containing primarily ammonium-N (USEPA, 1980; Canter and Knox,

1985).   One of the most effective means of ammonium removal is

through biological nitrification and denitrification. Ammonium is

converted to nitrate    (nitrification) and the nitrate is then

converted to nitrogen gas (denitrification) which is released to

the atmosphere.

     The nitrification and denitrification processes require a

variety of bacteria and environmental conditions.     In order for

these  processes   to  be  successful, an understanding     of  the

conditions necessary for each process is essential.

     Nitrification is commonly defined as the biological oxidation

of   ammonium  to   nitrate  with   nitrite  as  an   intermediate.

Autotrophic   microorganisms   are   largely,   if  not   entirely,

responsible for nitrification in natural systems. These nitrifying

autotrophs require oxygen and derive the carbon for cell synthesis

largely from C02, carbonates, or bicarbonates (Delwiche,1981).

      Oxidation of ammonium to nitrite by Nitrosomonas and the

 subsequent oxidation of nitrite to nitrate by Nitrobacter is

 usually represented by the following equations:

           NH4+ + 1.5 02 + 2 HC03 ---> N02 + 2 H2CO3 + H20

         NO2 + .5 02 ---> N03

      Nitrosomonas obtain energy from the oxidation of ammonium to

 nitrite while Nitrobacter obtain energy from the further oxidation
 of nitrite to nitrate.

      The overall reduction of ammonium to nitrate can be shown as

 (EPA , 1975) :