23

CHAPTER 2

LITERATURE REVIEW

This chapter covers extensive literature collection on tannery

industry, treatment methods of organic effluent, rotating disk electrode and

residence time distribution.

2.1 TANNERY INDUSTRY

Tanning industry contributes significantly towards exports,

employment generation and occupies an important role in Indian economy on

the other hand; tannery wastes are ranked as the highest pollutants among all

the industrial wastes. Tannery industries use a large number of chemicals (i.e.,

natural and synthetic tanning agents, surfactants, salts, etc.) for the treatment

of skins and consequently produce large quantities of effluents (about 10m3 of

waste per tonne of leather) which have to be purified before being discharged

into the environment. Since tannery wastewater contains both organic

compounds, mainly tannins that are polyphenolic molecules, and inorganic

compounds such as ammonia, sulfides, and chlorides, the combination of

conventional methods of physiochemical and biological methods do not

always meet the legal limits for waste discharge (Marcopanizza et al 2004,

Balakrishnan et al 2002, Iaconi et al 2002, Jochimsen et al 1997, Rao et al

2001, Dima et al 2006, Balakrishnan et al 2002, Lefebvrea et al 2005).

Physico-chemical treatment of tannery effluents consists of chemical

24

precipitation, adsorption, coagulation, flocculation, sedimentation, filtration,

ion exchange and chemical oxidation (Ramesh et al 2007, Kyung-Sok et al

2004, 2007, Lefebvre et al 2006, Metes et al 2004, EPA, 2004, UNEP, 2004,

Linda and Peter 1999).

Advanced Oxidation Processes (AOPs) have been developed to

overcome the problem of treating high refractory pollutants (Preethi et al

2009a,b, Lech Kos et al 2003, Giusy Lofrano et al 2009).

2.2 TREATMENT METHODS

2.2.1 Conventional Methods

Various treatment processes, such as sedimentation, coagulation–

flocculation, adsorption, chemical oxidation, and membrane processes, as

well as biological (both aerobic and anaerobic) processes and fungal

treatment, have been studied for the treatment of tannery wastewaters. The

conventional method includes Membrane filtration (Justina et al 2009),

Precipitation (Kabdasli et al 2003, Esmaeili et al 2005) and Coagulation

(Jing-Wei et al 2007, Haydar et al 2009, Zhi et al 2009, Espinoza-Quinones

et al 2009, Sengil et al 2009). Adsorption (Santosa et al 2008, Covarrubias

et al 2008), Ion exchange (Tiravanti et al 1997, Kabir and Ogbeide 2008) and

Biological methods studied by Martinez et al (2003), Farabegoli et al (2004),

Murat et al (2006), Lefebvre et al (2006), Banu and Kaliappan (2007), Munz

et al (2008), Zupancic and Jemec (2010). Schrank et al (2004) has reported

that many organic compounds contained in the effluent are resistant to

conventional chemical and/or biological treatment. The degradation of

inhibitor substance such as tannin during the anaerobic digestion was

investigated by Banu and Kaliappan (2007). Lombardo et al (1997) reported

a marginal increase in COD reduction and a substantial improvement in floc

25

settling when bentonite or organic flocculants were used together with ferric

chloride or polyaluminium salts.

In biological method, microorganisms are utilized to treat

wastewater because they can uptake organic matter and nutrients (nitrogen

and phosphorus) for energy source, metabolism and for building blocks

(cell synthesis) (Pittier and Chudoba 1990, Wiesmann et al 2007). Typical

biological treatment processes make use of trickling filters, activated sludge,

Sequencing Batch Reactor (SBR) and wetland as polishing system. Organic

pollutants and colloidal organics from wastewater can successfully be

removed by biological treatments (Benefield and Randall 1985, Seabloom and

Buchanan 2005). In this treatment technique, micro-organisms utilize the

organic matter for metabolism processes (Bashaar 2004, Wiesmann et al

2007, Seabloom and Buchanan 2005). Biodegradation of organic matter

during wastewater treatments occur either in the presence of oxygen

(aerobically) or in anoxic conditions by denitrification (Jeyaseelan 1997,

Nicholas 1996, USEPA 2004). Many investigators have reported the use of

biological treatments, specifically, sequencing batch reactors (SBR) in

tannery wastewater as an efficient way for the removal of organic matter

(Ganesh and Ramanujam 2009, Farabegoli et al 2004, Murt et al 2002,

Lefebvre et al 2006).

Even though the conventional methodologies are technically

feasible, on lab scale operations they are questionable for their technical and

economic feasibility in large scale operations. Thus, it can be observed that

the problem involving the proper management of the tannery effluent has not

been solved. Hence the evolution of AOP was originated as an alternative

method of treating this kind of problems.

26

2.2.2 Advanced Oxidation Process (AOP)

Tannery effluents are oxidized by four different reagents like

ozone, hydrogen peroxide, Fenton oxygen, and air, in precise,

pre-programmed dosages, sequences UV radiation. These procedures may

also be combined with any one of oxidation process or agents are known as

advanced oxidation process (AOP). Advanced oxidation processes (AOPs)

may become the most widely used water treatment technologies for organic

pollutants, which cannot be treated by conventional technologies due to the

high chemical stability and/or low biodegradability. Depending on the

operating temperature, the type of used oxidant, and the concentration of the

pollutants in the wastewater, the possible partial oxidation step can be

classified into three main categories: Incineration, Wet air oxidation process

(WAO) and Oxidation with strong oxidants. These processes involve

generation and subsequent reaction of hydroxyl radicals (·OH).

Schrank et al (2004) studied the various oxidation techniques (UV,

TiO2/UV, O3, H2O2/UV and O3/UV) to degrade tannery wastewater.

Degradation of xenobiotics originating from the textile preparation, dyeing,

and finishing industry using ozonation and advanced oxidation was reported

to be very effective (Arslan-Alaton et al 2004). The UV/O3 process is

expected to improve the treatment efficiency due to the activation of ozone

molecules by UV photons and thereby facilitating the formation of hydroxyl

radicals.

Awan et al (2003) was analysed that the recovery of chromium

from tannery wastewater by H2O2, sodium hypochlorite and calcium

hypochlorite oxidants to soluble chromate. H2O2 was potentially a suitable

oxidant as it could oxidize a suspension of Cr (OH)3 to chromate to 98% and

27

88% in the synthetic and real wastewater. Dantas et al (2003) was evaluated

that the efficiency of Fenton and Photo-Fenton processes for the treatment of

wastewater from leather industry, in a 4 hr time period 90% of COD removal

was achieved. The concentration of hydroxyl radicals (•OH) increases

removal efficiency also increased.

However, all these methods have some major drawbacks. For

example, in conventional treatment, filtration and adsorption are not always

sufficient to achieve the discharge limits; coagulation and flotation generate a

large amount of sludge; chemical oxidations have low capacity rates and need

transportation and storage of dangerous reactants; and advanced oxidation

processes require high investment costs. In this context, oxidative

electrochemical technologies offer an alternative solution to many

environmental problems (Marco Panizza and Giacomo Cerisola 2009).

2.3 ELECTROCHEMICAL TREATMENT

Electrochemical technologies have gained importance in waste

water treatment during the past two decades (Lin et al 1998). There are

different companies supplying facilities for metal recoveries, treating drinking

water or process water, treating various wastewaters resulting from tannery,

electroplating, dairy, textile processing, oil – in – water emulsion, etc. at the

present time, electrochemical technologies have reached such a state that they

are not only comparable with other technologies in terms of cost, but also

more efficient and compact. The development, design and application of

electrochemical technologies in water and wastewater treatment have been

focused particularly to electro oxidation, electro deposition, electro

coagulation and electro flocculation (Kobya et al 2003). An extensive

research has been done by many researchers in treating various wastewaters

by using electrochemical technologies. These techniques will be explained in

28

brief, being the electrochemical oxidation ours central point. Technical

feasibility for the treatment of various industrial effluents such as

electroplating waste water (Adhoum et al 2004b); oil mill wastewater

(Apostolos et al 2007); heavy metal laden wastewater (Lai et al 2003); nitrite

effluent (Koparal et al 2002); de-fluoridation (Zhu et al 2007); arsenic

removal (Balasubramanian et al 2009a, 2009b); textile dyes

(Saravanathamizhan et al 2007, Chithra et al 2008, Mohan et al 2007); landfill

leachate (Tsai et al 1997); restaurant wastewater (Chen et al 2000a, 2000b);

laundry wastewater (Ge et al 2004); surfactants (Ebru et al 2007); agro

industry wastewater (Patrick et al 2008) etc has been studied extensively.

2.3.1 ELECTRO OXIDATION

Electro oxidation of organic and inorganic compounds takes place

via two principle pathways direct oxidation and indirect oxidation. The direct

oxidation occurs at the anode in which the pollutants discharge its electron to

the anode electrode in order to maintain the flow of current in the bulk

solution. The indirect oxidation occurs as a result of the production of

powerful oxidizing agents in the bulk solution such as chlorine, hydrogen

peroxide and ozone. Short-lived intermediates, such as OH.2

, O.2

and HO.2

also form during the electro-oxidation and could effectively destroy the

organic and inorganic pollutants.

The electrochemical oxidation of phenolic compounds was studied

using different anodic materials by Comninellis et al 1993, Saracco et al 2000,

2001, Canizares et al 2002, 2004a, 2004b, 2005b, Iniesta et al 2001 and these

authours are reported, electro oxidation gives more removal efficiency. Lidia

et al (2005) studied the ability of the electro oxidation technique in improving

the biodegradability of the tannery wastewater. Extensive research has been

going on for the electro oxidation of wastewater treatment using various

electrodes.

29

Vijayalakshmi et al (2011) studied the electro-oxidation and

advanced oxidation as tertiary treatment technique for the purification of

tannery wastewater. The results indicated that the TOC removal by advanced

oxidation became sluggish, when the wastewater was processed initially by

electro-oxidation. However, the effluents processed by EO were found to be

completely disinfected.

Raoof et al (2011) investigated the electrochemical oxidation of

methanol on the surface of a new modified carbon paste electrode which was

prepared by incorporation of Ni (II)–zeolite in the carbon paste matrix. It was

found that methanol was oxidized by NiOOH generated with further

electrochemical oxidation of nickel hydroxide on the surface of this modified

electrode during the anodic potential sweep.

Xiuping Zhu et al (2011) studied the electrochemical oxidation of

biologically-pretreated dye wastewater which was performed in a boron

doped diamond (BDD) anode system. After electrolysis of 12 h, the COD was

decreased from 532 to 99 mg L1. The performance of BDD anode system in

degradation of large molecular organics such as humic substances makes it

very promising in practical applications as an advanced treatment of

biologically-pretreated wastewaters.

Hongbin Zhao et al (2011) synthesized ordered mesoporous carbon

(OMC) supported nanosized Pt and Pt Co alloy electro catalysts by ethylene

glycol hydrothermal reduction route and their electrochemical oxidation

activity toward several typical small organic molecules (SOMs) was

investigated. Electrochemical measurements confirmed that electrochemical

oxidation behaviors of SOMs on synthesized Pt/OMC and Pt Co/OMC

catalysts were complicated and the carbon chain length of SOMs and the

intermediates, especially COads oxidation behaviors on Pt active sites are two

main factors which affect the SOMs oxidation.

30

Nigel Bunce et al (2011) studied the electrochemical oxidation of

ammonia in alkaline solution at platinized platinum. Elemental nitrogen is

formed at mildly positive potentials with near quantitative current efficiency

through dimerization of partly dehydrogenated ammonia molecules NHx(ads).

The major intermediate, NH2(ads), is formed at Pt(1 0 0) domains on the

metal surface, where it dimerizes to hydrazine, and rapidly oxidizes to N2. At

somewhat more positive potentials, the formation of adsorbed nitrogen atoms

poisons the anode, and nitrogen evolution ceases. At potentials where water is

oxidized, the Pt anode is modified by a surface oxide. Under these conditions,

nitrogen evolution is accompanied by nitrogen oxides and oxy anions.

Elisabetta Turro et al (2011) studied the electrochemical oxidation

of stabilized landfill leachate with high COD value of 2960 mg L1, using

Ti/IrO2–RuO2 anode in the presence of supporting electrolyte (HClO4). The

important parameters of electrolysis time, temperature, current density, initial

effluent's pH and electrolyte concentration addition of NaCl or Na2SO4 were

studied. The performance was evaluated by COD, total carbon, total phenols

and color removal and anode properties were studied by scanning electron

microscopy and cyclic voltammeter. The author’s concluded that the main

parameters affecting the process were the effluent's pH and the addition of

salts.

Arseto et al (2011), electrochemical oxidation was investigated for

the treatment of reverse osmosis concentrate generated during the reclamation

of municipal wastewater effluent. Five different anodes (titanium coated with

IrO2–Ta2O5, RuO2–IrO2, Pt–IrO2, PbO2, and SnO2–Sb) were evaluated for the

electrochemical oxidation of reverse osmosis concentrate in a batch reactor.

Concluded that the highest formation of trihalomethanes (THMs) and

haloacetic acids (HAAs) was observed for Ti/Pt–IrO2 and Ti/SnO2–Sb. Also,

31

indirect oxidation by chlorine was a dominant oxidation mechanism for all

anodes.

Ilje Pikaar et al (2011) compared the performance of five different

mixed metal oxide (MMO) coated titanium electrode materials (Ta/Ir, Ru/Ir,

Pt/Ir, SnO2 and PbO2) for the electrochemical removal of sulfide from

domestic wastewater. All electrode materials performed similarly in terms of

sulfide removal (with ± 1% difference) at a constant current density. It

concluded that all the MMO coated titanium electrode materials studied are

suitable anodic materials for sulfide removal from wastewater.

Costa et al (2010) studied the electrochemical treatment of a

synthetic tannery wastewater with different type of anodes Si/BDD,

Ti/SnO(2)-Sb, and Ti/SnO(2)-Sb-Ir were evaluated. The level of organic

contaminant such as total phenols, COD, TOC and absorbance were measured

with the influence of pH and current density. Results showed different rate of

completion of process with anode type. The increase in current density

resulted in faster wastewater oxidation, with lower current efficiency and

higher energy consumption. In this aspect, Si/BBD proved to be the best

electrodic material for the direct electro oxidation of tannery wastewaters.

Sundarapandian et al (2010) studied the electrochemical treatment

of saline wastewater with organic load. The influence of the parameters such

as pH, period, salt concentration and current density on the reduction of

organic load was studied using graphite electrodes. It was found that current

density of 0.024 A/cm2 for a period of 2 h at pH 9.0 rendered best results in

terms of reduction in COD and TKN. The energy requirement for the

reduction of 1 kg of TKN and 1 kg of COD are 22.45 kWh and 0.80 kWh

respectively at pH 9 and 0.024 A/cm2. These experiments were reused at

commercial scale, on pickling process in leather manufacturing. The

32

characteristics of the waste stream and the quality of the leathers indicate that

the reuse of saline streams with intermittent electrochemical treatment is

feasible.

Ahmed Basha et al (2009) investigated the electrochemical

degradation of various process stages such as soak liquor, tanning effluent,

and post-tanning effluent using Ti/RuOx-TiOx-coated titanium substrate

insoluble anode (TSIA). The treatment was found to be effective for the soak

liquor in almost complete mineralizing the organic part. A high percentage of

sodium chloride makes the process effective by improving the rate,

completion, and energy efficiency of the process. The optimal operating point

giving maximum COD removal (94.8%) was found using RSM at a

circulation flow rate of 142.8 L h-1

, a current density of 5.8 A dm-2

, and a time

of 7.05 h. The remaining salt in the process was further used in the post –

tanning stage.

Rameshraja et al (2009) reviewed the various oxidations and

combined processes in the treatment of tannery wastewater such as

UV/H2O2/Hypochlorites, Fenton and Electro-oxidation, photo-chemical,

photo-catalytic, electro-catalytic oxidation, wet air oxidation, ozonation,

biological followed by ozone/UV/ H2O2, coagulation or electro-coagulation

and catalytic treatments. For the tannery effluent with sulphide as main

sources of pollutant, electro-coagulation is the best removal process whereas

for chromium, photo catalytic oxidation process using nano-TiO2 and wet air

oxidation in the presence of manganese sulphate and activated carbon as a

catalyst are efficient processes.

Davood Nematollahi and Leila Mohammadi-Behzad (2009) studied

the electrochemical oxidation of catechol in the presence of 1, 10, diaza-18-

crown, 1,7-diaza-15-crown-5 and aza-15-crown-5 and also, in the presence of

some transition metal ions (Zn+2

, Ni+2

and Cd+2

) in acetonitrile containing

33

tetra-n-butyl ammonium perchlorate as supporting electrolyte, by means of

cyclic voltammetry. The cyclic voltammetric data were analyzed by digital

simulation to confirm and to measure the homogeneous parameters for the

suggested electrode mechanism.

Dominguez et al (2008) described the electrochemical oxidation

using boron-doped diamond electrodes as an efficient alternative in the

treatment of biorefractory effluents. Total organic carbon (TOC) removal was

reported higher than 80 % under the selected experimental conditions, where

COD was in the range of 500- 1500 mg/l, current density was between 3 and

6 A/dm2, and the concentrations of sodium sulfate and sodium chloride

supporting electrolytes were 2500 and 5000 mg/l respectively. Experimental

conditions were selected to evaluate the technical suitability of the process

and to establish a kinetic model and parameters. A first-order kinetic model

and kinetic parameters are found out.

Saravanathamizhan et al (2008), attempted to study electrochemical

oxidation of formaldehyde at lower concentrations. Experiments were carried

out in a batch electrochemical reactor using commercially available RuO2

coated titanium and SS as anode and cathode respectively and covering a

wide range in operating conditions. It was observed from the statistical tool

(RSM) that the model predictions match well with experimental values with a

R2 value of 0.999.

Mohan and Balasubramanian (2006) experimented electrochemical

oxidation of textile dye wastewater using RuO2/Ti anode in batch

electrochemical reactor. The authors reported more than 90% COD removal

under optimum conditions. Later the authors (2007) attempted to reuse the

electrochemically treated textile effluent for dyeing application and reported

that the electrochemically treated wastewater can be effectively reused for

industrial application.

34

Lidia Szpyrkowicz et al (2005) studied the influence of anode

materials Ti/Pt–Ir, Ti/PbO2, Ti/PdO–Co3O4 and Ti/RhOx–TiO2 on

electrochemical oxidation for the treatment of tannery wastewater. The

decrease in time of chemical oxygen demand, nitrogen (TKN and ammonia),

Cr and sulphides was monitored. The study showed that the rate of pollutant

removal was significantly influenced by the type of anode material and

electrochemical parameters. Electrochemical oxidation can be applied as a

post-treatment after the conventional biological process in order to remove the

residual ammonia with low energy consumption of 0.4kWhm-3

with rate of

0.75 min-1

.

Marco Panizza et al (2004) investigated the electrochemical

oxidation of vegetable tannery wastewater as a tertiary treatment by using

lead dioxide (Ti/PbO2) and mixed titanium and ruthenium oxide (Ti/TiRuO2)

as anodes under different experimental conditions. In Ti/TiRuO2 anode, the

rate of tannery wastewater oxidation increased with operating parameters. The

results strongly indicate that electrochemical methods can be applied

effectively as a final treatment of vegetable tannery wastewater allowing the

complete removal of pollutant. Also, Ti/TiRuO2 was more stable and did not

release toxic ions, so it was the best candidate for industrial applications.

Canizares (2004) studied electro oxidation of 2, 4-dinitro phenol

using boron doped diamond electrode. The use of new anodic materials like

boron doped diamond electrode allowed to achieving higher efficiencies in

the use of electric energy and as a consequences to decrease the operating

cost.

Rao et al (2001) investigated the electrochemical treatment of

tannery wastewater using Ti/Pt, Ti/PbO2 and Ti/MnO2 anodes and a

Ti cathode in a two-electrode stirred batch reactor. The changes in color

35

concentration, chemical oxygen demand (COD), ammonia (NH4+), sulfide and

total chromium were determined as a function of treatment time and applied

current density. The efficiency of Ti/Pt was 0.802kg COD h1A

1m

2 and

0.270kgNH4+h

1A

1m

2, and the energy consumption was 5.77kWhkg

1 COD

and 16.63kWhkg1 of NH4

+. The order of efficiency of anodes was found to

be Ti/Pt Ti/PbO2>Ti/MnO2. The results indicate that the electro-oxidation

method could be used for effective oxidation of tannery wastewater and a

final effluent with substantially reduced pollution load can be obtained.

Juttner et al (2000) reported in their work that formation of active

chlorine is responsible for the oxidation of azo dyes. Azo dyes are commonly

encountered in leather and textile wastewaters.

Anna et al (1997) investigated the electrochemical removal of

2-chlorophenol and 2,6-dichlorophenol from aqueous solutions using porous

carbon felt anodes. Operating variables including current input, ratio of

electrode and solution volumes, and initial pollutant concentration were

considered in order to determine their influence on the faradic efficiency of

the process. The by-products of the oxidation reaction were identified, and

their concentration was determined during the electrolysis. The experimental

results showed a satisfactory detoxification, consisting of removal of cyclic

chlorinated compounds, could be accomplished by means of this

electrochemical method with a faradic efficiency of 30 % under optimized

conditions. A mathematical model based on the reaction between

chlorophenols adsorbed on the carbon fibers of the electrode and hydroxyl

radicals produced by anodic oxidation of water has been proposed to interpret

the experimental behavior of the system under different operating conditions.

Lidia Szpyrkowicz et al (1995) investigated the treatment of

tannery wastewater by the electrochemical method using Ti/Pt and Ti/Pt/Ir

36

electrodes. The aim of a satisfactory elimination of NH+

4 from wastewaters of

different strength was achieved using both types of electrodes. For both types

of the electrodes NH+

4 removal followed pseudo first order kinetics, with the

rate decreasing in function of the presence of organic substances. In

conclusion the electrochemical process can be applied successfully as a final

polishing or an alternative to biological nitrification, but cannot substitute

completely the traditional treatment of tannery wastewater.

Table 2.1 Literature collection on organic effluent, EO, Reactor

configurations

Reference Details of

Elctrodes /Reactor Inference

Vijayalaksmi

et al (2011)

Titanium mesh coated with

oxides of IrO2 , TaO2 and

TiO2 was used as electrodes.

Effluents processed by EO

were found to be

completely disinfected.

Ahmed Basha

et al (2009)

Ti/RuOx-TiOx-coated

titanium anode

Using Ti/TiRuO2 anode is

most effective.

Soloman

et al (2009)

Comparing the overall

performance of basic

electrochemical reactor

configurations such as batch,

batch recirculation, recycle

and single pass systems

A recycle reactor was the

best configuration, because

of its flexibility of

operation.

Ahmed Basha

et al (2009)

Compare overall

performance of the basic

electrochemical reactor

configurations such as batch,

batch recirculation and

continuous recycle reactors

Continuous recycle reactor

is found to be the better

configuration, because of its

flexibility of operation.

37

Table 2.1 (Continued)

Reference Details of

Elctrodes /Reactor Inference

Saravanathamizhan

et al (2008)

Batch electrochemical

reactor using commercially

available RuO2 coated

titanium

It was observed that the

model predictions match

well with experimental

values

Mohan et al (2007) Batch electrochemical cell

Ti/TiO2, RuO2 coated

Electrochemically treated

wastewater can be

effectively reused for

dyeing application.

Saravanathamizhan

et al (2007)

Undivided electrode cell,

Different oxide coated

titanium metal anode.

It has been observed from

the present analysis that

the predicted values are in

good agreement with

experimental data

Mohan et al (2006) Ruthenium/lead/tin oxide

coated titanium

The results indicate that

the electrochemical

method can be used to treat

dye house effluents.

Mohan et al (2001) Batch electrochemical

Cell, Ti / RuO2.

It is observed from the

present investigation that

the organic pollutant

present in the wastewater

can be oxidized by an

electrochemical technique.

38

2.4 RESIDENCE TIME DISTRIBUTION

The residence time distribution (RTD) analysis of a chemical

process reactor is very useful information on flow characterization. It is

projected for investigation of hydrodynamic flows in chemical apparatuses on

the base of stimulus - response technique. The RTD study gives how long

each element spent inside the reactor, as a quantitative measure of the degree

of back mixing within the system.

The knowledge on RTD plays an important for accurate kinetic

modeling of the system, and help reactor design to achieve the desired flow

pattern. This technique includes the injection of a small amount of the tagged

inert particles (tracer) in the inlet stream and the observation of the

corresponding response in the outlet stream of an apparatus. A flow model is

then selected by matching the experimental RTD curve with that obtained

from the mathematical model. It provides the convenient interface and

powerful tools for: initial experimental data processing, creation model of the

flow in reactor with using the predefined flow patterns as the building blocks,

estimate the flow model parameters, simulate the system for the response to

the input signals of different types and creation of a user-defined flow

patterns. One of the functions in the tool is estimation expert; user can

estimate the best values for hydrodynamic flow models that are nonlinear in

the parameters. Estimation is done in time domain by the least square method.

The convolution approach is used to do that, calculate model output signal at

values of unknown parameters as model transfer function (in time domain)

and experimental input signal. Then it is compared with experimental signal.

Parameter values are changed to reduce the lack of fit and the process is

repeated automatically, if it is necessary.

39

Egedy et al (2012) presented an algorithm based on qualitative

analysis that can be used to identify a compartment model structure based on

the results of physical experiments performed in a stirred reactor.

Furthermore, based on the compartment structure, dynamic simulation CFD

can be applied to identify and validate the unknown model parameters of the

compartment model. The most important aspect of this study is that a suitable

structure identification algorithm was created, and tested. The author

concluded from the study, the developed algorithm engineers can make

various compartment models which, can be used for on-line control,

engineering design and modeling systems at a more detailed level. Also, in

the future the algorithm will be improved to identify more complex structures,

and the basic compartment models will be extended to handle more

compartment classes such as dead zone.

Bahadori et al (2012) studied full-scale plug-flow reactors, in PFR,

the flow usually is non-ideal because of entrance and exit flow disturbances

and axial dispersion. To facilitate the design of treatment processes authors

have developed an analytical for plug flow reactors with dispersion numbers

varying from complete-mix to ideal plug-flow reactor. Also, a simple-to-use

method which is easier than existing approaches, less complicated with fewer

computations, for accurate and rapid estimation of effluent and influent

concentration ratios in plug flow reactor (percent remaining) as a function of

plug flow dispersion factor (from zero to infinity), first order reaction constant

and hydraulic detention time. The error between model and experimental data

were found to be 1.8%.

Militaru et al (2011) developed a mathematical model for

hydrodynamic behavior in the water treatment pilot plant equipments and it

has been validated by Residence Time Distribution (RTD). The author

combined the well known non–ideal flow models of real mixing

40

(Cholette-Cloutier) and axial dispersion in order to identify models structure

that gives the best fitting of the experimental data for each unit. Further, the

validated hydrodynamic models used in the evaluation and selection of the

process and optimum operating conditions, in order to improve process

economics and drinking water quality.

Saravanathamizhan et al (2010), Described the electrolyte flow

behaviour in the parallel plate electrochemical reactor by residence time

distribution models. The author used different models include single tank and

two/three tanks in series. The exit age distribution curves obtained under

various operating conditions was analyzed and compared with experimental

observations. Finally, the results predict that single CSTR model closely

matches with three tanks in series model with the volume ratio 1:18:1.

Capela et al (2009) studied the hydrodynamic characteristics of a

full-scale anaerobic contact reactor treating evaporator condensate from a

sulphite pulp mill. The methodology applied was based on the RTD technique

using lithium as a tracer. Different non-ideal hydraulic flow models were

tested and the best model fitting RTD data was the Gamma distribution model

with by-pass. It was concluded that the full-scale bioreactor presents a good

degree of mixing with about 22% of non-effective volume due to the presence

of high amounts of inorganic materials.

Wang et al (2009), recognized from the literature the flow profile

and hydraulic conditions of Membrane Bioreactor (MBR) that predicts the

performance of design was not evolved. The author used a concept of RTD to

characterize the hydraulic conditions in MBR. The technique would be used

as a tool to determine the impact of membrane geometry, orientation and

mixing efficiency on MBR performance. Lithium chloride was used as a

tracer to generate the RTD profile. The ‘‘Tanks-in-Series’’ model was used to

simulate the performance of a pilot plant MBR. However, it was found that

41

experimental RTD profiles could not be reproduced with this simple model

and conclude that a more fundamental approach, based on computational fluid

dynamics is needed to model the hydraulic behaviour in MBR’s.

Jones et al (2008), Studied the variations in CSTR feed geometry,

impeller speed and inlet flow rate through the residence time distribution.

The measured RTD also used to characterize the degree of plug flow

behaviour and short-circuiting. Using the ratio of the mean residence time to

the batch mode mixing time (t/tM) could not evaluate the performance of a

CSTR. Moreover, this approach can lead to process overdesign and excessive

energy requirements. Instead, it was shown that all process parameters can be

correlated using the ratio of the inlet jet momentum to the impeller discharge

momentum. The prediction of the degree of short circuiting and plug flow in

the CSTR could be used to improve process control. In addition, the results

can be used to identify the design of CSTR (inlet position or diameter) should

be modified to improve process performance.

Saravanathamizhan et al (2008) studied the electrolyte flow

characteristics in a Continuous Stirred Tank Electrochemical Reactor

(CSTER) using RTD. A three-parameter model has been developed to explain

the flow characteristic of electrolyte in a continuous stirred tank

electrochemical reactor and the model simulations are validated with the

experimental observations. From the model simulations, it explains that the

residence time distribution has been increased with increase in the exchange

flow ratio between active and dead zones and increase in the electrolyte flow

rate. Further the experiments were carried out for the effluent color removal

and the values are compared with theoretically calculated value and it is

observed that the values are satisfactorily matching with experimental

observations.

42

Lifeng Zhang et al (2007), studied the RTDs in a multistage

agitated contactor (MAC) for Newtonian fluids and presented a suitable

Computational Fluid Dynamics (CFD) simulation. The resulting mean

residence times and variances via the CFD simulation were in good

agreement with the experimental data. Various liquids are used to simulate the

RTD under different flow conditions. The simulation results show that a

cascade of stirred tanks with a back flow CTB model is more suitable to

describe the flow behavior in a MAC with few stages than an axial dispersion

model.

Qingfeng Yang et al (2007), used to diagnose the flow

characteristics in spiral wound Reverse Osmosis (RO) modules by RTD

techniques. In usual procedure, the steps (conductivity-concentration

transformation, baseline selection and the use of exit age distribution function

of Et, or dimensionless exit age distribution function of E ) in processing

tracer response conductivity and modeling of RTD curves often involve

complicated steps. In this study author developed a simple and direct method

for processing RTD signals from conductivity data for spiral wound

membrane RO system. Further, the direct method was tested using axial

dispersion (AD) model and Exponentially Modified Gaussian (EMG) model

and it shows that the present method provides a simple, fast and accurate RTD

data reduction.

Andre et al (2007), studied the RTD for industrial indirect Joule

Effect Heaters (JEH), with Smooth (ST) and Modified (MT) tubes to improve

treatment homogeneity in heated vessel used in food industry. Demonstration

and quantification of the efficiency of the geometrical modifications, and

Proposition of a single semi-empirical model including the flow regime (10 <

Re < 2000) and tube diameters (18 and 23 mm). The results obtained confirm

that the simple Dispersed Plug Flow (DPF) model is not adaptable to small

43

Reynolds numbers. Additional analysis demonstrate that certain geometrical

modification improve the treatment homogeneity by increasing the plug flow

contribution and reducing the value of the reduced variance. These beneficial

effects increase when the Reynolds number is increased, the nominal diameter

is reduced, and modified tubes were used. The proposed model enables the

prediction of the RTD in JEH with an accurate degree of confidence.

Polcaro et al (2007) studied the water disinfection processes in a

stirred tank electrochemical reactor using boron doped diamond electrode and

developed a model for E(t) distribution for a pulse input considering two

CSTRs in parallel

t)V(1

)Q(1exp

)V(1

Q)(1t

V

Qexp

V

QE(t)

22

(2.1)

represents the fraction of the total flow rate fed to a fraction of the

total volume V. The authors observed deviation from the ideal behavior for

low Reynolds number.

Lidia and Marta (2006), have reported the treatment of textile

effluent of reactive dye Red Procion H-EXGL using electrochemically

generated redox mediator in a filter press cell and developed a model for exit

age distribution E(t) was,

t

tM

1MM

t

tN

1N

NN

et

t

1)!(M

Mf)(1

t

1e

t

t

1)!(N

Nf

t

1E(t)

(2.2)

where, M, N, f and is a function of flow rate. The authors observed that the

electrolyte flow behavior was close to plug flow and compared the

decolorization of the dye using experimental and exit age distribution model.

44

Achim Heibel et al (2005) introduced an experimental method and

data analysis procedure to determine the liquid RTD of monoliths based on

the imperfect pulse injection of a dye tracer. Both the experimentally

determined liquid saturations values and predictions of CFD calculations, fits

well. Despite the reasonable agreement for the mean residence time the CFD

model based on uniform liquid distribution over the monolith channels and

the individual channel corners failed to describe the experimentally

determined reduced RTD curves. The authors go for a measured corner-scale

liquid distribution pattern in combination with the CFD model resulted in

good agreement with the experiments. Furthermore, this approach also

improved the agreement between measurement and model for the mean

residence time.

Bang Cheng Xu et al (2005), determined interstage back mixing

rates in an agitated, fully baffled, two-stage, compartmented column. The

back mixing rate was indirectly determined by introducing a tracer into one

stage and then measuring the tracer concentration with time in both

compartments as the tracer migrated from the injected stage to the noninjected

stage. The effect of flow through the column on interstage back mixing was

determined. The data were correlated in dimensionless form, and predictive

methods are presented that allow the prediction of the interstage back mixing

rate as a function of the impeller type, fluid properties, the interstage opening

geometry, and the forward flow rate. The correlations for the effect of the

forward flow rate on back mixing allow the design of a compartmented

column, using a draft tube attached to a center hole opening, which has no

back mixing. Concluded that, a compartmented column can be designed and

operated as a series of continuous stirred reactors (or compartments) in series,

without any interstage back mixing.

45

Andrade Lima and Hodouin (2005) have reported the residence

time distribution of electrolyte in a gold leaching tank. The author used

lithium chloride as a tracer and modeled the flow behavior in the tank consists

of two parallel streams single perfect mixed reactor and another consisting of

n perfect mixed reactors in series with by-pass.

David Olivet et al (2005), investigated the hydrodynamic behaviour

of a bioreactor treating municipal wastewater, through RTD technique using

lithium (chloride) as tracer. Different non-ideal models (namely axial

dispersion, tanks-in-series and some simple compartment models) were tested

for the modeling of the experimental RTD. The best model fitting RTD data,

tanks in series. The author observed that, the system under study, local RTDs

have been used for the qualitative determination of particular situations of

insufficient mixing, recirculation or channeling and the type of flow (plug,

dispersed or mixed) rather than global RTD. From the study the author

concluded that, local RTDs have permitted a qualitative description of the

hydraulic behaviour of the bioreactor

Olivier Potier et al (2005), performed RTD experiments in a

activated sludge 3000m3 channel reactor aerated by gas diffusion (for

different liquid flow rates under constant aeration rate and constant water

depth) and on a benchscalechannel reactor aerated from the bottom (for

different liquid and gas flow rates and water depths) in order to characterize

their hydrodynamics. Both units can be modeled as plug flow reactors with

axial dispersion. A general correlation has been obtained to predict the axial

dispersion coefficient as a function of the gas and liquid velocities and the

geometrical parameters of the full-scale and bench-scale reactors. Finally, to

facilitate the simulation of biological reactions in transient state, an equivalent

model based on tanks-in-series with variable back-mixing flow rate is

proposed.

46

Dudukovic (2004), discussed about the many models of non-ideal

reactors, cholette –coultier model are discussed in detail. Effect of dead

space, stagnancy, macromixing etc with impulse response is critically

discussed.

Hardin et al (2001) studied the residence time distribution of a

tracer gas (carbon monoxide) through a rotating drum bioreactor for

solid-state fermentation. The exit concentration profiles expected for plug

flow, plug flow with axial dispersion, and Continuous Stirred Tank Reactor

(CSTR) models. Model parameters were the dispersion coefficient in the

central plug flow region, the volumes of the dead regions, and the exchange

rates between the different regions. The superficial velocity of the gas through

the reactor has a large effect on parameter values. Increased superficial

velocity tends to decrease dead region volumes, inter region transfer rates,

and axial dispersion. The significant deviation from CSTR, plug flow, and

plug flow with axial dispersion of the residence time distribution of gas within

small-scale reactors can lead to underestimation of the calculation of mass

and heat transfer coefficients and hence has implications for reactor design

and scale up. The author’s concluded that, proper estimates of the driving

force will lead to a better understanding of mass transfer in moving beds of

particles with concomitant benefits not only in SSF with RDBs, but also in

non-SSF applications of rotating drums, such as drying and calcining.

Bengoa et al (2000), studied the hydrodynamics behavior in the

laboratory-scale commercial filter-press electrolyzer by flow visualization and

RTD. The direct visualization studied both divided and undivided modes,

with two injection points. The use of foams instead of turbulence promoters

as well as that of electrodes without turbulence promoters is tested. In the

membrane-partitioned mode, radial and axial dispersions are observed.

Dispersion is drastically extended in case of empty channel flow. A flow

47

distributor helps to high-flow asymmetry. RTD is implemented in the whole

cell and in the reaction area. The reactor working with a plane plate alone

gives a rather constant, low Peclet number, Pe, whatever the tested flow rates.

The low Pe values denote an important axial dispersion. The Pe increasing

significantly with turbulence promoters or foams, giving plug flow character.

Martin (2000) presented an alternative method for interpreting RTD

data, which is relatively easy to use and addresses some of the weaknesses of

more conventional methods. An extension to the tanks in series a concept was

used (ETIS) and combined with the reactor network formulation. The

suitability and appropriateness of the model is discussed and compared with

the closed dispersion model. It gives the model in conjunction with the reactor

network structure to be a versatile method of describing the characteristics of

a small but diverse group of reactors. The ETIS model has been compared

with the conventional tanks in series approach and has been found to be

superior.

Jose Gonza lez-Garc a et al (1998), studied the optimization of the

ratio between felt / compartment thickness of a three-dimensional electrode

by different techniques to get good flow dispersion, and good electrode

conductivity. Nevertheless, the RTD analysis gives information about the

hydrodynamic behavior of the system. This analysis yields an optimum ratio

close to 10/8. In these conditions, a dispersed plug-flow model, with almost

no dead zones, can represent the hydrodynamic behavior obtained. It is also

expected that no accumulation or retention of the liquid or gas phase inside

the reactor, due to its hydrodynamic behavior. The authors in 2002 have

reported the hydrodynamic behaviour of electrochemical reactors by

simulating stimulus–response experiments. The experiments were performed

with a simple experimental arrangement to generate RTD data from

electrolytic conductivity measurements. The multiparametric model proposed

48

and the Matlab program developed for electrochemical reactors, providing

values of porosity and compressibility parameters. This also permits modeling

of the electrochemical reactions that will be produced during the process.

David Wolf and William Resnick (1963), studied the measurement

and analysis of residence time distribution of continuous flow systems.

A study of available experimental data shows that the usual assumptions of

perfect mixing or plug flow do not correspond to the situation existing in real

flow systems. The residence time distribution for real systems can be

represented by an F-function

t

F(t) 1 exp for t9

(2.3)

F(t) 0for 0 t (2.4)

This equation results for a number of possible flow models that

include the additional possibilities of dead space, short-circuiting, error in

average residence time determination, and lag in response and any

combination of these models. This equation can be used to describe the

experimental results obtained far single as well as multistage systems.

2.5 ELECTROCHEMICAL REACTORS

Different types of electrochemical reactors ranging from conventional

plate and frame cell to advanced electrodes such as three-dimensional electrodes

are used for electrochemical processes (Saravanathamizhan et al 2008). The

design or selection of suitable electrochemical reactor is very important in

electrochemical process as the reactor geometry plays an important role in the

process yield. Extensive work has been reported on the analysis of

performance of various electrochemical reactors. Bengoa et al (1997) studied

49

the flow behavior of electrolyte in a filter press type electrochemical reactor

and reported residence time distribution using a commercial ElectroSyn cell.

2.5.1 Rotating Disk Electrode

The RDE is an extremely valuable method for characterizing

electrochemical reactions, mass transfer rate. RDE is a hydrodynamic

electrode system. The electrode rotates during experiments enhancing the

mixing of the effluent thereby increasing the rate of mass transfer. The RDE

reactor is an important investigational device because one can study the

effects of both convection and diffusion and its effects on chemical reactions.

This will aid in understanding the physical phenomena involved and

eventually lead to better equipments based on RDE; as a result it finds many

applications in electro synthesis, electro winning, electro oxidation, corrosion

detection etc. Ajith Wijayawardhana et al (1999) find application of RDE in

immune assay experiments. Thus the applications of RDE are practically

limitless.

Antonio Chaparro (2011) recently used rotating disc electrode to

study the electro catalysis. The authour studied the spillover effects of

catalysts on electrochemical reaction. Wojtowicz et al (1967) studied the

construction and operation of rotating disc electrode at high temperature.

Cesar Real-Ramirez et al (2010) said that electrochemical cells with

a rotating disc electrode are the preferred devices to characterize

electrochemical reactions. The three-dimensional numerical simulations of the

cell hydrodynamic behavior presented in this work shows that the fluid flow

pattern inside the cell was erroneously interpreted (Blurton and Riddiford

1965).

50

Pascal Viel et al (1999) studied the electro polymerization of

methacrylonitrile on a high spinning rotating disc electrode. Also, studied the

convection effects near and away from the electrode and concluded that

polymer film is initiated near the electrode region, rather than in the bulk

solution

Blurton et al (1959) described the shapes of many practical rotating

disc electrodes. Mass transfer and fluid flow in near disc regions were studied

and also opined that fluid flow below the working surface of the disc

electrode shall not interact with that in the upper region.

2.6 RESPONSE SURFACE METHODOLOGY

In conventional multifactor experiments, optimization is usually

carried out by varying a single factor while keeping all other factors fixed at a

specific set of conditions. It is not only time-consuming, but also usually

incapable of reaching the true optimum due to ignoring the interactions

among variables. On the other hand, the RSM has been proposed to determine

the influences of individual factors and their interactive influences. The RSM

is important in designing, formulating, developing, and analyzing new

scientific studying and products. It is also efficient in the improvement of

existing studies and products.

According to Hill and Hunter, RSM method was introduced by

G.E.P. Box and K.B. Wilson in 1951. Box and Wilson suggested to use a

first-degree polynomial model to approximate the response variable. Mead

and Pike stated origin of RSM starts 1930s with use of Response Curves

(Myers et al 1989).

51

One of the important facts is whether the system contains a

maximum or a minimum or a saddle point, which has a wide interest in

industry. Therefore, RSM is being increasingly used in the industry.

Statistical analysis widely used in various fields, dye removal by

absorbent (Ravikumar et al 2006), Biosorption of dye (Pavan et al 2007),

Fenton’s peroxidation on the removal of organic pollutants from olive oil mill

wastewater (Ahmadi et al 2005), decolorization of dye using enzyme

(Murugesan et al 2007), and nickel plating process (Oraon et al 2006).

Barbara Bianco et al (2011) studied the experimental factorial

analysis on wastewaters samples taken from a real treatment plant in different

periods, were used to construct a model response function to be applied in

operative conditions. The second-order model was modified to include the

initial COD as an independent variable in order to account for the wide range

of initial COD in the treatment plant. The regression gives an excellent

quantitative agreement between the predicted and measured values of COD

for different experimental conditions. For the optimum condition, the removal

of 80% COD was obtained.

Bahadir Korbahti (2007) studied the electrochemical degradation of

Levafix Blue CA, Levafix Red CA and Levafix Yellow CA reactive dyes in

textile dye wastewater using iron electrodes. The effect of operating

parameters on COD removal, dye removal and turbidity removal was

optimized using RSM, and the approximating functions of dye removal were

obtained with satisfactory degrees of fit (R2=0.93).

Arul Murugan et al (2009) studied the electrocoagulation of Acid

Blue 113 textile effluent in a batch electrochemical reactor covering wide

range of operating conditions. Experiments were designed using Response

52

Surface Method (RSM) to examine the combined effect of operating

parameters on the efficiency of electro coagulation process.

Saravanathamizhan et al (2008) studied electrochemical oxidation

of formaldehyde at lower concentrations. Experiments was carried out in a

batch electrochemical reactor using commercially available RuO2 coated

titanium and SS as anode and cathode respectively and covering a wide range

in operating conditions. Further, the statistical tool Response surface

methodology by Box-Behnken design was used to examine the influence of

individual parameters on electro-oxidation of formaldehyde, and the quadratic

model for formaldehyde removal efficiency was derived. It was observed that

the model predictions match well with experimental values with a R2 value of

0.999. Further, the author in (2007) studied the electrooxidation of textile

effluent using Response Surface Methods, electrochemical technique was

designed and analyzed using the Box-Behnken method. The influence of

individual parameters on electro-oxidation of textile effluent was critically

examined by author using the response surface method (RSM), and a

quadratic model for chemical oxygen demand (COD) reduction was

developed. The authour observed from the analysis that the predicted values

are in good agreement with experimental data with a correlation coefficient of

0.945.

It is observed from the literature that, though there are good amount

of work on electrochemical reactor, very few are on RTD. Hence, it is

proposed to study the RTD of electrolyte in Rotating disc electrochemical

reactor and extend the data for CFD simulation. Further, no literature is

available for RSM tool in RDE reactor. With help of this optimization tool,

lab scale reactor is to be scaled up into a pilot scale reactor. It is strongly

believed that the outcome of the research will have a real social and

commercial value.