RESULTS AND DISCUSSION
4.2 PERFORMANCE OF SEQUENTIAL ANAEROBIC–ANOXIC–AEROBIC FED BATCH MOVING BED RECTOR (FMBR) SYSTEM
4.2.3.2 Performance of anoxic FMBR (B2) at varied HRT
Table 4.19: Performance of anaerobic fed batch MBR (B1) at HRT variation
HRT (day) SCN– Phenol COD TVS
Total B1 S0 Se Rem S0 Se Rem S0 Se Rem
pH
(mg/L)
5 2.5 790
(0)
1.25 750
(0)
50.00 4035
(60)
25.27 6.5 10537 (300)
6 3 790
(0)
1.25 700
(36)
53.33 3587
(39)
33.57 6.9 10200 (335)
8 4 790
(0)
1.25 620
(0)
58.67 2808
(115)
48.0 6.6 10721 (330)
10 5
800
794 (12)
0.75
1500
558 (9.5)
62.80 5400
2527 (45)
53.20 6.8 10126 (300) S0: Influent (mg/L), Se: Effluent (mg/L), Rem: Removal (%),
Numbers in parenthesis indicate standard deviation values
Ramakrishnan and Gupta (2006) reported phenol and COD removal of 92% and 88%
while treating phenolic wastewater by a hybrid anaerobic reactor (bottom UASB and anaerobic filter on the end) at phenol and COD loading rate of 0.45 g/L.day and 2.24 g/L.day, respectively. Boubaker and Ridha (2007) have reported that a maximum COD removal of ~90% could be achieved by increasing the HRT to 36 days at lower OLR (0.67 g/L.day). Present study with high feed phenol concentration in presence of toxic compounds and at low HRT shows better performance than suspended system though it was less to the reported attached growth system. No ammonia was removed in B1 even at higher HRT and no SMA was detected through out the study. Reactor pH was always observed to decrease to 6.8–6.9. Suspended biomass and attached biomass was fluctuating 920–1000 mg/L and 9200–9700 mg/L, respectively in B1 during the study. Total biomass concentration through out the study was 10.1 –10.7 g/L showing no significant affect of HRT on it and the ratio of attached biomass to suspended biomass was 9.6–10.2.
Influent SCN– ranged from 396–399 mg/L and loading increased from 0.16–0.32 g SCN– /L.day with decreasing HRT from 2.5–1.25 days. B2 was the sole reactor efficiently degrading 88–97% of influent SCN–with increasing HRT and released 12–45 mg/L SCN– in its effluent [Table 4.20 (a)]. At minimum HRT, thiocyanate removal was only 88%
releasing maximum of 45 mg/L thiocyanate in effluent. However, SCN– removal significantly increased to 96–97% at increase in HRT of 1.5–2.5 days. It was 44–48% of total feed SCN– removal. The removal rate increased from 0.154 to 0.283 g SCN–/L.day with increased SCN– loading and decreased HRT. Maximum SCN– removal rate in B2 was 0.283 g/L.day at HRT 1.25 day at maximum loading indicating absence of substrate inhibition of thiocyanate in B2 (Figure 4.50).
Figure 4.50 Thiocyanate degradation at varied loading in B2 at varying reactor HRT
50 60 70 80 90 100
0 0.1 0.2 0.3 0.4
Thiocyanate loading (g/L.day)
Removal (%)
0.0 0.1 0.2 0.3 0.4
Rate (g/L.day)
Removal Rate
With decreasing HRT, B2 received increased influent phenol concentration 279–376 mg/L.
B2 removed ~92% influent phenol and released maximum of 30 mg/L phenol in effluent when HRT was minimum of 1.25 days. With increase in HRT to 1.5 days or more phenol removal in B2 increased to more than 99% and released only ~ 1 mg/L phenol. Phenol loading rate in B2 was 0.112–0.301 g/L.day and removal rate achieved was 0.111–0.278 g/L.day (Figure 4.51). Similarly, influent COD concentration in B2 increased from 1323 mg/L to 2135 mg/L with decreasing HRT and the corresponding loading rate was 0.529–
1.71 g/L.day. The COD removal efficiency increased from 54 to 82% with increase in HRT. B2 showed increased COD removal rate from 0.433–0.922 g/L.day at increasing
loading rate (Figure 4.51). Contribution of B2 in total feed phenol and COD removal was 18–23% and 20–23%, respectively and B2 was efficiently removing phenol/COD or thiocyanate at a wide range of HRT in present study.
Table 4.20 (a): Performance of anoxic fed batch MBR (B2) at HRT variation HRT
(d)
SCN– Phenol COD NH4+–N
B2 S0 Se Rem S0 Se Rem S0 Se Rem S0A Se Rem
pH
1.25 399 45 (0)
88.72 376 30 (0)
92.02 2135 983 (3.7)
54 435 390 (0)
10.10 8.4
1.5 396 16 (0)
95.96 350 1 (5)
99.71 1916 635 (8.9)
69 381 366 (0)
3.40 8.5
2 396 12
(2)
96.97 310 1 (0)
99.68 1525 450 (30)
70 395 380 (0)
6.20 8.3
2.5 398 12 (0)
96.98 279 1 (0)
99.64 1323 240 (22)
82 395 360 (0)
8.90 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)}.
Numbers in parenthesis indicate standard deviation values
Figure 4.51 Effect of Phenol and COD loading on removal rates in B2 at varying reactor HRT
0.00 0.05 0.10 0.15 0.20 0.25 0.30
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 Phenol loading (g/L.day)
Phenol removal rate (g/L.day)
0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.5 1.0 1.5 2.0
COD loading (g/L.day)
COD removal rate (g/L.day)
Phenol COD
Influent NH4+–N in B2 was 381–435 mg/L that constituted from effluent of B1 and recycle from B3 along with NH4+–N generated from degradation of SCN–in B2. At higher HRT slightly higher SCN– degradation occurred increasing NH4+–N generation and at short HRT, recycle from B3 was rich in NH4+–N resulting higher final influent NH4+–N in B2.
B2 removed 3–10% NH4+–N releasing 360–390 mg/L NH4+–N in effluent.
NO3––N externally added in the recycle was 1000 mg/L and influent NO3–N ranged between 590–715 mg/L. During higher HRT period, B3 released more NO3––N in its effluent which enriched the recycle and hence total influent NOx–N to B2 increased.
During short HRT 1.25–1.5 days NO2––N 0.2–1.0 mg/L was detected in effluent of B2 whereas no nitrite was detected in effluent during longer HRTs. NOx–N removal efficiency in B2 initially increased from 63–69% with increase in HRT 1.25 to 2 days and then decreased to 58% with further increase in HRT to 2.5 days. During lower HRT, influent COD was higher which might have facilitated increased NOx–N removal. NOx–N removal rate was found to increase from 0.175–0.321 g/L.day with a slope of 0.68 with increase in NOx–N loading 0.303–0.506 g/L.day (Figure 4.52).
Figure 4.52 Effect of nitrate and nitrite -N loading on denitrification rate in B2 at varying reactor HRT
y = 0.6833x - 0.0175 R2 = 0.9503 0.00
0.05 0.10 0.15 0.20 0.25 0.30 0.35
0.0 0.1 0.2 0.3 0.4 0.5 0.6
NOx-N loading (g/L.day) NOx-N Removal rate (g/L.day)
Contribution of B2 in total nitrogen removal increased from 26% to 31% with increase in reactor HRT (Figure 4.53). Also, the COD/Nrem ratio calculated using equation 4.5 was 2.0– 3.07 at HRT 1.25–2.5 days being low at higher HRT and the COD fraction calculated for biomass was low being 2–7% only. The biomass yield coefficient was only 0.01–0.04.
In B2, suspended biomass was 3800– 4730 mg/L being higher at low HRT and attached
biomass was 8800–9700 mg/L during the study. The attached to suspended biomass ratio increased from 1.9 to 2.6 with increase in HRT as suspended biomass concentration slightly decreased. Total biomass concentration in B2 was ~13–13.7 g/L through out the study with higher ratio of attached biomass to suspended biomass at higher HRT.
Figure 4.53 Contribution of B2 and B3 in nitrogen removal in FMBR
0 5 10 15 20 25 30 35
1.25 1.5 2 2.5
HRT (day) Fraction nitrogen removal (%)
B2 B3
Table 4.20 (b): Performance of anoxic fed batch MBR (B2) at HRT variation HRT
(d)
Nitrate Nitrite NOx–
N
Sulfate
B2 S0 Se S0 Se Rem S0 Se Gen Th
SO4–2 Err
COD:
Nrem
CODB TVS (mg/L)
1.25 590 230 (0.6)
42.5 1 (2)
63.50 405 760 (62)
355 584 –229 3.07 7
13740 1.5 664 245
(1.6)
36.0 0.2 65.00 485 932 (19)
447 626 –179 2.92 2
13172
2 678 225
(4.94)
47.3 0 68.91 488 960 (44)
471 633 –161 2.00 –
13110 2.5 715 319
(6.3)
42.5 0 58.00 644 1197 (36)
553 636 –82 2.40 –
13580 S0: Influent (mg/L), Se: Effluent (mg/L), Rem: Removal (%), Gen: Generation (mg/L);
Th SO4–2: Theoretical sulfate generation; Err: Error (mg/L) CODB: COD fraction (%) for biomass
Numbers in parenthesis indicate standard deviation values
Nearly 760–1197 mg/L sulfate was detected in effluent of B2 during the study and higher sulfate concentration was observed during higher HRT. Sulfate generation was 355–553 mg/L and increased with increase in HRT. However sulfate generation was lower to the theoretical sulfate generation through out the study. During lower HRT of 1.25– 2 days, though the reactor showed high thiocyanate removal as in higher HRT, the final product, sulfate generation was having large distance from theoretical sulfate generation.