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10 JOURNAL OF APPL/ED AQUACULTURE Note that R is constant over the entire range of half-order ammonia removal, only if C and the quantity C-E (i.e., ammonia drop in Fig- ure 2) are linearly related with a y-intercept of zero. The y-intercept for this relationship was not significantly different (P<0.05) from zero in the gravel +ozonation, granular carbon +ozonation, and fluidized granular carbon +ozonation biofilters, but it was signifi- cantly different from zero in the fluidized granular carbon + oxy- genation biofilter. The occurrence of a significant positive intercept for ammonia removal in this biofilter may reflect a decrease in nitri- ficationefficiency as ammonia concentrations increased. Such a de- crease could occur as a result of the accumulation of organic matter in the biofiltration system. The accumulation of organic matter is dependent upon the efficiency of organic matter removal from the system which, in turn, is partly dependent on system management and on other components of the recycle system (such as physical filters) intended specifically to remove organic matter. Ammonia removal (R) is a function of the retention time (i.e., time required to fill the biofilter [containing filter medium] with water}. Consider the effect of multiple passes through a biofilter with a hypothetical removal of 0.40 and retention time of one min- ute. If the initial influent ammonia concentration is 2.0 mg amm~- nia-nitrogen/l, the concentration following one pass through the biofilter will be 1.2 mg ammonia-nitrogen/l. The concentration fol- lowing asecond pass through the biofilter (assuming no further ad- dition of ammonia} will be 0.72 mg ammonia-nitrogen/1 or approxi- mately 0.35 of the initial concentration. This number (i.e., 0.36) is equal ta: (1 - 0.4)zii with the exponent being the ratio of the cumulative retention time (two minutes} and the original retention time (one minute}. Thus, the effects of a change in retention time on ammonia concentration reduction in a biofilter can be described as follows: where M =the new retention time in the biofilter and Mr =the original retention time (i.e., the retention time at which R was cal- culated).