Membrane and Desalination Technologies - TCE Moodle Website

490 P. Kajitvichyanukul et al.
nonporous membrane can be covered by a biofilm that alters its performance (60). The conventional
method of membrane cleaning may damage the microbial cells.
3. Covalent bonding and covalent crosslinking. The covalent bonding method involves the creation
of covalent bonds between the reactive groups on the outer surface of microbial cells and different
ligands on the bedding material. To activate the ligands on the microbial cells, coupling agents are
applied. The most commonly used coupling agents are glutaraldehyde, carbodiimine, isocyanate,
and amino silane (57). The covalent crosslinking method is support free and involves the joining
of the microorganisms to each other to form a large, three-dimensional complex structure, which
is used as a bedding material. The major problems of these two methods are the toxicity effects
associated with the covalent bonding method (48).
4. Entrapment within polymers. This method consists of trapping microorganisms within a threedimensional
polymer matrix. The pores in the matrix are smaller than the microbial cells, keeping
them trapped within the material. But the pores still allow the penetration of substrates through the
polymer matrix towards the trapped microorganisms (48).
4.1.2. Application of Biofiltration in Natural Organic Matter Removal
The factors affecting biofilter efficiency in removal of NOM include the use of preozonation,
NOM characteristics, water temperature, and biofilter backwashing (61).
In using preozonation prior to the biofilter, it was found that (61) the removal of organic
carbon by biodegradation is inversely proportional to the UV absorbance (254 nm)-to-TOC
ratio and directly proportional to the percentage of low molecular weight material (as
determined by ultrafiltration). The extent and rate of total organic carbon (TOC) removal
typically increased as ozone dose increased, but the effects were highly dependent on NOM
characteristics. NOM with a higher percentage of high molecular weight material provided
the greatest enhancement in biodegradability by ozonation.
In biofilters operating for OBPs removal, the efficiency of the biofilter is a function of
empty bed contact time (EBCT), or filter velocity, and influent concentration (62). In
biological potable water treatment, substrate concentrations in influent are relatively low
and biofilter performance can be modeled as a first-order process (62). Huck et al. (63) found
a first-order relationship for the influent concentration and the removal rate of assimilable
organic carbon (AOC), biodegradable dissolved organic carbon (BDOC) and THM formation
potential in biological filters treating ozonated water. Gagnon et al. (64) reported a first-order
relationship for the removal of carboxylic acids. For the relation between EBCT and rates, it
was reported that in most cases higher specific rates were obtained with shorter EBCT (63).
To evaluate the efficiency of biofilters, a simple respirometric method was developed and
applied for the measurement of biomass activity in bench-scale potable water biofilters (65).
The term “Biomass respiration potential” (BRP) was introduced based on the consumption of
DO resulting from the biodegradation of BOM by aerobic respiration in a water sample
containing a given amount of biomass-laden filter media. Such a method represents a useful
tool for water utilities that are operating their filters biologically. Recently, a numerical model
was developed to simulate the nonsteady state behavior of biologically active filters used for
drinking water treatment (49, 66). The biofilter simulation model is called “BIOFILT” that
simulates the substrate (normally BOM) and biomass (both attached and suspended) profiles