A variant of the standard BAF denitrification system with external carbon addition was developed specifically to service smaller flows with less maintenance and energy consumption than typically required. Instead of the standard submerged anoxic medium, however, a freedraining absorbent
trickle filter was adapted with stagnant air to produce an ‘asphyxiant absorbent filter’, or ‘AAF’. Test AAFs were inserted as retrofits into the treatment trains of two commercial sites in Ontario, a high-nitrogen factory and a high-strength truck stop.
A variety of flows, carbon addition rates, and three carbon source types were tested to find the limits and peculiarities of the AAF operation. This paper describes the results of the testing to date, which indicate that about 100% of the NO2,3-N can be removed within the
AAF, depending on flows and rates of carbon addition. The excess carbon is removed when the AAF effluent is returned to the existing nitrifying filters. The AAF unit recovers quickly if disturbed by tampering or carbon source depletion. Alkalinity addition varies directly
with the NO2,3-N removed. No backwashing or cleaning of the medium is required even after one year operation; the AAF medium can be cleaned of any excess biomat by stopping carbon addition for a few days. Given enough initial alkalinity for thorough nitrification, a constant
source of labile carbon, a polishing filter to remove excess carbon, and regular maintenance, expectations are that a total nitrogen concentration of <5 mg/L can be achieved in the final effluent.
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