TM 5-814-3/AFM 88-11, Volume III
d. Nitrification in trickling filter plants. Low biochemical oxygen demand loadings (less than 5 pounds
per day per 1,000 cubic feet) and high wastewater temperature (20 oC or higher) are necessary for good
nitrification (80-90 percent) in trickling filters although they are often run at 12 degrees Centigrade with some
loss of overall efficiency. A trickling filter filled with plastic packing can obtain 90 percent nitrification of
secondary effluent at a loading rate of 0.05 gallons per minute per cubic foot. Year-round nitrification
facilities must be designed for the lowest wastewater temperatures experienced in the winter months. In this
instance, the required filter volume will be much greater than that required for seasonal nitrification and will
require at least two-stage treatment.
e. Rotating biological contactors as nitrification units. Ninety percent nitrification can be accomplished
using rotating biological contactors if the influent biochemical oxygen demand is less than 150 milligrams per
liter and the hydraulic application rate is 2 gallons per day per square foot or less. For further information,
consult the EPA Process Design Manual for Nitrogen Control.
f. Air stripping. Ammonia, in the molecular form, is a gas which dissolves in water to an extent
controlled by the partial pressure of the ammonia in the air adjacent tot he water. Therefore, ammonia
removal from wastewater can be accomplished by contacting water droplets with large amounts of ammonia-
free air. This process is desorption, but is commonly referred to as ammonia stripping. For additional
information, refer to the EPA Process Design Manual for Nitrogen Control.
15-10. Denitrification.
a. Suspended growth denitrification. Denitrification is performed by heterotrophic anaerobic organisms
and, therefore, requires an organic carbon source and anaerobic conditions. Suspended growth denitrification
will provide gentle mixing (no aeration) with the mixed liquor being clarified. The effluent is aerated to
provide dissolved oxygen and to drive off entrained nitrogen, and the sludge is recycled to the contact tank.
Methanol is the most common organic carbon source used and is usually applied at a dosage of 2 to 4 pounds
methanol per pound of nitrate nitrogen removed. This dosage will be carefully adjusted according to nitrate
concentra tion and temperature in order to avoid the effects of under-dosage and over-dosage. Raw or
partially treated sewage and sludge can be used as a carbon source but yield lower denitrification efficiencies.
Typical design factors for this system include:
(1) Mixed liquor volatile suspended solids concentrations of 1,500 to 2,500 milligrams per liter;
(2) Detention time of 4 hours;
(3) Clarifier overflow rates not more than 600 gallons per square foot;
(4) Dissolved oxygen levels of up to 0.5 milligrams per liter (pH 7.0);
(5) Optimum pH from 6.5 to 7.5;
(6) 5 minutes of effluent aeration; and
(7) Sludge recycle from 50 to 100 percent of average flow.
Temperature effects are very significant and should be kept in mind when designing contact tanks since a 10
degress Centigrade decrease would require twice the tank capacity.
b. Fixed denitrification. Fixed denitrification utilizes a flooded packed column, with denitrifying
microbial populations attached on the media surface. Sand, activated carbon, gravel, coal, and plastic packing
can be used; with the finer media having shorter contact times. Columns can be either downflow or upflow
configuration. Upflow columns have much longer contact times and operate at lower hydraulic loadings, but
they are more efficient at lower temperatures than downflow columns. Temperature again is a very significant
factor affecting denitrification efficiency. Typical design factors for both downflow and upflow denitrification
are given in table 15-7. Fixed denitrification is capable of nitrate removal efficiencies of over 90 percent;
however, the efficiency of the overall nitrification-denitrification process is limited by the performance of the
nitrification system (i.e., around 90 percent nitrogen removal).
15-23
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