Membrane and Desalination Technologies - TCE Moodle Website

500 P. Kajitvichyanukul et al.
do not produce a residual that needs disposal, and require almost no modification of ambient
conditions. The only prerequisite for maximizing bacterial substrate utilization in filters is the
absence of disinfectant in the filter influent or backwash water. The filter media colonized by
bacteria can be sand, anthracite, or GAC. Anthracite and sand are considered inert because
neither interacts chemically with PM. Thus, any removal of PM would be solely to biological
activity. GAC that has been colonized by bacteria will initially remove DOM through
adsorption and biological substrate utilization. After the GAC’s adsorptive capacity has
been exhausted, PM removal is achieved only through substrate utilization, and the GAC is
defined as BAC. All drinking water filters will become biologically active in the absence of
applied disinfectant residuals. The process of biological colonization and substrate utilization
is enhanced by the ozonation of filter influent water.
In the US, preozonation is practiced to remove color, taste, and odor, to inactivate Giardia
and Cryptosporidium, and to serve as an alternative to chlorine disinfection. Ozonation
decreases the average molecular size and weight of the PM, allowing indigenous bacteria
to utilize more of it as substrate in a given amount of contact time. Some fraction of microbes
will always survive ozonation. As long as no liquid phase ozone residual is present in the filter
influent, the surviving microbes will eventually colonize the filter media. A preozonated
biological filter will achieve greater PM removals than one with influent that has not been
preozonated. PM is measured as TOC, DOC, THMFP, or haloacetic acid (HAA) formation
potential (HAAFP). All of the studies discussed in this section measure DBP control using
one or some combination of the parameters TOC, DOC, THMFP, and HAAFP. Because of its
potential to control DBP precursors and its economic advantages, the US EPA has performed
or funded a number of research studies to characterize the impact of biological filtration on
the control of DBPs (120).
5.1. Pilot-Scale Study, Shreveport, LA
A pilot-scale study was executed to investigate the combination of ozone and GAC for
THM precursor removal (121). Following conventional treatment (coagulation, flocculation,
and sedimentation), the process water was split to a GAC filter, and to an ozone contactor
followed by a GAC filter (O3–GAC). The results are summarized in Table 11.1. Analysis of
Table 11.1
TOC and THMFP removal in pilot-scale GAC columns, Shreveport, LA (121)
Week
% TOC removal % THMFP removal
GAC O 3–GAC Week GAC
0–21 82 83 0–21 82
22–52 55 55 22–52 55
53–62 10 19 53–62 10
65–83 24 27 65–83 24
Note: Average filter influent TOC = 30 mg/L.