MATERIALS AND METHODS
3.2 Experimental Methodologies
3.2.3 Reactor operation
3.2.3.1 Continuous moving bed reactor (CMBR) .1 Acclimatization of CMBR
Two glass aspirator bottle of (5 L working volume) labeled R1A and R2A were used for anaerobic acclimatization of 3L screened seed sludge added with 60 g sponge cube each and final volume was made to 5 L with tap water. In R1A and R2A, 500 mg/L of dextrose was added daily for initial 21 days. Along with dextrose, the synthetic feed also included ammonium chloride (50 mg NH4Cl/L) as nitrogen source; phosphate buffer (dissolving 104.5 g/L K2HPO4 and 72.3 g/L KH2PO4) of pH 6.8; trace metals solution (1mL/L) and
yeast extract 20 mg/L as nutrients. Phosphate buffer of 1 mL/L of feed was added as phosphorus source and to maintain reactor pH. Concentration of phosphate buffer, yeast extract and trace metal solution remained unchanged through out the study. Synthetic feed was dissolved in tap water and purged with nitrogen gas before addition to reactors.
For the first 21 days R1A and R2A were operated only with 500 mg/L of dextrose as carbon source. On day-22, feed dextrose concentration was increased to 1000 mg/L daily.
From day 30 onwards, phenol 10 mg/L and NH4+-N 10 mg/L, and thiocyanate 5 mg/L were introduced along with 1000 mg/L dextrose. Dextrose concentration was decreased gradually and dextrose concentration in feed became nil on day-90 onwards and R1A and R2A were operated only with phenol (250 mg/L), NH4+-N (100 mg/L), thiocyanate (50 mg/L) and other nutrients. With each feed R1A and R2A was operated for 3-5 days before going for next increment. In feed of R1A and R2A, phenol, thiocyanate and NH4+-N concentrations were gradually increased to 2500, 110 and 500 mg/L to overcome the toxic effect on microorganisms on 217 days through out acclimatization period.
Similarly for anoxic sludge acclimatization, two aspirator bottles R3A and R4A were prepared with sludge (3L), sponge cube (60 gm) and tap water to 5 L each. For the initial 21 days in anoxic reactors were fed daily with 500 mg/L of dextrose and 70 mg/L of nitrate-nitrogen. Same amount of phosphate buffer, yeast extracts and trace metal solutions were added in the feed of anoxic reactors like anaerobic reactors. On day-22 onwards, feed dextrose and nitrate (NO3--N) concentrations were increased to 1000 mg/L and 250 mg/L, respectively (COD:NO3--N of 4.24). From 30th day onwards, phenol of 5 mg/L, 5 mg/L SCN- and 5 mg/L NH4+-N were added in anoxic reactors along with 1000 mg/L of dextrose and 250 mg/L of NO3--N. Feed dextrose concentration was decreased in R3A and R4A along with increase in phenol concentration in the feed. On day-79, feed phenol, thiocyanate and NH4+-N concentration became 500 mg/L, 25 and 50 mg/L and dextrose concentration became 200 mg/L. On day-97 onwards anoxic reactors were operated with 500 mg/L phenol, 40 mg/L thiocyanate and 200 mg/L NH4+-N and 450 mg/L of NO3--N without any dextrose in the feed (COD:NO3--N ratio of 2.64). In three-stage system, effluent of anaerobic reactor is expected to be diluted with nitrified effluent of aerobic reactor so that denitrification can take place in anoxic reactor. Due to this, anoxic reactor was acclimatized up to maximum phenol concentration of 1250 mg/L, 50 mg/L SCN- and
250 mg/L NH4+-N and acclimatization period was 140 days like anoxic reactors. Feed to R3A and R4A was also purged with nitrogen gas to make oxygen free.
In aerobic reactors (R5A and R6A) sponge of 60 g in each reactor was added in 5L of screened sewage. Compressed air was added in R5A and R6A for supply of oxygen which caused mixing of reactor content. Synthetic feed consisting of ammonia-nitrogen (using ammonium chloride) was added in aerobic reactors. Concentration of ammonia-nitrogen, phenol and thiocyanate was increased gradually from 10 mg/L to 250 mg/L, 500 mg/L and 25 mg/L in 100 days.
3.2.3.1.2 Reactor assembly of CMBR system
Three PVC (polyvinyl chloride) columns, each of diameter 15 cm and height 118 cm were used as reactors for CMBR system. It consisted of three reactors in series, maintained under anaerobic (R1), anoxic (R2) and aerobic (R3) environments. After acclimatization contents (liquid + biomass + sponge) of two 5L batch anaerobic reactors (R1A and R2A) were added in anaerobic reactor (R1). Similarly contents of two anoxic (R3A and R4A) and aerobic (R5A and R6A) reactors were added in R2 and R3, respectively. A schematic representation of the experimental assembly of CMBR system is shown in Figure 3.1.
Figure 3.1 Schematic of the experimental set up of the moving bed reactor system with its various components: (1) Anaerobic reactor, (2) Anoxic reactor, (3) Aerobic reactor, (4) Sponge cube (5) Influent, (6) Peristaltic pump (7) Clarifier, (8) Biogas collection pipe, (9) Gas outlet (10) Effluent reservoir, (11) Recirculation to anoxic reactor with KNO3 addition,
(12) Separator with nylon mesh (13) Compressed air supplier
1 2 3
11
10 9 13
8
6 4
12 12
7 7 5 7
Working volume (liquid, sponge and biomass) of each reactor was made as 15 L with tap water. In each 15 L reactor total amount of sponge cubes were 120 g and total volume of sponge cube in each reactor was 2360 cm3, which was 15.7% of working volume of each reactor, giving number of sponge cubes in each reactor as 2353. All reactors were operated in up flow mode and influent were added 10 cm height from the bottom. Each reactor was connected with a clarifier of volume 500 mL. Mixed liquor from each reactor was allowed to flow to the clarifier for settling of biomass. From the clarifier clear supernatant was pumped to the next reactor and settled biomass was returned to the reactor. In R1 and R2, mixing was achieved only by the upflow motion of the influent. In R3, compressed air (0.15 L/min) was supplied for aeration which also provided mixing. Hydraulic retention time (HRT) was maintained adjusting the influent flow rate and calculated using equation (3.3a). Also, treated effluent from third reactor R3 was partially recycled to R2 at recycle ratio 1 through out the study (Eq.3.3b). In this recycle, 1000 mg/L NO3--N (as KNO3) was added, to supply adequate nitrate in anoxic reactor in order to maintain the anoxic condition. The effluent from R2 was pumped to third reactor R3 (flow rate according HRT). The influent flow rate to R2 and R3 were twice of feed flow rate. Dissolved oxygen concentrations (mg/L) in the reactors were: 0 (R1 and R2) and 4.0-4.8 (R3). The reactor system was maintained at a constant temperature (30 ± 3 °C) using a temperature controlled blower. Plastic pipes were connected to R1 and R2 which were maintained in a water seal for collection of biogas by water displacement method.
HRT (day) = Reactor volume(L)
Feed flow rate (L/day) (Eq.3.3 a)
Recycle ratio (R) = Recycled flow rate (L/day)
Influent Flow rate (L/day) (Eq.3.3 b)
Influent concentration to anoxic reactor =
Q (Effluent concentrations of anaerobic reactor) +RQ (Effluent concentrations of aerobic reactor) (Q+RQ)
(Eq.3.4)
3.2.3.1.3 Experiments in CMBR
Four sets of experiments were carried out in continuous moving bed system (CMBR)
the other parameter constant. The operating conditions and feed composition during the experiments were as shown in Table 3.1. In experiments, feed thiocyanate was varied from 0-600 mg/L; in experiment 2, HRT of reactors were varied (total HRT 3-8 days); in experiment 3, feed phenol concentration was varied from 1000-2500 mg/L and in experiment 4, feed NH4+-N was modified from 100-600 mg/L.
Table 3.1 Operational schedule and conditions of CMBR system
Feed (mg/L) HRT (day)
Experim
ent Days
Phenol NH4+-
N SCN-
Feed flow rate
(Q) (L/day)
Anaerobic (R1)
Anoxic (R2)
Aerobic
(R3) Total
140-190 2500 500 0
210-236 2500 500 110
237-285 2500 500 200
286-330 2500 500 450
1
331-388 2500 500 600
7.5 2 1 1 4
389-435 2500 500 600 10.0 4 2 2 8
436-465 2500 500 600 6.0 3 1.5 1.5 6
466-505 2500 500 600 5.0 2.5 1.25 1.25 5
2
540-592 2500 500 600 3.7 1.5 0.75 0.75 3
610-660 1000 500 800
661-710 1500 500 800
711-750 2000 500 800
3
751-794 2500 500 800
5.0 3 1.5 1.5 6
805-850 1500 100 800
851-893 1500 300 800
894-934 1500 500 800
4
935-985 1500 600 800
5.0 3 1.5 1.5 6
Feed thiocyanate in three-stage system was gradually increased from 110 to 200 mg/L with increment of 10-20 mg/L/day without changing other feed parameters. Feed SCN- was further increased to 450 and 600 mg/L in a likely manner and operated for almost 30 days.
Influent concentration to R2 was calculated using equation 3.4. Performance of the CMBR was also evaluated by varying the hydraulic retention time (HRT) from 3-8 days at constant feed of phenol, thiocyanate and ammonia concentration of 2500 mg/L, 600 mg/L and 500 mg/L, respectively. Similar to thiocyanate phenol concentration in the CMBR was varied from 1000 to 2500 mg/L maintaining thiocyanate and ammonia concentration fixed at 800 mg/L and 500 mg/L, respectively during the experiments. Feed ammonia in CMBR was varied from 100-600 mg/L and performance was evaluated in presence of thiocyanate 800 mg/L and phenol 1500 mg/L, respectively.
3.2.3.2 Fed batch moving bed reactor (FMBR)