of contamination and situated on groundwater recharge zones compared to groundwater discharge zones to eliminate collection of run-off.  Abandoned wells need to be sealed to prevent entry of nitrates into the groundwater.  In addition, sinkholes are direct routes to an aquifer and should never be used as garbage dumps.

For some communities it may be possible to find a new source of water from a new well, a deeper well or obtain water from a nearby waterway that has a lower concentration of nitrate.  Another possibility is to install a more effective treatment system.  However, this is not easily implemented in many parts of the world, so the best preventative measure is to improve fertilization management.

There are several ways to control the source of contamination including reducing the residual nitrate level at the completion of the growing season when the potential is high for nitrate loss.  In addition, the quantity and timing of water and nitrogen applications should be linked closely with actual crop requirements.  There is also nitrogen in the soil, irrigation water and manure that need to be taken into account and irrigation should be minimized.

Only now are we beginning to see the resulting contamination from agricultural chemical usage of thirty to forty years ago, the impact of the previous fifteen years is yet to be seen.  Therefore, preventative measures will not present immediate ramifications and nitrate removal is the only option.

Nitrate Removal

With the increasing concern of high concentrations of nitrate in drinking water, many methods have been developed in attempts to efficiently remove nitrate.  Nitrate is a stable and highly soluble ion and has a low potential for adsorption or co precipitation. These properties are what make this ion difficult to remove using conventional water treatment processes including lime softening and filtration.  The methods include reverse osmosis, ion exchange, biological denitrification, ion exchange with denitrification, catalytic reduction, chemical denitrification and electro dialysis.

Ecotech Solution
Ecotech Systems International Pakistan uses an exclusive Ion Exchange Media regenerated with common salt for selective removal of nitrites. This media is effective, robust and has over ten years of life. Plants installed in Rawalpindi Islamabad where nitrites are unusually high are running efficiently for past ten years.