Drinking Water Quality for the 21 st Century
285
such a system of ozonation with or without biofiltration routinely and quite often
consulting engineers are retained on site for this purpose.
In addition to the effects of ozonation on DBP precursors, ozonation byproducts can lead to the formation of secondary byproducts from postchlorination
of preozonated waters. McKnight and Reckhow (1992) studied secondary byproducts formed during ozonationlchlorination. Acetaldehyde produced by
ozonation can react with chlorine to form chloroacetaldehyde. When acetaldehyde
undergoes an initial chlorine substitution reaction, the reaction proceeds rapidly to
form the trichlorinated product chloral hydrate. According to Scully (1990),
formaldehyde - another aldehyde produced during ozonation - and
monochloramine may react under acidic conditions to form CNCI. In this
reaction, N-chloraldimines are likely to be intermediates, whereas nitriles are the
products. Because aldehydes (e.g., formaldehyde, acetaldehyde) can be removed
in biologically active filters (Weinberg et al. 1993), the formation of secondary
byproducts will depend on the mode of filter operation (whether the filters are
prechlorinated or postchlorinated, media type, etc.) (Coffey et al. 1996).
4.4 Granular Activated Carbon (GAC)
GAC treatment of a substance involves the adsorption of the substance onto the
solid carbon phase. When equilibrium is reached between the solid and aqueous
phases, no further sorption onto the carbon will occur. The spent carbon must then
be replaced or regenerated.
The time to reach equilibrium depends on
characteristics of the carbon used, such as surface area and pore size distribution,
and the surface chemistry between the wastewater and the carbon (AWWA 1999).
Adsorption isotherms such as Freundlich and Langmuir isotherms are used to
quantifY the relationship between the adsorbate and adsorbent.
GAC adsorption is a well-established technology. Contactors are designed as
columns of carbon, referred to as carbon beds, through which the water usually
flows in the vertical direction. The carbon bed is supported by an underdrain
system, which can be of various designs. Common contactors include pressure
filters (of up flow or downflow design) and gravity filters. Contactors can be used
alone, in series, or in parallel. Optimization of contactors in the system depends
on characteristics of the wastewater to be treated, contact time (empty bed contact
time), bed depth, and hydraulic loading rate.
GAC contactors require backwashing to dislodge any solids that become
trapped in the column, to maintain the desired hydraulic properties of the carbon
bed and to prevent the growth of a biomass if this is not desired. In the latter case,
disinfection can be applied ahead of the filters, as seen in Fig. 1. Proper
backwashing is essential, but should be minimized, since it can be detrimental to
the effectiveness of the system. Some of the concentrated chemical species
retained on the carbon are susceptible to desorption during backwashing, and
mixing of the carbon bed may occur, increasing the chances of desorption. All
possible steps should be taken to avoid compound desorption, since this would
lead to recontamination of the treated water.
285
such a system of ozonation with or without biofiltration routinely and quite often
consulting engineers are retained on site for this purpose.
In addition to the effects of ozonation on DBP precursors, ozonation byproducts can lead to the formation of secondary byproducts from postchlorination
of preozonated waters. McKnight and Reckhow (1992) studied secondary byproducts formed during ozonationlchlorination. Acetaldehyde produced by
ozonation can react with chlorine to form chloroacetaldehyde. When acetaldehyde
undergoes an initial chlorine substitution reaction, the reaction proceeds rapidly to
form the trichlorinated product chloral hydrate. According to Scully (1990),
formaldehyde - another aldehyde produced during ozonation - and
monochloramine may react under acidic conditions to form CNCI. In this
reaction, N-chloraldimines are likely to be intermediates, whereas nitriles are the
products. Because aldehydes (e.g., formaldehyde, acetaldehyde) can be removed
in biologically active filters (Weinberg et al. 1993), the formation of secondary
byproducts will depend on the mode of filter operation (whether the filters are
prechlorinated or postchlorinated, media type, etc.) (Coffey et al. 1996).
4.4 Granular Activated Carbon (GAC)
GAC treatment of a substance involves the adsorption of the substance onto the
solid carbon phase. When equilibrium is reached between the solid and aqueous
phases, no further sorption onto the carbon will occur. The spent carbon must then
be replaced or regenerated.
The time to reach equilibrium depends on
characteristics of the carbon used, such as surface area and pore size distribution,
and the surface chemistry between the wastewater and the carbon (AWWA 1999).
Adsorption isotherms such as Freundlich and Langmuir isotherms are used to
quantifY the relationship between the adsorbate and adsorbent.
GAC adsorption is a well-established technology. Contactors are designed as
columns of carbon, referred to as carbon beds, through which the water usually
flows in the vertical direction. The carbon bed is supported by an underdrain
system, which can be of various designs. Common contactors include pressure
filters (of up flow or downflow design) and gravity filters. Contactors can be used
alone, in series, or in parallel. Optimization of contactors in the system depends
on characteristics of the wastewater to be treated, contact time (empty bed contact
time), bed depth, and hydraulic loading rate.
GAC contactors require backwashing to dislodge any solids that become
trapped in the column, to maintain the desired hydraulic properties of the carbon
bed and to prevent the growth of a biomass if this is not desired. In the latter case,
disinfection can be applied ahead of the filters, as seen in Fig. 1. Proper
backwashing is essential, but should be minimized, since it can be detrimental to
the effectiveness of the system. Some of the concentrated chemical species
retained on the carbon are susceptible to desorption during backwashing, and
mixing of the carbon bed may occur, increasing the chances of desorption. All
possible steps should be taken to avoid compound desorption, since this would
lead to recontamination of the treated water.