RESULTS AND DISCUSSION
5.2 Performance Evaluation of Batch Shakes Flasks
5.2.3 Bioremoval Studies in Batch Shake Flasks .1 Batch Studies in Absence of Iron
Effects of initial arsenic concentration:
Figure 5.4, 5.5, 5.6, and 5.7 represents the performance of mixed bacterial culture in batch shake flasks at four different initial arsenic concentrations of 250, 350, 450 and 550 µg/L, respectively. Although sulphate and COD were gradually reduced during entire period of 7 days operation, reduction rate decreased after about 3-4 days of reaction. COD of only 22-24 mg/L and 16-18 mg/L remained in the effluent after 4 days and 7 days of operation respectively. pH of the effluent (Fig. 5.4) increased with the biodegradation of sulphate. This could have been due to formation of alkalinity due to reduction of sulphate.
Microbial mass (MLVSS) concentration started decreasing after initial increase for first 3- 4 days of operation. This might be due to unavailability of sufficient food (COD) in the media leading to net growth negative. In all the cases complete removal of nitrate from its initial 50 mg/L was noticed within first day of operation and therefore it has not been plotted in any of the above mentioned figures. With respect to arsenic, the mixed bacterial culture performed well in batch shake mode. Irrespective of initial concentration, arsenic in the treated water reduced below permissible limit (10 µg/L) in 5-6 days of reaction.
Figure 5.4 Performance of batch shake flasks in absence of iron at initial arsenic = 250 µg/L, nitrate = 50 mg/L and sulphate = 25 mg/L.
Figure 5.5 Performance of batch shake flasks in absence of iron at initial arsenic = 350 µg/L, nitrate = 50 mg/L and sulphate = 25 mg/L.
Figure 5.6 Performance of batch shake flasks in absence of iron at initial arsenic = 450 µg/L, nitrate = 50 mg/L and sulphate = 25 mg/L.
Figure 5.7 Performance of batch shake flasks in absence of iron at initial arsenic of = 550 µg/L, nitrate = 50 mg/L and sulphate = 25 mg/L.
Effects of initial nitrate concentration:
Figure 5.7, 5.8 and 5.9 represents effects of initial nitrate concentration of 50, 100 and 150 mg/L on performance of mixed bacterial culture in batch shake flasks.
Irrespective of initial nitrate concentration (up to 150 mg/L), arsenic in the treated water was found to be below 10 µg/L within first 4 days of operation. Nitrate in the treated water was always less than the detection limit and therefore, not shown in the figures.
Variation in sulphate and MLVSS concentration followed similar trend as observed at varying arsenic concentration. Little increase in pH with time would have been due to alkalinity formation during nitrate and sulphate reduction.
As arsenic cannot be removed from the system by biological processes, the reduction in arsenic concentration could be possible due to either adsorption on the microbial mass or precipitation as arsenosulphide. As arsenic removal by adsorption on biomass was almost nil, the only possibility of removal could be due to formation of arsenosulphides precipitation. However, formation of arsenosulphides precipitation could not be proved due to unavailability of sufficient amount of such solids in batch shake flasks, which were used for the study only with 50 ml of contaminated water. Analysis of
solids of a semi batch reactor (SmBR-1) and flow through reactor (AGR-1) were done to confirm the formation of arsenosulphides.
Figure 5.8 Performance of batch shake flasks in absence of iron at initial arsenic = 500 µg/L, nitrate = 100 mg/L and sulphate = 25 mg/L.
Figure 5.9 Performance of batch shake flasks in absence of iron at initial arsenic = 500 µg/L, nitrate = 150 mg/L and sulphate = 25 mg/L.
5.2.3.2 Batch Studies in Presence of Iron
Effects of initial iron and sulphate concentration:
Figure 5.10, 5.11, 5.12, 5.13 and 5.14 represents effects of initial iron concentration of 1.0, 2.0, 3.0, 4.0 and 5.0 mg/L, respectively. Irrespective of initial iron concentration (up to 5.0 mg/L), nitrate and arsenic removal was not affected in batch reactors. Up to an initial 3.0 mg/L, iron in the treated water was always below detection limit. However, 0.23 mg/L (94% removal) and 0.48 mg/L (90% removal) of iron concentration was observed in the treated water at 4.0 mg/L and 5.0 mg/L of initial iron.
The possible reason for this higher iron concentration (0.48 mg/L) in treated water might be due to unavailability of sufficient sulphides for precipitation as iron sulphides.
Therefore another set of experiment was conducted with an increased initial sulphate concentration of 50 mg/L (Figure 5.15). Iron in the treated water now reduced below 0.3 mg/L at initial sulphate of 50 mg/L. Better removal of iron with the increase in influent sulphate was due to availability of sufficient sulphide to react with iron to form iron sulphide precipitate. Although sulphate and COD were gradually reduced during entire period of 7 days operation, reduction rate decreased after about 3-4 days of reaction. COD of only 27-30 mg/L and 22-25 mg/L remained in the effluent after 4 days and 7 days of operation, respectively. The pH of the effluent increased with the biodegradation of sulphate. This could have been due to formation of alkalinity due to reduction of sulphate.
Microbial mass (MLVSS) concentration started decreasing after initial increase for first 2- 3 days of operation. This might be due to unavailability of sufficient food (COD) in the media leading to net growth negative. In all the cases complete removal of nitrate from its initial 50 mg/L was noticed within first day of operation and therefore it has not been plotted in any of the above given figures.
Figure 5.10 Performance of batch shake flasks in presence of iron at initial iron = 1.0 mg/L, arsenic = 500 µg/L, nitrate = 50 mg/L and sulphate = 25 mg/L.
Figure 5.11 Performance of batch shake flasks in presence of iron at initial iron = 2.0 mg/L, arsenic = 500 µg/L, nitrate = 50 mg/L and sulphate = 25 mg/L.
Figure 5.12 Performance of batch shake flasks in presence of iron at initial iron = 3.0 mg/L, arsenic = 500 µg/L, nitrate = 50 mg/L and sulphate = 25 mg/L.
Figure 5.13 Performance of batch shake flasks in presence of iron at initial iron = 4.0 mg/L, arsenic = 500 µg/L, nitrate = 50 mg/L and sulphate = 25 mg/L.
Figure 5.14 Performance of batch shake flasks in presence of iron at initial iron = 5.0 mg/L, arsenic = 500 µg/L, nitrate = 50 mg/L and sulphate = 25 mg/L.
Figure 5.15 Performance of batch shake flasks in presence of iron at initial iron = 5.0 mg/L, arsenic = 500 µg/L, nitrate = 50 mg/L and sulphate = 50 mg/L.
Effects of initial arsenic concentration:
Figure 5.12, 5.16, 5.17 and 5.18 represent the performance of mixed bacterial culture in batch shake flasks at four different initial arsenic concentrations of 500 µg/L, 600 µg/L, 750 µg/L and 1000 µg/L, respectively. Irrespective of initial concentration, arsenic in the treated water was reduced below permissible limit (10 µg/L) in 3-5 days of reaction and finally averaged 7±2 µg/L with 98.5% removal efficiency. Variation in
COD, sulphate, pH and MLVSS concentration followed similar trend as observed in previous experiments. The higher arsenic removal is possibly due to formation of arsenosulphides as well as co-precipitation and/or adsorption with iron sulphides in presence of iron (Altun et al., 2014).
Figure 5.16 Performance of batch shake flasks in presence of iron at initial iron = 3.0 mg/L, arsenic = 600 µg/L, nitrate = 50 mg/L and sulphate = 25 mg/L.
Figure 5.17 Performance of batch shake flasks in presence of iron at initial iron = 3.0 mg/L, arsenic = 750 µg/L, nitrate = 50 mg/L and sulphate = 25 mg/L.
Figure 5.18 Performance of batch shake flasks in presence of iron at initial iron = 3.0 mg/L, arsenic = 1000 µg/L, nitrate = 50 mg/L and sulphate = 25 mg/L.
Effects of initial nitrate concentration:
Figure 5.12, 5.19 and 5.20 represents effects of initial nitrate concentration of 50, 100 and 150 mg/L on performance of mixed bacterial culture in batch shake flasks.
Irrespective of initial nitrate concentration (up to 150 mg/L), arsenic in the treated water was found to be below 10 µg/L within first 4-6 days of operation. Nitrate in the treated water was always less than the detection limit within one day for all the tested concentrations and therefore, not shown in any of the figures. Variation in sulphate and MLVSS concentration followed similar trend as observed at varying arsenic concentration. Slight increase in MLVSS formation was observed and this is expected due to more biomass formation at higher nitrate concentrations of 100 and 150 mg/L.
Similarly little more increase in pH with time was also noticed due to more alkalinity formation at higher nitrate concentrations.
Figure 5.19 Performance of batch shake flasks in presence of iron at initial iron = 3.0 mg/L, arsenic = 1000 µg/L, nitrate = 100 mg/L and sulphate = 25 mg/L.
Figure 5.20 Performance of batch shake flasks in presence of iron at initial iron = 3.0 mg/L, arsenic = 1000 µg/L, nitrate = 150 mg/L and sulphate = 25 mg/L.