Catalytic thermal treatment (catalytic thermolysis) of a rice grain-based biodigester effluent of an alcohol distillery plant

Abstract

The catalytic thermolysis (CT) process is an effective and novel approach to treat rice grain based biodigester effluent (BDE) of distillery plant. Catalytic thermolysis treatment of rice grain-based distillery wastewater was carried out in a 0.5 dm(3) thermolytic batch reactor using different catalyst like CuO, copper sulphate and ferrous sulphate. With the CuO catalyst, a temperature 95(o)C, catalyst loading 4 g/dm(3) and pH 5 was found to be optimal, obtaining a maximum chemical oxygen demand (COD) and color removal of 80.4% and 72%, respectively. The initial pH (pHi) was an important parameter to remove COD and color from BDE. At higher pHi (pH 9.5) less COD and color reduction was observed. The settling characteristics of CT treated sludge were also analyzed at different temperature. It was noted that the treated slurry at temperature 80(o)C gave best settling characteristic. Characteristics of residues are also analyzed at different pH.

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Catalytic thermal treatment (catalytic thermolysis) of
a rice grain-based biodigester effluent of an alcohol
distillery plant
Abhinesh Kumar Prajapatia, Parmesh Kumar Chaudharib, Bidyut Mazumdarb & Rumi
Choudharyc
a Department of Chemical Engineering, Institute of Engineering and Science Indore, Indore
452001, India
b Department of Chemical Engineering, National Institute of Technology Raipur, Raipur
492001, India
c Department of Chemical Engineering, C.V.Raman College of Engineering, Bhubaneswar,
Odisha 752054, India
Accepted author version posted online: 02 Apr 2015.Published online: 24 Apr 2015.
To cite this article: Abhinesh Kumar Prajapati, Parmesh Kumar Chaudhari, Bidyut Mazumdar & Rumi Choudhary (2015)
Catalytic thermal treatment (catalytic thermolysis) of a rice grain-based biodigester effluent of an alcohol distillery plant,
Environmental Technology, 36:20, 2548-2555, DOI: 10.1080/09593330.2015.1036787
To link to this article: http://dx.doi.org/10.1080/09593330.2015.1036787
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Environmental Technology, 2015
Vol. 36, No. 20, 2548–2555, http://dx.doi.org/10.1080/09593330.2015.1036787
Catalytic thermal treatment (catalytic thermolysis) of a rice grain-based biodigester effluent of
an alcohol distillery plant
Abhinesh Kumar Prajapatia∗, Parmesh Kumar Chaudharib, Bidyut Mazumdarband Rumi Choudharyc
aDepartment of Chemical Engineering, Institute of Engineering and Science Indore, Indore 452001, India; bDepartment of Chemical
Engineering, National Institute of Technology Raipur, Raipur 492001, India; cDepartment of Chemical Engineering, C.V.Raman
College of Engineering, Bhubaneswar, Odisha 752054, India
(Received 29 May 2014; accepted 24 March 2015)
The catalytic thermolysis (CT) process is an effective and novel approach to treat rice grain-based biodigester effluent (BDE)
of the distillery plant. CT treatment of rice grain-based distillery wastewater was carried out in a 0.5 dm3thermolytic batch
reactor using different catalysts such as CuO, copper sulphate and ferrous sulphate. With the CuO catalyst, a temperature
of 95°C, catalyst loading of 4 g/dm3and pH 5 were found to be optimal, obtaining a maximum chemical oxygen demand
(COD) and colour removal of 80.4% and 72%, respectively. The initial pH (pHi) was an important parameter to remove COD
and colour from BDE. At higher pHi(pH 9.5), less COD and colour reduction were observed. The settling characteristics of
CT-treated sludge were also analysed at different temperatures. It was noted that the treated slurry at a temperature of 80°C
gave best settling characteristics. Characteristics of residues are also analysed at different pH.
Keywords: biodigester effluent; chemical oxygen demand; catalytic thermolysis; catalyst; CuO
1. Introduction
Distillery industries represent an important segment of the
world economy because ethanol is one of the important
raw materials for production of various intermediate chem-
icals and medicines. In addition, many countries use pure
ethanol and ethanol-blended gasoline as motor fuel in auto-
mobiles which can reduce the demand for petrol/diesel.
In India about 61% ethanol is produced from sugar cane
molasses. Due to the increasing demand, recently rice grain
is also used for the production of ethanol. Rice is one of the
major crops of India and available through the year.
Ethanol is produced by fermentation of raw material
(rice grain) in fermentation broth. The liquor, after fermen-
tation contains around 6–14% ethanol, which is separated
at top of the distillation columns and the remaining liq-
uid coming out from the bottom of the distillation column
is a waste. This liquid is known as spent wash (SW).
SW is dark brown in colour and has a very high chem-
ical oxygen demand (COD: 35,000–50,000 mg/dm3) and
a very high biochemical oxygen demand (BOD: 10,000–
25,000 mg/dm3).[1] The SW is first sent to the biodigester
for anaerobic biological treatment where 50–75% COD
and 65–85% biochemical oxygen demand (BOD) can be
reduced. The effluent that comes out from the biodigester
is commonly known as biodigester effluent (BDE). The
BDE still contains high COD (10,000–15,000 mg/dm3)
*Corresponding author. Email: abhineshchem@gmail.com
and BOD (2500–3000 mg/dm3) and is dark brown in
colour.
Due to the BDE still containing high organic load it
cannot be directly discharged into any water body. The
Central Pollution Control Board (CPCB) of India has fixed
standards for the distillery units to meet the effluent dis-
charge quality, for release of the wastewater into surface
waters (COD <0.1 kg/m3,BOD<0.03 kg/m3) and sew-
ers (COD <0.3 kg/m3,BOD<0.1 kg/m3).[2] Therefore,
a comprehensive strategy is required for maintaining the
desired standard of the effluents. Nowadays, most of the
units use a membrane separation process, which is effec-
tive but the cost of operation and installation is too high.
In addition, several physiochemical methods such as wet
oxidation,[3] coagulation,[4] adsorption,[5] and electro-
chemical degradation [6,7] have been reported to treat
BDE but all these processes have limitations.
The catalytic thermolysis (CT) process is a simple tech-
nique in which either high organic or inorganic strength
wastewater is heated at moderate or high temperatures
in the absence of air/oxygen. CT is an anoxic process
and also cheaper than the wet oxidation process because
during the CT process oxygen at high pressure is not
required.[8] The CT process is the chemical transformation
of the dissolved organics and inorganics with or without
metal compounds (salts) at moderate temperature and self
© 2015 Taylor & Francis
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Environmental Technology 2549
(autogenous) pressure. Thermo-chemical precipitation of
dissolved organics and some inorganics present in the BDE
remove the COD and colour from BDE.
During CT, dual mechanisms take place simultane-
ously. The organic molecules both smaller and larger
present in the BDE undergo chemical and thermal break-
down and create complexation. Finally, it converts into
insoluble particles, which settle down. In addition, larger
molecules also undergo a breakdown during thermolysis
into smaller molecules which are soluble. Due to the
formation of insoluble particles, substantial reduction in
the original values of reduced carbohydrates and proteins
Table 1. Typical composition of BDE before and after treatment
by CT at Cw=4 g/dm3, pH 5, thermolysis time 9 h.
Parameters BDE
CT treated BDE at
optimum condition
(Temperature 95°C)
COD 11,500 2254
TDS 43,245 1623
TSS 39,331 4615
TS 82,576 6238
Reduced carbohydrate 517 Not found
Protein 165 86
Chlorine 161 65
Phosphate 0.05 Not found
Total hardness 10,220 355
Sulphate 4718 724
pH 7.8 5
Colour Blackish Light black
brown (transparent)
Absorbence at wave-
length =475 nm
0.831 0.23268
Colour (PCU) 398 111.4
Note: All value in mg/dm3except pH and colour.
along with that of phosphates, carbonate, or bicarbonate of
Ca/Mg and sulphates occurred (Table 1). These reductions
also exemplify the reduction in COD (80.4% reduction
from COD =11,500) and the colour (83% reduction from
398 PCU) of the BDE.
The thermolysis process of the BDE can, thus, be
represented without catalyst as [8–10]
Complex organic matter +H2O
+heat
>Solid residue +lower molecular weight organics.
In the presence of the catalyst, the solid residue forma-
tion rate increased. The reaction equation can be written
as
Organics +catalyst
+H2O+heat
>Solid residue +lower molecular organics.
To the best of our knowledge, there are few studies for
treatment and removal of COD and colour reduction from
BDE by the CT process. Chaudhari et al. [9] have studied
COD and colour removal by a high pressure reactor.[9]In
another study, removal of organic load with the CT process
has been investigated. In this paper the initial concentra-
tion of COD was kept constant at 35,000 mg/dm3, and after
12 h of the CT process the removal per cent of COD was
measured.[10] Previous work done by several investigators
is presented in Table 2.[3,9,11–14]
The objective of the present work is to explore the effi-
ciency of the CT process in the reduction of COD and
colour of rice grain-based distillery BDE. Since Cu and
iron-based compounds (namely CuO, CuSO4) have been
Table 2. Results of thermolysis of SW and BDE.
Catalyst System used COD
Author(s) Substrate used for reaction reduction (%)
Daga et al. [11] Cane-based DWW Nil Stainless steel (S.S. 316) 55
COD0=65–100 kg/m3Autoclave
Lele et al. [12] Cane-based DWW Nil Stainless steel (S.S. 316) 53.7
COD0=108 kg/m3Autoclave
Dhale and Mahajani [13] BDE from the cane-based DWW treatment
plant
Nil Stainless steel (S.S. 316) 35
COD0=34.0 kg/m3Autoclave
Belkacemi et al. [13] Timothy grass-based DWW Nil Stainless steel (S.S. 316) 54
TOC0=22.5 kg/m3autoclave
Chaudhari et al. [9] BDE from the cane-based DWW treatment
plant
CuO Stainless steel (S.S. 316) 70
COD0=34.0 kg/m3Autoclave
Chaudhari et al. [10] SW from cane-based DWW treated plant
COD =108.0 kg/m3pH0=4.0
CuO Stainless steel (S.S. 316) 60
Autoclave
Chaudhari et al. [14] Molasses-based SW and BDE,
COD0=34 kg/m3CuO Glass reactor 58
Present work Rice grain-based distillery effluent BDE,
COD =1.15 kg/m3CuO Glass reactor 80.4
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2550 A.K. Prajapati et al.
found to be very effective in the WO of many recalci-
trant pollutants in aqueous streams as reported by several
authors,[15–17] CuO, CuSO4and FeSO4have been used
as the catalysts in the present study. The anoxic CT was
performed at a temperature range of 65–100°C and atmo-
spheric pressure. The settling characteristics of the residue
and the fixation of copper in the residue and its leach-
ing into the filtrate have also been examined and reported
in this study.
2. Experimental section
2.1. Materials and catalyst preparation
The BDE was obtained from Chhattisgarh Distillery Pvt.
Ltd. Kumhari, Chhattisgarh. To maintain constant char-
acteristics of BDE, the sample was stored at 4°C in a
deep freezer. Typical analysis of the effluent, before and
after thermal treatment, is presented in Table 1. Labora-
tory reagent grade chemicals obtained from Merck Ltd,
Mumbai, India, were used in the experiment. The CuO cat-
alyst was prepared in the laboratory from cupric nitrate by
alkali precipitation followed by drying and calcination. To
prepare 10 g CuO catalyst, 30.40 mg/l copper nitrate solu-
tion was prepared in distilled water and liquid ammonia
was added to the solution gradually (drop by drop) while
stirring the solution at a constant speed. The resultant pre-
cipitate was washed thoroughly with distilled water and
then it was dried in an oven at 110°C for 18 h. The dried
matter was calcined in a furnace at 400°C for 4 h. The cal-
cined solid was ground in a laboratory grinder and sieved.
The solid particles with an average size of 220 μm were
used in the experiments.
2.2. Procedure
The CT experiments were performed in a 0.5 dm3atmo-
spheric pressure glass reactor (AGR). The AGR was
equipped with a vertical condenser as shown in Figure 1.
The reactor contents were stirred using a magnetic stirrer
(the stirring speed cannot be determined, but the intensity
of stirring can be varied). After the start of an experimen-
tal run at a desired temperature, the effluent samples were
withdrawn from the reactor at definite time intervals. The
samples were filtered and the filtrate was analysed for its
COD value. Each COD run was repeated twice to check
the reproducibility of the results. Any run with a devia-
tion of more than 2.5% was further repeated to check its
reliability. The effect of variables such as pH0(3.5–9.5),
temperature (T=65–100°C) and catalyst mass loading
(Cw=2–5 kg/m3) on the COD removal efficiency was
studied. The experiments at atmospheric pressure and up
to 100°C were carried out in the AGR. The treated effluent
was filtered and centrifuged and the solid residue was dried
in an oven at 105°C until its weight became constant. The
oven-dried residue was analysed for its copper content.
Figure 1. Thermolysis process (set-up). (A) Condensers, (B)
heating mantle, (C) magnetic stirrer, (D) stand, (E) thermometer,
(F) water inlet, (G) water outlet, (H) sample collection.
2.3. Analytical procedure
The COD of the sample was determined by the close
reflux method. The samples were digested at 148°C and,
then, their absorbance was determined at 605 nm.[18] Sul-
phate and phosphate contents were determined by stan-
dard methods.[18] Protein was estimated by the Lowry
method.[19] Strength of the chloride in the sample
was determined by the standard titrametric method.[20]
The reduced carbohydrate was estimated by Fehling’s
method.[21] The amounts of metal ions leached out in the
solution and those fixed in the solid residue were deter-
mined by using an atomic absorption spectrometer (AAS)
GBC, model Avanta. The residue obtained was dried and
dissolved in aquarezia (1/3 concentrated nitric acid and
2/3 concentrated hydrochloric acid) while heating at about
80°C. After suitable dilution of the solution with double-
distilled water, the Cu content was determined by the
AAS.[10] pH of the sample was determined using a dig-
ital pH meter (EI Made, India). The colour of the samples
was measured in terms of the absorbance at λ=475 NM
using a UV-spectrophotometer (Thermo Fisher, Germany)
as reported by Migo et al. [22]
3. Results and discussions
3.1. Effect of pH
The pH of the solution plays a very important role in
influencing the CT process.[10,11]
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Environmental Technology 2551
To examine its effect, a series of experiments were
performed with the use of BDE and with an initial pH
varying in the range of 3.5–9.5. Figure 2(a) and 2(b)
shows the COD and colour removal efficiency of BDE
at different pH. It can be seen that the initial pH had a
significant effect on the removal efficiency especially at
acidic pH (pH 5) values. As illustrated in Figure 2,the
COD and colour reduction of BDE reached a value as
high as 70% and 54%, respectively, at pH 5 with CuO
catalyst. Thermal treatment of the BDE at 80–150°C with
a low molecular weight anionic synthetic flocculants can
reduce its COD and colour in the pH range of 5–12. How-
ever, the role of pH on the catalytic wet oxidation of the
BDE was not studied by previous investigators.[14,15]
CT without catalyst or coagulant addition does not
provide significant removal of COD and colour from
the BDE.
It is observed that other catalysts such as copper sul-
phate and ferric sulphate gave poor COD and colour reduc-
tion at their optimum pH (pH 6.5) as compared with CuO.
Maximum COD reduction of 63.24% and colour reduc-
tion of 45.24% with copper sulphate were achieved, simi-
larly 53.24% COD reduction and 33.28% colour reduction
were achieved, respectively, with ferric sulphate at this
pH. Chaudhari et al. reported [8] that the presence of
reduced carbohydrate in BDE is responsible for this result.
20
30
40
50
60
70
3579
% COD reduction
pH
Copper sulphate Ferrric sulphate
Copper oxide Without catalyst
0
10
20
30
40
50
60
3579
% color reduction
pH
Copper sulphate
Ferric sulphate
Copper oxide
Without catalyst
(a)
(b)
Figure 2. Effect of pH on (a) COD reduction (b) colour reduc-
tion of BDE (COD =11,500 mg/dm3,Cw=4 g/dm3, thermoly-
sis time =3h, T=95°C).
Carbohydrates contain hydroxyl groups that are highly
reactive at pH range 6–8 that convert to heavy flocs which
finally settle down. However, it must be included that the
nature of the catalytically active sites is unknown and that
the phenomenon of metal complexation with the reactive
groups in the BDE and the effect of the pH on complexa-
tion are not explained at present. pH either higher than 6.5
or lower than 6.5 did not have any significance change in
organic load reduction from BDE.
3.2. Effect of thermolysis time
The thermolysis time, being a surrogate parameter, is
found to influence the treatment efficiency of the CT pro-
cesses. The influence of electrolysis time on the removal
of COD and colour is shown in Figure 3(a) and 3(b). The
results showed that the COD removal efficiency increased
from 73% after 3 h up to 80.4% after 9 h while colour
removal efficiency increased from 57% after 3 h up to
72% after 9 h using CuO as a catalyst. The activation
of catalyst increases up till a certain period, thereafter
the deactivation rate of the catalyst starts increasing. It is
reported that during CWO of phenol using copper oxide
mounted on activated carbon as a catalyst, the leaching of
the active substance (CuO) is responsible for the catalyst
deactivation.[15]
30
40
50
60
70
80
90
246810
% COD reduction
Time (hr)
Copper sulphate Ferric chloride
Copper oxide Without catalyst
0
10
20
30
40
50
60
70
80
246810
% color reduction
Time (hr)
Copper sulphate Ferric sulphate
Copper oxide Without catalyst
(a)
(b)
Figure 3. Effect of thermolysis time on (a) COD reduc-
tion (b) colour reduction of BDE (COD =11,500 mg/dm3,
Cw=4 g/dm3,T=95°C) (pH of copper sulphate, ferrous sul-
phate, CuO and without catalyst is 6.5, 6.5, 5 and 5).
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2552 A.K. Prajapati et al.
Figure 4. Effect of catalyst doses on (a) COD reduction (b)
colour reduction of BDE (COD =11,500 mg/dm3,T=95 °C,
pH 5).
3.3. Effect of catalyst doses
Since the CuO catalyst is found to be the best among the
homogeneous copper sulphate and ferrous sulphate, so it
was selected for further studies. Effect of catalyst mass
loading is observed on COD and colour reduction of BDE.
The experiments were conducted with varying catalyst
doses 1–5 g/dm3and corresponding autogenous pressure,
pH 5, and temperature 95°C, for tR=9 h. It can be seen
from Figure 4(a) and 4(b) that maximum COD and colour
reduction of 80.4% and 72% was obtained with optimum
coagulant doses of 4 g/dm3. Coagulant dose 4g/dm3was
given better COD reduction as compared to other coagulant
doses. This catalyst mass loading is termed as the critical
chemical concentration (CCC) at which the precipitation
just starts. Addition of the catalyst may either increase or
decrease the final pH of the treated wastewater from its
initial value. This trend may be attributed to the enhanced
formation of carboxylic acids.[17]
3.4. Effect of temperature
Temperature is an important parameter that directly influ-
enced the efficiency of the CT process.[10–15,23]The
experiments were conducted at 65–100°C and correspond-
ing autogenous pressure, pH 5, and Cw=4g/dm
3,for
50
55
60
65
70
75
80
85
60 80 100
% COD reduction
Temperature (ºC)
3
h
r
30
35
40
45
50
55
60
65
70
75
60 80 100
% color reduction
Temperature (
o
C)
3 hr
6 hr
9 hr
(c)
(a) (b)
Figure 5. Effect of temperature on (a) COD reduction
(b) colour reduction (c) settling characteristic of BDE
(COD =11,500 mg/dm3,Cw=4 g/dm3, thermolysis
time =9h, pH5).
tR=9 h. The COD of BDE was 11500 mg/dm3.TheCOD
and colour reduction with different temperatures are shown
in Figure 5(a) and 5(b), during 9 h of operation. It can be
seen that the reaction is based on a two-step process – the
first step during the initial period (up to 3 h) being faster
than the slower second step. At 95°C, 73% COD reduc-
tion is obtained during th=3 h and another 7% reduction
is obtained during the next 6 h (tR). Similarly 57% colour
reduction is obtained during th=3 h and another 15%
reduction is obtained during the next 6 h (tR). During the
first step of thermolysis, large sized organic molecules of
organic matter break down into smaller molecules, along
with catalytic complexation and carbon sequestration lead-
ing to the formation of carbon-enriched solid residue which
was observed at bottom of the reactor after the experi-
ments. The thermolysis of the smaller molecules, there-
after, appears to be relatively difficult as shown by the
relatively much slower second step.[10]
With further increase at temperature upto 100°C, it was
observed that the percentage COD and colour reduction
decreased. Possibly this may be due to components such
as reduced carbohydrate, proteins present in the BDE are
not properly react at 100°C.
3.5. Settling characteristic
The slurry obtained from CT treatment was subjected to
sedimentation test. To determine the separation charac-
teristic by settling, the treated BDE sample after the CT
process was slowly mixed and taken in 0.5 dm3cylinder
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Environmental Technology 2553
having a diameter of 46 mm. Settling characteristics were
analysed at varying temperatures (65–100°C), thermolysis
time 9 h, pH 5 and catalyst doses of 4 g/l. For this pur-
pose, the treated BDE sample after the CT process was
slowly mixed and taken in a 0.5 dm3cylinder having a
diameter of 46 mm. The supernatant and solid interface are
noted down at different time intervals. Figure 5(c) shows
the time vs. height graph of settling sludge of effluent
treated at different temperatures. The settling rate is found
to be in the order of 80°C >95°C >100°C >65°C.
Settling characteristics are better at 65°C. This may
be due to the formation of heavy flocks at this tem-
perature, which settle down. However, poor COD and
colour reduction was observed at 65°C. The method
proposed by Richardson et al. (2003) is most suit-
able to design a continuous thickener based on batch
studies.[24]
3.6. Copper balance
The residue obtained after the CT process is dried and dis-
solved in aquarezia with slow heating at about 90°C until
the entire residue gets dissolved completely. After suitable
dilution, the metal content of the residue and the filtrate
were determined by AAS. The material balance for copper
is given in Table 3. CPCB of India has declared wastewa-
ter discharge limit of treated effluent in surface and sewer
(Cu <4 mg/l). Table 3indicates that the overall copper
balance was found to be maintained accurately (i.e. the
amount of copper ions used in the effluent is equal to sum-
mation of copper ions present in supernatant and residue).
It is also observed that CT cleaves carbon of the BDE and
through complexation with copper results in the formation
of an enriched carbonaceous residue.
3.7. Analysis of the filtrate
The wastewater treated at 95°C for 9 h was filtered by
Whatman filter paper (42 size) and some of the physical
and chemical characteristics of the filtrate are listed in
Table 1. It was observed that the filtrate contains little
amount of dissolved copper. However, large amounts of
COD (2254 mg/l) are still present in the treated effluent,
therefore it can be said that the thermolysis step is only
the pre-treatment step and the pretreated effluent has to be
treated further to meet the effluent standards before being
discharged into sewers or receiving water bodies.
3.8. Analysis of the residue
The solid residue obtained after filtration was dried in
an oven at 110°C until its weight became constant. Its
composition and some of its physical properties are pre-
sented in Table 4. It was observed that the colour of the
residue obtained was black upto pH 6.5 and then changes to
brown to greenish brown at pH 8 and 9.5, respectively. The
residue obtained from acidic pH treatment shows a free-
flowing powder property, while the residue from the basic
pH treatment shows a sticky character. It is also found that
the drying period for the residue increases from pH 3.5–5
and then after it decreases upto 9.5.
3.9. Economic analysis
Electricity and catalyst are considered to be the two
major costs involving components when the CT process is
applied. Economic cost analysis has been done for a plant
capacity of 100 m3wastewater/day. Economic data used
for the evaluation of operating cost are given in Table 5.
Table 3. Material balance for copper at different temperatures.
Initial copper in Copper in Copper in Total copper in
Temperature BDE (mg/dm3) residue (mg/dm3) filtrate (mg/dm3) residue and filtrate (mg/dm3)%error
65°C 1.0506 0.93 0.07 1.00 4.7
80°C 1.0506 0.94 0.05 0.99 5.7
95°C 1.0506 0.98 0.015 0.995 5.2
100°C 1.0506 0.94 0.03 0.97 7.6
Table 4. Analysis of residue after CT at different temperatures.
Parameter 65°C 80°C 95°C 100°C
Weight of residue (kg/m3) 10.53 11.11 11.32 11.88
Colour Brown Brown Brown Brown
Nature Bulky mass, easily
grindable
Bulky mass, easily
grindable
Bulky mass, easily
grindable
Bulky mass, easily
grindable
Approximated drying
period (h)
4 3.5 3 3.2
% convertible COD 66.28 69 80.34 79.57
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2554 A.K. Prajapati et al.
Table 5. Economical data used for calculating the operating
cost.
Item Cost ($)
Rectifier installation cost 800
Thermolysis tank installation cost 200
Maintanance (m−3) 0.006
Electricity (kwh−1) 0.2
Labour cost (m−3) 0.05
Sludge transportation cost and disposal cost (kg−1) 0.01
Material cost
CuO cost (/4 kg) 10
The electrical energy consumption was calculated
using the following equation:
Energy consumption (kWh/m3)=VIt
Treated volume (l),
where Vis the cell voltage in volt, Iis the current in ampere
(A) and tis the treatment time (h). The energy consumption
was calculated under optimized conditions (a temperature
95°C, catalyst loading 4 g/dm3and pH 5 was found to
be optimal). The energy consumption was observed to be
50 kWh/m3.
The operating cost was calculated using the following
equation:
Operating cost (US $/m3)=aCenergy +bCcatalyst,
where Cenergy is the energy consumption (kWh/m3) and
Ccatalyst is the chemical consumption (kg/m3) of wastewater
treated. Unit prices aand bgiven for the India market are
as follows: (a) electrical energy price 0.2 US $/kWh and
(b) CuO price 10 US $/4 kg. The operating cost was cal-
culated under optimum conditions and it was found to be
17 $/m3. However, after the regeneration of CuO, the cost
of process can be more reduced.
4. Conclusions
The CT process for BDE treatment using CuO as the cata-
lyst has been found to be very effective in COD reduction at
moderate temperature (80–95°C) and autogenous pressure
compared with higher temperature and pressure. Treatment
at 95°C with Cw=4g/dm
3and pH 5 results in a maxi-
mum of 80.4% COD and 72% colour removal, mainly due
to the formation of insoluble and settleable solid residues.
The slurry obtained after thermolysis has good settling
characteristics. The residue may be used as a fuel in com-
bustion furnaces. Ash containing Cu may be blended with
organic manure and used as fertilizer for agricultural and
horticultural purposes. However, the thermolysis step is
the only pre-treatment step. The pretreated effluent has
to be treated further to meet the effluent standards before
being discharged into sewers or receiving water bodies.
Thermolysis followed by the membrane process can be
used for the complete treatment of BDE.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Funding
Authors gratefully acknowledge Chhattisgarh Council of Science
and Technology (CGCOST), Raipur, India for providing research
grants to support this work.
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