3. Results and discussion
3.1. Removal of dissolved organic carbon
In the first campaign, the DOC in the feed water (non-ozonated) was 11.2 0.4 mg L1. With an EBCT of 120 min, SAND 1
reduced the DOC by 22 3% (Fig. 2). This is in agreement with
what has been previously observed by other researchers in
sand columns simulating riverbank filtration or managed
aquifer recharge: Maeng et al. (2008) observed up to 20% DOC
removal for an EBCT of 4 days; Rauch and Drewes (2004)
obtained a removal of 25% after 18 hours of residence time.
The investigators observed a faster removal at the top of the
columns (corresponding to shorter EBCTs) which is consistent
with the result of the present study. The effect of the full scale
sand filter preceding the pilot scale filter is assumed to be
negligible since the EBCT there is only 15 minutes and backwashes are performed typically every hour preventing the
establishment of a biologically active layer. It is therefore
suggested that the fraction of DOC removed here corresponds
to the more easily biodegradable fraction of the effluent
organic matter (EfOM).
In BAC 1, the DOC influent level was reduced by 63 1%,
which is much higher than what was observed in SAND 1
suggesting that the biodegradable fraction of EfOM is not
totally removed in SAND 1. It is also possible that the higher
removal observed is due to adsorption of EfOM onto activated
carbon. However, a previous study on these filters showed
that DOC removal had reached a steady state one year prior to
the collection of the samples for the present study (PipeMartin et al., 2010). This indicates that complete breakthrough of EfOM has been reached and it is therefore suggested that biodegradation is responsible for the removal
observed even though adsorption might still play a role in the
mechanism. Indeed, the surface of the activated carbon is not
totally covered by the biofilm and the free areas might still
take part in adsorption/desorption processes leading to an
increased flux of pollutants to the biofilm (Herzberg et a