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Electrochemical treatment of rice grain-based distillery effluent: chemical oxygen demand and colour removal a
Abhinesh Kumar Prajapati & Parmesh Kumar Chaudhari
a
a
Department of Chemical Engineering, National Institute of Technology Raipur, Raipur 492001, India Published online: 25 Aug 2013.
To cite this article: Environmental Technology (2013): Electrochemical treatment of rice grain-based distillery effluent: chemical oxygen demand and colour removal, Environmental Technology, DOI: 10.1080/09593330.2013.824507 To link to this article: http://dx.doi.org/10.1080/09593330.2013.824507
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Environmental Technology, 2013 http://dx.doi.org/10.1080/09593330.2013.824507
Electrochemical treatment of rice grain-based distillery effluent: chemical oxygen demand and colour removal Abhinesh Kumar Prajapati∗ and Parmesh Kumar Chaudhari Department of Chemical Engineering, National Institute of Technology Raipur, Raipur 492001, India
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(Received 6 October 2012; final version received 25 June 2013 ) The electrochemical (EC) treatment of rice grain-based distillery wastewater was carried out in a 1.5 dm3 electrolytic batch reactor using aluminium plate electrodes. With the four-plate configurations, a current density (j) of 89.3 A/m2 and pH 8 was found to be optimal, obtaining a maximum chemical oxygen demand (COD) and colour removal of 93% and 87%, respectively. The chemical dissolution of aluminium was strongly influenced by initial pH (pHi ). At higher pHi (pH 9.5) anode consumption decreased while energy consumption increased. At the optimal current density 89.3 A/m2 , the aluminium electrode consumption was 16.855 g/dm3 wastewater and energy consumption was 31.4 Wh/dm3 achieving a maximum COD removal of 87%. The settling and filterability characteristics of electrochemically treated sludge were also analysed at different pH. It was noted that treated slurry at pHi 9.5 gave best settling characteristic, which decreased with increase in pH. EC-treated effluent at pHi 8 had provided best filterability. Characteristics of scum and residues are also analysed at different pH. Keywords: rice grain-based biodigester effluent; chemical oxygen demand; colour reduction; energy consumption; filterability
Introduction In India, alcohol distillery is a main agro-based industry. About 61% ethanol is produced from the sugar cane molasses. Ethanol may also be produced from straight sugarcane juice, sugar beets, starch crops (corn, wheat, rice, cassava, etc.), and dairy products (whey). India is the largest rice-producing country, but still, around 10% ethanol is produced from the rice grain, whereas enormous amount of poor quality rice grain may also be used as raw material for ethanol production. Therefore, rice grain can be a major supplement raw material at the place of sugar cane molasses. From the distilleries high volume of wastewater is generated and is highly polluted. Rice grain-based distillery provides less harmful wastewater in comparison with sugar cane molasses-based distilleries. Ethanol is produced by fermentation of raw material in fermentation broth. The liquor, after fermentation contains around 7–12% ethanol, which is separated in a series of distillation columns. The ethanol is recovered from the condensation system at top and the hot liquid coming out from the bottom of the distillation column is a waste. This hot liquid is commonly known as spent wash (SW) distillery wastewater and vinasses depends on its organic load. SW is dark brown in colour, acidic, and toxic. The rice grain-based SW is putrifiable because of high concentration of organics (COD = 35–50 kg/m3 , BOD = 10–20 kg/m3 ). The SW is first sent to the biodigester for anaerobic ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
biological treatment. Biodigester can reduce 50–70% chemical oxygen demand (COD) and 70–90% Biochemical oxygen demand (BOD) from the SW. The untreated effluent comes out of biodigester is commonly known as biodigester effluent (BDE). The BDE still contains high COD (10,000–15,000 mg/dm3 ) and BOD (2500–3000 mg/dm3 ) and is dark brown in colour. Due to these reasons, extensive treatment of BDE is essential. The common process used to treat BDE is biological aerobic treatment. In this process, submersed air bubble or surface aerator is used. This process is not cost intensive and also is able to reduce COD up to 70% and BOD up to 80%. Thus, 3000–5000 mg/dm3 COD and 500–600 mg/dm3 BOD still remains. The Central Pollution Control Board (CPCB) of India has fixed standard for the distillery units to meet the effluent discharge quality, for release of the wastewater into surface waters (COD < 0.1 kg/m3 , BOD < 0.03 kg/m3 ) and sewers (COD < 0.3 kg/m3 , BOD < 0.1 kg/m3 ).[1] Therefore, comprehensive strategy is required for maintaining desired standard of the effluents. Nowadays, most units use a membrane separation process, which is effective but cost of operation and installation cost is too high. Several physico-chemical processes have been proposed to treat distillery effluent such as coagulation with chemical coagulants such as alum, ferric chloride, poly aluminium chloride, and natural coagulant like Moringa Oleifera,[2,3] thermolysis with catalyst and
2
A.K. Prajapati and P.K. Chaudhari Table 1.
Studies to treat distillery effluents by the EC process.
Investigator
Electrode
Effluent
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Prasad and Srivastava [9] Ruthenium oxide-coated titanium mesh Sugar cane molasses SW acting as anode and stainless steel as cathode Kumar et al.[10] Stainless steel Sugar cane molasses BDE Thakur et al.[11] Iron Sugar cane molasses BDE Manisankar et al.[12] Graphite electrodes Sugar cane molasses SW Yavuz [13] Lead dioxide coated on titanium Sugar cane molasses SW Present studies Aluminium Rice grain-based BDE
without catalyst,[4–6] wet oxidation,[7,8] etc. These processes have several drawbacks. The Coagulation process needs large amount of chemicals and generates sludge which has disposal problems. Thermolysis and wet oxidation processes need rigorous operating condition and also they are not cost intensive. Electrocoagulation as a treatment method provides better result as compared with the traditional coagulation/ flocculation method due to less sludge generation and better colour and COD removal. Prasad and Srivastava [9] reported the optimal removal of colour of 83.31% and COD of 39.66% with ruthenium oxide-coated titanium mesh acting as anode and stainless steel as cathode at the pH of 5.5. A 50.5% COD reduction and 95.5% colour reduction were obtained using an iron electrode at their optimum pH.[10] Thakur et al. reported 61.6% COD reduction and 98.4% colour reduction with the iron electrode at their optimum pH.[11] Comparative studies for treatment of distillery effluents have been presented in Table 1. From the table, it can be seen our results are better to other investigators. The aim of this study is to derive experimental data on EC treatment of BDE of the rice grain-based distillery at the different conditions. The energy consumption and electrode loss have been determined at different pH. The settling and filterability characteristics of the treated BDE have also been presented in the paper. The scum and residues characteristic of treated BDE are also analysed.
Experimental section Effluent and its characterization The BDE used in this study was obtained from Chhattisgarh Distillery Pvt. Ltd. Kumhari, Chhattisgarh. To maintain constant characteristics of BDE, the sample was stored at 4◦ C in a deep freezer. Aluminium plate was used as electrode. Reactor was made up of Perspex glass. The effluent was characterized for various parameters, namely COD, colour, total solids, total dissolved solids, total suspended solid reduced carbohydrate, sulphate, chloride, etc., as per standard method of analysis.[14] Furthermore, the COD and colour of treated effluent was also analysed. The main characteristic of the original and treated effluent used for this study is given in Table 2.
% COD removal % colour removal 39.66
83.39
50.5 61.4 80.6 90.8 93
95.2 98.4 95.6 98.5 87
Table 2. Typical composition of BDE before and after treatment by EC at current density = 89.3 A/m2 , g = 20 mm. Parameters COD TDS TSS TS Reduced carbohydrate Protein Chlorine Phosphate Total hardness Sulphate pH Colour Absorbence at wavelength = 475 nm Colour (PCU)
BDE 13,800 46,245 40,000 86,245 416 185 124 0.05 9000 4920 7.8 Blackish brown
EC-treated BDE at optimum condition
0.932
966 1700 5231 6931 Not found 77 50 Nil 110 526 8.0 Light yellow (transparent) 0.149
450
72
Note: All values in mg/dm3 except pH and colour. PCU, platinum cobalt unit.
Experimental methods The lab-scale batch experimental setup used for the EC studies is shown in Figure 1 and characteristics of EC cell is presented in Table 3. The 1.4 dm3 BDE was taken in a 1.5 dm3 reactor, made up of Perspex glass. To maintain the pH of BDE, H2 SO4 (2 M) and NaOH (2 M) was used. Four electrodes of aluminium plates 2 mm thickness were used for the experiment. Electrodes shapes were rectangular and dimensions of each electrode were 8 cm × 7 cm. The area of electrodes dipped into the BDE was 8 cm × 7 cm. Therefore, total effective surface area of each electrode was 56 cm2 . A 1.5 cm gap was provided between the bottom of the electrodes and the bottom of the reactor for easy stirring. The gap between two electrodes in EC reactor was varied from 1.5 to 2.5 cm. Magnetic stirrer was used for proper mixing of BDE sample. The electrodes were cleaned with 10% diluted HCl solution before each run. Following each run, the electrodes were washed with distilled water, dried at room temperature and then weighted. After this it was reused for the next experiment. The current density was adjusted constant by means of a digital direct current (DC)
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Environmental Technology
Figure 1. Table 3.
3
Schematic diagram of the experimental setup used for the electrocoagulation study. Characteristics of the EC cell.
Electrodes Materials (anode and cathode) Shape Size Thickness (mm) Plate arrangement Effective electrode surface area (cm2 ) Reactor characteristics Make Reactor type Dimensions (cm) volume (dm3 ) Electrode gap (mm) Stirring mechanism Power supply Voltage range (V) Current range (A)
Aluminium Rectangular plate 8 cm × 7 cm 2 Parallel 56 Perspex glass Batch mode 10.7 × 10.7 × 13.7 1.5 20 Magnetic bar DC 0–30 0–5
power supply (0–30 V, 0–5 A). All experiments were done at a room temperature (30◦ C) and atmospheric pressure (101.1 kPa). The current density was controlled by using regulator. At the end of the experiments, sample was filtered with Whatman (41 size) filter paper to remove sludge. The filtered liquid was used for analysis of parameters (COD and colour). Settling and filterability studies were also performed in a 0.5 dm3 measuring cylinder.
Results and discussion The BDE of rice grain-based distillery contains COD and BOD material. The COD and BOD are due to the presence of various organics. Melonidian provides dark brown colour and passes net negative charge.[2] The BDE also contains colloidal particles. The colloidal surfaces have negative charges which take part in the electrochemical (EC) process. When potential is applied through external
DC power surface across the electrode, Al3+ ions released from anode which gets attached to the negative ions of BDE, causing small colloids to convert into the heavy flocks. It is similar to the coagulation process where aluminium salt (such as alum and AlCl3 ) releases Al3+ ions. When electro generated Al cations are liberated in solution it hydrolyses to form monomeric and polymeric species: Al(OH)+ , Al 5+ − 0 OH2+ , Al2 (OH)4+ 2 , Al(OH)4 , Al(OH)2 (s), and Al(OH)4 , etc.[15] The general equation is xM3+ + yH2 O = Mx (OH)y(3x−y)+ + yH+ .
(1)
The colloidal particles have net negative charges, which gets entrapped in amorphous ions hydroxides and gets neutralized. The neutralized mass, as well as Al(OH)3 formed, promotes sweep flocculation. Effect of pH The pH of the solution plays a significant role in influencing the EC treatment. The effect of pH in the range of 3.5–9.5 was studied. The effect of electrolysis time on COD and colour reduction was also observed and the results are presented in Figure 2(a) and 2(b). It can be seen that 78%, 80%, 81%, 88%, and 82% COD reduction is obtained at pH of 3.5, 5, 6.5, 8, and 9.5, respectively, in 20 min which increases to 89%, 90%, 91%, 93%, and 91% in 120 min. The colour reductions of 75%, 76%, 73%, 77%, and 65% are obtained at pH of 3.5, 5, 6.5, 8, and 9.5, respectively, in 20 min which enhance to 84%, 85%, 83%, 87%, and 76%. The results reflect that about 10% COD and colour reduction increase with increase in treatment time to 100 min. The working mechanism of EC treatment using aluminium electrodes is similar to that of chemical coagulation using Alum or AlCl3 as a coagulant. The COD and colour reduction at different pH depend on types of metal hydroxide formed and behaviours of organics contained in the wastewater.
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A.K. Prajapati and P.K. Chaudhari
Figure 2. Effect of pH on: (a) percentage COD reduction, (b) colour reduction, (c) electrode energy consumption, and (d) anode consumption (CODi = 13,800 mg/dm3 , colour i = 450 PCU, j = 89.3 A/m2 , g = 20 mm).
It can be seen that pH 8 provides best result during EC treatment. The 93% COD reduction and 87% colour reduction are achieved in 120 min. Since the BDE has pH 8, thus, no pH adjustment is needed for treatment. For the optimum design of EC treatment, energy consumption has also been determined. It was calculated with the help of the following equation [10]: Energy consumption (Wh/dm3 ) =
VIt
, Treated volume (dm3 ) (2)
where V is the cell voltage in volt, I is the current in ampere (A), and t is the electrolysis time (h). The electrical energy consumption during the EC of BDE was determined at different pH (3.5–9.5) current density
j = 89 A/m2 , electrode gap (g) = 20 mm, treatment time (tR = 120 min), and stirring speed = 130 rpm. The results are shown in Figure 2(c). It may be seen that energy consumption increases with increase in pH. The energy consumption of 17.4, 22.8, 24.3, 31.4, and 48.6 Wh/dm3 BDE is noted at pH 3.5, 5, 6.5, 8, and 9.5, respectively. It has been reported that the aluminium electrode consumes more energy in comparison with the iron electrode. This may be due to large amount of flocks adheres to the surface of the aluminium electrode which further works as an insulator causing an increase in voltage. The anode consumption at different pH was also calculated. Two anodes and two cathodes were used in the experiment. The values are shown in Figure 2(d) at similar operating condition and pH 3.5, 5, 6.5, 8, and 9.5. At these pH anodes consume to 1.67, 1.0154, 1.3763, 1.6855, and
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where K is the cell specific conductance and A is electrode surface area. When the value of g increases after the certain limit the aluminium oxidation rate decreases, therefore the COD and colour removal efficiency also decrease. In our studies, COD and colour reduction are found in order of electrode gap (g) 20 mm > 25 mm > 15 mm.
Figure 3. Effect of electrode gap on: (a) COD reduction and (b) colour reduction. (CODi = 13,800 mg/dm3 , colour i = 450 PCU, pH = 8, j = 89.3 A/m2 ).
1.0598 g/dm3 of BDE treated. It is observed that electrode consumption decreases from pH 3.5 to 5 and then increases from pH 5 to 8 but decreases from pH 8 to 9.5.
Effect of electrode gap and electrolysis time The effect of electrode gap (g) was studied by varying it 15–25 mm and keeping other parameters pH 8, j = 89.3 and stirrer speed 130 rpm. The result is presented in Figure 3(a) and 3(b). The COD reduction seems to list at g = 25 mm, crosses to g = 15 mm in 90 min with 89% COD reduction, which increase to 92% in 120 min. At g = 15 mm COD reduce to 91%. The highest COD reduction 93% obtained at g = 20 mm in 120 min of EC. The colour reduction of 65%, 66%, and 76% is obtained in 20 min, which increase to 80%, 82%, and 87% at g = 15, 25, 20 mm, respectively. The relation between the electrode gap and resistance is given by Faraday’s law. As the gap (g) between the anode and the cathode increases, resistance (R) offered by the cell also increases.[11] The relation is expressed as R=
g , KA
(3)
Effect of current density The current density (j) is an important parameter which highly influences to the treatment efficiency of the EC treatment processes. Therefore, the applied current density was varied to examine its effect on the COD and colour removal. An increase in current density from 44.6 to 178.5 A/m2 for a four-plate configuration causes an increase in voltage from 9.5 to 21.3 V. The effect of current density on COD and colour removal is presented in Figure 4(a) and 4(b). At current density 44.6, 89.3, 133.8 and 178.5 A/m2 , COD reduces to 85%, 88%, 89%, and 90% after 20 min. With further increase in electrolysis time up to 120 min, a COD reduction of 92%, 93%, 96%, and 97%, respectively, is achieved at their optimum pH 8. An increase in current density the COD removal efficiency increases because when current density increases more amount of Al3+ ions gets released and more aluminium hydroxide cations are formed. Which attaches with negative ions of BDE, thereby causing organic matter to convert in heavy flocks and ultimately it settles down due to its gravity. The colour removal efficiency is also increases with increase in current density. About 65%, 70%, 72%, and 74% of colour is removed after 20 min during the EC process which increases up to 80%, 84%, 83%, and 88%, respectively, after 120 min. It can be seen that the colour removal efficiency of BDE is poor compared with COD removal efficiency, this may be due to small colloidal particles still present in the treated BDE. The electrolysis time also plays a very significant role for the treatment of effluent. Amount of ions produced is the function of electrolysis time, i.e. amount of ions production increases with respect to electrolysis time. Therefore, increases in t, causes increase in concentration of aluminium ions and their hydroxide flocs. Consequently, when t is increased the COD and colour removal efficiency also increases, which can be seen from Figures 2–4. Settling characteristics The slurry obtained from EC treatment was subjected to sedimentation test. To determine the separation characteristic by settling, the treated BDE sample after the EC process was slowly mixed and taken in 0.5 dm3 cylinder having diameter 46 mm. The settling characteristics were performed at different pH. Figure 5(a) shows the time vs. dimension less height (H/Ho ) as a function of settling time. At the beginning, for a very short period, sludge is relatively slow settling, because of the Brownian motion of the particle, this is followed by steady-state decrease in the height
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Figure 5. Effect of pH on: (a) settling characteristic and (b) filterability characteristic of the EC-treated slurry (j = 89.3 A/m2 , g = 20 mm). Figure 4. Effect of current density (j) on: (a) COD, (b) the colour removal of BDE (CODi = 13,800 mg/dm3 , colour i = 450 PCU, g = 20 mm, pH = 8).
of the solid/liquid interface, known as the zone-settling region. Later on a compressed layer occur on the bottom of the cylinder known as compression settling zone. The settling rate is found in order of pH 9.5 > pH 6.5 > pH 8 > pH 5 > pH 3.5. It can be seen that the settling rate was found best at basic condition (pH 9.5) which decrease at the acidic condition (pH 3.5). This may be due to formation of heavy flocks at basic pH, which settle down. But the poor COD and colour removal limits the EC treatment at pH 9.5. At pH 6.5 and 8, 60% settling occurs in 40 min. The COD reduction of 93% is obtained at the pH 8, j = 89.3 A/m2 with 60% settling in 40 min. Due to this, pH 8 is optimum pH for the process. Method proposed by Richardson et al. [16] is mostly used in practice to design a continuous thickener, based on batch studies. Filterability characteristics To analyse the filtration characteristic of treated BDE, gravity filtration was conducted on Whatman filter paper (41 size) supported on Buchner funnel. The filtration experiment was conducted at atmospheric pressure and room
temperature (at 30◦ C and 101.3 kPa). At different times, different volumes were noted. Necessary parameters like filtration resistance of the filter media and filter cake resistance for the constant pressure filtration were evaluated using the following filtration equation [17]: dt = Kp V + β, dV cαμ kp = 2 , A (−p)
(4) (5)
and β=
μRm , A(−p)
(6)
where t is the time taken for filtration (s), V is the volume of filtrate collected in t time (m3 ), kp is the slope for the plot of Equation (4) (s/m6 ), β is the intercept for the plot of Equation (4) (s/m3 ), c is the concentration of sludge (kg/m3 ), α is the specific cake resistance (m/kg), μ is the viscosity of the filtrate (Pa s), A is the area of the filter media (m2 ), p is the pressure drop across the filter (Pa), and Rm is the filter medium resistance (m−1 ). After the experiments data are plotted as (dt/dV ) vs. V presented in Figure 5(b). The value of β (intercept) and kp
Environmental Technology Table 4.
Filterability of EC-treated wastewater at different pH.
pH
kp × 10−12 (s/m6 )
β × 10−6 (s/m3 )
C (kg/m3 )
α × 1013 (m/kg)
Rm × 109 (m−1 )
12.3 12.35 10.09 7.34 9.26
1.82 96.37 17.57 12.76 17.64
21.85 18.46 9.08 50.32 24.67
8.41 9.95 16.6 2.17 5.6
2.71 144 26.25 19.06 26.38
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pH 3.5 pH 5 pH 6.5 pH 8 pH 9.5
(slope) is calculated by best fit of experimental data. The data are presented in Table 4. The cake resistance is found in order of 16.6 > 9.95 > 8.41 > 5.6 > 2.17 × 1013 (m/kg) at the pH 6.5, 5, 3.5, 9.5, and 8, respectively. Therefore, it can be seen that EC-treated BDE at the pH 8 provides best filterability. Any change from pH 8 reduces the filterability of EC-treated BDE. The reason for variation in cake resistance is due to porosity of the cake which seemed to be highest at pH 8 and lowest at pH 6.5. The filter medium resistance is found to be in decreasing order 144, 26.38, 26.25, 19.06, and 2.71 × 109 m−1 at pH 5, 9.5, 6.5, 8, and 3, respectively. The filter medium resistance of clarifying filter plays a role during the early state of filtration. The approximated values of specific cake resistance for different sludges are also reported by Barnes et al. [18] these values are 3–30 × 1013 m/kg for the digester sludge and 4–12 × 1013 m/kg for the activated sludge. Cake resistance of EC-treated paper mill effluent has been reported by Mahesh et al. [19] The values were 3.242–6.89 × 1011 . Kumar et al. reported cake resistance 1.34–10.67 × 1010 m/kg of composite textile mill.[20] Our studies give results near to activated sludge.
Analysis of filtrate The EC-treated BDE was filtrated by Whatman filter paper (41 size) and then analysed. The results are presented in Table 2. The 93% COD and 85% colour have reduced at current density 89.3 A/m2 (2A), i.e. COD reduces to a value Table 5. Parameter
7
of 966 mg/dm3 from initial value of 13,800 mg/dm3 and colour reduces to 72 PCU from 450 PCU. After treatment many functional group and chemicals (organic and inorganic) such as reduced carbohydrate, proteins, chloride, sulphate, total hardness, and total solid are also satisfactory reduced from the BDE. The treatment at j = 178.5 A/m2 provided COD 414 mg/dm3 and colour reduction of 54 PCU (97% COD and 88% colour reduction) at the cost of high energy consumption. Optimum result is achieved at pH 8 (93% COD and 85% colour reduction) with current density 89.3 A/m2 at electrode distance 20 mm. From the results, it is clear that the treated effluent cannot be directly discharged in land or water body due to large amount of COD (966 mg/dm3 ) still present in the ECtreated BDE. Hence, further treatment is required for this EC-treated BDE. Many physico-chemical methods such as wet oxidation, adsorption, and membrane separation may be used as possible options to treat EC-treated BDE. Recently, many industries directly treat the BDE (COD 12,000–18,000 mg/dm3 ) by membrane separation, which utilizes high energy and higher capital cost. For the good results, BDE should first be treated by EC, which should be followed by membrane separation.
Analysis of scum and residue For the analysis of solid residues and scum, it was first separated from the treated BDE and later on dried it at 105◦ C,
Analysis of residue and foam obtained after EC treatment at different pH. At pH 3.5
At pH 5.0
At pH 6.5
At pH 8.0
At pH 9.5
Analysis of residue obtained after EC at different pH 8.53 36.7 28.12 55.63 39.2 Weight of residue (kg/m3 ) Colour White White White White White Nature Bulky mass, difficult Bulky mass, difficult Bulky mass, difficult Bulky mass, difficult Bulky mass, difficult to grinding to grinding to grinding to grinding to grinding Approximated 13 20 16 22 15 drying period % convertible COD 89 90 91 93 91 Analysis of foam obtained after EC at different pH Colour White White Nature Soft mass, easy Soft mass, easy to grinding to grinding Approximated 5 7 drying period (h) % convertible COD 89 90
White Soft mass, easy to grinding 9
White Soft mass, easy to grinding 8
White Soft mass, easy to grinding 7
91
93
91
8
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until its weight became constant. Characteristics of foam and solid residues were also determined. Data of analysis are presented in Table 5. It can be seen that the weight of residue and foam obtained at pH 8 have highest mass. Drying period lies between 13 and 22 h. Residues of ECtreated BDE are hard and difficult to grind, whereas scum has soft and easy grindable mater. Possibly these residues organics have good heating value as presented by various authors.[5,6,19,20] After incineration, the ash may be blended with soil to make bricks, which can be used as building material. The ash blended bricks will have good strength. Conclusions The EC process with aluminium as the sacrificial electrode had provided good results to treat BDE of a rice grain-based distillery. The COD reduction of 89%, 90%, 91%, 93%, and 91% and colour reduction of 85%, 86%, 83%, 84%, and 77% were obtained at the pH of 3.5, 5, 6.5, 8, and 9.5, respectively, and at current density 89.3 A/m2 . The COD and colour reduction were found to increase with increase in current density. The COD reduction of 92%, 93% 96%, and 97% and colour reduction of 80%, 85%, 83%, and 88% were noted at pH 8 and at current density 44.6, 89.3, 133.8, and 178.5 A/m2 . At pH 3.5, 5, 6.5, 8, and 9.5, the 17.4, 22.8, 24.3, 31.4, and 48.6 Wh energy have consumed for per dm3 treatment of BDE. The weight losses of electrodes obtained at these pH are 16.7, 10.15, 13.76, 16.855, and 10.6 g per dm3 BDE treatment. The settling characteristic of treated BDE was found to be best at pH 9.5, while filterability characteristic was best at the pH 8. The scum and residues obtained from the EC-treated BDE sample may have good heating values, therefore, it can be used as a fuel. COD and colour were not completely removed by the EC process. Therefore, post treatment by using aeration, wet oxidation, adsorption, coagulation, or membrane separation needs to reach discharge standard prescribed by pollution control agencies. Many distillery treat BDE directly by membrane separation process is not suitable because BDE contains high organic and inorganics, fouls the membrane and need high operation cost to treat it. The EC-treated BDE will be surely good feed for membrane separation process. Thus, the treatment of BDE by EC followed by membrane separation will be good combination to achieve zero discharge. Acknowledgements Authors gratefully acknowledge Chhattisgarh Council of Science and Technology (CGCOST), Raipur, India for providing research grants to support this work.
References [1] CPCB. Pollution Control Acts, rules and Notifications Issued Thereunder. Delhi, India: Central Pollution Control Board; 2006.
[2] Chaudhari PK, Mishra IM, Chand S. Decolorization and removal of chemical oxygen demand (COD) with energy recovery: treatment of biodigester effluent of a molassesbased alcohol distillery using inorganic coagulants. Colloids Surf A: Physicochem Eng Aspects. 2007;296:238–247. [3] Prasad RK. Color removal from distillery spent wash through coagulation using Moringa oleifera seeds: use of optimum response surface methodology. J Hazard Mater. 2009;165:804–811. [4] Chaudhari PK, Singh R, Mishra IM, Chand S. Kinetics of catalytic treatment (catalytic thermolysis) of distillery wastewater and bio-digester effluent of alcohol production plant at atmospheric pressure. Int J Chem Reactor Eng. 2010;8:1–22. [5] Chaudhari PK, Mishra IM, Chand S. Catalytic thermal treatment (catalytic thermolysis) of a biodigester effluent of an alcohol distillery. Plant Ind Eng Chem Res. 2005;44:5518– 5525. [6] Chudhari PK, Mishra IM, Chand S. Effluent treatment of alcohol distillery: catalytic thermal pretreatment (catalytic thermolysis) with energy recovery. Chem Eng J. 2008;136:14–24. [7] Belkacemi K, Larachi F, Hamoudi S, Turcotte G, Sayari A. Inhibition and deactivation effects in catalytic wet oxidation of high-strength alcohol-distillary liquors. Ind Eng Chem Res. 1999;38:2268–2274. [8] Belkacemi K, Larachi F, Hamoudi S, Sayari A. Catalytic wet oxidation of high strength alcohol-distillery liquors. Appl Catal A: General. 2000;199:199–209. [9] Prasad RK, Srivastava SN. Electrochemical degradation of distillery spent wash using catalytic anode: factorial design of experiments. Chem Eng J. 2009;146:22–29. [10] Kumar M, Infant F, Ponselvan A, Malviya JR, Srivastava VC, Mall ID. Treatment of bio-digester effluent by electrocoagulation using iron electrodes. J Hazard Mater. 2009;165:345–352. [11] Thakur C, Srivastava VC, Mall ID. Electrochemical treatment of a distillery wastewater: parametric and residue disposal study. Chem Eng J. 2009;148:496–505. [12] Manisankar P, Viswanathan S, Rani C. Electrochemical treatment of distillery effluent using catalytic anodes. Green Chem. 2003;5:270–274. [13] Yavuz Y. EC and EF processes for the treatment of alcohol distillery wastewater. Sep Purif Technol. 2007;53: 135–140. [14] Clesceri L, Greenberg AE, Trussell RR, editors. Standard methods for water and waste water examination. 17th ed. New York: APHA; 1989. [15] Ching HW, Tanaka TS, Limelech ME. Dynamics of coagulation of kaolin particles with ferric chloride. Water Res. 1994;28(3):559–569. [16] Richardson JF, Harker JH, Backhurst JR. Chemical engineering, particle technology & separation processes. Vol. 2, 5th ed. Chennai: Elsevier India Pvt. Ltd.; 2003. [17] MaCabe WL, Smith JC, Harriot P. Unit operations of chemical engineering, 6th ed. New York: McGraw Hill; 2001. [18] Barnes D, Bliss PJ, Gould BW, Vallentine HR. Water wastewater engineering systems. New York: Pitman Publishing Inc.; 1981. [19] Mahesh S, Prasad B, Mall ID, Mishra IM. Electrochemical degradation of pulp and paper mill wastewater. Part 2. Characterization and analysis of sludge. Ind Eng Chem Res. 2006;45:5766–5774. [20] Kumar P, Prasad B, Mishra IM, Chand S. Treatment of composite wastewater of a cotton textile mill by thermolysis and coagulation. J Hazard Mater. 2008;151:770–779.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Electrochemical treatment of rice grain-based distillery effluent: chemical oxygen demand and colour removal a
Abhinesh Kumar Prajapati & Parmesh Kumar Chaudhari
a
a
Department of Chemical Engineering, National Institute of Technology Raipur, Raipur 492001, India Published online: 25 Aug 2013.
To cite this article: Environmental Technology (2013): Electrochemical treatment of rice grain-based distillery effluent: chemical oxygen demand and colour removal, Environmental Technology, DOI: 10.1080/09593330.2013.824507 To link to this article: http://dx.doi.org/10.1080/09593330.2013.824507
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Environmental Technology, 2013 http://dx.doi.org/10.1080/09593330.2013.824507
Electrochemical treatment of rice grain-based distillery effluent: chemical oxygen demand and colour removal Abhinesh Kumar Prajapati∗ and Parmesh Kumar Chaudhari Department of Chemical Engineering, National Institute of Technology Raipur, Raipur 492001, India
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(Received 6 October 2012; final version received 25 June 2013 ) The electrochemical (EC) treatment of rice grain-based distillery wastewater was carried out in a 1.5 dm3 electrolytic batch reactor using aluminium plate electrodes. With the four-plate configurations, a current density (j) of 89.3 A/m2 and pH 8 was found to be optimal, obtaining a maximum chemical oxygen demand (COD) and colour removal of 93% and 87%, respectively. The chemical dissolution of aluminium was strongly influenced by initial pH (pHi ). At higher pHi (pH 9.5) anode consumption decreased while energy consumption increased. At the optimal current density 89.3 A/m2 , the aluminium electrode consumption was 16.855 g/dm3 wastewater and energy consumption was 31.4 Wh/dm3 achieving a maximum COD removal of 87%. The settling and filterability characteristics of electrochemically treated sludge were also analysed at different pH. It was noted that treated slurry at pHi 9.5 gave best settling characteristic, which decreased with increase in pH. EC-treated effluent at pHi 8 had provided best filterability. Characteristics of scum and residues are also analysed at different pH. Keywords: rice grain-based biodigester effluent; chemical oxygen demand; colour reduction; energy consumption; filterability
Introduction In India, alcohol distillery is a main agro-based industry. About 61% ethanol is produced from the sugar cane molasses. Ethanol may also be produced from straight sugarcane juice, sugar beets, starch crops (corn, wheat, rice, cassava, etc.), and dairy products (whey). India is the largest rice-producing country, but still, around 10% ethanol is produced from the rice grain, whereas enormous amount of poor quality rice grain may also be used as raw material for ethanol production. Therefore, rice grain can be a major supplement raw material at the place of sugar cane molasses. From the distilleries high volume of wastewater is generated and is highly polluted. Rice grain-based distillery provides less harmful wastewater in comparison with sugar cane molasses-based distilleries. Ethanol is produced by fermentation of raw material in fermentation broth. The liquor, after fermentation contains around 7–12% ethanol, which is separated in a series of distillation columns. The ethanol is recovered from the condensation system at top and the hot liquid coming out from the bottom of the distillation column is a waste. This hot liquid is commonly known as spent wash (SW) distillery wastewater and vinasses depends on its organic load. SW is dark brown in colour, acidic, and toxic. The rice grain-based SW is putrifiable because of high concentration of organics (COD = 35–50 kg/m3 , BOD = 10–20 kg/m3 ). The SW is first sent to the biodigester for anaerobic ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
biological treatment. Biodigester can reduce 50–70% chemical oxygen demand (COD) and 70–90% Biochemical oxygen demand (BOD) from the SW. The untreated effluent comes out of biodigester is commonly known as biodigester effluent (BDE). The BDE still contains high COD (10,000–15,000 mg/dm3 ) and BOD (2500–3000 mg/dm3 ) and is dark brown in colour. Due to these reasons, extensive treatment of BDE is essential. The common process used to treat BDE is biological aerobic treatment. In this process, submersed air bubble or surface aerator is used. This process is not cost intensive and also is able to reduce COD up to 70% and BOD up to 80%. Thus, 3000–5000 mg/dm3 COD and 500–600 mg/dm3 BOD still remains. The Central Pollution Control Board (CPCB) of India has fixed standard for the distillery units to meet the effluent discharge quality, for release of the wastewater into surface waters (COD < 0.1 kg/m3 , BOD < 0.03 kg/m3 ) and sewers (COD < 0.3 kg/m3 , BOD < 0.1 kg/m3 ).[1] Therefore, comprehensive strategy is required for maintaining desired standard of the effluents. Nowadays, most units use a membrane separation process, which is effective but cost of operation and installation cost is too high. Several physico-chemical processes have been proposed to treat distillery effluent such as coagulation with chemical coagulants such as alum, ferric chloride, poly aluminium chloride, and natural coagulant like Moringa Oleifera,[2,3] thermolysis with catalyst and
2
A.K. Prajapati and P.K. Chaudhari Table 1.
Studies to treat distillery effluents by the EC process.
Investigator
Electrode
Effluent
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Prasad and Srivastava [9] Ruthenium oxide-coated titanium mesh Sugar cane molasses SW acting as anode and stainless steel as cathode Kumar et al.[10] Stainless steel Sugar cane molasses BDE Thakur et al.[11] Iron Sugar cane molasses BDE Manisankar et al.[12] Graphite electrodes Sugar cane molasses SW Yavuz [13] Lead dioxide coated on titanium Sugar cane molasses SW Present studies Aluminium Rice grain-based BDE
without catalyst,[4–6] wet oxidation,[7,8] etc. These processes have several drawbacks. The Coagulation process needs large amount of chemicals and generates sludge which has disposal problems. Thermolysis and wet oxidation processes need rigorous operating condition and also they are not cost intensive. Electrocoagulation as a treatment method provides better result as compared with the traditional coagulation/ flocculation method due to less sludge generation and better colour and COD removal. Prasad and Srivastava [9] reported the optimal removal of colour of 83.31% and COD of 39.66% with ruthenium oxide-coated titanium mesh acting as anode and stainless steel as cathode at the pH of 5.5. A 50.5% COD reduction and 95.5% colour reduction were obtained using an iron electrode at their optimum pH.[10] Thakur et al. reported 61.6% COD reduction and 98.4% colour reduction with the iron electrode at their optimum pH.[11] Comparative studies for treatment of distillery effluents have been presented in Table 1. From the table, it can be seen our results are better to other investigators. The aim of this study is to derive experimental data on EC treatment of BDE of the rice grain-based distillery at the different conditions. The energy consumption and electrode loss have been determined at different pH. The settling and filterability characteristics of the treated BDE have also been presented in the paper. The scum and residues characteristic of treated BDE are also analysed.
Experimental section Effluent and its characterization The BDE used in this study was obtained from Chhattisgarh Distillery Pvt. Ltd. Kumhari, Chhattisgarh. To maintain constant characteristics of BDE, the sample was stored at 4◦ C in a deep freezer. Aluminium plate was used as electrode. Reactor was made up of Perspex glass. The effluent was characterized for various parameters, namely COD, colour, total solids, total dissolved solids, total suspended solid reduced carbohydrate, sulphate, chloride, etc., as per standard method of analysis.[14] Furthermore, the COD and colour of treated effluent was also analysed. The main characteristic of the original and treated effluent used for this study is given in Table 2.
% COD removal % colour removal 39.66
83.39
50.5 61.4 80.6 90.8 93
95.2 98.4 95.6 98.5 87
Table 2. Typical composition of BDE before and after treatment by EC at current density = 89.3 A/m2 , g = 20 mm. Parameters COD TDS TSS TS Reduced carbohydrate Protein Chlorine Phosphate Total hardness Sulphate pH Colour Absorbence at wavelength = 475 nm Colour (PCU)
BDE 13,800 46,245 40,000 86,245 416 185 124 0.05 9000 4920 7.8 Blackish brown
EC-treated BDE at optimum condition
0.932
966 1700 5231 6931 Not found 77 50 Nil 110 526 8.0 Light yellow (transparent) 0.149
450
72
Note: All values in mg/dm3 except pH and colour. PCU, platinum cobalt unit.
Experimental methods The lab-scale batch experimental setup used for the EC studies is shown in Figure 1 and characteristics of EC cell is presented in Table 3. The 1.4 dm3 BDE was taken in a 1.5 dm3 reactor, made up of Perspex glass. To maintain the pH of BDE, H2 SO4 (2 M) and NaOH (2 M) was used. Four electrodes of aluminium plates 2 mm thickness were used for the experiment. Electrodes shapes were rectangular and dimensions of each electrode were 8 cm × 7 cm. The area of electrodes dipped into the BDE was 8 cm × 7 cm. Therefore, total effective surface area of each electrode was 56 cm2 . A 1.5 cm gap was provided between the bottom of the electrodes and the bottom of the reactor for easy stirring. The gap between two electrodes in EC reactor was varied from 1.5 to 2.5 cm. Magnetic stirrer was used for proper mixing of BDE sample. The electrodes were cleaned with 10% diluted HCl solution before each run. Following each run, the electrodes were washed with distilled water, dried at room temperature and then weighted. After this it was reused for the next experiment. The current density was adjusted constant by means of a digital direct current (DC)
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Environmental Technology
Figure 1. Table 3.
3
Schematic diagram of the experimental setup used for the electrocoagulation study. Characteristics of the EC cell.
Electrodes Materials (anode and cathode) Shape Size Thickness (mm) Plate arrangement Effective electrode surface area (cm2 ) Reactor characteristics Make Reactor type Dimensions (cm) volume (dm3 ) Electrode gap (mm) Stirring mechanism Power supply Voltage range (V) Current range (A)
Aluminium Rectangular plate 8 cm × 7 cm 2 Parallel 56 Perspex glass Batch mode 10.7 × 10.7 × 13.7 1.5 20 Magnetic bar DC 0–30 0–5
power supply (0–30 V, 0–5 A). All experiments were done at a room temperature (30◦ C) and atmospheric pressure (101.1 kPa). The current density was controlled by using regulator. At the end of the experiments, sample was filtered with Whatman (41 size) filter paper to remove sludge. The filtered liquid was used for analysis of parameters (COD and colour). Settling and filterability studies were also performed in a 0.5 dm3 measuring cylinder.
Results and discussion The BDE of rice grain-based distillery contains COD and BOD material. The COD and BOD are due to the presence of various organics. Melonidian provides dark brown colour and passes net negative charge.[2] The BDE also contains colloidal particles. The colloidal surfaces have negative charges which take part in the electrochemical (EC) process. When potential is applied through external
DC power surface across the electrode, Al3+ ions released from anode which gets attached to the negative ions of BDE, causing small colloids to convert into the heavy flocks. It is similar to the coagulation process where aluminium salt (such as alum and AlCl3 ) releases Al3+ ions. When electro generated Al cations are liberated in solution it hydrolyses to form monomeric and polymeric species: Al(OH)+ , Al 5+ − 0 OH2+ , Al2 (OH)4+ 2 , Al(OH)4 , Al(OH)2 (s), and Al(OH)4 , etc.[15] The general equation is xM3+ + yH2 O = Mx (OH)y(3x−y)+ + yH+ .
(1)
The colloidal particles have net negative charges, which gets entrapped in amorphous ions hydroxides and gets neutralized. The neutralized mass, as well as Al(OH)3 formed, promotes sweep flocculation. Effect of pH The pH of the solution plays a significant role in influencing the EC treatment. The effect of pH in the range of 3.5–9.5 was studied. The effect of electrolysis time on COD and colour reduction was also observed and the results are presented in Figure 2(a) and 2(b). It can be seen that 78%, 80%, 81%, 88%, and 82% COD reduction is obtained at pH of 3.5, 5, 6.5, 8, and 9.5, respectively, in 20 min which increases to 89%, 90%, 91%, 93%, and 91% in 120 min. The colour reductions of 75%, 76%, 73%, 77%, and 65% are obtained at pH of 3.5, 5, 6.5, 8, and 9.5, respectively, in 20 min which enhance to 84%, 85%, 83%, 87%, and 76%. The results reflect that about 10% COD and colour reduction increase with increase in treatment time to 100 min. The working mechanism of EC treatment using aluminium electrodes is similar to that of chemical coagulation using Alum or AlCl3 as a coagulant. The COD and colour reduction at different pH depend on types of metal hydroxide formed and behaviours of organics contained in the wastewater.
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A.K. Prajapati and P.K. Chaudhari
Figure 2. Effect of pH on: (a) percentage COD reduction, (b) colour reduction, (c) electrode energy consumption, and (d) anode consumption (CODi = 13,800 mg/dm3 , colour i = 450 PCU, j = 89.3 A/m2 , g = 20 mm).
It can be seen that pH 8 provides best result during EC treatment. The 93% COD reduction and 87% colour reduction are achieved in 120 min. Since the BDE has pH 8, thus, no pH adjustment is needed for treatment. For the optimum design of EC treatment, energy consumption has also been determined. It was calculated with the help of the following equation [10]: Energy consumption (Wh/dm3 ) =
VIt
, Treated volume (dm3 ) (2)
where V is the cell voltage in volt, I is the current in ampere (A), and t is the electrolysis time (h). The electrical energy consumption during the EC of BDE was determined at different pH (3.5–9.5) current density
j = 89 A/m2 , electrode gap (g) = 20 mm, treatment time (tR = 120 min), and stirring speed = 130 rpm. The results are shown in Figure 2(c). It may be seen that energy consumption increases with increase in pH. The energy consumption of 17.4, 22.8, 24.3, 31.4, and 48.6 Wh/dm3 BDE is noted at pH 3.5, 5, 6.5, 8, and 9.5, respectively. It has been reported that the aluminium electrode consumes more energy in comparison with the iron electrode. This may be due to large amount of flocks adheres to the surface of the aluminium electrode which further works as an insulator causing an increase in voltage. The anode consumption at different pH was also calculated. Two anodes and two cathodes were used in the experiment. The values are shown in Figure 2(d) at similar operating condition and pH 3.5, 5, 6.5, 8, and 9.5. At these pH anodes consume to 1.67, 1.0154, 1.3763, 1.6855, and
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where K is the cell specific conductance and A is electrode surface area. When the value of g increases after the certain limit the aluminium oxidation rate decreases, therefore the COD and colour removal efficiency also decrease. In our studies, COD and colour reduction are found in order of electrode gap (g) 20 mm > 25 mm > 15 mm.
Figure 3. Effect of electrode gap on: (a) COD reduction and (b) colour reduction. (CODi = 13,800 mg/dm3 , colour i = 450 PCU, pH = 8, j = 89.3 A/m2 ).
1.0598 g/dm3 of BDE treated. It is observed that electrode consumption decreases from pH 3.5 to 5 and then increases from pH 5 to 8 but decreases from pH 8 to 9.5.
Effect of electrode gap and electrolysis time The effect of electrode gap (g) was studied by varying it 15–25 mm and keeping other parameters pH 8, j = 89.3 and stirrer speed 130 rpm. The result is presented in Figure 3(a) and 3(b). The COD reduction seems to list at g = 25 mm, crosses to g = 15 mm in 90 min with 89% COD reduction, which increase to 92% in 120 min. At g = 15 mm COD reduce to 91%. The highest COD reduction 93% obtained at g = 20 mm in 120 min of EC. The colour reduction of 65%, 66%, and 76% is obtained in 20 min, which increase to 80%, 82%, and 87% at g = 15, 25, 20 mm, respectively. The relation between the electrode gap and resistance is given by Faraday’s law. As the gap (g) between the anode and the cathode increases, resistance (R) offered by the cell also increases.[11] The relation is expressed as R=
g , KA
(3)
Effect of current density The current density (j) is an important parameter which highly influences to the treatment efficiency of the EC treatment processes. Therefore, the applied current density was varied to examine its effect on the COD and colour removal. An increase in current density from 44.6 to 178.5 A/m2 for a four-plate configuration causes an increase in voltage from 9.5 to 21.3 V. The effect of current density on COD and colour removal is presented in Figure 4(a) and 4(b). At current density 44.6, 89.3, 133.8 and 178.5 A/m2 , COD reduces to 85%, 88%, 89%, and 90% after 20 min. With further increase in electrolysis time up to 120 min, a COD reduction of 92%, 93%, 96%, and 97%, respectively, is achieved at their optimum pH 8. An increase in current density the COD removal efficiency increases because when current density increases more amount of Al3+ ions gets released and more aluminium hydroxide cations are formed. Which attaches with negative ions of BDE, thereby causing organic matter to convert in heavy flocks and ultimately it settles down due to its gravity. The colour removal efficiency is also increases with increase in current density. About 65%, 70%, 72%, and 74% of colour is removed after 20 min during the EC process which increases up to 80%, 84%, 83%, and 88%, respectively, after 120 min. It can be seen that the colour removal efficiency of BDE is poor compared with COD removal efficiency, this may be due to small colloidal particles still present in the treated BDE. The electrolysis time also plays a very significant role for the treatment of effluent. Amount of ions produced is the function of electrolysis time, i.e. amount of ions production increases with respect to electrolysis time. Therefore, increases in t, causes increase in concentration of aluminium ions and their hydroxide flocs. Consequently, when t is increased the COD and colour removal efficiency also increases, which can be seen from Figures 2–4. Settling characteristics The slurry obtained from EC treatment was subjected to sedimentation test. To determine the separation characteristic by settling, the treated BDE sample after the EC process was slowly mixed and taken in 0.5 dm3 cylinder having diameter 46 mm. The settling characteristics were performed at different pH. Figure 5(a) shows the time vs. dimension less height (H/Ho ) as a function of settling time. At the beginning, for a very short period, sludge is relatively slow settling, because of the Brownian motion of the particle, this is followed by steady-state decrease in the height
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A.K. Prajapati and P.K. Chaudhari
Figure 5. Effect of pH on: (a) settling characteristic and (b) filterability characteristic of the EC-treated slurry (j = 89.3 A/m2 , g = 20 mm). Figure 4. Effect of current density (j) on: (a) COD, (b) the colour removal of BDE (CODi = 13,800 mg/dm3 , colour i = 450 PCU, g = 20 mm, pH = 8).
of the solid/liquid interface, known as the zone-settling region. Later on a compressed layer occur on the bottom of the cylinder known as compression settling zone. The settling rate is found in order of pH 9.5 > pH 6.5 > pH 8 > pH 5 > pH 3.5. It can be seen that the settling rate was found best at basic condition (pH 9.5) which decrease at the acidic condition (pH 3.5). This may be due to formation of heavy flocks at basic pH, which settle down. But the poor COD and colour removal limits the EC treatment at pH 9.5. At pH 6.5 and 8, 60% settling occurs in 40 min. The COD reduction of 93% is obtained at the pH 8, j = 89.3 A/m2 with 60% settling in 40 min. Due to this, pH 8 is optimum pH for the process. Method proposed by Richardson et al. [16] is mostly used in practice to design a continuous thickener, based on batch studies. Filterability characteristics To analyse the filtration characteristic of treated BDE, gravity filtration was conducted on Whatman filter paper (41 size) supported on Buchner funnel. The filtration experiment was conducted at atmospheric pressure and room
temperature (at 30◦ C and 101.3 kPa). At different times, different volumes were noted. Necessary parameters like filtration resistance of the filter media and filter cake resistance for the constant pressure filtration were evaluated using the following filtration equation [17]: dt = Kp V + β, dV cαμ kp = 2 , A (−p)
(4) (5)
and β=
μRm , A(−p)
(6)
where t is the time taken for filtration (s), V is the volume of filtrate collected in t time (m3 ), kp is the slope for the plot of Equation (4) (s/m6 ), β is the intercept for the plot of Equation (4) (s/m3 ), c is the concentration of sludge (kg/m3 ), α is the specific cake resistance (m/kg), μ is the viscosity of the filtrate (Pa s), A is the area of the filter media (m2 ), p is the pressure drop across the filter (Pa), and Rm is the filter medium resistance (m−1 ). After the experiments data are plotted as (dt/dV ) vs. V presented in Figure 5(b). The value of β (intercept) and kp
Environmental Technology Table 4.
Filterability of EC-treated wastewater at different pH.
pH
kp × 10−12 (s/m6 )
β × 10−6 (s/m3 )
C (kg/m3 )
α × 1013 (m/kg)
Rm × 109 (m−1 )
12.3 12.35 10.09 7.34 9.26
1.82 96.37 17.57 12.76 17.64
21.85 18.46 9.08 50.32 24.67
8.41 9.95 16.6 2.17 5.6
2.71 144 26.25 19.06 26.38
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pH 3.5 pH 5 pH 6.5 pH 8 pH 9.5
(slope) is calculated by best fit of experimental data. The data are presented in Table 4. The cake resistance is found in order of 16.6 > 9.95 > 8.41 > 5.6 > 2.17 × 1013 (m/kg) at the pH 6.5, 5, 3.5, 9.5, and 8, respectively. Therefore, it can be seen that EC-treated BDE at the pH 8 provides best filterability. Any change from pH 8 reduces the filterability of EC-treated BDE. The reason for variation in cake resistance is due to porosity of the cake which seemed to be highest at pH 8 and lowest at pH 6.5. The filter medium resistance is found to be in decreasing order 144, 26.38, 26.25, 19.06, and 2.71 × 109 m−1 at pH 5, 9.5, 6.5, 8, and 3, respectively. The filter medium resistance of clarifying filter plays a role during the early state of filtration. The approximated values of specific cake resistance for different sludges are also reported by Barnes et al. [18] these values are 3–30 × 1013 m/kg for the digester sludge and 4–12 × 1013 m/kg for the activated sludge. Cake resistance of EC-treated paper mill effluent has been reported by Mahesh et al. [19] The values were 3.242–6.89 × 1011 . Kumar et al. reported cake resistance 1.34–10.67 × 1010 m/kg of composite textile mill.[20] Our studies give results near to activated sludge.
Analysis of filtrate The EC-treated BDE was filtrated by Whatman filter paper (41 size) and then analysed. The results are presented in Table 2. The 93% COD and 85% colour have reduced at current density 89.3 A/m2 (2A), i.e. COD reduces to a value Table 5. Parameter
7
of 966 mg/dm3 from initial value of 13,800 mg/dm3 and colour reduces to 72 PCU from 450 PCU. After treatment many functional group and chemicals (organic and inorganic) such as reduced carbohydrate, proteins, chloride, sulphate, total hardness, and total solid are also satisfactory reduced from the BDE. The treatment at j = 178.5 A/m2 provided COD 414 mg/dm3 and colour reduction of 54 PCU (97% COD and 88% colour reduction) at the cost of high energy consumption. Optimum result is achieved at pH 8 (93% COD and 85% colour reduction) with current density 89.3 A/m2 at electrode distance 20 mm. From the results, it is clear that the treated effluent cannot be directly discharged in land or water body due to large amount of COD (966 mg/dm3 ) still present in the ECtreated BDE. Hence, further treatment is required for this EC-treated BDE. Many physico-chemical methods such as wet oxidation, adsorption, and membrane separation may be used as possible options to treat EC-treated BDE. Recently, many industries directly treat the BDE (COD 12,000–18,000 mg/dm3 ) by membrane separation, which utilizes high energy and higher capital cost. For the good results, BDE should first be treated by EC, which should be followed by membrane separation.
Analysis of scum and residue For the analysis of solid residues and scum, it was first separated from the treated BDE and later on dried it at 105◦ C,
Analysis of residue and foam obtained after EC treatment at different pH. At pH 3.5
At pH 5.0
At pH 6.5
At pH 8.0
At pH 9.5
Analysis of residue obtained after EC at different pH 8.53 36.7 28.12 55.63 39.2 Weight of residue (kg/m3 ) Colour White White White White White Nature Bulky mass, difficult Bulky mass, difficult Bulky mass, difficult Bulky mass, difficult Bulky mass, difficult to grinding to grinding to grinding to grinding to grinding Approximated 13 20 16 22 15 drying period % convertible COD 89 90 91 93 91 Analysis of foam obtained after EC at different pH Colour White White Nature Soft mass, easy Soft mass, easy to grinding to grinding Approximated 5 7 drying period (h) % convertible COD 89 90
White Soft mass, easy to grinding 9
White Soft mass, easy to grinding 8
White Soft mass, easy to grinding 7
91
93
91
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A.K. Prajapati and P.K. Chaudhari
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until its weight became constant. Characteristics of foam and solid residues were also determined. Data of analysis are presented in Table 5. It can be seen that the weight of residue and foam obtained at pH 8 have highest mass. Drying period lies between 13 and 22 h. Residues of ECtreated BDE are hard and difficult to grind, whereas scum has soft and easy grindable mater. Possibly these residues organics have good heating value as presented by various authors.[5,6,19,20] After incineration, the ash may be blended with soil to make bricks, which can be used as building material. The ash blended bricks will have good strength. Conclusions The EC process with aluminium as the sacrificial electrode had provided good results to treat BDE of a rice grain-based distillery. The COD reduction of 89%, 90%, 91%, 93%, and 91% and colour reduction of 85%, 86%, 83%, 84%, and 77% were obtained at the pH of 3.5, 5, 6.5, 8, and 9.5, respectively, and at current density 89.3 A/m2 . The COD and colour reduction were found to increase with increase in current density. The COD reduction of 92%, 93% 96%, and 97% and colour reduction of 80%, 85%, 83%, and 88% were noted at pH 8 and at current density 44.6, 89.3, 133.8, and 178.5 A/m2 . At pH 3.5, 5, 6.5, 8, and 9.5, the 17.4, 22.8, 24.3, 31.4, and 48.6 Wh energy have consumed for per dm3 treatment of BDE. The weight losses of electrodes obtained at these pH are 16.7, 10.15, 13.76, 16.855, and 10.6 g per dm3 BDE treatment. The settling characteristic of treated BDE was found to be best at pH 9.5, while filterability characteristic was best at the pH 8. The scum and residues obtained from the EC-treated BDE sample may have good heating values, therefore, it can be used as a fuel. COD and colour were not completely removed by the EC process. Therefore, post treatment by using aeration, wet oxidation, adsorption, coagulation, or membrane separation needs to reach discharge standard prescribed by pollution control agencies. Many distillery treat BDE directly by membrane separation process is not suitable because BDE contains high organic and inorganics, fouls the membrane and need high operation cost to treat it. The EC-treated BDE will be surely good feed for membrane separation process. Thus, the treatment of BDE by EC followed by membrane separation will be good combination to achieve zero discharge. Acknowledgements Authors gratefully acknowledge Chhattisgarh Council of Science and Technology (CGCOST), Raipur, India for providing research grants to support this work.
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