RESULTS AND DISCUSSION
5.3 Performance Evaluation of Semi-batch Bioreactor SmBR-1 in Absence of Iron
5.3 Performance Evaluation of Semi-batch Bioreactor SmBR-1 in
After attaining the stability in the reactor performance, the HRT was changed to 3 days.
The SmBR-1 performance was not negatively affected at 3 days HRT from day 34 to day 56. This might be due to well adaption of microbial communities in the reactor. The maximum and minimum arsenic and COD in the treated water were 8 and 4 µg/L and 13 and 11 mg/L during last 20 days of operation. The average values were 6±1.5 µg/L and 12±0.5 mg/L, respectively. The pH of treated water remained between 7 and 7.5 during the entire period of operation.
5.3.2 SmBR-1 Phase-2: Effect of Initial Arsenic Concentration
Figure 5.22 represents the effects of initial arsenic concentration of 200, 300, 400, 500 600, 700 and 800 µg/L in SmBR-1. As observed at 200 µg/L of initial arsenic, complete nitrate removal occurred within 24 hours of operation concludes no adverse effects even up to an initial arsenic concentration 800 µg/L. Performance on SO42− and COD removal was also remained same to that in the previous phase with only 6±0.5 mg/L and 12.5±0.6 mg/L in the treated water, irrespective of the initial arsenic concentrations studied.
Figure 5.22 Performance evaluation of SmBR-1 in phase-2 at initial arsenic = 200-800 µg/L, nitrate = 50 mg/L and sulphate = 25 mg/L.
Arsenic in the treated water remained below permissible limit of 10 µg/L when the initial arsenic was 600 µg/L or less. Whereas arsenic in treated water averaged 55 and 138 µg/L with 92% and 83% removal efficiency at initial arsenic concentration of 700 and 800 µg/L respectively. The pH remained between 7.0 and 7.3 throughout the experiment.
5.3.3 SmBR-1 Phase-3: Effect of Initial Nitrate Concentration
The effects of initial nitrate concentration of 50, 100, 150, 200 and 250 mg/L is summarised in Figure 5.23. Irrespective of initial NO3− concentration (up to of 250 mg/L) arsenic in the treated water was found always below 10 µg/L. Efficient arsenic removal (up to 98%) was seen even at highest NO3− concentration of 250 mg/L. However, the nitrate in the treated water was always less than the detection limit and therefore, not shown in the figure. SO42− and COD concentration in the treated water was remained between 6 and 8 mg/L, and 12 and 17 mg/L, respectively, in the treated water. Increase in pH was more in the reactor when initial nitrate concentration was higher justifying alkalinity formation during denitrification. However, pH in the reactor always remained below 7.5.
Figure 5.23 Performance evaluation of SmBR-1 in phase-3 at initial nitrate = 50-250 mg/L, arsenic = 600 µg/L and sulphate = 25 mg/L.
5.3.4 SmBR-1 Phase-4: Effect of Different Carbon Sources
Among the five carbon sources used in reactor acetate, malate, succinate, lactate and glucose, the mixed bacterial culture showed the best performance by utilizing glucose as the sole carbon source. The performance of SmBR-1 with the five carbon sources on arsenic, sulphate, nitrate and COD removal is shown in Figure 5.24. Nitrate in the treated water was below detection limits with all the tested carbon sources. Arsenic removal was the best to below detection limit when glucose was used as the carbon source. The SO42−
and COD removal was 90% and 89.5%, respectively. Lactate and succinate was the second best carbon sources resulted in 98%, 84%, and 94% arsenic, sulphate and COD removal, respectively. Malate is also utilized effectively in our experiment for arsenic removal. Only 68% sulphate removal was seen with malate as carbon source, however, it was not affecting the arsenic and nitrate removal in SmBR-1. Lactate, succinate and malate are reported as preferred carbon sources for SRB among organic acids under mesophilic operating conditions (Costa et al., 1996; Hao et al., 1996; Liamleam &
Annachhatre, 2007). Kesserű et al. (2002) reported succinic acid as best carbon source for denitrification among succinic acid, acetic acid and ethanol.
Figure 5.24 Performance evaluation of SmBR-1 in phase-4 at initial nitrate = 200 mg/L, arsenic = 600 µg/L and sulphate = 25 mg/L.
Around 72% sulphate removal efficiency was observed in the case of using acetate as the sole carbon source provided 98% arsenic removals. The poor efficiency of the mixed consortium to reduce sulphate using acetate could be due to inability of the SRB to completely oxidize acetate even with excess sulphate levels (Lens et al., 2002). SRB are generally poor competitors of methanogenic archaea (MA) for acetate. However, in a long-term operation, SRB gradually out compete MA in a sulphidogenic reactor due to their higher affinity for the substrate and higher substrate removal rate. Moreover, SRB also have a thermodynamic advantage over methanogens and acetogens in terms of the standard free energy change of the acetate oxidation. Desulfotomaculum genus among SRB generally consumes acetate. In addition most of the published research regarding drinking water denitrification involves the use of methanol, ethanol and acetic acid (Mohseni-Bandpi et al., 2013; Park & Yoo, 2009). Different species of Pseudomonas were found to be the dominant bacterial species in acetate fed denitrifying bioreactors.
High sulphate and nitrate removal efficiency in the experimental results indicates the presence and growth of sulphate and nitrate reducers. The pH of the treated water was in the range of 7.2-7.5 for all carbon sources studied during the entire phase. For glucose as carbon source showing the best performance may be because of the fact that sugar is an effective electron donor that is easily degraded under anaerobic conditions and glucose supports the growth of a wide variety of nitrate as well as sulphate reducing bacteria leading to increased microbial diversity and treatment system resilience (Akunna et al., 1993; Liamleam & Annachhatre, 2007; Mohseni-Bandpi et al., 2013). Similar performance in presence of other carbon sources might be due to the fact that the anaerobic degradation pathway of sugar such as glucose, fructose, and dextrose are also similar to that of many other organic compounds (Liamleam & Annachhatre, 2007).