Page 700 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Oxidation 655

in trace amounts include: polychlorinated biphenyls (PCBs), may be removed as a colloid or particle in flocculation=
polychlorinated dibenzo-p-dioxins (PCDDs), and polychlor- sedimentation or filtration. In taste and odor control, the
inated dibenzofurans (PCDFs). Landfill disposal was prac- oxidant is added at the end of the process so that only residual
ticed but was becoming less feasible because of both real concentrations are removed. In most cases, the oxidation
and perceived toxic substances in the sludge. Incineration occurs as a part of an existing treatment train rather than in
was thought to have been feasible, but the risk of toxic a designed reactor.
substances in gas emissions and ash was an issue that made In oxidizing SOCs, as in groundwater remediation, oxida-
it less attractive. tion is most likely one of the main unit processes, as
The waste stream from the pulp and paper plant was about opposed to being ancillary. Either a complete mix or a plug-
5%–7% solids (0.30 fraction inorganics of clays and fillers, flow reactor may be used, with the latter being favored. In
0.70 fraction organic cellulosic fiber fines, residual pulping, mathematical modeling, kinetics is the underpinning. The
and de-inking chemicals), diluted to 2.5% solids for feed-flow principle of plug-flow reactor modeling is encompassed in
Equation 4.18. As with other processes, a pilot plant is the
to the pilot plant. Reactor temperature was T(reactor) ¼
4508C, but was increased to T(reactor) ¼ 5008C after adding surest way to reduce the uncertainty. Key questions have to do
methanol, 2% by weight; also, for Q(reactor) ¼ 76 L=h with stoichiometry, kinetics, mixing, dispersion, detention
(20 gph), u(reactor) 50 s. At T(reactor) ¼ 5008C, the reactor time, etc. For gases, for example, ozone, the method of
destroyed > 0.99 fraction of most dioxin-type compounds, introduction can be, for example, by fine-bubble diffusers, a
0.96 fraction of 2,3,7,8-TCDD and 2,3,7,8-TCDF, Venturi intake, a proprietary mixer such as Water Champt
and >0.90 fraction PCBs. The destruction of organics was (U.S. Filter, Inc.).
surmised to be mass-transfer limited, that is, in terms of
reactor turbulence; the latter would be at a higher level in a
full-scale reactor. PROBLEMS
Cost estimates for alternative technologies for processing
20.1 Balancing a Redox Reaction by Half-Reaction
the wastes were
Method
Suppose a redox reaction is between Fe 3þ and Mg.
a. Write the half-reactions (always include the standard
Cost (Dry Sludge)
potential).
($=metric ton)
b. Write the overall reaction, balancing the equation
Treatment Option Low High (include the resultant standard potential).
1 Land fill—dewater to 0.50 fraction solids 113 353 c. Illustrate with a cell depiction.
2 Fluidized bed incineration, GAC off-gas 129 198 d. Label the oxidants=oxidized species and reductants=
treatment, ash to landfill reduced species.
3 SCWO—0.10 fraction solids: pure O 2 oxidant; 187 215 20.2 Balancing a Redox Reaction by Half-Reaction
p(reactor) ¼ 245 atm; T ¼ 5008C; u ¼ 50 s; Method
ash to landfill is an oxidant considered for use to oxidize
MnO 4
4 SCWO—0.12 fraction solids: pure O 2 oxidant; 120 140 Mn 2þ to MnO 2 .
p(reactor) ¼ 135 atm; T ¼ 5008C; u ¼ 50 s;
a. Write the half-reactions (always include the standard
recover salvage value of ash
potential).
b. Write the overall reaction, balancing the equation
(include the resultant standard potential).
As seen in the comparisons, the SCWO option (3) has higher c. Illustrate with a cell depiction.
costs, in general. Process modifications, for example, thick- d. Label the oxidants=oxidized species and reductants=
ening the sludge to 0.12 fraction solids and obtaining reduced species.
$55=metric ton credit for calcium carbonate recovery as seen 20.3 Balancing a Redox Reaction by Oxidation Number
in (4), makes the cost competitive with the other processes. Method
On the other hand, as landfill regulations have become For the oxidation of Mn 2þ to MnO 4 by PbO 2 , show

more stringent and the cost of incineration has increased, the balancing by the oxidation number method. Note:
40%–45%, SCWO becomes more attractive. Stumm and Morgan (1996, p. 428) do this by the half-
reaction method.
20.4 Balancing a Redox Reaction by Oxidation Number
20.3 PRACTICE
Method
In most cases in drinking water treatment, oxidants are added Balance the reaction in which Fe 2þ is oxidized to Fe 3þ
to pipes or channels to suit the case at hand. For example, an by MnO 4 ; the latter is reduced to manganese

oxidant for iron or manganese removal is usually added either dioxide MnO 2 (s). The reaction takes place in an
before the rapid mix or in the rapid mix so that the products alkaline solution.