286
H. S. Weinberg
Eventually, all carbon beds require reactivation by either replacement with
virgin carbon or by regeneration of the carbon. The choice of replacement or
regeneration will depend on plant scale, type of activated carbon, and water
characteristics. Regeneration is conducted by a thermal process, in which the
activated carbon is first dried and then oxidized at temperatures around 700 °C. A
loss of carbon mass during regeneration is inevitable, although the amount lost can
be minimized by optimizing the regeneration process. A change in pore size and
distribution can also occur during thermal regeneration, changing the effectiveness
of the carbon. In addition, thermal regeneration can create gaseous byproducts
which may pollute the atmosphere. Proper control of these by-products is
necessary and often expensive.
GAC is often used as a part of the granular filter media in rapid filters to form
hybrid filter-adsorbers or for treatment of the off-gas from aeration towers where
its adsorptive sites do not have to compete with the NOM present in the water.
Such fixed-bed GAC adsorption is also gaining increased use through point-of-use
devices in home filtration systems.
4.5 Biofiltration
Ozonation generally renders natural organic matter (NOM) more hydrophilic (i.e.,
more nonhumic and lower in molecular weight). Ozone can convert NOM in
water to biodegradable organic matter (BOM). Surrogate measurements for BOM
include assimilable organic carbon (AOC) and biodegradable organic carbon
(BDOC). Specific components of the BOM include low-molecular-weight
compounds such as aldehydes and carboxylic acids, each of which could, if
delivered into distribution systems, render the drinking water a nutrient potential
for microbial regrowth. Such components can be removed through biological
filtration but through a fear of introducing additional microorganisms into the
water supply, the process has taken off slowly in the designs of water-treatment
systems. GAC or the less expensive anthracite are the preferred medium for
successful controlled growth of a biofilm. However, with its higher surface area,
GAC is generally more efficient and certainly the more effective in colder waters
(Coffey et al. 1997). In each of Figs. 2 and 3, the potential for biological filtration
exists, since the schemes are shown without a disinfectant applied ahead of the
filters and assuming ozone residual has dissipated through the empty third
contactor. In its most frequent design, a shallow 6- to 12-inch sand layer is placed
under the carbon media to prevent breakthrough of the biomass into the effluent
water. Some plants have found that occasional use of small doses of chlorine to
the backwash water can help control the biological culture in the filter and overall
maintain the filter's effectiveness. What is clear about this technology is that it
does have the potential to control chemical contamination of the finished water
but, as with ozonation, it requires individual study at each treatment facility to
tune its effectiveness for a specific source water.
H. S. Weinberg
Eventually, all carbon beds require reactivation by either replacement with
virgin carbon or by regeneration of the carbon. The choice of replacement or
regeneration will depend on plant scale, type of activated carbon, and water
characteristics. Regeneration is conducted by a thermal process, in which the
activated carbon is first dried and then oxidized at temperatures around 700 °C. A
loss of carbon mass during regeneration is inevitable, although the amount lost can
be minimized by optimizing the regeneration process. A change in pore size and
distribution can also occur during thermal regeneration, changing the effectiveness
of the carbon. In addition, thermal regeneration can create gaseous byproducts
which may pollute the atmosphere. Proper control of these by-products is
necessary and often expensive.
GAC is often used as a part of the granular filter media in rapid filters to form
hybrid filter-adsorbers or for treatment of the off-gas from aeration towers where
its adsorptive sites do not have to compete with the NOM present in the water.
Such fixed-bed GAC adsorption is also gaining increased use through point-of-use
devices in home filtration systems.
4.5 Biofiltration
Ozonation generally renders natural organic matter (NOM) more hydrophilic (i.e.,
more nonhumic and lower in molecular weight). Ozone can convert NOM in
water to biodegradable organic matter (BOM). Surrogate measurements for BOM
include assimilable organic carbon (AOC) and biodegradable organic carbon
(BDOC). Specific components of the BOM include low-molecular-weight
compounds such as aldehydes and carboxylic acids, each of which could, if
delivered into distribution systems, render the drinking water a nutrient potential
for microbial regrowth. Such components can be removed through biological
filtration but through a fear of introducing additional microorganisms into the
water supply, the process has taken off slowly in the designs of water-treatment
systems. GAC or the less expensive anthracite are the preferred medium for
successful controlled growth of a biofilm. However, with its higher surface area,
GAC is generally more efficient and certainly the more effective in colder waters
(Coffey et al. 1997). In each of Figs. 2 and 3, the potential for biological filtration
exists, since the schemes are shown without a disinfectant applied ahead of the
filters and assuming ozone residual has dissipated through the empty third
contactor. In its most frequent design, a shallow 6- to 12-inch sand layer is placed
under the carbon media to prevent breakthrough of the biomass into the effluent
water. Some plants have found that occasional use of small doses of chlorine to
the backwash water can help control the biological culture in the filter and overall
maintain the filter's effectiveness. What is clear about this technology is that it
does have the potential to control chemical contamination of the finished water
but, as with ozonation, it requires individual study at each treatment facility to
tune its effectiveness for a specific source water.