Proceedings of the 13th International Conference on Environmental Science and Technology
Athens, Greece, 5-7 September 2013
ROLE OF STRONG OXIDANTS IN REDUCING COD: CASE STUDY AT
COMMON EFFLUENT TREATMENT PLANT VAPI, GUJ., INDIA.
ABHISHEK JAIN1and Mrs. ANJALI KHAMBETE2
1
M.Tech Scholar, SVNIT, Surat, Gujarat, INDIA (email: [email protected])
2
Associate Professor, CED, SVNIT, Surat, Gujarat, INDIA
ABSTRACT:
At presently majority of Common Effluent Treatment Plants (CETPs) in India are running
below desired level of emission which results in increase of the pollution load on
receiving bodies. As the secondary effluent has high COD which is well above
discharging norms it is essential to treat the same for betterment of the environment.
This study discusses the role of strong oxidants like Sodium Hypochlorite, Calcium
Hypochlorite, Hydrogen Peroxide; and their Combination; on secondary effluent at
CETP, Vapi, Gujarat, India. These strong oxidants were study for reducing COD with
different concentrations and retention time. The result shows that 200 ppm Sodium
hypochlorite and 200 ppm Calcium hypochlorite with one hour retention time gives
36.36% and 45.45% reduction of COD. While10ml Hydrogen peroxide with one hour
retention time shows 49.09% reduction of COD. The Combination of 100 ppm Sodium
Hypochlorite, 100 ppm Calcium Hypochlorite and 10 ml of Hydrogen Peroxide with 2
hours reaction time reduce COD around 59.20%.
KEYWORDS: Common Effluent Treatment Plant (CETP), COD Reduction, Hydrogen
Peroxide, Sodium Hypochlorite, Calcium Hypochlorite, Strong oxidants.
1.
INTRODUCTION
In India almost 70 per cent of its surface water resources and a growing percentage of
groundwater reserves were contaminated by biological, toxic, organic, and inorganic
pollutants which show water scarcity for both human use and for the ecosystem (Murty,
2011).Overall, some 5–20 per cent of total water usage goes to industry and it
generates a major proportion of total wastewater(Corcoran, 2010).During the past few
decades Indian industries have registered a quantum jump, which has contributed to
high economic growth but simultaneously it has also given rise to severe environmental
pollution (Mishra A). Industrial wastewater entering a water body represent a heavy
source of environmental pollution in rivers (Kanu, 2011).The bulk of industrial pollution in
India is cause by the small and medium scale industrial (SMIs) sector which are almost
3 million and are widely scattered throughout the country (Nalini Bhatt, 2009). To deal
with the effluent in these SSIs the concept of Common Effluent Treatment Plan (CETP)
was introduced with a hope that it would help the industries in abating the pollution.The
concept of CETP has only compounded the toxic content to larger volumes and various
standards formulated for inlet and outlet effluent for CETPs has no mention of such toxic
contents, thus it goes beyond the capacity of primary and secondary treatment in
CETPs (Manjari, 2000).Hence there is an urgent need for Tertiary treatment options for
Common effluent treatment processes to deal with the high volume of effluent loading.
2. OXIDATION PROCESSES IN WASTEWATER TREATMENT
The inability of conventional biological wastewater treatment to remove many industrial
toxic pollutants evidences that efficient tertiary treatment like chemical oxidation systems
are required (Anuta, 2001). Various chemical wet oxidation techniques to remove toxic
non-biodegradable pollutants from wastewater were found efficient in reducing the
refractory organics in wastewater (Rakholiya V.V., 2012). The use of titanium dioxide/UV
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light process, hydrogen peroxide/UV light process and Fenton’s reactions in wastewater
treatment found effective in COD degradation. Initial concentration of the target
compounds, amount of oxidation agents, catalysts and nature of the wastewater affects
the oxidation processes in wastewater treatment (Stasinakis; 2008). Advanced oxidation
processes (AOP) have received considerable attention because it is possible to degrade
organic compounds and colour from wastewater. (Schrank, 2005).
Application of chlorine dioxide, chlorine mediated electrochemical oxidation, and
aqueous oxidants like Hydrogen peroxide, Sodium Hypochlorite and Calcium
Hypochlorite at different temperatures & reaction durations reduces COD effectively
(Vaezi F.2004; Rajkumar D., 2006; Ali Awan M, 2004). Various oxidation and combined
processes such as UV/H2O2/Hypochlorites, Fenton and Electro-oxidation, photochemical, photo-catalytic, electro-catalytic oxidation, wet air oxidation, ozonation,
biological followed by ozone/UV/H2O2, coagulation or electro-coagulation and catalytic
treatments have been considered effective in treating wastewater of non-biodegrable
nature (Rameshraja D, 2011). Ferric chloride (FeCl3•6H2O), alum (Al2(SO4)3•18H2O),
ferrous sulphate (FeSO4•6H2O) and Poly Aluminium Chloride also found effective in
reducing the COD non-biodegradable wastewater(El-Gohary F. 2010).
The study presents the application of Sodium hypochlorite (NaOCl), Calcium
hypochlorite (Ca (OCl)2), Hydrogen peroxide (H2O2) and their Combination for polishing
a secondary effluent at Common Effluent treatment plant at Vapi, Gujarat, India.
3.
MATERIALS AND METHODOLOGY
3.1. Vapi, CETP and Secondary Effluent
Vapi CETP is one of the largest running wastewater treatment plants in India. It provides
effluent treatment to Vapi town as well as to 1400 industrial units in Vapi Industrial
Estate. CETP was designed and built by the National Environmental Engineering and
Research Institute (NEERI) in 1997.Effluents are received from 700 member industries
in the GIDC estate through a network of drains and pipelines after primary treatment at
the individual company. Received effluents are treated through five stages: mechanical
processes to remove gross solids and grit, equalization, primary chemical treatment to
flocculate and settle colloidal suspended solids, biological digestion to reduce COD
(Khambete A. 2012). The key challenge for Vapi CETP is to reduce COD in the final
output about 250 mg/l. The samples collected from CETP after secondary clarifier was
analyzed for Chemical Oxygen Demand, Total Suspended Solid and Chlorine before
and after treating with different oxidants, using Standard Methods(Eaton. 1995).
3.2. Sodium hypochlorite (NaOCl)
Sodium hypochlorite, the active ingredient in household bleach, was discovered by the
French chemist Berthollet, in Javel on the outskirts of Paris, in 1787. Its ability to
effectively whiten textiles was quickly discovered and put to commercial use with great
success (Odyssey, 2007).Commercial grades of NaOCl contain about 10% available
chlorine generally. Many consumers are replacing chlorine gas with sodium hypochlorite
as the oxidizing or disinfecting agent because of safe handling measures (John,
2001).The reaction between sodium hypochlorite and water is shown in Equation 1.
NaOCl + H2O
HOCl + Na+ + OH- ………………………………………
(1)
3.2. Calcium hypochlorite (Ca (OCl)2)
Calcium hypochlorite (Ca(OCl)2) is essentially a solid that is used in place of Chlorine
gas. Commercial grades of Ca (OCl)2 generally contain about 70% available chlorine. It
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is appropriate for wastewater applications (Leslie, 1998).The reaction between calcium
hypochlorite and water is shown in Equation 2.
Ca(OCl)2 + 2H2O
2HOCl + Ca++ + 2OH-………………………………...................(2)
The application of calcium hypochlorite to water also produces Hypochlorous acid,
similar to sodium hypochlorite as shown in equation 2. Hypochlorous acid is a weak acid
it dissociates slightly into hydrogen and hypochlorite ions as noted in Equation 3.
HOCl
H+ + OCl-……………………………………………………………................. (3)
The Hypochlorous acid is the prime disinfecting agent in wastewater treatment. The sum
of the OCl- and HOCl concentration is called the free available chlorine (EPA 1999).
3.3. Hydrogen Peroxide (H2O2)
Hydrogen peroxide has been used to reduce the BOD and COD of industrial wastewater
for many years. The oxidation process involves the production of reactive hydroxyl
radicals (*OH) that are ultimately capable of mineralizing organic contaminants which
may occur via one of three general pathways: (1) hydrogen abstraction; (2)
electrontransfer and (3) radical addition(Schrank, 2005).Hydrogen peroxide has been
used in different experiments to improve the oxygen supply and oxidation rate and
behave as oxidizing auto catalyst for treatment of halogenated hydrocarbon in waste
water treatment (Soji A., 1998). Reaction mechanism of hydrogen peroxide is shown in
Equation 4 and 5 respectively (Yun W.K. 1999). Hydrogen peroxide reaction chemistry
is complex, but potentially capable of degrading a wide range of organic contaminants
depending on conditions (Benjamin 2011).
H2O2
RH + 0OH
OH- + 0OH……………………………………………………………............... (4)
R0 + H2O ……………………………………………………….........
(5)
3.4. Methodology
The 1 L wastewater sample was reacted with different concentrations of Merck Sodium
hypochlorite having 5% Chlorine content, Merck 3.3% (w/v) diluted Calcium hypochlorite
having 30% Chlorine content, Merck 2% (v/v) diluted Hydrogen peroxide for 1 hour
reaction time and Combination of each of three oxidant for 2 hour reaction time. The
samples were agitated with the help of magnetic stirrer.
Samples were analysed with three strong oxidants for the period of four months, from
August 2012 to November 2012. The COD reduction by different concentration was
presented by averaging the results of four trails per week for all three oxidants. The best
suitable concentration for optimum COD reduction was analyzed based on the average
of the four months results for each oxidant. The best concentration so determined for
each oxidant was analyzed for period of one and half months in order to check the
consistency. From this best suitable concentration of each oxidant, Combination of all
three oxidants was formulated and examined for a period of one month for optimum
COD reduction.
4.
RESULTS AND DISCUSSION
4.1. COD Reduction using Sodium Hypochlorite
The analysis of COD before and after treatment with various concentrations of sodium
hypochlorite for period of four months trials is shown in Figure 1. Each trail in the figure
shows the average of four results per week.
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800
COD Conc.
700
600
COD Conc.
500
50ppm
400
100ppm
300
200ppm
200
500ppm
100
750ppm
1000ppm
0
1
2
3
4
5
6
7
8
9
10
11
No. of Trials
Figure 1 : COD Reduction with different Sodium Hypochlorite concentrations
It can be seen from the analysis that 1000 ppm concentration of sodium hypochlorite
reduces the CODwith an average about 52.78% during four month trails. But with the
increase in oxidants concentrations there is the increase in Total Suspended Solids
(TSS) and Residual Chlorine (Cl2) concentration hence optimum oxidants concentration
is decided taking into consideration the effect on other parameters as shown in Figure 2.
Figure 2: Reduction of different parameters with
different sodium hypochlorite concentrations
Figure 3 : COD Reduction at 200 ppm sodium
hypochlorite concentration
From analysis it was observed that 1000 ppm concentration can reduced the COD from
519 mg/L to 234 mg/L but increases TSS from 196 mg/L to 1186 mg/L and free Cl2 from
0 mg/L to 443.13 mg/L while comparing the results at different concentration it was
found out that 200 ppm concentration reduces COD from 519 mg/L to 337 mg/L with a
little increase in TSS from 196 mg/L to 252 mg/L and free Cl2 from 0 mg/L to 44.31 mg/L
which can be easily dechlorinated. The 200 ppm sodium hypochlorite solution gives
optimum result for all parameters. Figure 3 shows the COD reduction at 200 ppm
concentration for one and half month which on an average shows 36.36% COD
reduction.
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4.2. Reduction of COD using Calcium Hypochlorite
The analysis of COD before and after treatment with different concentrations of calcium
hypochlorite for the period of four months is shown in Figure 4. Each trail in the figure
shows the average of four results per week.
800
700
COD Conc.
COD Conc.
600
50ppm
500
400
100ppm
300
200ppm
200
500ppm
100
750ppm
1000ppm
0
1
2
3
4
5
6
7
8
9
10
11
No. of Trails
Figure 4 : COD Reduction with different Calcium Hypochlorite concentrations
It can be seen from the analysis that 1000 ppm concentration of calcium hypochlorite
reduces the COD with an average about 54.90% in four month trails. But with the
increase in oxidants concentrations there is the increase in Total Suspended Solids
(TSS) and Residual Chlorine (Cl2) concentration hence optimum oxidants concentration
is decided taking into consideration the effect on other parameters as shown in Figure 5.
Figure 5 : Reduction of different parameters with
different calcium hypochlorite concentrations
Figure 6: Reduction of COD at 200ppm calcium
hypochlorite concentrations
From analysis it was observed that 1000 ppm concentration can reduced the COD from
524 mg/L to 247 mg/L but increases TSS from 196 mg/L to 1192 mg/L and free Cl2from
0 mg/L to 472.23 mg/L while comparing the results at different concentration it was
observed that 200 ppm concentration reduces COD from 524 mg/L to 356 mg/L with a
little increase in TSS from 196 mg/L to 320 mg/L and free Cl2 from 0 mg/L to 44.31 mg/L
which can be easily dechlorinated.The optimum results was obtained with 200 ppm
calcium hypochlorite solution for all parameters. Figure 6 shows the reduction of COD at
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200 ppm concentration for one and half month which shows average 45.45% COD
reduction.
4.3. Reduction of COD using Hydrogen peroxide
The results for COD analysis before and after treatment with different concentrations of
Hydrogen Peroxide for the period of four months are shown in Figure 7. Each trail in the
figure shows the average of four results per week.It can be seen from the analysis that
with the increase in concentration of hydrogen peroxide from 10 ml, there was increase
in the COD concentration; hence optimum oxidants concentration considered is 10 ml
hydrogen peroxide solution per litre of wastewater. Figure 8 shows the reduction of COD
at 10 ml concentration for one and half month analysis where average COD reduction
recorded is 49.09%.
Figure 7: COD Reduction with different Hydrogen
peroxide concentrations
Figure 8: COD Reduction at 10 ml Hydrogen
Peroxide concentration
4.4. Reduction of COD using Combination of Three Oxidants
The 1 L volume wastewater samples were reacted with combination of 100 ppm Merck
Sodium hypochlorite having 5% Chlorine content, 100 ppm Merck 3.3% (w/v) diluted
Calcium Hypochlorite having 30% Chlorine content, 10 ml Merck 2% (v/v) diluted
Hydrogen peroxide for retention time of 2 hour and with the help of magnetic stirrer the
samples were agitated. Figure 9 shows the result of the COD reduction in secondary
effluent of CETP,before and after treatment. It shows that combination of three oxidants
gives 59.20% reduction of COD.
Figure 9: COD Reduction with Combination of Oxidants
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5. CONCLUSIONS
Oxidizing the wastewater of CETP, Vapi after secondary treatment with strong oxidants
helps in reducing the COD of the wastewater effectively, and this process can bring
down COD strength near the discharging norms, With the application of Merck Sodium
hypochlorite having 5% Chlorine content, Merck 3.3% (w/v) diluted Calcium hypochlorite
having 30% Chlorine content, Merck 2% (v/v) diluted Hydrogen peroxide its was found
that:

With 200 ppm Sodium hypochlorite and 1 hour detention time the COD reduction
was 36.36%.

While with 200 ppm Calcium hypochlorite and 1 hour retention time shows 45.45%
reduction of COD.

And the 10 ml Hydrogen peroxide with 1 hour detention time shows reduction of
COD by 49.09%

Mixture of 100 ppm Sodium hypochlorite, 100 ppm Calcium hypochlorite and 10 ml
Hydrogen peroxide with 2 hour retention time gives 59.20% reduction of COD.
Ca(OCl)2 is safer than NaOCl and chlorine gas. It also has excellent stability when kept
in dry storage, maintaining its potency well over time. Although solid Ca(OCl) 2 is more
stable and safer to handle than its liquid counterpart NaOCl, it is corrosive and
hygroscopic (i.e., readily absorbs moisture), reacting slowly with moisture in the air to
form chlorine gas if not stored in air-tight containers.
Decomposition of hypochlorite over time can affect the feed rate and dosage, as well as
produce undesirable byproducts such as chlorite ions or chlorate.The addition of
hypochlorite to water yields a hydroxyl ion that will increase the pH of the water.
These oxidation processes increases the Total Suspended Solids in wastewater which
eventually increases the amount of final sludge generation in plant by 10 to 12%
Consistent strength of Sodium hypochlorite and Calcium hypochlorite solution gives
consistent chlorine residual which is to be dechlorinated before discharging it to natural
water body.
ACKNOWLEDGMENT
The Authors would like to thank Mr.Davda, G.M.Tech, CETP, Vapi for his valuable
suggestion & comments and thankful to the laboratory staffs for their assistance
throughout experimental work.
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