wet air oxidation of hazardous waste

1. WET AIR OXIDATION OF HAZARDOUS WASTE
Professor I. M. Mishra
Department of Chemical Engineering,
Indian Institute of Technology, Roorkee, Roorkee-247 667.
1. Wet Air Oxidation
Wet oxidation is a hydrothermal treatment of aqueous solutions of biologically
recalcitrant and hazardous chemicals/wastes. It is the oxidation of dissolved or suspended
matter in water using an oxidant such as ozone, oxygen, hydrogen peroxide, air etc. It is
referred to as Wet Air Oxidation (WAO) when air is used as an oxidant. The oxidation
reactions generally occur at temperatures above the normal boiling point of water (100
°C) but below its critical point (374 °C). The system must also be maintained under
pressure i) to maintain the solution in liquid form; ii) to avoid excessive evaporation of
water and also iii) to conserve energy, as the evaporation needs latent heat of
vaporization. Under wet conditions, many compounds get oxidized which would
otherwise not oxidize under dry (not wet) conditions, even at the same temperature and
pressure.
When the hydrogen peroxide is used as the oxidant, the oxidation is referred to as
the Wet Peroxide Oxidation (WPO). Due to high cost of peroxides, it is used only to
create free initiation radicals as a pretreatment step followed by either air oxidation or
oxygen oxidation.
Wet air oxidation (WAO) is a well-established technique of importance for
wastewater treatment particularly toxic and highly organic or inorganic wastewaters
[Zimmermann (1950, 1954a, b, 1958a, b, 1961), Laughlin et al. (1983), Joshi et al.
(1985), Copa and Gitchel (1989), and Joshi and Mishra (2006)]. Wet air oxidation
involves the liquid phase oxidation of organics or oxidizable inorganic components at
elevated temperatures (125-320 °C) and pressures (0.5-20 MPa) using a gaseous source
of oxygen (usually air). Enhanced solubility of oxygen in aqueous solutions at elevated
temperatures and pressures provides a strong driving force for oxidation.
The elevated pressures are required to keep water in the liquid state. Water also
acts as a moderant by providing a medium for heat transfer and removing excess heat by

2. evaporation. WAO has been demonstrated to oxidize organic compounds to C02 and their
innocuous end products.
Carbon is oxidized to CO2; nitrogen is converted to NH3, NOx or elemental
nitrogen; halogen and sulfur are converted to inorganic halides and sulfates. The higher
the temperature the higher is the extent of oxidation achieved, and the effluent contains
mainly low molecular weight oxygenated compounds, predominantly carboxylic acids.
The degree of oxidation is mainly a function of temperature, oxygen partial pressure,
residence time, and the oxidizability of the pollutants under consideration. The oxidation
conditions depend on the treatment objective.
For instance, in the case of sewage sludges, mild oxidation conditions can be used
to achieve 5-15% COD reduction resulting in a sludge which is sterile, is biologically
stable, and has very good settling and drainage characteristics. On the other hand, in the
case of oxidation of caustic scrubbing liquors, more than 99.9% of the waste components
are oxidized. Fig. 1 depicts a typical WAO treatment system.
WASTE
STORAGE
TANK
REACTOR
SEPARATOR
GAS
HEAT
EXCHANGER
OXIDIZED
LIQUID
AIR
AIR
COMPRESSOR
PUMP
Fig. 1: Basic Wet Oxidation flow Sheet

3. Wet air oxidation requires much less fuel than other thermal oxidation processes
such as incineration. This is because, for WAO, the only energy required is the difference
in enthalpy between the incoming and outgoing streams. However, for incineration, not
only the sensible enthalpy (combustion products and excess air to be heated to the
combustion temperature of about 1000 °C) is to be provided but also it is required to
supply heat for the complete evaporation of water. The capital cost of a WAO system is
higher and depends on the flow, oxygen demand of the effluent, severity of the oxidation
conditions, and the material of construction required.
2. Chemistry of Wet Oxidation
Before discussing the various aspects of CWO in detail, it is necessary to discuss
the aspects of noncatalyzed WO. In most systems, both WO and CWO occur
simultaneously and reaction pathways (and, hence, the reactions that occur) for both
processes are usually very similar, because the main partial oxidation product of both
WO and CWO processes is usually acetic acid. Although the main aim of WO (the
removal of organic compound(s) by conversion to carbon dioxide) and methods for
evaluating WO (percentage of TOC and/or COD removed) are usually explained in very
simple terms, the chemistry that occurs during WO of single organic compounds and
mixtures of organic compounds is quite complex. This complexity is partially due to the
different types of chemical reactions (both oxidative and non-oxidative) that can occur
for various organic compounds under typical WO conditions and the high total number of
reactions that can occur during the WO of even a single organic compound. Different
types of chemical reactions that can cause/lead to oxidation of organic compounds under
typical WO conditions include auto-oxidation (free radical reactions involving oxygen),
heterolytic/homolytic cleavage (oxidative or non-oxidative thermal degradation),
hydrolysis, decarboxylation, alkoxide formation followed by subsequent oxidation
(alkaline solution), and carbanion formation followed by subsequent oxidation (alkaline
solution). The total number of reactions that can occur during the WO of a single organic
compound can be extremely high, even for a simple low molecular-weight organic
compound such as propionic acid. For example, Day et al. (1973) proposed a 16-step
free-radical reaction mechanism for the WO of propionic acid.

4. 3. The Two Main Stages of Wet Oxidation (WO).
WO of an organic compound involves two main stages: (i) a physical stage, which
involves transfer of oxygen from the gas phase to the liquid phase, and (ii) a chemical
stage, involving the reaction between the transferred oxygen (or an active species formed
from oxygen) and the organic compound. Although various other phenomena can
influence and/or cause the WO of an organic compound, such as co-oxidation (oxidation
of an organic compound via a free-radical intermediate produced during oxidation of
another compound), the two main stages either directly or indirectly (as in the case of
cooxidation) determine the rate at which an organic compound undergoes WO.
3.1 Physical Stage.
The physical stage of WO, which involves the transfer of oxygen from the gas
phase to the liquid phase, has been described in detail by Debellefontaine and Foussard,
2000. According to their study, the only significant resistance to oxygen transfer is
located at the gas/liquid interface (film model), with the three limiting cases being (i)
oxygen reacts within the film because of a rapid chemical reaction (in this way, the
oxygen transfer rate is enhanced); (ii) oxygen reacts rapidly within the bulk liquid, where
its concentration is close to zero (the overall rate is equal to the rate at which oxygen is
transferred); and (iii) the oxygen concentration within the bulk liquid is equal to the
interface (or equilibrium) concentration (the overall rate is the chemical step rate, and it is
usually low). Fig. 2 shows the path of gaseous reactant to catalyst surface in slurry
reactor.
Fig 2. Path of gaseous reactant to catalyst surface in slurry reactor

5. According to Debellefontaine and Foussard (2000) the effect that the rate of
oxygen transfer has on the overall rate can often be eliminated through high mixing
efficiency, which then enables unencumbered chemical kinetic rates to be determined.
4. Wet Oxidation Kinetics.
The effect of reaction temperature, oxygen partial pressure, and organic(s)
concentration on WO reaction rates have been described in detail in recent reviews.
Briefly, the effect of these parameters on the reaction rate of non-catalytic wet oxidation
in simple solutions can be described by the following kinetic model (Kolaczkowski et al.,
1999):
where, rr is the reaction rate, A the pre-exponential factor, E the activation Energy, R the
universal gas constant, T the reaction temperature, Corg the organic compound
concentration in the bulk liquid, and CO2, L the oxygen concentration in the bulk liquid.
The superscripts m and n are the orders of reaction. The partial order, with respect to the
organic compound, is usually 1, whereas the order with respect to dissolved oxygen is
~0.4. For complex wastestreams that contain a mixture of organic compounds, more-detailed
kinetic models are required to explain the effects of the main reaction parameters
on the rate of WO.
These models are usually based on the existence of two general types of
compounds/intermediate compounds present in complex solutions:
(i) compounds and intermediate compounds that undergo relatively fast oxidation to
carbon dioxide and water, and
(ii) compounds and intermediate compounds that are difficult to oxidize (mostly acetic
acid).
Most researchers refer to these classes of compounds as classes “A” and “B”.
Generalized kinetic models for various complex solutions have been developed by
several researchers, using the aforementioned class descriptions. Kinetic models are

6. important for determination of the effects of various reaction parameters on rates and for
the design of reactors.
5. Chemical Reaction Aspects.
To improve the rates of the chemical reactions that occur during WO, a better
understanding of the types of reactions that are occurring is required. The general
consensus among researchers in the field is that the chemical reaction stage of WO occurs
mostly via free-radical chemical reactions.
Numerous free-radical chemical reactions, from each of the three main types of
free-radical chemical reactions (initiation, propagation, and termination) have been
proposed to occur during WO of various individual organic compounds and mixtures of
organic compounds.
5.1. Initiation.
Bimolecular reactions:
Unimolecular reactions:
Trimolecular reaction:
Alkaline solution only:
where X carboxylate group,

7. 5.2. Propagation.
Alkaline solution only:
where X represents a carboxylate group.
5.3. Termination.
Alkaline solution only:
where X represents a carboxylate group,
The general material balance for the WAO process is as follows (Debellfontaine
and Foussard, 2000):
CmHnOkClwNxSyPz + (m + 0.25(n - 3x)) - 0.5k + 2(y + z))O2

8. 2 2 3 + - + + - + - + - +
® mCO 0.5(n 3x)H O xNH wCl ySO zPO3 Heat
4
24
The heat evolved in the above reaction is found to be around 435 kJ (mole O2 reacted)-1.
6. Mechanism of the Reaction
The oxidation of the organic compounds takes place according to a chain reaction
mechanism. The following reaction steps are involved in the WAO process:
Organic compounds + O2 ¾¾® Hydroperoxides
Hydroperoxides ¾¾® Alcohol
Alcohols + O2 ¾¾® Ketones (or aldehydes)
Ketones (or aldehydes) + O2 ¾¾® Acids
Acids + O2 ¾¾® CO2 + H2O
Actually, an organic radical R· is coupled with molecular oxygen to propagate
the reaction in the WAO reaction. R· radical is originated as a result of the reaction
between weakest C-H bonds and oxygen, which forms ·
HO2 , this ·
HO2 then combines
with RH forming hydrogen peroxide. The hydrogen peroxide obtained decomposes
readily to hydroxyl radicals due to temperature. The last reaction is a propagating step
leading to oxidized species. The mechanism of WAO can be understood better according
to the following reactions:
O - O + R· ¾¾®ROO·
+ ¾¾® · + ·RH O2 R HO2
RH + HO2 ¾¾® R + H2O2 · ·
H2O2 M 2HO M + ¾¾® · +
ROO· + RH ¾¾® R· + ROOH
As for most of the molecules, reaction, the initiation step is a limiting step too,
which depends on the temperature with an activation energy, which can exceed 100 or
200 kJ mol-1. That is why the WAO does not take place at room temperature, but requires
high temperatures (> 250 or 300 oC). As this mechanism shows the importance of free

9. radicals, so the use of catalysts and promoters can reduce the severity of the operating
conditions required for the reaction.
The overall WAO mechanism includes two steps. One step has been discussed
above, i.e., chemical reaction between the organic matter and the dissolved oxygen. The
second step involves the transfer of oxygen from the gas phase to the liquid one and the
transfer of CO2 from liquid to gas phase. During designing of a wet air oxidation
reaction, it is considered that gases must be diffused rapidly within the gas phase.
Li et al. (1991) presented a generalized kinetic model based on a simplified
reaction scheme with acetic acid as the rate-limiting intermediate product as shown in
Fig. 3.
Organic Compound + O2 ¾¾k¾1 ® CO2
k1 k2
CH3COOH + O2
+ -
+
[O.C. CH COOH] - - +
(k k ) t
(k k )
k t 1 3
k
3 3 e 1 2
1 2 3
2
1 2 3
3 o
(k k k )
e
(k k k )
[O.C. CH COOH]
+ -
+ -
=
+
where, O.C. = Organic compound
Fig. 3: Simplified kinetic model for wet air oxidation process
Table 1. Gives an idea of the basic reaction mechanisms involved in the WAO
process.

10. Table 1: Wet air oxidation process reaction mechanisms (Liu and Bela, 1995)
Reaction Mechanism Typical Effects Strongest Influences
Hydrolysis Dissolved solids splits long-chain
hydrocarbons
pH, temperature
Mass Transfer Dissolves, absorbs oxygen Pressure, Presence of
liquid gas interface
Chemical Kinetics Oxidizes organic chemicals Temperature, Catalysts,
Oxygen activity
7. Commercial Catalytic Wet Air Oxidation (CWAO) Processes
The operating conditions of the WAO can be much reduced by the use of
catalysts, which allow substantial gains on temperature, pressure and residence time. The
same amount of destruction of compound/ COD can be achieved at low temperatures by
the use of catalysts. CWAO can be divided in two types:
(a) Heterogeneous CWO process
(b) Homogeneous CWO process
7.1 Heterogeneous CWAO Process
Since the mid-eighties, three CWAO technologies have been developed using
heterogeneous catalysts containing precious metals supported on titania or titania-zirconia.
In comparison to standard WAO process, these processes were able to oxidize
two refractory compounds, i.e., acetic acid and ammonia. The technologies were
developed after studying the important aspects of heterogeneous catalysts, such as,
chemical and physical stability of the heterogeneous catalysts during WAO, which
include leaching and sintering of the active phase and reduction in surface area of the
support. Leaching of the metal can be controlled by pH adjustment and proper choice of
catalyst. The running commercial processes employ catalysts comprising of precious

11. metals, such as Pt and Pd or a mixture of precious and base metals. The support used is a
mixture of titania and zirconia.
7.2 Homogeneous CWAO Process
In the last decade, wet air oxidation of the toxic waste streams has been carried
out in the presence of homogeneous transition metal catalysts. The problem encountered
with such catalysts is to separate before disposing. So there is a need to develop such
methods, which can separate the active metal ions, so that the catalyst can be recycled
and reused.
8. Applications of Wet Air Oxidation
Non-catalytic and catalytic WAO is an attractive alternative process to biological
oxidation. This process is particularly useful for toxic and refractory pollutants, and can
also be used as an alternative to incineration. This method is useful for the following
situations (Joshi et al., 1985):
(i) For the treatment of pulp and paper mill effluent, where excess energy in
the form of steam can be recovered due to high chemical oxygen demand
(COD).
(ii) Activated carbon can be regenerated. Using this treatment method, the
charcoal loss is much lower than by the thermal regeneration process.
(iii) Filterability and dewaterability properties of municipal sewage can be
improved and the pathogens present in sludge can be destroyed.
(iv) Several non-biodegradable wastewater streams can be treated. Recovery
of chemicals is also possible.
8.1 Wet Air Oxidation of Pure Compound Solutions
Carboxylic acids are very valuable commercial products as they find their use in a
large number of synthetic organic products. Several dicarboxylic acids are also of
commercial importance because of their use in synthetic polymers. Among
monocarboxylic acids, formic acid is used as a disinfectant, as a preservative, to make
formates and cellulose esters and in the textile and leather industries. Acetic acid is an
important solvent in organic processes apart from its major use in cellulose acetates.

12. Other acids also find their use in preparation of pharmaceuticals, dyes, flavoring
ingredients, perfumery esters, etc. During manufacture and during their use in synthetic
processes, carboxylic acids find their way into the waste streams. Sometimes these acids
are formed as byproducts in a process and part invariably find their way in the waste
streams. For example, the caprolactam plant waste stream, petrochemical waste stream,
and pharmaceutical plant waste stream contain appreciable quantities of carboxylic acids.
8.2 Wet Air Oxidation of Phenol and Substituted Phenols.
Phenol and substituted phenols are very important chemicals commercially.
Phenol, cresylic acids, and cresols are used for making phenolformaldehyde resins and
tricresyl phosphates. Phenol, alkylphenol, and polyphenols are important raw materials
for the wide variety of organic compounds, dyes, pharmaceuticals, plasticizers,
antioxidants, etc. Phenols are mainly of coal tar origin and hence present in the effluent
from coke ovens, blast furnaces, and shale oil processing. Phenols are also present in the
effluent from the chemical process industries which are either manufacturing or using
them. The importance of phenols in water pollution stems from their extreme toxicity to
the aquatic life and resistance to biodegradation. Phenols impart a strong disagreeable
odor and taste to water even in very small concentrations.
8.3 Wet Air Oxidation of Cyanides and Nitriles.
Widespread use of cyanides and nitriles has increased the probability that they
will be found in significant concentrations in surface waters and effluents. Alkali metal
cyanides (NaCN, KCN, etc.) are used in extraction of silver and gold from their ores,
electroplating, germicidal sprays in agriculture, pharmaceuticals, preparation of organic
cyanides, etc. Cyanamide is used to produce calcium cyanide as intermediate for
pesticides and is a raw material for dicyandiamide and melamine. Calcium cyanamide is
used for steel nitridation and also in agriculture in defoliants, fungicides, and weed
killers. Calcium cyanide is used in the preparation of fumigants, rodenticides, and
ferrocyanides. Organic cyanides are used in the production of polymers, synthetic rubber

13. (acrylonitrile), textiles (nylon via adiponitrile), plastics, pesticides, dyes, solvents
(acetonitrile), etc. Cyanides and nitriles (particularly the unsaturated ones) are highly
toxic and nonbiodegradable at the concentrations normally encountered in effluents.
8.4 Catalytic wet air oxidation of olive mill wastewater
Catalytic wet air oxidation (CWAO) for the treatment of olive mill wastewater
(OMW). Experiments were performed in a high pressure reactor at 100 and 200 8C under
an oxygen partial pressure of 6.9 bar, using carbon supported platinum (1 wt.% Pt) and
iridium (5 wt.% Ir) catalysts prepared by incipient wetness impregnation. At 100 8C,
refractory organic compounds persisted even after prolonged reaction time (8 h). At 200
8C, complete total organic carbon and colour removal was obtained with the Pt/C catalyst
after 8 h of reaction. A kinetic model was developed taking into account catalytic and
non-catalytic reactions, formation of refractory compounds and catalyst deactivation.
Very good agreement between the proposed model and CWAO experimental data at 200
oC was found.
8.5 Biological treatment of Industrial Wastewater containing Sodium
Dodecylbenzene Sulfonate (DBS)
Wet air oxidation (WAO) and catalytic wet air oxidation (CWAO) were
investigated as suitable precursors for the biological treatment of industrial wastewater
containing sodium dodecylbenzene sulfonate (DBS). Two hours WAO semi-batch
experiments were conducted at 15 bar of oxygen partial pressure (PO2 ) and at 180, 200
and 220 C. It was found that the highest temperature provides appreciable total organic
carbon (TOC) and chemical oxygen demand (COD) abatement of about 42 and 47%,
correspondingly. Based on the main identified intermediates (acetic acid and
sulfobenzoic acid) a reaction pathway for DBS and a kinetic model inWAO were
proposed. In the case of CWAO experiments, seventy-two hours tests were done in a
fixed bed reactor in continuous trickle flow regime, using a commercial activated carbon
(AC) as catalyst. The temperature and PO2 were 140–160 C and 2–9 bar, respectively.
The influence of the operating conditions on the DBS oxidation, the occurrence of
oxidative coupling reactions over the AC, and the catalytic activity (in terms of substrate

14. removal) were established. The results show that the AC without any supported active
metal behaves bi-functional as adsorbent and catalyst, giving TOC conversions up to
52% at 160 C and 2 bar of PO2 , which were comparable to those obtained in WAO
experiments. Respirometric tests were completed before and after CWAO and to the
main intermediates identified through the WAO and CWAO oxidation route. Then, the
readily biodegradable COD (CODRB) of the CWAO and WAO effluents were found.
Taking into account these results it was possible to compare whether or not the CWAO
or WAO effluents were suitable for a conventional activated sludge plant inoculated with
non adapted culture.
9. Industrial Applications of Wet Air Oxidation
9.1 Wet Air Oxidation of Alcohol Distillery Waste.
The molasses generated in sugar manufacture (from sugar cane) is a prime raw
material for ethyl alcohol production all over the world. This process is of particular
importance to India and Brazil, major sugar-producing countries. The molasses is
fermented by yeast after suitable dilution. The fermented solution contains about 6-12%
ethyl alcohol which is recovered by distillation. The effluent remaining after alcohol
recovery (spent wash or stillage) is dark brown in color and about 6-15 times by volume
of the alcohol produced. The spent wash has a very high organic content (COD = 60-200
kg/m3) and is very complex in nature. Treatment of this spent wash is a major pollution
problem faced by distilleries. The spent wash also has high sulfate content due to SO2
used for bleaching of sugar, making it difficult to biotreat without dilution. The effluents
generated by distilleries using beet molasses are less concentrated and comparatively
easily treatable.
10. Catalytic wet oxidation of the pretreated synthetic pulp and paper mill effluent
under moderate conditions
The black liquor originating from the chemical cooking of wood and other raw
materials in the pulping process contains lignin, organic acids, unsaturated fatty acids,
resin acids, phenolics, terpenes, sulfur compounds, like sulphides, thiosulphate, etc.
Besides dissolved substances, it also contains high suspended solids, colloids and has

15. high biochemical oxygen demand (BOD) and chemical oxygen demand (COD). Lignin
and other dissolved compounds impart intense black colour to the black liquor. The
amount of wastewater generated from an integrated pulp and paper mill having chemical
recovery units varies from 20 to 250 m3 ton-1 of pulp produced, depending upon the raw
materials and the process conditions employed (World Bank Group, 1998; Garg et al,
2005). In India, most of the paper mills discharge an effluent having a COD value of
600–6250 mg l-1 and have a COD/BOD ratio in the range of 4.0–7.0. Several authors
have performed the treatment of the pulp and paper mill effluents having a COD value of
around 7000 mg l-1 (Stephenson and Duff, 1996; Laari et al., 1999; Garg et al., 2005).
Diluted black liquor from kraft pulp and paper mill can be used as substrate for bench
scale studies to simulate the final pulp mill effluent (Peralta-Zamora et al., 1998; Buzzini
et al., 2005). The pulp and paper mill wastewater cannot be treated directly by biological
methods, as it contains non-biodegradable, refractory organics and toxic inorganics. Wet
air oxidation (WAO) can be an alternative method to treat such wastewaters. It is carried
out at elevated temperature (398–593 K) and pressure (0.5–20 MPa) in the presence of
oxygen (viz. air, oxygen, ozone, hydrogen peroxide) (Mishra et al., 1995). To reduce the
severity of oxidation conditions, various heterogeneous catalysts like metal oxides (ZnO,
CuO, MnO2, SeO2, TiO2, ZrO2, etc.), noble metals on alumina support (Pt, Pd, Ce singly
or in mixed form) and metal impregnated on activated carbon (Cu, Co, Bi, Fe, Mn) have
been used in wet oxidation (WO) studies (Prasad and Joshi, 1987; Pintar et al., 2001).
Apart from heterogeneous catalysts, homogeneous copper sulfate catalyst has also been
used in the oxidation of pure toxic compounds such as glyoxalic and oxalic acids and
sulfide bearing spent caustic from petroleum refineries and concentrated waters from
paper mills at moderate operating conditions (Shende and Mahajani, 1994; Jagushte and
Mahajani, 1999). The catalyst was found quite effective exhibiting almost complete
conversion of thiosulfate into sulfate within 8 min at a temperature and oxygen partial
pressure of 393 K and 0.69 MPa, respectively. Verenich et al. (2000) have used copper
sulfate for the WO of concentrated wastewaters and obtained around 75% COD reduction
at 473 K and 1 MPa oxygen partial pressure.

16. 10.1 Effect of Different Catalysts on CWO of the Filtered Effluent at 423 K
Temperature
The effectiveness of different catalysts on COD reduction by destructive
oxidation of the pretreated paper mill effluent (pH0 = 8) at 423 K and 0.85 MPa total
pressure is shown in Fig. 4. The oxygen partial pressure was kept at 0.4 MPa. The
catalysts used in WO were CuSO4, 5%CuO/95% activated carbon, 60%CuO/40% MnO2
and 60%CuO/40% CeO2. The catalyst concentration used for the reaction was 8 g l-1. The
results show that among all catalysts, C and E show the best activity under the operating
Figure 4. COD reduction of the catalytic thermolysis treated paper mill effluent (i.e.
filtrate) due to wet oxidation (COD0 = 2700 mg l-1, T = 423 K, P = 0.85 MPa, pO2 =
0.4 MPa, Cw0 = 8 g l-1, pH0 = 8.0).
conditions, exhibiting a COD reduction of around 78%. The COD reduction without any
additional catalyst (A) not exhibit significant improvement in COD reduction. It can also
be seen from the figure that an induction period exists in the CWO reaction with 5%
CuO/95% activated carbon, showing a very small COD reduction in first 2 h (10%).
However, it was difficult to identify any induction period in all other cases.
10.2 WO with additional copper sulfate
10.2.1. Effect of catalyst loading.
The effect of fresh addition of copper sulfate on wet oxidation of the filtrate solution is
shown in Fig. 5a. The results are also compared with the COD reduction obtained in the

17. absence of the fresh catalyst during wet oxidation. It is found that at Cw = 8 and 10 g l-1,
the COD reduction is higher (63% and 67% respectively), but the reduction is not
significant when these are compared to 59% COD reduction without any fresh catalyst.
The COD reductions at Cw = 3 g l-1 and 5 g l-1 are lower (i.e. about 32% and 57%,
respectively) than that without any fresh catalyst. These values of copper sulfate
correspond to an addition of 0.76 g l-1 and 1.27 g l-1 fresh copper ions, respectively. From
the results, it appears that the concentration of the active Cu2+ (0.876 g l-1), already
present in the effluent, is sufficient to remove about 60% of the COD. The addition of
fresh copper sulfate upto 5 g l-1 does not enhance the COD It can also be seen from the
figure that an induction period exists in the CWO reaction with 5% CuO/95% activated
carbon, showing a very small COD reduction in first 2 h (10%). However, it was
difficult to identify any induction period in all other cases.
10.2.2 Effect of temperature
Fig. 5b shows the effect of temperature on the COD reduction as a function of treatment
time, tR at a fresh copper sulfate mass loading of 8 g l-1. The temperature of reaction was
varied from 383 to 443 K at 0.85 MPa pressure. The COD of the pre-treated wastewater
is reduced by 78% from COD0 = 2700 mg l-1 at 443 K than that of 63% at 423 K in 4 h
treatment time. At 383 K, the COD reduction is quite low (~33%) in comparison to those
at higher temperatures. A portion of the total COD reduction (~22%), however, is
obtained during the pre-heating period of the filtrate from room temperature to 383 K and
the WO treatment for 4 h at 383 K results in only 11% COD reduction from the zero
time.

18. Figure 5. Effect of (a) catalyst loading (T = 423 K), and (b) temperature (Cw0 = 8 g l-
1) on the COD reduction of the treated pulp and paper mill effluent using copper
sulfate as catalyst during wet oxidation (COD0 =2700 mg l-1, P = 0.85 MPa, pO2 =
0.4 MPa, pH0 = 8.0, tR = 4 h).
10.3. CWO with 5% CuO/95% activated carbon as the catalyst
10.3.1. Effect of catalyst loading.
Fig. 6a show the effect of Cw on the COD reduction of the wastewater was found
to be shown as a function of the reaction time, tR. CWO was performed at 423 K, 0.85
MPa pressure for tR = 4 h. The ordinate shows the COD of the effluent at zero time for
different Cw. The effect of the pre-heating period is clearly visible, as the COD at zero
time is much lower than the initial COD of the filtrate (2700 mg l-1). About 50% COD
reduction is obtained at zero time with catalyst loadings higher than 5 g l-1 due to
preheating of the filtrate to 423 K. The COD values at zero time were 2000, 1900, 1500,
1300, 1200 mg l-1 at Cw = 0, 3, 5, 8 and 10 g l-1, respectively. The increase in the catalyst
loading results in greater COD reduction at zero time. Pre-heating caused a reduction in
COD from 2700 to 2000 mg l-1 and further COD reduction was due to the adsorption on
activated carbon catalyst. COD reduction increases with the increase in catalyst loading
due to the presence of more adsorption sites for organic components. From the figure,

19. this is clear that no significant decrease in COD can be observed during the first hour of
WO reaction (especially for higher catalyst concentration runs). This can be interpreted
that any further reduction in COD after 1 h is only due to the oxidation. Catalyst loading
above 8 g l-1 does not show any appreciable increase in the COD reduction (78% at 8 g l-1
to 82% at 10 g l-1). Therefore, further CWO experiments were carried out at a catalyst
loading of 8 g l-1.
10.3.2 Effect of temperature.
The reaction temperature for WO was varied from 403 to 443 K. Fig. 6b shows
the effect of temperature on the COD reduction as a function of reaction time, tR. At 443
K and 0.85 MPa total pressure the COD reduced from 2700 to 300 mg l-1 in 4 h reaction
time. It can be seen that at ‘zero time’, the COD reduction decreases as the reaction
temperature increases. The corresponding COD values at zero time were found to be
1100, 1200, 1300 and 1400 mg l-1 at 403, 413, 423 and 443 K, respectively. Again, pre-heating
caused a COD reduction of around 26% and the rest of the reduction was due to
adsorption. Also, the increase in temperature reduces the amount of adsorption of
reaction species, which is evident from the result. However, total COD reduction
increases with increase in the reaction temperature at tR = 4 h. At 403 K, the COD
reduction was very low at tR = 4 h (from 1100 mg l-1 at tR = 0 to 1000 mg l-1 at tR = 4 h).

20. Figure 6. COD reduction of catalytic thermolysis treated paper mill effluent using
5% CuO/95% activated carbon as catalyst during wet oxidation as a function of (a)
catalyst loading (T = 423 K), and (b) temperature (Cw0 = 8 g l l-1) (COD0 = 2700 mg l-
1, T = 423 K, P = 0.85 MPa, pO2 = 0.4 MPa, pH0 = 8.0, tR = 4 h).
10.3.3 Without Additional Catalyst
WO of the filtrate solution was also performed at 423 K and 443 K without adding any
fresh catalyst. The COD reductions were 59% and 78%, respectively, after tR = 4 h. Thus,
the COD of the effluent reduces to 1100 and 600 mg l-1, respectively from an initial value
of 2700 mg l-1.
10.3.4 Effect of different catalysts on CWO of the pretreated effluent filtrate at 443
K temperature
The COD reduction of the effluent with 5% CuO/95% activated carbon and
copper sulfate at Cw = 8 g l-1were compared with that obtained without any fresh
catalyst. The experiments were carried out at 443 K and a total pressure of 0.85 MPa with
oxygen partial pressure being 0.1 MPa. The COD of the effluent is reduced by about 89%
from 2700 to 300 mg l-1 with 5% CuO/95% activated carbon, whereas with fresh copper
sulfate (Cw = 8 g l-1) and without any fresh catalyst COD reduction was equal (78%) in 4
h treatment time. Since the effluent from the catalytic thermal pretreatment of the
wastewater has sufficient amount of Cu ions, further addition of copper sulfate to the
effluent does not show enhanced COD removal.
10.3.5 Change in the pH value with time
Fig. 7 shows the change in final pH of the solution after treatment with different
catalysts, as a function of time. The pH of the solution after zero time first reduces and
then increases. The same trend has also been reported by Zhang and Chuang (1999). At
zero time, the pH of the solution with copper sulfate goes down to about 5.0, which drops
further to about 2.5 in 1 h at 443 K and thereafter increases with time. The decrease in the
pH of the filtrate on heating to 443 K with 5% CuO/95% activated carbon and without
addition of any other catalyst is only slight (from pH 8.0 to pH 7.4). Further heating at
443 K from zero time reduces pH slightly. pH variations of Fig. 7 indicate that the
heating of the filtrate solution with or without addition of fresh catalyst in the presence of

21. oxygen results in breakdown of organics into lower molecular weight species and
possibly carboxylic acids. With the passage of treatment time, the carboxylic acids are
oxidized to form CO2 and water with the resultant rise in pH. Lowering of the pH after
WO of softwood kraft pulp mill effluent has also been reported by others (Zhang and
Chuang, 1998a, b).
Figure 7. Variation in final pH with time using different catalysts during wet
oxidation (pH0 = 8.0, P = 0.85 MPa, Cw0 = 8 g l-1, COD0 = 2700 mg l-1).
11. Sulphidic Refinery Spent Caustic
Using typical conditions of 23O °C, 4 MPa and one hour retention time, 88%
destruction of chemical oxygen demand was achieved after wet oxidation of a sulphidic
refinery spent caustic. The reduced sulphur compounds in the waste were oxidized
completely to sulphate, thus eliminating the odour associated with this waste product.
The odour was not present in samples taken as early as 5 minutes after the start of
reaction. This waste is illustrative of the fact that, in the wet oxidation system, carbon-sulphur
or sulphm-hydrogen bonds appear to be broken preferentially, and sulphur more
readily oxidized than either carbon or hydrogen. Advantage of this fact is taken in
applying wet oxidation techniques to the problem of chemical desulphurization of coal.
11.1 Chlorinated hydrocarbon pesticide residues
In this test a 90% COD reduction was achieved after one hour’s residence time.
One particular component of concern in this residue was DDT. A destruction efficiency
of greater than 98% was measured for DDT.

22. 11.2 Cyanide wastes from electroplating operations
A number of different plating wastes containing up to 50,000 mg/litre of cyanide
were oxidized. Oxidation occurs quite readily for all but the most resistant ferricyanides.
Residual cyanide levels of less than 1 mg l-1 or destruction efficiencies of greater than
99.998% were achieved. Since the nitrogen in the cyanide appears in the effluent as
ammonia, much of the cyanide destruction must, in fact, be a hydrolysis reaction. In
subsequent experiments, it was shown that the reaction could be achieved with no
addition of oxygen or air to the system, but just by heating the reactor up to about 230°C
and maintaining that temperature for l-2 hours. This hydrolysis process is being
developed and is presently the subject of a commercial demonstration being supported by
the American Electroplaters’ Society. Wood preservative liquor Many wood preservative
liquors contain pentachlorophenol which is a toxic compound of some concern to
environmental regulatory agencies around the world. Earlier work undertaken by ORF
showed a reduction of 99.99% in pentachlorophenol with an equivalent overall COD
reduction in the stream of 76%. In a recent study we have examined both a wood
preservative waste containing pentachlorophenol (PCP) and pure PCP. In this study, the
effluent streams from the oxidation were examined to try to determine the nature of any
breakdown product. PCP destructions of up to 99.96% were observed. Of this, 99.4% of
the chlorine was accounted for as inorganic chloride ion. Trace quantities of other
chlorinated organic materials were observed in GClnass spectrometry analysis of these
effluents. PCP destruction via wet air oxidation has also been investigated in a Zimpro
high-temperature, high-pressure system. At 320” C and 275” C, PCP removal efficiencies
of 99.88% and 81.96% respectively were achieved. At 275”C, and employing a copper
catalyst, PCP removal efficiency was increased to 97.30%. Other sulphur and nitrogen
containing chemicals In addition to the work on PCP described above, work was recently
undertaken on two other pure compounds as examples of organic compounds containing
nitrogen and sulphur. The compounds chosen were mercaptobenzothiazole (MBT) and
diphenylamine (DPA). With mercaptobenzothiazole, a destruction efficiency of 99.99%
was observed. The major by-product observed in the oxidized effluent of batch testing of
MBT was nitrobenzene at a concentration of 0.03% of the original MBT. Traces of

23. azobenzene, aniline and azoxybenzene were also observed. In tests run in the continuous-flow
pilot plant, rather high concentrations of aniline and nitrobenzene were observed in
the vapour phase effluent from the reactor. These compounds are steam strippable, and it
would suggest that, since only trace quantities of aniline were observed in the batch
reactor effluent, had the aniline not been stripped from the reactor, it would have further
oxidized to nitrobenzene and other products. The wet oxidation of pure diphenylamine
indicated a 99.99% reduction in DPA. Traces of aniline, nitrobenzene, azobenzene,
dimethylazobenzene and chloro-aniline were detected by GC/MS in the effluent from the
batch oxidation tests. In the continuous flow tests carried out on DPA, aniline and
nitrobenzene were again detected in the vapour phase effluent from the reactor in
quantities equivalent to -20% of the original DPA in the influent to the reactor. In a
programme to be undertaken later, Ontario Research will be examining vapour phase
catalysis as a means of “polishing” the destruction of steam strippable components in the
vapour phase effluent from a continuous flow Wetox reactor.
11.3 Spent Caustic Oxidation CATOx2
Environment division of EIL (Mishra and Joshi, 2006) in association with
Department of Chemical Engineering, IIT Roorkee, has developed a technology for the
treatment of spent caustic waste. Process is based on catalytic oxidation of spent caustic
waste at pressure temperature levels that are consistent with the levels of
pressure/temperature of plant air and steam that are usually available in the plant. The
initial trial run on lab-scale was conducted at IIT, Roorkee and the results were quite
satisfactory in terms of sulphide reduction. The Process is found to be economical and
environment friendly when compared with other technologies using chemical oxidation
with H2O2 or wet air oxidation using patented processes.
Lab scale studies were conducted in order to critically examine the effect of
various process parameters. One of the highlights of lab scale tests was that actual spent
caustic samples from Panipat refinery were used to carry out tests in addition to synthetic
samples, which were prepared to examine the effect of following listed parameters:

24. 1. Effect of phenol in spent caustics
2. Kinetics determination
3. Catalytic oxidation
4. Reaction order with respect to Air
5. Kinetic regimes for a first order gaseous reaction
6. Reaction order with respect to sodium sulfide
7. Reaction order with respect to Copper based Catalyst
8. Temperature effect on reaction rate constant
9. Comparison of air and oxidants
The information thus generated was used to develop a flow scheme for the process,
after which EIL developed a skid mounted unit at their RD center. This was designed
for a flow of 100-400 lph on a continuous basis. This skid was moved to Panipat refinery
for demonstration purposes and to test the process performanc while handling actual
samples of spent caustic generated in the refinery with the following objectives:
1. To check the efficacy of the process at actual site conditions and variable loads.
2. To minimize H2O2 consumption for the treatment of spent caustic streams.
3. Develop Wet Air Oxidation technology indigenously as patented technologies are
very expensive and are unable to treat spent caustic if sulphide levels are more
than 35000 ppm and COD more than 100000 ppm.
4. Start with a hybrid approach consisting of a combination of H2O2 oxidation, wet
air oxidation and biological treatment.
5. Stagewise development of technology so that users are benefited right at the
beginning.
Other aspects, which are related to test the process at site conditions, are as
follows:
1. To establish efficacy of catalyst based Wet Air Oxidation process at pilot scale
Plant.
2. To re-establish kinetics for the medium pressure Wet Air Oxidation process as
developed in lab scale plant.
3. To find out effect of different process variables on sulphide removal efficiency.

25. 11.3.1 Distinct Features of EIL’s Process:
1. Approximately 50% sulfide reduction is achieved before spent caustic is oxidized
in the oxidation tower.
2. Approximately 65% conversion of sulfide is achieved after the addition of
catalyst before oxidation tower.
3. Addition of water helps in enhancing the reaction rate by lowering the sulfide
concentration.
4. It can treat high concentration of sulfide. During the pilot plant test run, sulfide
level as high as 80000 ppm was treated successfully.
5. Approx. 99.8% of conversion of sulfides is achieved at the outlet of oxidation
tower.
11.3.2 EIL’s process(CATOx2)
Sulphuric spent caustic is stored in a tank and subjected to oil removal. After intial free
oil removal, it is subjected to further removal of aoil thoriugh a media coalescer so that
oil left in the spent caustic is less than 10 ppm. This oil free spent caustic is aerated for 2
hours in aeration tank. The air flow rates are adjusted in such a way tha tagitation and
mixing are minimized that helps in lowering the foam formation if small quatities of
nephthenates are present. After aeration, spent caustic is transferred to another tank i.e.
the oxidation tank where copper based catalyst is added and again aeration is done for
another 30 minutes. The aeration is required for providing intimate mixing of spent
caustic with the catalyst and air for oxidation. This spent caustic is heated to 50° C with
the help of condensate recovered in the process. First stage of oxidation is achieved at
this stage and 65% of conversion of S is achieved.
The spent caustic is pumped to a guard filter and is passed through a pre-heater
where oxidized spent caustic exchanges the heat with the incoming feed of spent
caustic. This preheated spent caustic is transferred to start uo heater where desired
temperature 150- 170° C for the oxidation reaction is achieved. This spent caustic is now
oxidized in the oxidation tower where air is also mixed and released through sparger at
the bottom of the reactor. The top of the reactor contains packing and provides intimate

26. mixing and space for the breaking of bubbles generated if any during the reaction in the
process. The reactions in the oxidation tower are exothermic and self sustaining and the
need for start-up heater isonly at the beginning. The temperature and pressure maintained
in the reactor are 150-170° C and pressure up to 10 bars.
The oxidized spent caustic after transferring its heat to the feed is cooled down in
a cooler to 50° C and depressurized in a separator. Off gases released from the process
are passed through an activated carbon column and oxidized spent caustic is released to
WWTP for further treatment/disposal. The following table provides a comparison of
various technologies. Table 2 below shows the potential benefits through replacement of
existing process with EIL’s CATOx process.
Table: 2 Potential benefits through replacement of existing process with EIL’s CATOx
process

27. Conclusions:
Most of the compounds are amenable to WAO except low molecular weight
carboxylic acids (particularly acetic and propionic acid) and polychlorinated biphenyls
(PCB’s). However, during WAO, pollutant molecules are broken down to low molecular
weight carboxylic acids. The slow rate of oxidation of the low molecular weight
carboxylic acids is a major limitation of the WAO technique. Some catalyst systems have
shown promise for WAO of these acids under less severe conditions.
Wet air oxidation is a very attractive technique for the treatment of waste streams
which are toxic and dilute. When the feed COD is higher than 20 000 mg/L, WAO
becomes energetically self-sustaining with no auxiliary fuel requirement and may in fact
produce energy in the form of high-pressure steam at sufficiently higher feed COD.
Wet air oxidation system is capital intensive, although the operating costs are
almost entirely for the power requirement of the air compressor and the high-pressure
liquid pumping. The capital cost of a WAO system depends on the flow, oxygen demand
of the effluent, severity of oxidation conditions, and the material of construction required.
The severity of oxidation conditions can be reduced by use of a suitable catalyst system.
Wet air oxidation has been tested to treat the waste streams generated by various
industries such as distilleries, pulp and paper manufacturing units, cyanidelnitrile bearing
wastes, and a host of other waste streams. Wet air oxidation regenerates the spent carbon
with relatively less carbon loss (14 %) and at the same time destroys the adsorbed
organics thus avoiding the need for the separate treatment step. Oxydesulfurization of
coal is another promising application of WAO for removal of pyritic as well as organic
sulfur present in coal. Wet air oxidation has been suggested for energy and resource
generation from low-grade fuels and waste biomass such as peat, forestry, and municipal
residues.
The potential benefits of the CWO process over other conventional water
treatment processes, such as low reaction temperatures and residence times and the
formation of harmless products, will be a key driver for more research in the field. The
main challenge faced in the development of successful industrial-scale CWO processes
for treating specific wastewaters seems to be the development/discovery of suitable
catalysts, i.e., a catalyst that is highly active, economical and environmentally friendly.
The total saving of Rs. 78.34 crores can be made in cost if EIL’s CATOx process
is used in all 17 refineries in India.

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