AMMONIA REMOVAL BY AIR STRIPPING
1. OBJECTIVE AND IMPORTANCE OF EXPERIMENT
Air stripping is a process by which a liquid, usually water or wastewater, is brought into
intimate contact with a gas, usually air, so that some undesirable substances present in the
liquid phase can be released and carried away by the gas.
In the past, the major objectives of wastewater treatment were the removal of SS (suspended
solids), BOD (biochemical oxygen demand), and coliform bacteria. It is only very recently
that the removal of inorganic nutrients, such as nitrogen and phosphorus, has been brought
into focus. This is because it has been realized that the discharge of these nutrients into
surface waters can result in excessive growths of algae and other aquatic plants, a
phenomenon commonly referred to as “eutrophication.”
Municipal wastewater and many industrial wastes are among the principal contributors of
these nutrients to surface waters. For example, the average concentrations of nitrogen and
phosphorus in typical domestic wastewater are, respectively, about 35–45 mg/L as N and 10–
15 mg/L as P. Yet, nutrient concentrations of as low as 0.3–0.5 mg/L of nitrogen and 0.01–
0.05 mg/L of phosphorus have been reported to cause eutrophication. Therefore, to eliminate
this problem, a high efficiency of nutrient removal in the waste treatment process must be
achieved. Conventional waste treatment processes are effective in removing only about 40–
50% of the nitrogen and 25–30% of the phosphorus. Therefore, new treatment technologies
must supplement conventional methods in order to improve the nutrient removal efficiencies.
Ammonia stripping is a simple desorption process used to lower the ammonia content of a
wastewater stream. Some wastewaters contain large amounts of ammonia and/or nitrogencontaining compounds that may readily form ammonia. It is often easier and less expensive to
remove nitrogen from wastewater in the form of ammonia than to convert it to nitratenitrogen before removing it.
Ammonia (a weak base) reacts with water (a weak acid) to form ammonium hydroxide. In
ammonia stripping, lime or caustic is added to the wastewater until the pH reaches 10.8 to
11.5 standard units which converts ammonium hydroxide ions to ammonia gas according to
the following reaction(s):
NH4+ + OH−  NH3 + H2O
The efficiency of an ammonia stripping operation depends primarily on five factors:
1. pH: As shown in Fig. 1, the relative distribution of the dissolved NH3 gas vs the NH4+ ions
in true solution depends greatly on pH. Because only the dissolved gas can be removed from
solution, it is important to raise the pH to a value of 11 or higher so that at least 95% of the
ammonia nitrogen is converted to the gas form. In full-scale operation, lime is usually the
most economical, and thus is the most commonly used material for raising the pH. The
amount of lime required to raise the pH depends on the characteristics of the water, primarily
its bicarbonate concentration.
Fig 1. Effect of initial pH on removal of ammonia in bubble aeration air stripping (Air flow of
10 L/min, strippingtime of 24 h).
2. Temperature: The liquid temperature can affect the ammonia stripping efficiency in two
different ways. First, at a given pH, the percentage of ammonia nitrogen present as a
dissolved gas increases with temperature. For example, at pH 10, at a temperature of 40°C
about 95% of the ammonia nitrogen is present as the gas, but at 0°C only about 50% is
present in the gaseous form. Second, the solubility of ammonia gas in water increases with
decreasing temperature. The greater the solubility, the greater the amount of air required to
remove a given amount of ammonia gas.
3. Rate of Gas Transfer: In order to remove ammonia from water, the dissolved NH3
molecules must first move from the bulk liquid solution to the air–water interface, and then
from the interface to the stripping air flow.
4. Air Supply Rate: Because the difference in the ammonia pressures between the liquid and
gaseous phases is the force for ammonia to transfer from the liquid to the air flow, an
sufficient supply of air flow through the ammonia tower will dilute the concentration of the
ammonia released thereby reducing its partial pressure in the gaseous phase and maximizing
the ammonia release rate.
5. Hydraulic Loading Rate: The hydraulic loading rate on the stripping tower can affect the
ammonia removal in two ways. First, for a fixed tower depth, the larger the hydraulic loading
rate, the shorter is the air–water contact period. Below a certain critical contact time the
ammonia-stripping efficiency will be reduced drastically. Second, for a given internal packing
configuration, if the hydraulic loading rate is too high, it may cause sheeting of the water,
which reduces the intensity of droplet formation, thus decreasing the ammonia-stripping
efficiency. For most ammonia-stripping operations, using a 6–7 m (20–24 ft) tower with an
internal packing of 3.8 × 5 cm (1.5 × 2 in.), a hydraulic loading rate between 0.04 and 0.12
m3/min/m2 is recommended.
Fig 2. Effect of air flow on removal of ammonia in bubble aeration air stripping (Initial pH
12, stripping time of 24 h).
2. EXPERIMENTAL PROCEDURE
2.1 Materials and Equipments
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Beaker
Diffuser (Aquarium stone)
Air pump
Spectrophotometer (at 640 nm)
Cuvettes (1 cm)
Erlenmeyer flask
Paraffin wrapper film
Measuring cylinder
2.2 Chemicals
a. 0.02 N or 0.1 N NaOH solution
b. Phenol solution (Mix 11.1 mL liquefied phenol with 95% v/v ethyl alcohol to a final
volume of 100 mL, prepare weekly)
c. Sodium nitroprusside, 0.5% w/v (Dissolve 0.5 g sodium nitroprusside in 100 mL
deionized water, store in amber bottle for up to 1 month)
d. Alkaline citrate (Dissolve 200 g trisodium citrate and 10 g sodium hydroxide in
deionized water, dilute to 1000 mL)
e. Sodium hypochlorite, commercial solution, 5% (This solution slowly decomposes
once the seal on the bottle cap is broken, replace about every 2 months)
f. Oxidizing solution (Mix 100 mL alkaline citrate solution with 25 mL sodium
hypoclorite, prepare fresh daily)
g. Stock ammonium solution (Dissolve 3.819 g anhydrous NH4Cl (dried at 100oC) in
water and dilute to 100 mL; 1.00 mL = 1.00 mg N = 1.22 mg NH3)
2.3 Steps of the Experiment
1. Measure pH value of your sample.
2. Add NaOH solution until the pH reaches 12.
3. Place the sample into a measuring cylinder (thin and tall reactors preffered) and apply air
in a proper rate and for a proper time.
4. Take 25 mL sample from measuring cylinder and place it into 50 mL erlenmeyer flask.
Use “Manuel Phenate Method” from Standard Methods. This method is applicable to both
fresh water and seawater and is linear to 0.6 mg NH3-N/L (Dilute the sample if the
estimated value for NH3-N is more than 0.6 mg/L).
a) Add 1 mL phenol solution, 1 mL sodium nitroprusside solution, and 2.5 mL
oxidizing solution through mixing after each addition.
b) Cover samples with plastic wrap or paraffin wrapper film.
c) Let color develop at room temperature (22 to 27oC) in subdued light for at least
1 h. An intensely blue compound, indophenol, is formed by the reaction of
ammonia, hypochlorite. Color is stable for 24 h.
d) Measure absorbance at 640 nm.
2.5 Calculations:
Prepare a solution curve by plotting absorbance readings of standards against ammonium
concentrations of standards. Compute sample concentration by comparing sample absorbance
with the standard curve.
p.s. 1 Nesslerization has been dropped as a standard method, although it has been considered
a classic water quality measurement for more than a century. The use of mercury in this test
warrants its deletion because of the disposal problems.
p.s. 2 The ammonia-selective electrode method is applicable over the range 0.03 to 1400 mg
NH3-N/L.