What Is Ammonia Nitrogen? A Detailed Explanation of a Wastewater-Treatment Term
Natural surface water and groundwater are mainly dominated by nitrate nitrogen (NO3-); the nitrogen in polluted water bodies that exists as free ammonia (NH3) and ammonium ions (NH4+) is called ammoniacal nitrogen (also known as ammonia nitrogen); the fraction that exists as hydrated ammonia, also called un-ionised ammonia. Un-ionised ammonia is the main factor causing toxicity to aquatic organisms, whereas ammonium ions are relatively essentially non-toxic. For Class III surface water under the national standard, the concentration of un-ionised ammonia nitrogen is <= 1 mg/L.
Ammonia nitrogen is a nutrient in water bodies that can cause eutrophication and is a major oxygen-consuming pollutant in water; it is toxic to fish and certain aquatic organisms.
In an environment with pH greater than 11, ammonium ions convert to ammonia; the ammonia transfers through the hydrophobic membrane of the ammonia-sensitive electrode, causing a change in the electromotive force of the ammonia electrode, and the instrument measures the ammonia-nitrogen concentration based on this change in electromotive force.
Rinse the containers for the measured sample and reagent volumes and the electrode mounting tube with fresh sample water.
Sample introduction uses a peristaltic pump. The sample water does not directly contact the peristaltic pump tube — there is an air buffer. The volume of the introduced sample is controlled by a visual measurement system.
As with sample introduction, the auxiliary reagent is also dosed by the peristaltic pump and its volume is controlled by the visual measurement system.
Mix the sample water and reagent by bubbling.
The reaction time is automatically controlled by the measurement system.
Within a user-defined measurement cycle, the analyser automatically performs calibration and cleaning using its built-in calibration standard solution and cleaning solution.
Weigh separately 7 g of potassium iodide and mercuric iodide (HgI2) and dissolve in water; then, with stirring, slowly pour this solution into sodium hydroxide solution, dilute with water to 100 mL, and store in a polyethylene bottle, tightly stoppered.
1. Range: the ammonia-nitrogen range specifications of the electrode method are: 0-1200; 0-2000; 0-3000; 0-10000, etc. The range can be freely switched; a larger range indicates that the electrode used by the instrument has stronger adaptability.
2. Minimum detection limit: the lower the instrument's minimum detection limit, the better the electrode quality; it is generally 0.05 mg/L.
The alkaline solution of mercuric iodide and potassium iodide reacts with ammonia to form a light reddish-brown colloidal compound whose colour intensity is proportional to the ammonia-nitrogen content; the absorbance is usually measured in the wavelength range 410-425 nm to determine the content.
The minimum detectable concentration of this method is 0.025 mg/L (photometric method), with an upper determination limit of 2 mg/L. Using visual colorimetry, the minimum detectable concentration is 0.02 mg/L. After appropriate pre-treatment of the water sample, this method can be used for the determination of ammonia nitrogen in surface water, groundwater, industrial wastewater and domestic sewage.
2.1 Nitrogen-ball Kjeldahl distillation apparatus: 500 mL Kjeldahl flask, nitrogen bulb, straight condenser tube and delivery tube.
Water used for preparing reagents should be ammonia-free water.
3.1 Ammonia-free water may be prepared by one of the following methods:
Distillation method: add 0.1 mL of sulfuric acid per litre of distilled water, re-distil in an all-glass distiller, discard the first 50 mL of distillate, collect the remaining distillate in a ground-glass-stoppered glass bottle and store tightly stoppered.
Ion-exchange method: pass distilled water through a strong-acid cation-exchange resin column.
3.2 1 mol/L hydrochloric acid solution.
3.3 1 mol/L sodium hydroxide solution.
3.4 Light magnesium oxide (MgO): heat magnesium oxide at 500 C to remove carbonates.
3.5 0.05% bromothymol blue indicator solution: pH 6.0-7.6.
3.6 Antifoaming agent, e.g. paraffin chips.
Boric acid solution: weigh 20 g of boric acid, dissolve in water and dilute to 1 L.
0.01 mol/L sulfuric acid solution.
3.8 Nessler's reagent: may be prepared by one of the following methods:
Weigh 20 g of potassium iodide and dissolve in about 100 mL of water; while stirring, add mercuric chloride (HgCl2) crystal powder (about 10 g) in small portions several times until a vermilion precipitate appears that does not dissolve easily; then switch to dropwise addition of saturated mercuric chloride solution with thorough stirring, and stop adding mercuric chloride solution when a trace amount of vermilion precipitate no longer dissolves.
Weigh separately 60 g of potassium hydroxide, dissolve in water and dilute to 250 mL; after cooling to room temperature, slowly pour the above solution into the potassium hydroxide solution, dilute with water to 400 mL and mix well. Let stand overnight, then transfer the supernatant to a polyethylene bottle and store tightly stoppered.
Weigh 16 g of sodium hydroxide, dissolve in 50 mL of water and cool thoroughly to room temperature.
3 How to assess the performance of an ammonia-sensitive-electrode-method instrument
3.9 Potassium sodium tartrate solution: weigh 50 g of potassium sodium tartrate (KNaC4H4O6-4H2O), dissolve in 100 mL of water, boil to remove ammonia, cool, and make up to 100 mL.
3.10 Ammonium standard stock solution: weigh 3.819 g of superior-grade ammonium chloride (NH4Cl) dried at 100 C, dissolve in water, transfer to a 1000 mL volumetric flask and dilute to the mark. This solution contains 1.00 mg of ammonia nitrogen per mL.
3.11 Ammonium standard working solution: pipette 5.00 mL of the ammonium standard stock solution into a 500 mL volumetric flask and dilute to the mark with water. This solution contains 0.010 mg of ammonia nitrogen per mL.
4.1 Sample pre-treatment: take 250 mL of water sample (if the ammonia-nitrogen content is high, take an appropriate volume and add water to 250 mL so that the ammonia-nitrogen content does not exceed 2.5 mg), transfer to a Kjeldahl flask, add a few drops of bromothymol blue indicator solution, and adjust to about pH 7 with sodium hydroxide solution or hydrochloric acid solution. Add 0.25 g of light magnesium oxide and a few glass beads, connect immediately to the nitrogen bulb and condenser, with the lower end of the delivery tube inserted below the surface of the absorbing solution. Heat and distil until the distillate reaches 200 mL, then stop distillation and make up to 250 mL.
When the acid-titration method or Nessler colorimetric method is used, 50 mL of boric acid solution is used as the absorbing solution; when the salicylate-hypochlorite colorimetric method is used, 50 mL of 0.01 mol/L sulfuric acid solution is used instead as the absorbing solution.
4.2 Drawing the calibration curve: pipette 0, 0.50, 1.00, 3.00, 7.00 and 10.0 mL of the ammonium standard working solution into 50 mL colorimetric tubes respectively, add water to the mark, add 1.0 mL of potassium tartrate solution and mix well. Add 1.5 mL of Nessler's reagent and mix well. After standing for 10 min, measure the absorbance at a wavelength of 420 nm using a 20 mm path-length cuvette with water as the reference. Subtract the absorbance of the zero-concentration blank tube from the measured absorbance to obtain the corrected absorbance, and draw a calibration curve of ammonia-nitrogen content (mg) versus corrected absorbance.
4.3 Determination of the water sample:
Take an appropriate portion of the water sample pre-treated by flocculation-sedimentation (so that the ammonia-nitrogen content does not exceed 0.1 mg), add to a 50 mL colorimetric tube, dilute to the mark, and add 0.1 mL of potassium sodium tartrate solution. The rest follows the drawing of the calibration curve.
Take an appropriate portion of the distillate pre-treated by distillation, add to a 50 mL colorimetric tube, add a certain amount of 1 mol/L sodium hydroxide solution to neutralise the boric acid, and dilute to the mark. Add 1.5 mL of Nessler's reagent and mix well. After standing for 10 min, measure the absorbance following the calibration-curve procedure.
4.4 Blank experiment: use ammonia-free water instead of the water sample to perform a whole-procedure blank determination.
After subtracting the absorbance of the blank experiment from the absorbance measured for the water sample, read the ammonia-nitrogen amount (mg) from the calibration curve, then
Ammonia nitrogen (N, mg/L) = m/V x 1000
where: m — the ammonia-nitrogen amount read from the calibration curve, mg;
V — the volume of the water sample, mL.
6.1 The ratio of mercuric iodide to potassium iodide in Nessler's reagent has a greater influence on the sensitivity of the colour-development reaction. The precipitate formed after standing should be removed.
6.2 Filter paper often contains trace ammonium salts; when used, wash it with ammonia-free water. The glassware used should be protected from contamination by ammonia in the laboratory air.
The composition of ammonia nitrogen in wastewater mainly falls into two categories: ammonia nitrogen formed from ammonia water, and ammonia nitrogen formed from inorganic ammonia, mainly ammonium sulfate, ammonium chloride, etc. Altogether there are four forms: organic nitrogen, ammonia nitrogen, nitrite nitrogen (NO2-) and nitrate nitrogen (NO3-).
Natural surface water and groundwater, however, are mainly dominated by nitrate nitrogen (NO3-).
The general formation of high-ammonia-nitrogen wastewater is caused by the co-existence of ammonia water and inorganic ammonia,
Generally, for wastewater with pH above neutral, the main source of ammonia nitrogen is the combined action of inorganic ammonia and ammonia water,
while under acidic conditions, the ammonia nitrogen in wastewater is mainly caused by inorganic ammonia.
Ammonia nitrogen in water can, under certain conditions, be converted into nitrite; if consumed over a long period, the nitrite in the water will combine with proteins to form nitrosamines, which are strong carcinogens, extremely harmful to human health.
Process flow diagram of high-concentration ammonia-nitrogen wastewater system
The main form of ammonia nitrogen that harms aquatic organisms is free ammonia, whose toxicity is tens of times greater than that of ammonium salts and increases with rising alkalinity. The toxicity of ammonia nitrogen is closely related to the pH and temperature of the pond water; generally, the higher the pH and water temperature, the stronger the toxicity, and its harm to fish is similar to that of nitrite.
The harm of ammonia nitrogen to aquatic organisms is divided into acute and chronic. Chronic ammonia-nitrogen poisoning causes: reduced feeding, slowed growth, tissue damage and reduced oxygen transport between tissues. Fish are relatively sensitive to ammonia nitrogen in water, and high ammonia-nitrogen levels can cause fish death. Acute ammonia-nitrogen poisoning causes: aquatic organisms become excited, lose balance in the water and convulse, and in severe cases even die.
