What Is Ammonia Nitrogen? A Detailed Explanation of Wastewater Treatment Terminology
In natural surface water bodies and groundwater, nitrate nitrogen (NO3-) predominates; the nitrogen existing in the forms of free ammonia (NH3) and ammonium ions (NH4+) in polluted water is called aqueous ammonia, also known as non-ionized ammonia. Non-ionized ammonia is the main factor causing toxicity to aquatic organisms, while ammonium ions are relatively essentially non-toxic. For Class III surface water under the national standard, the concentration of non-ionized ammonia nitrogen is ≤1 mg/L.
Ammonia nitrogen is a nutrient in water bodies that can cause eutrophication, and is the main oxygen-consuming pollutant in water bodies, toxic to fish and some aquatic organisms.
In an environment with pH > 11, ammonium ions are converted to ammonia; the ammonia passes through the hydrophobic membrane of the ammonia-sensitive electrode, causing a change in the electromotive force of the ammonia-sensitive electrode. The instrument measures the ammonia-nitrogen concentration based on the change in electromotive force.
Flush the containers for the measured water sample, the reagent volume, and the electrode mounting tube with the new water sample.
Use a peristaltic pump for sample introduction. The water sample does not directly contact the peristaltic pump tube — there is an air buffer zone. The volume of sample introduced is controlled by a visual measurement system.
Same as the sample introduction, the auxiliary reagent is also dosed through the peristaltic pump and its dosing volume is controlled by the visual measurement system.
Mix the water sample and reagent by bubbling.
The reaction time is automatically controlled by the measurement system.
Within the user-defined measurement cycle, the analyzer automatically performs calibration and cleaning using the built-in calibration standard solution and cleaning solution.
3. How to distinguish the performance of ammonia-sensitive-electrode-method instruments
1. Range: The ammonia-nitrogen range specifications of the electrode method are divided into: 0–1200; 0–2000; 0–3000; 0–10000, etc. The range can be switched freely; the larger the range, the stronger the adaptability of the electrode used by the instrument.
2. Minimum detection limit: The lower the instrument's minimum detection limit, the better the quality of the electrode, generally 0.05 mg/L.
An alkaline solution of mercuric iodide and potassium iodide reacts with ammonia to form a light red-brown colloidal compound, whose color intensity is proportional to the ammonia-nitrogen content. Its absorbance can usually be measured in the wavelength range of 410–425 nm, and its content calculated.
The minimum detection concentration of this method is 0.025 mg/L (photometric method), and the upper limit of determination is 2 mg/L. Using the visual colorimetric method, the minimum detection concentration is 0.02 mg/L. After appropriate pretreatment 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 distillation apparatus for ammonia determination: 500 mL Kjeldahl flask, nitrogen ball, straight condenser tube and delivery tube.
All water used for preparing reagents should be ammonia-free water.
3.1 Ammonia-free water can be prepared by one of the following methods:
Distillation method: Add 0.1 mL sulfuric acid per liter of distilled water, redistill in an all-glass distiller, discard the first 50 mL of distillate, and collect the rest of the distillate in a stoppered ground-glass bottle and store tightly sealed.
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, such as paraffin flakes.
Boric acid solution: Weigh 20 g boric acid, dissolve in water, dilute to 1 L.
0.01 mol/L sulfuric acid solution.
3.8 Nessler's reagent: One of the following methods may be selected for preparation:
Weigh 20 g potassium iodide, dissolve in about 100 mL water, and while stirring add mercuric chloride (HgCl2) crystalline powder (about 10 g) in small portions several times, until a vermilion precipitate that is difficult to dissolve appears, then switch to dropwise addition of saturated mercuric chloride solution and stir fully; when a trace amount of vermilion precipitate no longer dissolves, stop adding the mercuric chloride solution.
Separately weigh 60 g potassium hydroxide, dissolve in water and dilute to 250 mL, cool to room temperature, then slowly pour the above solution into the potassium hydroxide solution, dilute with water to 400 mL, mix well. Let stand overnight, transfer the supernatant to a polyethylene bottle, and store tightly sealed.
Weigh 16 g sodium hydroxide, dissolve in 50 mL water, and cool thoroughly to room temperature.
Separately weigh 7 g potassium iodide and mercuric iodide (HgI2), dissolve in water, then slowly pour this solution into the sodium hydroxide solution with stirring, dilute with water to 100 mL, store in a polyethylene bottle, and keep tightly sealed.
3.9 Potassium sodium tartrate solution: Weigh 50 g potassium sodium tartrate (KNaC4H4O6·4H2O), dissolve in 100 mL water, heat to boiling to remove ammonia, cool, and make up to 100 mL.
3.10 Ammonium standard stock solution: Weigh 3.819 g of superior-grade pure 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 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, dilute with water to the mark. This solution contains 0.010 mg ammonia nitrogen per mL.
4.1 Water sample pretreatment: Take 250 mL water sample (if the ammonia-nitrogen content is high, take an appropriate amount 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 pH about 7 with sodium hydroxide solution or hydrochloric acid solution. Add 0.25 g light magnesium oxide and a few glass beads, immediately connect the nitrogen ball and condenser tube, and insert the lower end of the delivery tube below the surface of the absorption liquid. Heat to distill; when the distillate reaches 200 mL, stop distillation and make up to 250 mL.
When using the acid-titration method or Nessler colorimetric method, use 50 mL boric acid solution as the absorption liquid; when using the salicylic acid–hypochlorite colorimetric method, use 50 mL 0.01 mol/L sulfuric acid solution as the absorption liquid instead.
4.2 Drawing the standard 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 potassium sodium tartrate solution, mix well. Add 1.5 mL Nessler's reagent, mix well. After standing for 10 min, at a wavelength of 420 nm, using a 20 mm optical-path cuvette and water as reference, measure the absorbance. From the measured absorbance, subtract the absorbance of the zero-concentration blank tube to obtain the corrected absorbance, and draw the standard curve with ammonia-nitrogen content (mg) against the corrected absorbance.
4.3 Determination of water samples:
Take an appropriate amount of water sample after flocculation-sedimentation pretreatment (so that the ammonia-nitrogen content does not exceed 0.1 mg), add to a 50 mL colorimetric tube, dilute to the mark, add 0.1 mL potassium sodium tartrate solution. The rest is the same as drawing the standard curve.
Take an appropriate amount of distillate after distillation pretreatment, add to a 50 mL colorimetric tube, add a certain amount of 1 mol/L sodium hydroxide solution to neutralize the boric acid, dilute to the mark. Add 1.5 mL Nessler's reagent, mix well. After standing for 10 min, measure the absorbance following the standard-curve procedure.
Ammonia nitrogen in water can be converted into nitrite under certain conditions; if drunk over a long period, the nitrite in the water will combine with proteins to form nitrosamines, which are strong carcinogens and extremely harmful to human health.
After subtracting the absorbance of the blank experiment from the absorbance measured for the water sample, and looking up the ammonia-nitrogen amount (mg) from the standard curve,
Ammonia nitrogen (N, mg/L) = m/V × 1000
where: m — the ammonia-nitrogen amount looked up from the standard curve, mg;
V — the water sample volume, mL.
6.1 The ratio of mercuric iodide to potassium iodide in Nessler's reagent has a great influence on the sensitivity of the color-developing reaction. The precipitate formed after standing should be removed.
6.2 Filter paper often contains trace amounts of ammonium salts; when using it, note to wash with ammonia-free water. The glassware used should avoid contamination by ammonia in the laboratory air.
The composition of ammonia nitrogen in wastewater mainly has two categories: one is ammonia nitrogen formed from ammonia water, and the other is ammonia nitrogen formed from inorganic ammonia, mainly ammonium sulfate, ammonium chloride, etc. It is divided into four types in total: organic nitrogen, ammonia nitrogen, nitrite nitrogen (NO2-) and nitrate nitrogen (NO3-).
In natural surface water bodies and groundwater, nitrate nitrogen (NO3-) predominates.
The general formation of high-ammonia-nitrogen wastewater is due to the coexistence 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.
Under acidic conditions, the ammonia nitrogen in wastewater is mainly caused by inorganic ammonia.
4.4 Blank experiment: Use ammonia-free water instead of the water sample, and perform a full-procedure blank determination.
High-concentration ammonia-nitrogen wastewater system flow chart
Free ammonia is the main hazard of ammonia nitrogen to aquatic organisms; its toxicity is tens of times that of ammonium salts and increases with increasing alkalinity. The toxicity of ammonia nitrogen is closely related to the pH value and water temperature of the pond water; generally, the higher the pH value 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 harms by: reduced feeding, slowed growth, tissue damage, and reduced oxygen transport between tissues. Fish are relatively sensitive to ammonia nitrogen in water; when ammonia-nitrogen content is high it can cause fish death. Acute ammonia-nitrogen poisoning harms by: aquatic organisms becoming hyperactive, losing balance in the water, convulsing, and in severe cases even dying.
