United States

Environmental Protection

Agency

Office of Water

Washington, D.C.

EPA 832-F-00-019

September 2000

Wastewater

Technology Fact Sheet

Ammonia Stripping

DESCRIPTION

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 nitrogen-containing 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 nitrate-nitrogen before removing it (Culp et al.,

1978). 

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-6 H2O + NH38

Figure 1 illustrates two variations of ammonia

stripping towers, cross-flow and countercurrent. In

a cross-flow tower, the solvent gas (air) enters

along the entire depth of fill and flows through the

packing, as the alkaline wastewater flows

downward. A countercurrent tower draws air

through openings at the bottom, as wastewater is

pumped to the top of a packed tower. Free

ammonia (NH3) is stripped from falling water

droplets into the air stream, then discharged to the

atmosphere. 

APPLICABILITY

Ammonia stripping works well with wastewater

that has ammonia contents between 10 to 100mg/l.

For higher ammonia content (more than 100mg/l),

it may be more economical to use alternate

ammonia removal techniques, such as steam

stripping or biological methods. Air stripping may

also be used to remove many hydrophobic organic

molecules (Nutrient Control, 1983).

ADVANTAGES AND DISADVANTAGES

The following advantages and disadvantages should

be considered when comparing ammonia stripping

with other ammonia removal systems.

Source: Culp, et. al, 1978.

FIGURE 1 TWO TYPES OF STRIPPING

TOWERS
Advantages

C The operation is relatively simple and not

affected by wastewater fluctuation if pH and

air temperature remain stable.

C Ammonia stripping is a mechanical

procedure and creates no backwash or

regeneration.

C Ammonia stripping is unaffected by toxic

compounds that could disrupt the

performance of a biological system.

C Ammonia stripping is a controlled process for

selected ammonia removals.

Disadvantages

C Water must be re-pumped to the stripping

tower. Pumping requires higher maintenance

and power requirements.

C Scale formation can be removed hydraulically

in most cases but not all, resulting in a need

to pilot test at most locations.

C Ammonia stripping cannot be performed in

freezing conditions (unless sufficient heated

air is available). Fogging and icing result in

a significant reduction in ammonia removal.

C While ammonia is usually discharged to the

atmosphere at low level (6 mg/m3), this may

be unacceptable in certain locations due to air

quality concerns or regulations.

C Ammonia stripping does not remove nitrite

and organic nitrogen.

C Air pollution problems may result from

ammonia and sulfur dioxide reaction.

C Air stripping often requires the addition of

lime to control pH, which may create

operation and maintenance concerns.

C Noise may be a problem.

C High pH wastewater will corrupt the wood

packing of the stripping tower.

DESIGN CRITERIA

The following criteria should be considered when

designing ammonia stripping systems. Optimum

conditions are noted in parentheses.

• Hydraulic wastewater loading (0.1 to 0.2

l/min/m3 or 1 to 2 gal/min/ft2).

• Stripping air flow rate (32 to 54 l/min/m3 or

300 to 500 ft3/min/gal).

• Packing depth (6.1 to 7.6 meters or 20-25

ft).

• pH of wastewater (10.8-11.5).

• Air pressure drop (0.015” – 0.019” of

water/ft).

• Blower type.

• Site and land requirements.

• Packing material (plastic or wood).

• Packing spacing (approx. 5 cm or 2”

horizontal and vertical).

• Water temperature.

• Plant capacity.

• Ammonia concentration of the wastewater.

• Water distribution uniformity.

• Scale removal and ease of cleaning up.

The introduction of air into the system is the major

design difference in the two basic types of towers.

Air enters from the side in the cross-flow tower,

which is less efficient than the countercurrent tower

where the air enters from the bottom of the tower.
PERFORMANCE

Ammonia stripping performance is highly

dependent on air temperature and air/water ratios.

Efficiency decreases significantly as air temperature

decreases. At 20EC (68EF), there is a 90 to 95%

ammonia removal efficiency, while at 10EC (50EF),

efficiency decreases to 75 percent. 

Lake Tahoe EPA Research Project

Lake Tahoe found that the removal rate was 95

percent ammonia nitrogen at 11.5 pH using 53,460

l/m3 (400 gal/ft3) wastewater during warmer

weather (Culp et al., 1978). Erected in 1969, the

7.3 meter (24-foot) tower uses a cross-flow design

to treat a flow of 28,390 m3/day (7.5 MGD).

OPERATION AND MAINTENANCE

A routine O&M schedule should be developed and

implemented for any ammonia stripping system.

Regular O&M includes the following activities:

C Following all manufacturer O&M

recommendations. 

C Testing and calibrating equipment.

C Maintaining pumps and blowers.

C Inspecting the tower periodically for fouling.

C Maintain proper air and water flows.

C Proper pH adjustment with lime requires safe

handling.

C Clarifying the influent before stripping.

C Monitoring and controlling noise from the

stripping equipment. 

Table 1 lists sources and solutions to noise

problems.

COSTS

The cost of ammonia stripping depends on the

manufacturer, the site, the capacity of the plant, the

ammonia concentration of the wastewater, the flow

rate desired, the types of blowers, and the water

temperature (water temperature affects design

which affects price). Price comparisons are

possible with a specific set of design criteria.

Operation and maintenance include power,

materials, chemical, and labor.

REFERENCES

Other Related Fact Sheets

Other EPA Fact Sheets can be found at the

following web address:

http://www.epa.gov/owmitnet/mtbfact.htm

1. “Air Stripping” [http://www.scana.com/sce

%26g/business_solutions/technology/ewt

wair.htm].

2. Cornwell, David A., 1990. Air Stripping

and Aeration. In Water Quality and

Treatment: A Handbook of Community

Water Supplies. Ed Pontius, Frederick W.,

AWWA 4th Ed. McGraw-Hill, Inc., NY. 

3. Culp, Russel L.; Wesner, George Mack; and

Culp, Gordon L., 1978. Handbook of

Advanced Wastewater Treatment. 2nd Ed.

Van Nostrand Reinhold Co., NY.

4. Nutrient Control, Manual of Practice FD-7

Facilities Design, 1983. Water Pollution

Control Federation.

TABLE 1 SOURCES OF AND SOLUTIONS

TO POTENTIAL NOISE PROBLEMS

Noise Source Possible Solutions

Motors Proper installation, maintenance,

and insulation

Fans Reduction in tip speed and

installation of exhaust silencers

Water Splashing Water shielding of the tower

packing and air inlet plenum

Source Culp et al., 1978.
For more information contact:

Municipal Technology Branch

U.S. EPA

Mail Code 4204

1200 Pennsylvania Avenue, N.W.

Washington, D.C. 20460

5. U.S. EPA, Wickramanayake, G.B.; Evers,

D.; Kittel, J.A.; Gavaskar, A., 1991. BenchScale Evaluation of Ammonia Removal

from Wastewater by Steam Stripping. EPA

600/2-91-046, Washington, D.C.

6. U.S. EPA, 1980. Innovative and

Alternative Technology Assessment Manual.

EPA 430/9-78-009, Washington, D.C. 

7. Water Engineering & Management

Ammonia Removal Suppliers

[http://www.waterem.com].

ADDITIONAL INFORMATION

ResinTech, Inc.

Frank DeSilva

1980 Old Cuthbert Rd.

Cherry Hill, NJ 08034

Water Equipment Services

Mark Gorrell

6389 Tower Lane

Sarasota, FL 34240