Removal of NOM, Nutrients, and Micropollutants in BAC Filtration - Activated Carbon for Water and Wastewater Treatment - page 273

BOM removal within biofilters is reported to be related to temperature [39]. This
effect became more pronounced under unfavorable conditions, namely in the case
of chlorine in backwash water, anthracite media, or refractory organic matter.
Comparative bench-scale sand biofiltration at 5

1 C, 20 1 C, and 35 1 C revealed that

1

at 5 C significantly lower NOM removal was achieved than at the other tem-

peratures [40].

9.2.6
Effect of Oxidant Residuals

In water treatment, preoxidation steps leave some residuals such as ozone,
hydrogen peroxide, chlorine, chlorine dioxide, and monochloramine, which then
enter biofilters and lead to a reduction in biological removal [13]. For example,
ozone residuals of 0.1-0.2mg L 1 were shown to inhibit bacterial development in
pilot-scale anthracite-sand filters and to reduce their performance [41].
In this regard, GAC serves another important function in that it decomposes
chlorine and other oxidants through redox reactions [42]. Therefore, establishment
of biological activity would still be possible in such filters. However, in the long
term the presence of oxidants in influent water can lead to structural deterioration
of GAC. To avoid this, fluidized-bed GAC filtration was proposed as a process for
the safe destruction of ozone residuals [43].

9.3
Performance of BAC Filters: Organics Removal

The total removal of organic matter across a BAC filter is commonly monitored by
the parameters TOC, DOC, and UV absorbance. Table 9.5 exemplifies the typical
concentration ranges and the extent of removal at each treatment stage.
Of all the parameters, DOC is the most widely employed one. In units preceding
BAC filtration, such as coagulation, flocculation, and sedimentation, the reduction in
DOC is due to physicochemical mechanisms. In contrast to this, the reduction
observed in BAC filtration is attributed to biodegradation as well as adsorption.
It is also important to know which fractions in NOM are removed in BAC fil-
tration. Depending on water source and treatment conditions, the extent of
removal achieved in DOC, UV 254 , BDOC, and AOC may vary. In that respect,
fractionation of these parameters with regard to molecular size is also important.
In examining the fate of the biodegradable fraction in BAC filtration the best
procedure is to monitor the change in BDOC and/or AOC parameters. The con-
centration ranges of BDOC and AOC vary appreciably from one source to another
as shown in Table 9.5.
The organics contributing to the AOC parameter consist of small molecules
having a MW o 1 kDa. Therefore, the AOC concentration in the influent water is
only slightly changed in conventional water treatment consisting of flocculation,
sedimentation, and filtration units. In contrast to this, in most cases, biological