In this treatment the biofilters were operated as in treatment 1
except that instead of a fresh batch of wastewater being added each
day, water collected after passing through the biofilter was used to
refill the supply tank. Water was circulated through the biofilters
for seven days. At the start of each week, 3 L of wastewater was
added to each supply tank, to replace the water already in the biofilter
from the previous week (which was discarded). The following
day an additional 3 L of wastewater was added to all biofilters. The
displaced water was used to supply the biofilter on the following
day. This new fixed volume of water was then recirculated for a
7-day period. Each week during treatment 2, water samples were
taken for analysis from the inlet at the start of the week and outlet
of all biofilters on day 7 (i.e. water that had made four passages
through the biofilter).