Sources of Nitrates in Nature

Ion Exchange Resins for Nitrate Removal

Table of Contents

Nitrate in water       

Nitrates are inorganic compounds that form Nitrogen that can be found in different forms in the ecosystem. Ammonia (NH3), nitrates (NO3), and nitrites (NO2) are the most common Nitrogen compound types. Nitrates are essential in plants, but excess amounts of Nitrates & Phosphorus may result in eutrophication and some severe syndromes in humans.

Ion Exchange Resins for Nitrate Removal due to natural resources and human activities, high nitrate levels in natural water are present. Water utility companies are affected by increasing nitrate levels in drinking water sources. Aquifers under the arable lands are breached water quality permissible levels of nitrates. Agricultural sources of nitrates are common reasons. Fertilizer runoff, farm animal manure, and septic tank discharge percolate into the ground water bed. Fertilizers are most available in ammonia or anhydrous ammonium nitrate form. Rather than the agricultural field, industrial wastes are highly involved in the high level of nitrates in water. Naturally occurring nitrate sources such as saltpeter (KNO3) deposits, atmospheric precipitations such as ammonia, and nitrifying bacteria in decomposition matters are a few factors of nitrate addition to the water cycle. The overall contribution of human activities badly affected the Nitrogen cycle and increased the nitrate levels more than permissible levels. Excess ions will contaminate and damage the ecological system and the natural environment.

Do nitrate ions are harmful to humans?

When nitrates turn hemoglobin in the human body into methemoglobin and result in blue baby syndrome in infants, higher rates in pregnant women can affect congenital disabilities. Also, it may cause cancers and thyroid disease.

Can we easily identify nitrates in water?

Nitrates can be detected through laboratory-based methods because they have no taste, odor, and color. Electrochemical sensors, chromatography, bio-sensors, flow injection analysis, optical fiber sensors, and electromagnetic sensors are some methods of nitrate identification.

How to remove nitrate from water?

Nitrates are highly soluble and stable ions in water. They are much more complicated to eliminate in water sources. Nitrates couldn’t remove with carbon filters, sediment filters, and UV filtration. Ion exchange, reverse osmosis (RO), and distillation are the best common methods accepted worldwide.

This article will discuss how ion exchange resin removes nitrates from water.

How to use Ion Exchange Resins for Nitrate Removal?

One of the most effective technology to remove nitrates is ion exchange. The ion exchange method is used in many areas in the water purification industry. It is common in conventional water softeners, but it removes cations (ions cause water hardness) in water. Ion exchanging systems are more popular than RO systems because RO units generate a large quantity of waste, are less efficient and require high energy.

An ion exchange system requires low operating pressure that decreases electrical power consumption. Continued research improves the traditional ion exchange system into the continuous countercurrent ion exchange system to produce fewer wastages and increase the treated water quality.

Nitrate removal process description

Ion exchange is done by the tiny, spherical plastic beads called resins. These resins are manufactured from styrene crosslinking with divinyl benzene (DVB). The DVB is the binding agent, and it holds the resin together. It is also styrene and has a reactive vinyl (-HC=CH2) on both ends. DVB amount used while manufacturing performs the crosslinking percentage, and in simple meaning, resin bead strength depends on DVB. The polymer beads are coated with functional groups, and we can identify four main types of resins.

  1. Strong acid cations (SAC) – functional group is sulfonic
  2. Weak acid cations (WAC) – functional group is carboxylic
  3. Strong base anions (type – 1 & type – 2) (SBA) – functional group is quaternary ammonium
  4. Weak base anions (WBA) – functionality with primary, secondary, and tertiary amine groups

Cation resins have electrostatically bonded positive ions (Na+, H+) in their functional groups. When water passes through these cation resins, the bonded positive ions are released into the water, and positive ions in the water are captured.

Anion resins have negative ions (OH, Cl) bonded to their functional group. As in the similar procedure of cation resins, electrostatically bonded anions in the resins are released into the water and captured negatively charged particles dissolved in water.

Most design engineers select nitrate-selective resins to remove nitrates, but they have the exact mechanism to eliminate the nitrate ions. We can use standard resins – SBA and nitrate-selective SBA resins to remove nitrate. But, the performance is different with an equal level or higher level of sulfate concentration to the nitrate concentration.

Selectivity of nitrate-selective SBA resin

NO3 > SO42- > Cl > HCO3

Selectivity of type-1 and type-2 Standard SBA resins

SO42 > NO3 > Cl> HCO3

But strong base anion resins have a high affinity to nitrate ions over sulfate ions. Many industries use macro-porous type SBA resins in the form of Cl for nitrate removal systems. These are stable in the entire pH range and have a max limit of high temperature of around 80℃.

Strong base anion resin - polymer bead with its functional group

Nitrate removal ion-exchange systems can be identified as large duty/standby systems and a series of smaller column systems. These new technological systems improve resin utilization. Regeneration of the resin bed is started by ensuring the low-pressure drops through the resins protect the resin bed. The continuous water flow through the resin bed attaches nitrate ions to the beads and releases the Cl- ion into the water. Therefore, effluent water can perform a slight increase in conductivity.

Continuous service flow through the resin bed increases the saturation of the resin bed due to exchanging chloride ions with nitrates. Hence, the SBA resin bed cannot adhere to nitrates and other negative ions further. The regeneration procedure must recover the resin bed to its originated form.

Regeneration of nitrate-selective resins

Strong base anion resins in the form of chloride ions can be regenerated using Sodium chloride solution (NaCl) or Potassium chloride (KCl) solution. As in the similar way of ion exchanging, when NaCl flows through the bed, the resin beads substitute chloride ions and release nitrate ions.

NO3+ Resin – Cl  ←→ Resin-NO3 + Cl(the reaction is reversible)

Some points need to pay attention to the regeneration of the nitrate unit.

  1. The backwash has a low flow rate than softeners due to the less dense anion resins.
  2. Salt concentration must be prepared to as not to float the anion resins.
  3. The brine solution levels can remove nitrates to ensure the low nitrate leakages levels.

Usually, a few general steps can be identified in the regeneration process.

  1. Backwashing – This process helps to remove accumulated particles and fluidizes the system. Up-flow raw water expands the resin bed up to 60% by volume and continuous flow for 10 – 15 minutes.
  2. Regeneration with brine – NaCl solution can be used to regenerate the SBA resin bed. Due to the reversible ion exchange process, nitrates are released into the regenerant waste, and chloride ions substitute the polymer beads. The usable concentration of brine varies from 5% – to 12%.
  3. Displacement – slow rinse releases the brine solution after brine injection is complete.
  4. Bicarbonate wash (optional) – this process isn’t often used, but it is used to adjust the pH and alkalinity in treated water. (Consultation is recommended).
  5. Fast rinsing – The excess brine is thoroughly washed out with quick rinsing.

Commonly used regeneration designs.

Commonly used regeneration designs

pH effect and leakages

The anion resins used to remove nitrates also remove sulfate and alkalinity. The removal of alkalinity may cause a pH drop in treated water. After regeneration, the remaining nitrates can be expressed in the following service flow water as a nitrate leakage. This nitrate leakage is actual for both standard resins and nitrate-selective resins.

Conclusion

  • Water is a universal solvent, and there are many dissolved ions. Nitrate is an anion that is common in water sources.
  • Human activities are the main path of the addition of nitrates to the groundwater body. Naturally occurring sources also involve the nitrate concentration in water sources.
  • Nitrate in higher levels results in critical issues for human and aquatic lives. Water that has elevated levels of nitrates needs to be treated before consumption.
  • Reverse Osmosis and Ion exchange methods are common in the field, but ion exchange is more popular.
  • Resins in the form of chloride exchanged their chlorides ions into the nitrates in the water and released Cl into the water.
  • Standard gel-type SBA resins and nitrate-selective SBA resins use for nitrate removal. Still, specialists recommend nitrate-selective resins with macro-porous structure because of their high affinity to nitrate ions in the same concentrations of sulfates and other anions.
  • When the resin bed gets saturated, regeneration is required. NaCl solution can remove nitrate and refresh the resin bed with chloride ions.
  • There are a few different engineering designs for regeneration.
  • Always contact your service provider and strictly adhere to their guidelines before regeneration.

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