MATERIALS AND METHODS
4.4 Experimental Methodologies
4.5.2 Operation of AGR-1
Figure 4.5 Photographs of the lab scale reactor set-up of flow through reactors AGR-1 and AGR-2.
temperature and fluoride. For discussion and better understanding, the whole reactor operation has been divided into the following eight phases:
4.5.2.2 Phase-1: Effects of EBCT and Backwash Frequency
The AGR-1 was operated at varying EBCT of 90, 60 and 45 min to get the optimum EBCT. EBCT was changed by adjusting influent flow rate to AGR-1.
Backwashing operation is necessary for effective removal of excess biomass sludge and biogenic precipitates which may led to head loss development and clogging of WAC bed (Slavik et al., 2013). Backwashing reduces/minimizes the head loss development in the reactor which is an important consideration in AGR operation. Hence, backwashing of reactor was carried out regularly under mixed flow of deoxygenated-DI water (60 mL/min) and nitrogen gas to fully fluidize the WAC bed for 2-3 min followed by a flow of only (deoxygenated) DI water (500 mL/min) for 2-3 min to maintain the treatment efficiency. Performance of AGR-1 was evaluated at different backwash frequencies of 2, 3, 4 and 7 days starting from day 115 of reactor run. The backwash frequencies were selected based on previous findings where backwash frequencies of 1 to 7 days were practiced (Boller et al., 1997; Choi et al., 2007; Upadhyaya et al., 2010).
4.5.2.3 Phase-2: Effects of Influent Arsenic Concentration
In this phase of operation the AGR-1 was fed with influent containing varying arsenic concentration of 200, 250, 350, 450, 550, 650 and 750 µg/L. The reactor was operated at an EBCT of 45 min, nitrate 50mg/L, sulphate 25 mg/L and a carbon dose of 105 mg/L COD. At 750 µg/L of influent arsenic concentration, the reactor performance was also evaluated at elevated dose of sulphate (50 mg/L) and longer EBCT of 60 min.
The COD was kept 121 mg/L during this period of operation.
4.5.2.4 Phase-3: Effects of Influent Nitrate Concentration
In the 3rd phase of operation, the AGR-1 was fed with influent containing varying nitrate concentration of 50, 100, 150 and 200 mg/L. The reactor was operated at an EBCT of 45 min, arsenic 500 µg/L, sulphate 25 mg/L and a varying carbon dose as per the requirement for increased nitrate concentration. At 200 mg/L of influent arsenic concentration, the reactor performance was also evaluated at longer EBCT of 60 min.
Table 4.6 Operational schedule of AGR-1.
Purpose of operation Days of operation
Influent characteristics
EBCT
(Min.) Remarks
Arsenic (µg/L)
Nitrate (mg/L)
Sulphate (mg/L)
COD (mg/L)
Start-up 1-43
200 50 25
91 90
44-114 105 90 First backwashing - day 49
(Phase-1)
Effects of EBCT and backwash frequency
44-114
105
90
115-136 60 Backwash frequency 2 days
137-168
45
Backwash frequency 3 days
169-196 Backwash frequency 4 days
197-231 Backwash frequency 7 days
(Phase-2)
Effect of influent arsenic concentration
197-231 200
50 25 105 45
Profile sampling on day 228
232-263 250
264-290 350
291-313 450
314-340 550
341-369 650
370-398 750 Profile sampling on day 398
399-426 750 60
427-457 750 50 50 121 60 Profile sampling on day 457
(Phase-3) Effect of influent nitrate
concentration
458-480
500
50
25
105
45
Profile sampling on day 480
481-510 100 150 Profile sampling on day 508
511-541 150 194 Profile sampling on day 540
542-572
200 240 Profile sampling on day 570
573-604 60 Profile sampling on day 603
* AGR-1 was operated at 30°C and pH of 6.8 throughout experiment except during effect of temperature and pH studies.
(Phase-4) Effect of pH
605-637
500 50 25 105 45
pH = 6.6
638-668 pH = 7.2
669-699 pH = 8.4
(Phase-5)
Effect of temperature (°C)
700-732
500 50 25 105 45
Temp.= 20°C
733-763 Temp.= 30°C
764-791 Temp.= 40°C
792-839 Temp.= 50°C
840-861 Temp.= 30°C
(Phase-6)
Removal and effects of fluoride 862-896 500 50 25 105 45 5 mg/L of F− is introduced
(Phase-7)
Performance at Low EBCT 897-934 500 50 25 105 30
(Phase-8)
Performance after shutdown 942-969 500 50 25 105 30
4.5.2.5 Phase-4: Effects of Influent pH
The normal range for pH in surface water systems is 6.5 to 8.5 and for groundwater systems 6.0 to 8.5 (Jordana & Piera, 2004). Although, microorganisms can grow in a wide range pH, their performance depends upon the pH at which they are subjected to. In the 4th phase, performance of the AGR-1 was evaluated by introducing influent of pH 6.5, 7.0 and 7.5. The pH was adjusted with varying sodium bicarbonate and potassium carbonate in the influent solution.
4.5.2.6 Phase-5: Effects of Operating Temperature
The change in temperature is one of the common diurnal and seasonal variable parameter of water. Temperature plays a key role in the selection of both the overall bacterial diversity present and the efficiency of the biological process. The impact of these variations with temperature-specific bacterial consortia on the efficiency of anaerobic water treatment remains unknown. The objectives of this study were to investigate the effects of temperature on the performance of the AGR-1. The AGR-1 was operated at 20°C, 30°C, 40°C and 50°C in phase-5. The reactor was operated at an EBCT of 45 min, arsenic 500 µg/L, nitrate 50 mg/L, sulphate 25 mg/L and COD of 105 mg/L.
After the study at 50°C, the temperature was reduced to 30°C to bring back the AGR-1 in normal condition for future operation.
4.5.2.7 Phase-6: Effects of Influent Fluoride Concentration
The objective of this experiment was to evaluate the efficiency of the bioreactor on fluoride removal as well as its effects on removal of other pollutants. Fluoride of 5 mg/L was introduced into the AGR-1 for this purpose.
4.5.2.8 Phase-7: Performance at Low EBCT
AGR-1 was operated at lower EBCT of 30 min. in order to evaluate its effect on arsenic removal and overall performance of the reactor.
4.5.2.9 Phase-8: Performance After Shut down
In order to test the response of the bioreactor system to any emergent and/or accidental shutdown periods, the influent feed was stopped on day 935 and remained off for seven days. The reactor was restarted after a seven day shutdown and fed with influent arsenic 500 µg/L, nitrate 50 mg/L, and sulphate 25 mg/L, at an EBCT of 30 min.