RESULTS AND DISCUSSION
5.4 Performance Evaluation of Semi-batch Bioreactor SmBR-2 in Presence of Iron
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).
5.4 Performance Evaluation of Semi-batch Bioreactor SmBR-2 in
5.4.1 SmBR-2 Phase-1: Effect of HRT
Figure 5.25 represents the performance of SmBR-2 during phase-1 operation. The SmBR-2 was started with a HRT of 6 days and initial arsenic, NO3− and SO42−
concentration of 250 µg/L, 100 mg/L and 25 mg/L respectively with COD of 153 mg/L.
Like SmBR-1, complete removal of NO3− was observed within one day of operation.
Some instability in the treated water iron concentration observed in the initial periods of start-up might be associated with poor sulphate reduction in SmBR-2. Once sulphate reduction was well stabilized the iron in the treated water was always below detection limit. SmBR-2 showed a maximum arsenic, SO42− and COD removal of 99%, 78% and 92% respectively at 6 days HRT. After attainment of steady state in terms of arsenic removal the HRT was changed to 3 days. The SmBR-2 performance was not negatively affected due to reduction in HRT (to 3 d) might be due to better adaptation of microbial population and attainment of steady state in the reactor. The treated water arsenic, SO42−
and COD values were averaged 1.5±1 µg/L, 5.7±0.3 mg/L and 11.5±0.5 mg/L corresponding to 99.4%, 77% and 92.5% removal respectively. The treated water pH remained between 7.0 and 7.4.
Figure 5.25 Performance evaluation of SmBR-2 in phase-1 at initial nitrate = 100 mg/L, arsenic = 250 µg/L, iron = 2 mg/L and sulphate = 25 mg/L.
5.4.2 SmBR-2 Phase-2: Effect of Initial Arsenic Concentration
Figure 5.26 represents the effects of initial arsenic concentration of 250, 350, 450, 550, 750 and 1000 µg/L in SmBR-2. The SmBR-2 performance remained stable at all tested influent arsenic concentrations. Arsenic in the treated water was always reduced to below 10 µg/L (99.5% removal) whereas iron and nitrate remained below detection limits. Moreover, increased initial arsenic concentration did not affect iron and/or NO3− removal. The COD and SO42− removal did not showed significant differences during this phase. However, the SO42− reduction was improved during this phase this might be due to well establishment of sulphate reducing communities in the reactor. The SO42− and COD of only 5±0.6 mg/L and 11.6±0.6 mg/L remained in effluent at end of phase-2 operation of SmBR-2. The pH of treated water remained between 7.23 and 7.35 during the entire phase.
5.4.3 SmBR-2 Phase-3: Effect of Initial Nitrate Concentration
The effects of initial nitrate concentration of 100, 150, 200 and 250 mg/L is summarised in Figure 5.27. Irrespective of initial NO3− concentration (up to 250 mg/L), arsenic in the treated water was found below 10 µg/L, with more than 99% removal efficiency. Nitrate and iron in the treated water was always less than the detection limits.
Figure 5.26 Performance evaluation of SmBR-2 in phase-2 at initial nitrate = 100 mg/L, arsenic = 250-1000 µg/L, iron = 2 mg/L and sulphate = 25 mg/L.
Thus the performance of SmBR-2 was stable in terms of arsenic and iron removal with the increase of the influent nitrate. Variation in SO42− and COD concentration followed similar trend as observed in phase-2. The SO42− and COD removal efficiencies remained on average values of 79.6% and 92.5% respectively. pH trend was varied between 7.3 and 7.5.
5.4.4 SmBR-2 Phase-4: Effect of Different Carbon Sources
The performance of SmBR-2 on arsenic, iron and nitrate removal with the five different carbon sources is shown in Figure 5.28. Among the five carbon sources used in SmBR-2, the mixed bacterial culture showed the best performance with glucose. Almost 100% nitrate, iron and arsenic removal was achieved with all the carbon sources. Glucose was found to be best electron donor in terms of sulphate and COD removal with 93% and 96% removal respectively. Despite of higher sulphate reduction with glucose, the arsenic, iron and nitrate removal remained unaffected with other carbon sources.
Figure 5.27 Performance evaluation of SmBR-2 in phase-3 at initial nitrate = 100-250 mg/L, arsenic = 1000 µg/L, iron = 2 mg/L and sulphate = 25 mg/L.
Figure 5.28 Performance evaluation of SmBR-2 in phase-4 at initial nitrate = 250 mg/L, arsenic = 1000 µg/L, iron = 2 mg/L and sulphate = 25 mg/L.