Electrolyte Containing One Or More Redox Couples Biology Essay

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The MEO process involves an electrolyte containing one or more redox couples, wherein the oxidized form of atleast one redox couple is produced by anodic at the anode of an electrochemical cell. The oxidized forms of any redox couples present are produced either by similar anodic oxidation or reaction with the oxidized form of other redox couples present, capable of affecting the required redox reaction. The anodic oxidation in the electrochemical cell is driven by an externally induced electrical potential induced between the anode(s) and cathode(s) of the cell. The oxidized species of the redox couple oxidize the organic waste molecules and are themselves converted to their reduced form, where upon they are reoxidized by either of the afore mentioned mechanisms and the redox cycle continues until all oxidisable waste species, including intermediate reaction products, have undergone the desired degree of oxidation. The redox species, i.e., the ions are thus seem to "mediate" the transfer of electrons from the waste molecules to the anode, (i.e., oxidation of the waste) (Bremer 2003).

3.2 TOC analysis

To determine the quality of organically bound carbon, the organic molecules must be broken down into single carbon units and converted to a single molecular form that can be measured quantitatively. Samples were withdrawn from the electrolytic solution at different intervals. They were filtered and acidified by adding HCl and brought to a pH of 2 prior to analysis. Its injection volumes were 50/100µL. The temperature in the oven was 680oC in combination with a Pt catalyst. Calibration of the analyzer was achieved with potassium hydrogen phthalate standards by using standard analyser Merck. (Oturan et al. 2009)

Total Organic Carbon = Total carbon - Inorganic carbon.

3.3 COD analysis

In order to determine the extent of degradation of the effluent, COD was measured. COD determination was estimated as per standard methods (APHA 1997). The COD as the name implies is the oxygen requirement of a sample for oxidation of both biologically active and inert organic matter. COD is generally considered as the oxygen equivalent of the amount of organic matter oxidizable by potassium dichromate. The organic matter of the sample is oxidized with a known excess of potassium dichromate in a 50percent sulfuric acid solution. The excess dichromate is titrated with a standard solution of ferrous ammonium sulfate solution.

3.4 BOD analysis

The Winkler's test is used to determine the concentration of dissolved oxygen in water samples. An excess of manganese(II) salt, iodide(I-) and hydroxide(HO-) ions are added to a water sample causing a white precipitate of Mn(OH)2 to form. This precipitate is then oxidized by the dissolved oxygen in the water sample into a brown manganese precipitate. In the next step, a strong acid (either hydrochloric acid or sulfuric acid) is added to acidify the solution. The brown precipitate then converts the iodide ion(I-) to iodine. The amount of dissolved oxygen is directly proportional to the titration of Iodine with a thiosulfate solution.

BOD5

BOD determination was estimated as per standard methods (APHA 1998). To determine a five-day biological oxygen demand (BOD5), several dilutions of a sample are analyzed for dissolved oxygen before and after a five-day incubation period at 20OC in the dark. In some cases, bacteria are used to provide a source of oxygen to the sample; these bacteria are known as "seed". The difference in dissolved oxygen and the dilution factor are used to calculate BOD5. The resulting number (usually reported in parts per million or milligrams per liter) is useful in determining the relative organic strength of sewage or other polluted waters. The BOD5 test is an example of analysis that determines classes of materials in a sample.

3.5 Biodegradability index

The biodegradability index is defined as the ratio of BOD to COD. The value ranges from 0 to 1. Samples with biodegradability index smaller than 0.3 are not appropriate for biological degradation and for complete bio-degradation, the effluent must have a biodegradability index of atleast 0.4.

3.6 Specific energy consumption

The actual utilization of energy in fulfilling unit quantity of the targeted reaction (specific energy consumption, (E) Kwh(kgCOD)−1), is found by monitoring the cell voltage and extent of COD removal.

Specific Energy Consumption (E) =

3.7 Batch and batch recirculation process

Electrolysis can occur in a batch system or in a flow cell where the target compounds are continuously flowed past on the surface of the electrode. In an un-separated cell (flow or batch), the target compounds can be exposed to either the cathode or anode and hence both reduction and oxidation takes place. Therefore, in order to study single reactions, most researchers have chosen to use divided cells. Regardless of the configuration, the efficiency of the electrochemical processes will depend on the relationship between mass transfer of the substrate and electron transfer from the electrode or mediator.

Batch recirculation process pumps a fixed volume of feed solution (effluent) from a reservoir to electrochemical cell through a flow meter and back to the reservoir until the required COD removal is achieved. The power requirement for such a batch process depends on the degree of recirculation and mixing.

The organic destruction in the MEO process can be carried out in either a batch or in a continuous feeding mode. In the case of batch type reaction, the organic is added at one time (zero time) in the reactor and the process is carried out with or without oxidant regeneration. However, in real applications, the continuous organic addition is used mainly for minimizing the oxidant usage by simultaneous regeneration and by this way more quantity of the organic materials can be destructed than in the batch process. Usually, in the continuous process, an organic substance is added for a long time (e.g. hours, days etc.) at a particular flow rate and the oxidant concentration is maintained nearly at the same level by in situ electrochemical regeneration.

3.8 Response surface methodology

RSM is a collection of statistical and mathematical methods that are useful for the modelling and analyzing of engineering problems. In this technique, the main objective is to optimize the response surface that is influenced by various process parameters. RSM also quantifies the relationship between the controllable input parameters and the obtained response surfaces (Gunaraj and Murugan 1999; Kwak 2005). In the present study in Hydroquinone effluent part, the RSM has been used to determine the relation between the percentage of COD removal, power consumption and important operating parameters such as current density (CD), flow rate (Q), concentration of hydroquinone (HQ) and supporting electrolyte (SE).

Table 3.7 gives the parameters and the operating ranges covered. The concentration of Hydroquinone and supporting electrolyte, flow rate and current density are referred by uncoded variables as A, B, C and D respectively. The variables in uncoded form are converted to coded form: a, b, c and d using the following equation 1.

-------------------------- (1)

The Box-Behnken experimental design of RSM has been chosen to find the relationship between the response functions and variables using the statistical software package Design Expert Software-7.1.2, (Stat- Ease, Inc., Minneapolis, USA). The Box-Behnken design can be considered as a highly fractionalized three-level factorial design where the treatment combinations are the midpoints of edges of factor levels and the center point. These designs are rotatable (or nearly rotatable) and require three levels of each factor under study. Box-Behnken designs can fit full quadratic response surface models and offer advantages over other designs. The advantages of the Box-Behnken design over other response surface designs are: (a) it needs fewer experiments than central composite design and similar ones used for Doehlert designs; (b) in contrast to central composite and Doehlert designs, it has only three levels; (c) it is easier to arrange and interpret than other designs; (d) it can be expanded, contracted or even translated; and (e) it avoids combined factor extremes since midpoints of edges of factors are always used (de Oliveira et al. 2007).

The three level second order design demands comparatively lesser number of experimental data for precise prediction. Here, a total number of 29 experiments, including 3 centre points are carried out for 4 parameters. The interaction between the variables and the analysis of variance (ANOVA) has been studied by using RSM. The quality of the fit of this model is expressed by the coefficient of determination R2.The fit is confirmed by means of the absolute average deviation (AAD) defined as

-----------(2)

Where yi,exp and yi,pred refers to the experimental and predicted responses and n refers to the number of experimental runs.

3.9 Resin effluent

3.9.1 Batch process with recirculation

The synthetic resin effluent was prepared by dissolving an appropriate amount of cationic resin (Amberlite strong acid styrene based cation exchange resin - functional group - SO3H) in water in the presence of ferrous sulphate (Fe2-/Fe3-) as catalyst, with drop wise addition of H2O2 by maintaining the temperature of the reaction mixture at 95-100°C.

3.9.1.1 Experimental setup

Experiments were carried out under galvanostatic condition at different current densities and flow rates using RuO2 coated Titanium expanded mesh anodes and stainless steel cathodes. The electrolyser was of filter press type reactor (Figure. 3.1). The fluid flow circuit consists of a reservoir, a magnetically driven self priming centrifugal pump, a flow meter and the electrolytic cell. The electrical circuit consists of a regulated direct current power supply ammeter and the cells with the voltmeter are connected in parallel to the reactor.

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Reservoir 6.Voltmeter

Pump 7. DC Power Supply

Rota meter 8. Anode

Electrochemical Reactor 9. Cathode

Ammeter

Figure 3.1: Filter press type of reactor (continuous batch recirculation)

3.9.1.2 Experimental procedure

All experiments were carried out under batch recirculation conditions, first treating 2L of the solution per batch of electrolysis by passing a quantity of electricity corresponding to the current densities. The process of electrolysis was followed by continuous monitoring of the variation of pH, cell voltage and TOC values. For the purpose of determination of TOC, samples were drawn at predetermined intervals and the estimation was carried out using TOC analyzer.

NaCl was used as a supporting electrolyte with various concentrations and was applied for the oxidative destruction of organic compounds. The electrolysis was carried out at different flow rates of 20, 40, 60, 80, 100L/h TOC was determined at 1hour interval. Based on the results obtained, it was decided to continue the investigation using FeSO4 as the mediator along with NaCl as the supporting electrolyte with only 500mL of the electrolyte at a flow rate of 20L/h. In order to provide a good conductivity, concentrated H2SO4 (0.025N) was added and these experiments were carried out again in batch recirculation at different current densities such as 1.25, 2.50, 3.75, 5.00, 6.25, 7.50, 8.75, 10.00Adm-2. The concentration of the mediator was maintained at 0.37g/L and for the purpose of generation of OH free radical, H2O2 was added at the rate of 1mL/h.

The conditions are given below:

Type of mediator : FeSO4 with 0.37g/L

Type of supporting electrolyte : NaCl with 5g/L

Volume of the electrolyte : 0.5L

Flow rate : 20L/h

Duration of electrolysis : 8h

By taking 10Adm-2 as the optimum current density, experiments were carried out with different flow rates such as 20, 40, 60, 80, 100L/h.

Table 3.1: Experimental conditions for batch process with recirculation

Parameters

Remark

Type of effluent

Synthetic organic ion-exchange resins (Amberlite -IR400 Styrene- Divinylbenzene strong Base anionic resin)

Effluent color

Black

Initial TOC (mg/L)

(1100-1500) mg/L

Total content of electricity passed, Q(Amp.hr) for 3.75 Adm-2

5.78, 8.82, 9.85

Current density (Adm-2 )

1.25, 2.50, 3.75, 5.00

Reactor hold up, dm3

0.2, 0.3, 0.4

Volume of effluent taken (L)

0.2, 0.3, 0.4

Table 3.2: Parameters studied for batch process with recirculation

Parameters

Remark

Current density (Adm-2)

1.25, 2.50, 3.75, 5.00

Mediator

In-situ generated OCL-

Concentration of NaCl (g/L)

8.00

Table 3.3: Experimental parameters measured for batch process with recirculation

Sl.No

Parameters

1

Current (A)

2

Cell Voltage (V)

3

Electrode Potentials (V)

4

TOC (mg/L)

5

Temperature (OC)

6

pH

3.9.2 Batch process

The synthetic anion exchange resins (Amberlite R IRA 400 - strong base styrenic divinly benzene anion exchange resin) of different weights, say 2, 3, 4grams were mixed with 0.01M FeSO4 solutions of 100, 150, 200mL in a reaction flask provided with a reflux condenser. The temperature of the reaction mixture was maintained at around 95oC by immersing the reaction flask in a boiling water bath. 50percent (w/v) H2O2 was added to the hot reaction mixture drop wise using peristaltic pump. The dissolved resin solution was then made up to 200, 300, 400mL respectively with distilled water.

3.9.2.1 Experimental setup

Batch electrolyte cell used in the electrooxidation process is shown in Figure 3.2. The cell consists of a glass beaker of 250, 350, 450mL capacity closed with a PVC lid having provision to fit a cathode and an anode. Salt bridge with reference electrode was inserted through the holes provided in the lid. The current was supplied by an electric power source. Stirring was done with a magnetic stirrer.

The electrochemical destruction was carried with volumes (200, 300, 400mL) of dissolved resin solution which act as electrolyte in the electrochemical cell. In the electrochemical cell, the electrolytes used are

Anode : Ti/RuO2

Cathode : Stainless steel

Mediator : NaCl (g/L)

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Ammeter 6. Anode

Voltmeter 7. Cathode

DC Power Supply 8. Standard Electrode Potential Setup

Batch Reactor 9. Magnetic Stirrer Setup

Salt Bridge 10. Sample Collector

Figure 3.2: Constant stirring batch reactor set up

3.9.2.2 Experimental procedure

The experiment was carried out in batch set up with different current densities of 1.25, 2.5, 3.75, 5Adm-2 and it was run for 7hours. Sample of about 1mL was collected for every one hour and the temperature, pH, cell voltage, electrode potentials were recorded. The effluent volume of 200, 300, 400mL was taken up for the experiments. Electrolysis was carried out at different parameters such as current densities of 1.25, 2.5, 3.75 and 5.00Adm-2 with NaCl concentration as 8g/L. During the electrolysis sample were collected at different time intervals and the TOC concentration was measured.

Table 3.4: Experimental conditions for batch process

Parameters

Remark

Type of effluent

Synthetic organic ion-exchange resins (Amberlite -IR400 Styrene- Divinylbenzene strong Base anionic resin)

Effluent color

Black

Initial TOC (mg/L)

(1700-1900) mg/L

Current density (Adm-2 )

1.25, 2.50, 3.75, 5.00, 6.25, 7.50, 8.75, 10.00

Reactor hold up, dm3

0.00588

Volume of effluent taken (L)

0.5,2.0

Table 3.5: Parameters studied for batch process

Parameters

Remark

Current density (Adm-2)

1.25,2.50,3.75,5.00,6.25,7.50,8.75,10.00

Mediator

In-situ generated OCL-

Concentration of NaCl (g/L)

8.00

Flow rate (L/hr)

20,40,60,80,100

Table 3.6: Experimental parameters measured for batch process

Sl.No

Parameters

1

Current (A)

2

Cell Voltage (V)

3

TOC (mg/L)

4

Temperature (OC)

5

pH

3.10 Hydroquinone effluent

3.10.1 Monopolar configuration

The laboratory scale experimental setup used for the electrocoagulation studies is shown in Figure 3.3. The setup consists of an electrochemical cell; regulated multi output DC power supply, peristaltic pump, and reservoir with the volume of 1L. A schematic view of monopolar cell configuration is shown in Figure 3.4. Two aluminum electrodes were used as anode and cathode. PVC frame is used as a middle compartment. The effective surface area of anode and cathode were 7cmÃ-7cm. The electrodes were positioned vertically and parallel to each other with an inter electrode gap of 10mm. Electrolysis was carried out under batch recirculation mode. The effluent was taken in the reservoir, which was allowed to flow from it and was re circulated through the reactor using a peristaltic pump. The specified flow rate was adjusted. The required current was then passed using regulated power supply and cell voltage was noted. The experiments were carried out for six hours at different operating parameters. A one milliliter sample was drawn for every hour from the reservoir and COD was analyzed.

Figure 3.3: Experimental setup of flow reactor

Figure 3.4: Schematic view of monopolar electrocoagulation flow cell configuration

Table 3.7: Experimental range and levels of independent process variables for both monopolar and bipolar configuration for electrocoagulation

Factor

Variable

Unit

Range and levels

-1

0

+1

A

Hydroquinone concentration

mg/L

250

625

1000

B

Supporting electrolyte concentration

g/L

1.00

2.50

4.00

C

Flow rate, Q

mL/min

25

35

45

D

Current density, CD

Adm-2

0.20

0.60

1.00

3.10.2 Bipolar configuration

The setup (Figure 3.4) consists of electrochemical cell; regulated multi output DC power supply, peristaltic pump, and reservoir. A schematic view of bipolar cell configuration is shown in Figure 3.5. The bipolar cell consists of 2 compartments A and B with an inlet and outlet, respectively in A and B impart. Here, 4 aluminum electrodes were used with the dimensions of 9cmÃ-9cmÃ-0.1 cm. The working area of each of the electrode surface area is 7cmÃ-7cm. The assembly of the cell is in following manner. Two compartments were adjacent to each other; among the 4 electrodes, 2 were placed between the interface of 2 compartments, and another 2 electrodes were placed in the outer face of the compartments A and B, respectively. The volume of the electrolytic cell is 50mL whereas the reservoir capacity is 1L.

Figure 3.5: Schematic view of bipolar electrocoagulation flow cell configuration

Electrolysis was carried out under batch continuous recirculation mode. The cyclic pathway of the effluent is as follows, the effluent is pumped by peristaltic pump into compartment A. Then it undergoes upward travels over the working area of the electrode, outlet is passed into the bottom of the compartment B, there it gets upward travels over the working area of the electrodes. After this course, it returns to the reservoir tank. The required current is passed using regulated power supply and cell voltage is noted. The experiments are carried out for six hours at different operating parameters. Every hour the sample is collected from the reservoir and COD is analyzed.

3.10.3 Reaction mechanism of electrocoagulation

Electrocoagulation is based on the in situ formation of the coagulant as the sacrificial anode corrodes due to an applied current, while the simultaneous evolution of hydrogen at the cathode allows for pollutant removal by flotation. This technique combines three main interdependent processes, operating synergistically to remove pollutants: electrochemistry, coagulation and hydrodynamics.

In an electrocoagulation process, the coagulating ions are produced in situ involving three successive stages: (i) formation of coagulants by electrolytic oxidation of the "sacrificial electrode", such as aluminum, (ii) destabilization of the contaminants, particulate suspension and breaking of emulsions, (iii) aggregation of the destabilized phases to form flocs. Al gets dissolved from the anode generating corresponding metal ions, which almost immediately hydrolyze to polymeric aluminum xyhydroxides. These polymeric oxyhydroxides are excellent coagulating agents. Figure 3.6 shows a conceptual diagram of the electrocoagulation mechanism. The anodic reaction involves the dissolution of metal, and the cathodic reaction involves the formation of hydrogen gas and hydroxide ions.

At the Anode:

Al(s) → Al3+ + 3e− -------------------------------- (3)

At the Cathode

3H2O + 3e− → (3/2) H2 (g) + 3OH- -------------------- (4)

In bulk solution:

Al3+ (aq) + 3H2O → Al (OH) 3+(s) + 3H+ -------------------(5)

The other reactions that may be encountered in anodic compartment are

2OH− → 1/2O2 + H2O + 2e− ------------------- (6)

2Cl− → Cl2 + 2e− ------------------- (7)

Cl− + 2OH− → OCl− + H2O + 2e− ------------------- (8)

The main reaction encountered in the cathodic compartment is as follows:

2H2O + 2e− → H2 + 2OH− --------------------------- (9)

Al3+ and OH− ions generated via electrode reactions (1) and (2) react to form various monomeric species, such as Al(OH)2+, Al(OH)2+, Al2(OH)24+, Al(OH)4−; and polymeric species, such as Al6(OH)153+, Al7(OH)174+, Al8(OH)204+, Al13O4(OH)247+, Al13(OH)345+ which transform finally into Al(OH)3+(s), which are partly soluble in the water under definite pH values. This step results in the formation of colloidal particles and its structures is

------ (10)

Al (OH) 3+(s) form the nucleus of a colloidal particle. Around the nucleus, the adsorption layer of cations and anions is being organized. The nucleus and adsorption layer form a granule of the colloidal particle, which has a small positive charge. To compensate the charge, a diffusion layer is being formed around the granule, which makes the particle a neutral one. Aluminum hydroxides that are formed in the process of the electrocoagulation possess very high ability for absorption. Coagulated particles attract and absorb different ions and micro-colloidal particles from the wastewater. The flocks formed in the water are transported to the surface by the bubbles of gases (H2 and O2, etc.) produced in the process of electrolysis. As the solution contains Cl− ions, Cl2 will be liberated on the anode. According to following complex precipitation kinetics, Al (OH)3+(s) adsorb the organic particles from the wastewater by electrocoagulation process and reduce COD in wastewater. In surface complexation mode, the pollutant acts as a ligand (LD) to chemically bind hydrous aluminum:

LD − H (aq)(OH)OAl(s) → LD・ OAl(s) + H2O -----------(11)

The prehydrolysis of Al3+ cations also leads to the formation of reactive clusters for treatment. The contribution of separation by agglomeration of the particles by precipitation mechanism is predominant when pH is low, while adsorption plays a major role of separation mechanism in the case of operation at neutral to higher pH levels. The strong oxidizing agents produced in situ can take part in destructing the organic matter present in the waste by oxidizing them:

2Cl− → Cl2 + 2e− --------------- (7)

Cl2 + H2O → H+ + Cl− + HOCl -------------- (12)

HOCl ↔ H+ + OCl− -------------- (13)

HQ + OCl− → CO2 + H2O + Cl− --------------------- (14)

In cathode the following reactions takes place

2H3O+ + 2e− → H2 + 2H2O pH > 7 -------------------- (15)

2H2O + 2e− → H2 + 2OH− pH < 7 -------------------- (16)

The specific energy consumption (E) for flow reactors is obtained using the following expressions.

-------------------------------- (17)

ΔC is the difference in COD in mg/L, due to the treatment where I is the current for t seconds. V is the voltage (V), Q represents the volumetric flow rate in L/s.

Figure 3.6: A conceptual diagram of the electrocoagulation mechanism

3.10.4 Experimental design and optimization

Hydroquinone concentration, supporting electrolyte concentration, flow rate, current density were chosen as independent variables and the percentage of COD removal, energy consumption as dependent output response variables. Independent variables, experimental range and levels for COD removal are given in Table 3.7. The behavior of the system is explained by the following quadratic equation

------------- (18)

Where η is the response, xi and xj are coded independent variables (i=1 to k), β0 is the constant coefficient, βi, βii and βij (i and j=1 to k) are the regression coefficients for the intercept, linear, quadratic and the interaction terms, respectively, k is the number of independent parameters and ε is the statistical error. The results of the experimental design were studied and interpreted by Design Expert Software-7.1.2, Stat-Ease, Inc., Minneapolis, USA, to estimate the response of the dependent variable.

3.11 2,4-Dichlorophenol effluent

3.11.1 Preparation of the effluent

The synthetic effluent was prepared by dissolving an appropriate amount of 2,4-Dichlorophenol in water. The mixture was then kept at dark place in a closed container for 24 to 48hours. The resulting clear liquid was used for the analysis. Since the electrical conductivity of this solution was very low, Nacl was used as a mediator to increase the electrical conductivity. The effluent was characterized before and after the combined electrochemical oxidation and biological oxidation treatment, mainly in terms of COD which according to the American Public Health Association (APHA) are the standard methods for the examination of water and wastewater methods.

3.11.2 Microorganism preparation

The bacterial culture of Pseudomonas aeruginosa isolated from the paper mill wastewater was obtained from Microbiology laboratory Bharathiyar University, Coimbatore was used in the experimental studies. Nutrient medium (5g/L peptone, 3g/L Beef extract) had been taken and it was dissolved in distilled water followed by sterilization of medium in autoclave at 1210C for 15minutes at 15lb/inch2. The mother culture (Pseudomonas aeruginosa) had been taken for inoculation by using sterilized loop from the incubated Petri plate. After inoculation, the subculture was maintained for 24hours at 350C in an orbital shaker for mixing.

3.11.3 Electrochemical oxidation system

A batch electrolytic cell with recirculation (Schematic Figure 3.7 and Experimental setup 3.11) was used for the electrochemical oxidation process. The experimental setup consists of an undivided electrolytic cell of 300mL working capacity, closed with a PVC lid having provisions to fix a cathode and an anode keeping 2.5cm inter-electrode distance. The electrode used was noble metal oxide MOX (RuO2) coated on Ti as anode, an expanded mesh (of area 39.2cm2) was employed and a stainless steel plate (of dimension 8.0cmÃ-8.0cmÃ-0.2cm) was used as the cathode. A multi-output 2A and 30V (DC regulated) power source (with ammeter and voltmeter) was connected to the cell. Recirculation through electrochemical oxidation system was done with centrifugal pump and the flow rate measured by rotameter. The electrolyte taken was synthetic effluent containing 2,4-Dichlorophenol in water.

3.11.4 Experimental procedure

Electrochemical oxidation process carried out at room temperature using a colorless synthetic effluent, containing 2,4-Dichlorophenol with about 625 to 690mg/L of COD. A known quantity (2 to 6L) of effluent has been taken in electrooxidation reactor and subjected to input of electricity (current density 1 to 5Adm-2), and the flow rate of 10 to 65L/h as demanded by Box-Behnken method in the first step of operation to improve biodegradability. The volume of the reactor cell is 0.098dm3. The effluent treated in this reactor cell is re-circulated to the reservoir. In electrolysis, NaCl has been added to the effluent prior to electrolysis as a supporting electrolyte with the concentration of 2g/L. The concentration of the reactant (COD) and products in the batch reactor are function of time. The electrolysis may be carried out in any of the two modes- galvanostatic and potentiostatic, which are constant current and constant electrode potential process respectively. Galvanostatic process may be followed in industries using rectifiers. After electrolysis, to subside all the chemical reactions, the content of the reactor was kept idle for 12hours. The effluent from a well-mixed reservoir is rapidly re-circulated through the reactor.

Table 3.8: Experimental range and levels of independent process variables for the effluent Dichlorophenol electroconfiguration

Factor

Variable

Unit

Range and levels

-1

0

+1

A

Flow rate

L/h

10

35

60

B

Current density

Adm-2

1.00

3.00

5.00

C

Volume of reactant

L

2.00

4.00

6.00

D

Time of reaction

h

1.00

1.50

2.00

In the second step, biochemical oxidation experiment has been started with known inoculum concentrations and nutrients. A known volume (50mL/L, 75mL/L, 100mL/L) of pure Pseudomonas aeruginosa bacterial broth culture and known volume of plain nutrient broth (0mL/L, 25mL/L, 50mL/L) is taken in nine different proportions and was added into 1000mL capacity beaker containing a 500mL 2,4-Dichlorophenol effluent (electrochemically pre-treated). Further the beaker being aerated with an aqua pump aerator for a period of 5days in aerobic condition.

In the third step of operation, the beaker is (added with 25mL/L of pure bacterial broth culture if required) subjected to anoxic degradation for 5days in static condition. A sample of 5mL has been collected at the initial stage and at the end of electro-oxidation experiments and subjected to COD analysis. In bio-oxidation studies, 5mL of sample has been taken for every 24hours and subjected to COD and BOD5 analysis. In biochemical operation 0.1mL of sample has been collected from the effluent every day and spread on agar plates to ensure the presence of microbes.

1. Reservoir 2. Centrifugal Pump

3. Rota meter 4. Voltmeter

5. Ammeter 6. Power supply

7. Cathode (stainless steel) 8. Anode (Ti/RuO2)

Figure 3.7: Schematic representation of Electrochemical Oxidation System

ime1

Figure 3.8: Electrochemical reactor

Figure 3.9: Stainless steel cathode

Figure 3.10: Ru-TiO2 anode

Figure 3.11: Experimental setup of electrochemical reactor (2,4-Dichlorophenol)

3.11.5 Biochemical oxidation

All the bio-degradation experiments have been carried out at room temperature in batch mode. The experiment was carried out in a batch setup of 1000mL capacity beaker (with working capacity of 650mL). The total volume of 1000mL capacity of batch setup was being aerated with an aqua pump aerator for a period of 10days. The samples were being collected for every 24hours and it was subjected to COD and BOD5 analysis. Anoxic biochemical oxidation process is carried out using aerobically treated effluent. Beaker containing aerobically treated effluent is (added with 25mL/L of pure bacterial broth culture if required) subjected to anoxic degradation for 5days in static condition. Anoxic conditions will occur if the rate of oxidation of organic matter by bacteria is greater than the supply of dissolved oxygen. When the sampling is done the flasks were agitated. Only to restrict the oxygen supply agitation was not done during the anoxic degradation stage for 5days. The samples were being collected for every 24hours and were subjected to COD and BOD5 analysis.

3.12 para nitrophenol effluent

3.12.1 Preparation of effluent

The synthetic effluent was prepared by dissolving an appropriate amount of para nitrophenol in water. The mixture was then kept at dark place in a closed container for 24 to 48hours. Clear liquid results which were used for the analysis. The electrical conductivity of this solution was very low, hence a mediator was used to increase the electrical conductivity. In this present study NaCl was used as the mediator. The effluent was characterized before and after the combined electrochemical oxidation and biological oxidation treatment, mainly in terms of COD.

3.12.2 Microorganism preparation

The bacterial culture of Pseudomonas aeruginosa isolated from paper mill wastewater obtained from Microbiology laboratory Bharathiyar University, Coimbatore was used in the experimental studies. Nutrient medium (5g/L peptone, 3g/L Beef extract) had been taken and it was dissolved in distilled water followed by sterilization of medium in autoclave at 1210C for 1minute at 15lb/inch2. The mother culture (Pseudomonas aeruginosa) had been taken for inoculation by using sterilized loop from the incubated Petri plate. After inoculation, the subculture was maintained for 24hours at 350C in an orbital shaker for mixing.

3.12.3 Electrochemical oxidation system

A batch electrolytic cell with recirculation (Schematic Figure 3.12 and Experimental Figure 3.13) was used for the electrochemical oxidation process. The experimental setup consists of an undivided electrolytic cell of 300mL working capacity, closed with a PVC lid having provisions to fix a cathode and an anode keeping 2.5cm inter-electrode distance. The electrode used was lead oxide (PbO2) as anode, an expanded mesh (of area 39.2cm2) was employed and a stainless steel plate (of dimension 8.0cmÃ-8.0cmÃ-0.2cm) was used as the cathode. A multi-output 2A and 30V (direct current) power source (with ammeter and voltmeter) was connected to the cell. Recirculation through electrochemical oxidation system was done with centrifugal pump and the flow rate was measured by rotameter. The electrolyte taken was synthetic effluent containing para nitrophenol in water.

3.12.4 Experimental Procedure

Electrochemical oxidation process was carried out at room temperature using a colorless synthetic effluent, para nitrophenol with about 2500 to 2600mg/L of COD. A known quantity (2 to 6L) of effluent has been taken in electrooxidation reactor and subjected to input of electricity (current density 1 to 5Adm-2), and the flow rate of 10 to 65L/h as demanded by Box-Behnken method in the first step of operation to improve biodegradability. The volume of the reactor cell was 0.098dm3. The effluent treated in this reactor cell is re-circulated to the reservoir. In electrolysis, NaCl has been added to the effluent prior to electrolysis as a supporting electrolyte with the concentration of 2g/L. The concentration of the reactant (COD) and products in the batch reactor are function of time. The electrolysis may be carried out in any of the two modes; galvanostatic and potentiostatic, which are constant current and constant electrode potential process respectively. Galvanostatic process may be followed in industries using rectifiers. After electrolysis, to subside all the chemical reactions, the content of the reactor was kept idle for 12hours. The effluent from a well-mixed reservoir is rapidly re-circulated through the reactor.

Table 3.9: Experimental range and levels of independent process variables for para nitrophenol effluent electroconfiguration

Factor

Variable

Unit

Range and levels

-1

0

+1

A

Flow rate

L/h

10

35

60

B

Current density

Adm-2

1.00

3.00

5.00

C

Volume of reactant

L

2.00

4.00

6.00

D

Time of reaction

h

1.00

1.50

2.00

In the second step, biochemical oxidation experiment has been started with known inoculum concentrations and nutrients. A known volume (50mL/L, 75mL/L, 100mL/L) of pure Pseudomonas aeruginosa bacterial broth culture and known volume of plain nutrient broth (0mL/L, 25mL/L, 50mL/L) is taken in nine different proportions and was added into 1000mL capacity beaker containing a 500mL para nitrophenol effluent (electrochemically pre-treated). Further the beaker being aerated with an aqua pump aerator for a period of 5days in aerobic condition.

In the third step of operation, the beaker is (added with 25mL/L of pure bacterial broth culture if required) subjected to anoxic degradation for 5days in static condition. The sample of 5mL has been collected at the initial stage and at the end of electrooxidation experiments and subjected to COD analysis. In bio-oxidation studies, 5mL of sample has been taken for every 24hours and subjected to COD and BOD5 analysis. In biochemical operation 0.1mL of sample has been collected from the effluent every day and spread on agar plates to ensure the presence of microbes.

1. Reservoir 2. Centrifugal Pump

3. Rota meter 4. Voltmeter

5. Ammeter 6. Power supply

7. Cathode (stainless steel) 8. Anode (PbO2)

Figure 3.12: Schematic representation of electrochemical oxidation system

Figure 3.13: Experimental setup of electrochemical reactor (para nitrophenol)

3.12.5 Biochemical oxidation

All the bio-degradation experiments were carried out at room temperature in batch mode. The experiment was carried out in a batch setup of 1000mL capacity beaker (with working capacity of 650mL). The total volume of 1000mL capacity of batch setup was being aerated with an aqua pump aerator for a period of 10days. The samples were being collected for every 24hours and were subjected to COD and BOD5 analysis. Anoxic biochemical oxidation process is carried out using aerobically treated effluent. Beaker containing aerobically treated effluent is (added with 25mL/L of pure bacterial broth culture if required) subjected to anoxic degradation for 5days in static condition. Anoxic conditions will occur if the rate of oxidation of organic matter by bacteria is greater than the supply of dissolved oxygen. When the sampling was done the flasks were agitated. Only to restrict the oxygen supply agitation was not done during the anoxic degradation stage for 5days. The samples were being collected every 24hours and it was subjected to COD and BOD5 analysis.

3.13 Miscellaneous Equipments

Peristaltic pump: A peristaltic pump is a type of positive displacement pump used for pumping an effluent. The effluent is contained within a flexible tube fitted inside a circular pump casing. A rotor with a number of "rollers", "shoes" or "wipers" attached to the external circumference compresses the flexible tube. As the rotor turns, the part of tube under compression closes (or "occludes") thus forcing the effluent to be pumped to move through the tube. Additionally, as the tube opens to its natural state after the passing of the cam ("restitution" or "resilience") effluent flow is induced to the pump.

fig12

Figure 3.14: Peristaltic pump

pH Meter: The pH of the solution was varied by adding HCl or NaOH solution as per requirement and measured using ELICO, India, model no.LI 120 pH meter.

Autoclave: An autoclave is a device to sterilize equipment and supplies by subjecting them to high pressure steam at 121°C or more, typically for 15 to 20minutes depending on the size of the load and the contents. Typical loads include glassware, medical waste, utensils, animal cage bedding and lysogeny broth.

E:\Pictures\DP Experimental Setup\IMG_0503.JPG

Figure 3.15: BOD (Incubator)

E:\Pictures\DP Experimental Setup\IMG_0507.JPG

Figure 3.16: COD (reflux method, water condenser)

E:\Pictures\DP Experimental Setup\IMG_0519.JPG

Figure 3.17: pH meter

E:\Pictures\DP Experimental Setup\IMG_0527.JPG

Figure 3.18: Experimental setup of electrochemical reactor

E:\Pictures\DP Experimental Setup\IMG_0522.JPG

Figure 3.19: U-V Spectrometer

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