Continuous heavy metal removal experiments were conducted using downflow column reactor (DFCR) with immobilized SRB beads.
3.5.1 DFCR setup
The DFCR used in this study was made from a perspex tube of inner diameter (ID) 25.4 mm and effective length (L) 304.8 mm. Two sampling ports were provided at a distance of 10 cm each along the column length. The reactor was packed with immobilized SRB beads following its activation and washing with distilled water as mentioned earlier in Section 3.4.1. A schematic of the DFCR setup is shown in Fig. 3.3. The reactor was initially operated at an ambient temperature of 25 ± 2 °C for few days to attain steady state condition
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prior to the continuous metal removal experiments. Photograph of the DFCR packed with immobilized SRB beads is shown in Fig. 3.4. Pressure drop along the reactor was calculated as 0.83 psi or 0.0583 Kg/cm2.
Figure 3. 3 Schematic of the DFCR with immobilized SRB beads.
Figure 3. 4 Photograph of experimental setup showing downflow column reactor with immobilized SRB beads for heavy metal removal under continuous operation mode.
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3.5.2 Single component system 3.5.2.1 Metal removal experiments
For continuous metal removal using the DFCR, medium supplemented with the respective metals was fed into the reactor at a constant flow rate. The input metal concentration levels of Ni(II), Zn(II), Pb(II), Fe(III) and Cd(II) were chosen as 50, 75 and 90 mg/L. In the case of Cu(II), these values were 100, 150 and 175 mg/L. All these initial levels of the heavy metals were chosen based on the results obtained from previous batch heavy metal removal study using the immobilized SRB beads. As metal precipitation by SRB occurs in the range of within a few days to 5 days (Kuyucak and St-Germain, 2006), two different hydraulic retention time (HRT) values were chosen for this continuous metal removal study (24 h and 48 h). Phase wise operational conditions followed for continuous metal removal using the DFCR are presented in Table 3.2. The reactor was operated at each experimental condition for a period until three steady state values of effluent heavy metal concentration at the respective HRT were obtained (Villa-Gomez et al., 2015).
Table 3. 2 Operational conditions followed with the DFCR for continuous metal removal experiments
Parameter Experimental phase
I II III
HRT (h) 24 48 24 48 24 48
Inlet metal
concentration (mg/L) Cd(II), Ni(II), Fe(III), Pb(II) and Zn(II)
50 50 75 75 90 90
Cu(II) 100 100 150 150 175 175
Effect of metal loading on the performance of the DFCR reactor
The combined effect of inlet metal concentration and HRT on metal removal was examined by calculating the inlet metal loading rate (ILR) (mg/L∙h) and the corresponding metal removal rate (mg/L∙h) as per the following equations:
V QC
= (ILR) rate loading
metal
Inlet i (3.1)
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V ) C - Q(C
= (RR) rate
Removal i o (3.2)
100 Ci
o) C i- (C
= removal M etal
% (3.3)
where, Q is the inlet flow rate (L/h), Ci and Co are the inlet and outlet metal concentrations (mg/L), respectively, and V is the working volume of the reactor.
Samples collected at regular intervals of time were centrifuged at 8000 × g for 5 min (Remi, C24-L or R-24, India), and the supernatant obtained was analyzed for COD, sulfate, metal and sulfide concentrations.
3.5.3 Multi-component system
All experiments in this mixture study were carried out as per the statistically valid fractional factorial design (FFD) consisting of nineteen experimental runs with different combination levels of Ni(II), Cd(II), Fe(III), Zn(II), Pb(II) and Cu(II). Input metal concentration levels used in FFD of experiments are presented in Table 3.3 and these concentration levels shown are chosen based on the results obtained from batch study and the results obtained from single component system using the same bioreactor system.
Table 3. 3 Input concentration levels used in fractional factorial design of experiments Heavy metal Range and levels of metals
Low (-1) Centre point (0) High (+1)
Cd(II) 50 70 90
Cu(II) 100 137.5 175
Ni(II) 50 70 90
Fe(III) 75 82.5 90
Pb(II) 50 62.5 75
Zn(II) 50 62.5 75
Individual metal stock solutions of Fe(III), Ni(II), Cu(II), Zn(II), Pb(II) and Cd(II) of 100 g/L concentration each were prepared using FeCl3∙6H2O, NiCl2∙6H2O, CuCl2∙2H2O, Zn(NO3)2∙6H2O, PbNO3 and Cd(NO3)2∙4H2O, respectively. Desired metal concentration of the heavy metals in each of the experimental runs was obtained by adding corresponding metal stock solution to the modified Postgate medium as described earlier. All the
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experiments in this study were performed at 48 h HRT. The reactors were operated under continuous mode by supplying the medium at a constant flow rate at an ambient temperature of 25 ± 2 °C. The reactor was operated for a period until three steady state values of effluent heavy metal concentration. Samples collected at regular time intervals were centrifuged (8000 × g) for 5 min (Remi, C24-L or R-24, India), and the supernatant obtained was analyzed for sulfate, COD, metal and sulfide concentrations. The experimental design followed in this study with different metal combination levels is presented in Table 3.4, in which +1 and -1 indicate the high and low level of the metals, whereas 0 indicate centre point level of the metals.
Table 3. 4 Fractional factorial experimental design matrix showing different combination levels of the heavy metals in mixture using the DFCR
Experimental
runs Cd Cu Ni Fe Pb Zn
1 +1 -1 -1 -1 +1 -1
2 +1 -1 +1 +1 -1 -1
3 -1 +1 -1 +1 +1 -1
4 +1 +1 +1 +1 +1 +1
5 -1 -1 -1 +1 -1 +1
6 +1 -1 -1 +1 +1 +1
7 -1 +1 -1 -1 +1 +1
8 0 0 0 0 0 0
9 0 0 0 0 0 0
10 +1 +1 -1 -1 -1 +1
11 -1 +1 +1 +1 -1 +1
12 -1 +1 +1 -1 -1 -1
13 +1 -1 +1 -1 -1 +1
14 +1 +1 -1 +1 -1 -1
15 -1 -1 +1 +1 +1 -1
16 0 0 0 0 0 0
17 +1 +1 +1 -1 +1 -1
18 -1 -1 -1 -1 -1 -1
19 -1 -1 +1 -1 +1 +1
Precipitates obtained from experimental run 18 performed using the DFCR which yielded a maximum heavy metal removal efficiency in this study was analyzed for elemental composition and morphology by means of FESEM-EDX (Zeiss, Sigma, Germany) as per the procedure described earlier in the Section 3.3.1.1. Results reported are average of three
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steady state values. For the assessment of individual and collective effect of the metals, the results obtained from the fractional factorial design were analyzed in the form of student’s t test and ANOVA. The statistical software, Minitab (Version16, PA, USA) was used for designing the fractional factorial experiments and for statistical analysis of the results obtained. The effect due to a particular factor was defined as either reduction (negative) or improvement (positive) in the responses.
3.6 Continuous Heavy Metal Removal Using The Anaerobic Rotating Biological