NH4+ + 2 02 + 2 HC03" ---> N03- + 2 H2CO3 + H20

     The above equation shows that alkalinity is destroyed by the

oxidation of ammonia and that carbon dioxide (H2CO3 in the aqueous

phase) is produced. Past studies have shown that 6.3 to 7.4 mg of

alkalinity are destroyed for every mg of NH4+-N oxidized in attached

growth systems (EPA, 1975).     Thus the process of nitrification

tends to lower the pH. The significance of this pH depression is

that nitrification rates can be rapidly depressed.       Almost all

nitrifying bacteria have an optimum pH in the alkaline range,

usually near 8.0, and grow only slowly at pH values much below

neutral (Gaudy & Gaudy, 1980). Lamb et. al. (1990) reported that

alkalinity in the septic tank effluent appeared to limit the

nitrification process in a sand filter during warm weather.       If

sufficient alkalinity is not available, the pH of the system can

drop below 5.5 at which point nitrification could be inhibited

(Loudon et. al., 1989). Thus, it is recommended that the level of

alkalinity as CaCO3 be maintained above 40 mg/l (Sandy, 1987).

  The above equation also shows that dissolved oxygen is required

  for the nitrification process. An oxygen requirement of 4.6 mg of

02 for every mg NH4+ oxidized has been theorized to be sufficient

for aeration requirements (EPA, 1975). Several investigations have

provided indirect evidence of the importance of the effect of DO on
nitrification rates    (EPA, 1975).    Low DO levels can inhibit
nitrification and thus it is recommended that DO levels should be

maintained above 2 mg/l for nitrification systems (Grady and Lim,