When considering total nitrogen, both submerged and
flood/drain biofilters were remarkably similar in all treatments,
since any increase in TKN removal was matched by a decrease
in nitrite/nitrate removal. With a single passage of water through
these biofilters, the flood/drain biofilters offer no advantages over
submerged biofilters. The application of repeated passages demonstrated
the advantage of the flood/drain biofilters for the removal
of ammonia and organic nitrogen and the advantage of submerged
biofilters for the treatment of nitrates/nitrites regardless of the
length of the drain period. The combined biofilter showed the
exceptional ammonia removal that was observed in the flood/drain
biofilters in treatments 2 and 3 and showed only a relatively small
increase in nitrite.