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Potable Water Biotechnology, Membrane Filtration and Biofiltration 491
in a biofilter as a function of time through incorporation of the fundamental processes of
biofiltration as follows (48):
(a) deposition of suspended biomass onto filter media
(b) transport of substrate through the filter by advection and dispersion
(c) diffusion of soluble substrate across a liquid thin film layer
(d) diffusion of substrate in the biofilm
(e) biodegradation of substrate by immobilized (in biofilm matrix) bacteria
(f) biomass growth
(g) biomass decay
(h) biomass loss from the biofilm due to fluid shear
(i) biomass loss during a backwashing event
Hozalski and Bouwer (66) reported that BIOFILT appears to be an effective model for
simulation of the biofiltration process and is the first model to effectively simulate the
highly nonsteady-state behavior encountered in a periodically backwashed drinking water
biofilter. The information for biofiltration design based on the “BIOFILT” model is reported
as follows (66):
(a) BOM with a greater diffusivity or with faster degradation kinetics experienced greater pseudo
steady state BOM removals and also contributed to shorter biofilter start-up times.
(b) The presence of readily degradable substrate can significantly enhance the removal of slowly
degradable material primarily due to the ability to maintain greater biomass levels in the biofilters.
(c) A temperature decrease from 22.5 to 3 C resulted in declines in pseudo steady state BOM
removal and increases in biofilter start-up time.
(d) Periodic backwashing should not significantly impair the BOM removal performance of
biofiltration.
Recently, biofiltration technologies in NOM removal are progressively advanced. Various
methods were developed to evaluate the performance of biofiltration in removal of NOM from
water. GAC and expanded clay (EC) were used to examine biofiltration of surface water (67).
Particle removal was measured by flow cytometry, which enabled discrimination between totaland
autofluorescent particles (microalgae) in size ranges of 0.4–1 and 1–15 mm, and measured
by on-line particle counting. It was found that biofilters were also challenged with 1 mm
fluorescent microspheres with hydrophobic and hydrophilic surface characteristics and bacteriophages
(Salmonella typhimurium 28B) (67). Yavich et al. (68) developed a simple procedure
in order to describe the kinetics of biodegradation of NOM in drinking water and also used this
procedure to evaluate changes in the concentration of BOM during ozonation and biotreatment.
The results showed that ozonation of NOM depends on source of water that might result in
either minimal formation of biodegradable organic carbon, or the formation of predominantly
rapidly biodegradable NOM, or in the formation of both rapidly and slowly biodegradable
NOM (68). The distribution, composition, and activity of microbial communities developing in
biofilters treating nonozonated and ozonated drinking water were evaluated by Fonseca et al.
(69) using tetrazolium reduction assays, phospholipid analysis, and 16S rRNA (rDNA)
sequence analysis. The response of media-attached biomass to operating temperature (3 vs.
>12 C) and ozone application point was also evaluated. It was found that phospholipid and