Combination of Activated Carbon Adsorption and Biological Processes in Drinking Water Treatment - Activated Carbon for Water and Wastewater Treatment - page 245

treatment systems such as slow sand filtration (SSF) and biological rapid sand
filtration (BSF) have been in use for a long time, particularly in Western Europe.
These treatment systems employing microbial activity to varying extents can also be
considered as biological filtration processes [43]. However, they have been excluded
fromthe scope of this topic in viewof the fact that they do not employ activated carbon.
However, in some places in this topic activated carbon filtration is compared with
sand and anthracite/sand filtration, a very common operation in water treatment.

8.6.2
Combination of Ozonation and BAC Filtration

Ozone finds wide application in the pretreatment of water, in order to fulfill dif-
ferent requirements such as color, odor, and taste removal, oxidation of inorganics
such as iron and manganese, and as an aid in coagulation. Ozonation can also lead
to the removal of many micropollutants. Since ozone is a powerful oxidant it acts
against many microorganisms including viruses.
Ozonation usually precedes BAC filtration. As outlined in detail in Section 8.7.2,
ozonation converts slowly degradable or nonbiodegradable compounds into bio-
degradable ones, thus enabling their biological removal. Otherwise, these com-
pounds would create nuisances in treatment steps such as disinfection.
Depending on water characteristics and the aim of treatment, ozonation units
may be placed at the head of water treatment only (preozonation) or at inter-
mediate stages (Figure 8.4). As discussed in Section 8.7.2, pre- and post-ozonation
applications change the adsorbability and biodegradability of organic compounds
in water; therefore, they also affect BAC filtration.
In Europe, multiple applications of small ozone doses at several points of treatment
is more common than single-stage preozonation [44]. Experiences show the super-
iority of multistage ozonation over single-stage ozonation in the removal of DOC.
Ozone has been used for a long time as a last-stage disinfectant and was not
regarded as leading to the formation of DBPs. However, nowadays it is recognized
that ozonation also leads to by-products such as bromate that are of health con-
cern. Other preoxidation alternatives are also available in water treatment. For
example, oxidation with UV/H 2 O 2 does not lead to bromate formation. However,
this application has the disadvantage of nitrite formation [45].

8.6.3
Current Use of BAC Filtration in Water Treatment

GAC and BAC filtration may be applied in the case of surface and groundwater
treatment, mainly for the purpose of removing NOM and specific organics. Over

Pre-
ozonation

Rapid
mixing

Rapid
filtration

BAC
filtration

Flocculation Sedimentation Ozonation Disinfection

Figure 8.4 Positioning of ozonation and BAC filtration in drinking water treatment.