RESULTS AND DISCUSSION
5.6 Performance Evaluation of the Flow through Attached Growth Reactor, AGR-2
5.6.2 AGR-2 Phase-1: Effects of Influent Iron Concentration
The performance of AGR-2 on effects of initial iron concentration of 2, 3, 4, 5, 7.5 and 10 mg/L is summarised in Figure 5.47. Irrespective of initial iron concentration (up to 10 mg/L) nitrate in the treated water was always less than detection limits and therefore, not plotted in given figure. This also concludes no adverse effects on nitrate removal in reactor even at iron concentration up to 10 mg/L. Iron in the treated water remained below permissible limits of 0.3 mg/L when the influent iron was 4 mg/L or less whereas treated water iron averaged 0.82 mg/L with 83% removal efficiency at influent iron of 5 mg/L during day 155 to day 181. The possible reason for this higher iron concentration (0.82 mg/L) in treated water might be due to unavailability of sufficient sulphides for precipitation as iron sulphides. The average values of SO42− and COD were 3.2±0.8 mg/L and 10.8±1.0 mg/L with 87% and 89.5% removal efficiency, respectively. The maximum and minimum SO42− and COD in the treated water were 5.0 and 1.7 mg/L, 16.2 and 9.6 mg/L, respectively, during AGR-2 operation from day 79 to 181. The maximum and minimum values of arsenic in the treated water were 7.6 and 1.2 µg/L with average values of 4.0±1.8 µg/L (99.2% removal) during above period. pH of treated water remained between 7.05 and 7.25 during this period.
Figure 5.47 Performance evaluation of AGR-2 in phase-1 at initial arsenic = 500 µg/L, iron = 2.0 -10 mg/L, nitrate = 50 mg/L and sulphate = 25-75 mg/L.
Profile sampling: The profile sampling results of the day 181 is shown in Figure 5.48.
Complete nitrate removal was observed in 1st port. Sulphate removal started from 1st port and almost 89% sulphate removal was observed in 5th port. Iron removal followed sulphate reduction and only 82% iron removal was occurred in 5th port. The iron concentration in treated water was found to be 0.9 mg/L. This might be due to unavailability of sulphides for iron sulphide precipitation. The arsenic in the treated water was below 10 µg/L in 4th port and remained undetected in the 5th port. Based on the profile sampling results, future experiments were conducted with increased influent sulphate of 50 mg/L in order to ensure the availability of sufficient sulphide for iron sulphides precipitation. Iron in the treated water was again resumed below 0.3 mg/L when the initial sulphate was raised to 50 mg/L from the day 182 to day 208.
The profile sampling results of the day 208 is shown in Figure 5.49. Sixty five percent of iron removal was observed in 3rd sampling port and became less then detection limits in port 4. Iron in treated water averaged 1.35±0.08 mg/L from day 209 to day 233 of AGR-2 operation when influent iron was increased to 7.5 mg/L. However, reactor performance remained stable in terms of arsenic removal and other parameters such as SO42− and COD removal during this period with average values of 4.3±2.4 µg/L, 3.0±0.5
mg/L and 11.5±1.3mg/L respectively. Iron concentration was again reduced to below permissible limits when influent sulphate was increased to 75 mg/L from day 234 till day 264.
Figure 5.48 Sampling profile of the AGR-2 on day 181 (Iron = 5.0 mg/L, arsenic = 500 µg/L, nitrate = 50 mg/L, sulphate = 25 mg/L and EBCT 45 Min.).
Figure 5.49 Sampling profile of the AGR-2 on day 208 (sulphate = 50 mg/L, iron 5.0 = mg/L, arsenic = 500 µg/L, nitrate = 50 mg/L and EBCT 45 Min.).
The profile sampling results of the day 264 is shown in Figure 5.50. These result showed that only 83% SO42− removal was seen in the effluent indicating lower EBCT a reason for poor sulphate reduction in AGR-2. However, a total of 98% iron removal was observed in 5th port and only 0.16 mg/L iron was left out in final treated water from initial 7.5 mg/L. Arsenic in treated water was 5.7 µg/L corresponding to 98.8% removal in the 5th port. Iron in treated water was found between 0.35-0.7 mg/L with an average value of 0.52±0.1 mg/L between day 265 and 294 when influent iron was 10 mg/L. SO42− and COD of only 6.3±1.8 mg/L and 15.7±2.8 mg/L, respectively, remained in the treated water during this period, whereas the maximum and minimum SO42− and COD were between 9.3 and 2.5 mg/L, and 19.5 and 10.5 mg/L respectively. The change in EBCT to 60 min improved in SO42− reduction with time as a result of which better COD removal was also seen in the reactor from day 266 to 294. Further increase in the EBCT to 90 min provided even better SO42− removal leaving only an average of 2.4 mg/L in the treated water between day 295 and day 326. In treated water, iron concentration was dropped to below the drinking standards of 0.3 mg/L, and was found in the range of 0.18-0.26 mg/L during last 20 days of operation at 90 min EBCT. This was probably due to generation of enough sulphide to form iron sulphides in the system thus promoting iron removal in the system. Moreover, sulphate addition up to 75 mg/L to the synthetic feed did not adversely affect the system performance. Arsenic removal was stable during the entire period of operation.
Figure 5.50 Sampling profile of the AGR-2 on day 264 (sulphate = 75 mg/L, iron = 7.5 mg/L, arsenic = 500 µg/L, nitrate = 50 mg/L and EBCT 60 Min.).
Figure 5.51 Sampling profile of the AGR-2 on day 326 (sulphate = 75 mg/L, iron = 10.0 mg/L, arsenic = 500 µg/L, nitrate = 50 mg/L and EBCT 90 Min.).
The pH of the treated water remained between 7.25 and 7.60 from day 182 to day 326. Slight increase in pH might be due to alkalinity generation resulting from reduction of increased SO42− in influent. Higher EBCT improved the reactor performance in terms of iron removal because it prolongs the TEAP zone which results in improve sulphate reduction, which in turn provides more sulphides to form iron and arsenic sulphides.
The profile sampling results of the day 326 is shown in Figure 5.51. The results indicated better sulphate removal of up to 95% efficiency at higher EBCT of 90 min and only 3.1 mg/L of SO42− was left out in treated water. Average iron concentration in the treated water was found to be 0.26 mg/L (97% removal) from an initial of 10 mg/L. Thus, increase in EBCT provided sufficient residence time for influent media to contact with biofilm which in turn resulted in enhanced sulphate and therefore higher iron removal in the reactor.
At the end of phase-1, the treated water arsenic, iron and nitrate was within the drinking water permissible limits with around 99.5%, 97.5% and 100% removal efficiency. Moreover, high iron concentration (up to 10 mg/L) did not adversely affect reactor performance in terms of arsenic removal. The SO42− and COD removal efficiencies by mixed microbial consortia was around 96.5% and 91.5% during the last 20 days of operation. These results conclude that the microbial reduction became the predominant process in the system.