RESULTS AND DISCUSSION
4.2 PERFORMANCE OF SEQUENTIAL ANAEROBIC–ANOXIC–AEROBIC FED BATCH MOVING BED RECTOR (FMBR) SYSTEM
4.2.1.2 Performance of anoxic FMBR (B2) at varied influent thiocyanate
R1 culture. In the present study, the higher thiocyanate concentration was found to have direct effect on biogas production as biogas generation was 50–85 mL/h in presence of 100 and 200 mg/L SCN–, respectively and beyond it was absent. Biomass yield was 0.1-0.7 gVSS/gCODremoved /day. Biomass concentration through out the study was almost same at ~10000 mg/L. The ratio of attached biomass to suspended biomass decreased from 10.9 at 100 mg/L feed thiocyanate to 7.3 and 9.9 at higher feed SCN– of 400 and 800 mg/L, respectively. Volatile fatty acid (VFA) concentration was 625–740 mg/L as acetic acid through out the study.
99% influent phenol releasing ~1 mg/L phenol in effluent. Increased phenol removal rate 0.07–0.110 g/L.day was observed in presence of increased influent SCN–.
Figure 4.35 Thiocyanate removal by B2 and B3 at varied feed thiocyanate concentration
0 10 20 30 40 50 60
100 200 400 800
Feed thiocyanate (mg/L)
Fraction removal (%)
B1 B2 B3
Figure 4.36 (a) Effect of thiocyanate loading on thiocyanate removal rate in B2
y = 0.9814x - 0.0013 R2 = 0.9996 0
0.03 0.06 0.09 0.12 0.15 0.18
0 0.05 0.1 0.15 0.2
Thiocyanate loading (g/L.day) Thiocyanate removal rate (g/L.day)
Influent COD to B2 was 1253–1823 mg/L with increased COD loading of 0.50–0.73 g/L.day. B2 showed increased COD removal efficiency of 70–86% with increase in influent SCN–. Figure 4.36 (b) shows that COD removal rate in B2 increased from 0.361 to 0.633 g/L.day with increase in influent COD loading. B2 contributed 12–18% and 20–29%
of total phenol and COD removal, respectively that increased with increased SCN–, indicating thiocyanate inhibition on B2 in terms of phenol or COD removal was nil [Figure
4.34 (a) and (b)]. This was probably due to diluted concentration of SCN–in influent as compared to B1.
Figure 4.36 (b) Phenol and COD removal rates in B2 0.00
0.03 0.06 0.09 0.12 0.15
0.06 0.07 0.08 0.09 0.10 0.11 0.12
Phenol loading (g/L.day) Phenol removal rate (g/L.day)
0.0 0.2 0.4 0.6 0.8 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75
COD loading (g/L.day)
COD removal rate (g/L.day)
Phenol COD
Table 4.14 (a): Performance of anoxic FMBR (B2) at feed thiocyanate variation
Thiocyanate Phenol COD NH4+–N
S0 Se Rem S0 Se Rem S0 Se Rem S0A Se Rem
TVS pH
44.6 4.0 (1)
90.9 175.5 1 99.43 1253 350 (55)
72.06 271 250 (0)
7.8 10580 8.4
91 7.9 (1)
91.33 196.5 1 99.49 1293 380 (9)
70.62 282 260 (5)
7.9 11120 8.5
189 2.2 (0.5)
98.82 240 1 99.58 1527 405 (39)
73.47 308 288 (0)
6.4 12300 8.3
398 12 (0)
96.9 279.3 1 99.65 1823 240 (0)
86.8 395 360 (0)
8.9 13580 8.4
S0: Influent (mg/L), Se: Effluent (mg/L), Rem: Removal (%),
A Influent NH4+–N of B2 = {Effluent NH4+–N of (B1+B3)/2 + 0.24x (SCN– removed in B2)}.
TVS: Biomass as Total volatile solids in sponge + suspension, (mg/L);
Numbers in parenthesis indicate standard deviation values.
Influent NH4+–N in B2 was 271–395 mg/L along with NH4+–N generated from degradation of SCN–(0.24 mg ammonia from 1 mg SCN–) in B2. At higher SCN– loading
in B2, with higher SCN–degradation more NH4+–N generation occurred and resulted in higher NH4+–N concentration. B2 removed 6.4–8.9% NH4+–N releasing higher NH4+–N concentration of 280–360 mg/L in effluent towards higher influent thiocyanate study.
The mixed liquor from B3 was added with external 750 mg/L NO3––N and recycled to B2 to give complete anoxic environment. The influent NO3––N and NO2––N to B2 was 627–
580 mg/L and 22–43 mg/L, respectively resulting total NOx–N loading rate 0.249–0.260 g/L.day in B2. B2 showed almost complete nitrite removal though nearly 249–370 mg/L nitrate–nitrogen remained in effluent. NOx–N removal increased from 43–60% with increase in influent SCN–. NOx–N removal rate also increased from 0.112 to 0.149 g/L.day with increase in SCN– loading (Figure 4.37). The main cause of incomplete denitrification in B2 might be due to lower availability of carbon source as compared to NO3––N (Tam et al. 1992). However in the present study NOx–N removal was the main process of nitrogen removal from the system as contribution of B2 was significantly higher in total nitrogen removal being 24–29% compared to B3 (Figure 4.38).
Table 4.14 (b): Performance of anoxic FMBR (B2) at influent thiocyanate variation SCN- NO3––N NO2––N NOx–
–N
SO4–2
S0 S0# Se S0 Se Rem
COD:
Nrem
CODB
S0 Se Gen Th SO4-2 Err 44.6 627 370
(36)
22 1
(0.5)
42.9 3.4 15 80 150
(3)
70 67 3
91 599 340
(37)
27 1.4 (0.2)
45.5 3.4 16 160 305
(19)
145 137 8
189 597 310 (23)
40 1.0 (0)
51.2 3.9 27 265 520
(38)
255 308 -53
398 580 249 (34)
42 0 (0) 60.0 5.8 51 620 1180 (37)
560 636 -76
S0: Influent (mg/L), Se: Effluent (mg/L), Rem: Removal (%), Gen: Generation (mg/L);
CODB: COD fraction (%) for biomass;
#: NO3–N 750 mg/L was added externally in the recycle from B3.
Th SO4–2: Theoretical sulfate generation (mg/L); Err: Error (mg/L) Numbers in parenthesis indicate standard deviation values
Figure 4.37 NOx-N removal (solid line) and removal rate (dotted line) in B2 at varied SCN- loading
0.08 0.1 0.12 0.14 0.16
0.00 0.05 0.10 0.15 0.20
Thiocyanate loading (g/L.day) NOx-N removal rate (g/L.day)
30 40 50 60 70
NOx-N removal (%)
Figure 4.38 Nitrogen removal by B2 and B3 at varied feed thiocyanate concentration
0 10 20 30 40
100 200 400 800
Feed thiocyanate (mg/L)
Fraction removal (%)
B2 B3
In the present study COD/Nrem ratio calculated using equation 4.5 was 3.4–5.8 and found to increase with influent SCN– in B2 and increased COD removal [Table 4.14 (b)]. By dividing with stoichiometric ratio 2.86 (Sarfaraz et al. 2004; Zhu et al. 2006), it was found that 52–85% of removed COD was utilized for nitrate reduction and remaining 15–51%
COD was utilized for biomass synthesis in B2. Considering 1.42 as COD of the biomass (C5H7NO2), the observed yield of biomass in B2 was 0.10–0.35 (Metcalf and Eddy, 2003).
At influent SCN–concentration 44 mg/L, suspended biomass in B2 was 1400 mg/L and increased to 3200–4000 mg/L at influent SCN– 91 mg/L or above. Attached biomass
concentration initially decreased from 9100 to 7800 mg/L when influent SCN– increased from 44 to 91 mg/L and then gradually increased to 8000–9780 mg/L in presence of 190–
398 mg/L SCN–. The ratio between attached to suspended biomass decreased from 6.4 to 2–2.5 with increase in with increased suspended biomass in liquor at high influent SCN–. Total biomass concentration measured found to increase from 10.5 g/L to 13.5 g/L in presence of higher SCN–concentration.
Nearly 70 to 560 mg/L sulfate generation occurred in B2. Increased sulfate concentration was observed in effluent with increased SCN– removal in B2. However sulfate generation at higher influent thiocyanate concentration was lower to theoretical value and showed higher difference as was observed in case of R2 in CMBR with higher thiocyanate removal rate which might be due to accumulation of other intermediate compounds like thiosulfate and polysulfides etc.. The difference between experimental sulfate generation and theoretical sulfate generation is presented as error in Table 4.14 (b).