RESULTS AND DISCUSSION
4.1 PERFORMANCE OF SEQUENTIAL ANAEROBIC–ANOXIC–AEROBIC CONTINUOUS MOVING BED REACTOR (CMBR) SYSTEM
4.1.1.1 Performance of anaerobic CMBR (R1) at varied feed thiocyanate
Steady state performance of R1 at varied influent thiocyanate concentrations is presented in Tables 4.1 (a) and (b) as average values along with standard deviation. Thiocyanate loading rates were 0.055, 0.1, 0.225 and 0.30, g SCN–/L.day. In R1 SCN– removal increased from 4.5% to 7.7% with increase in SCN– loading up to 0.225 g SCN–/L.day and thereafter decreased to 4.7% at SCN– loading of 0.30 g/L.day. Figure 4.2 shows that maximum SCN– removal rate of 0.017 g SCN–/L.day was achieved in R1 at loading of 0.225 g SCN–/L.day. Hung and Pavlostathis (1998) observed no degradation of SCN– by anaerobic microorganisms at feed SCN– of 145 mg/L and the probable reason was reported as absence of appropriate enzymes. Present result was better than literature findings, though removal of SCN– was low in R1.
0 100 200 300 400 500 600 700
200 250 300 350 400
Thiocyanate (mg/L) a)
0 500 1000 1500 2000 2500 3000
150 200 250 300 350 400
Phenol (mg/L)
b)
0 2000 4000 6000 8000
150 200 250 300 350 400
COD (mg/L)
c)
Figure 4.1 Pollutant profile in sequential CMBR a) Thiocyanate, b) Phenol, c) COD and d) NH4+-N during feed
-
0 100 200 300 400 500 600
150 200 250 300 350 400
Time (days) NH4+ -N (mg/L)
Feed R1 Inf R2 Eff R2 Eff R3 d)
Influent phenol concentration was fixed at 2500 mg/L with loading rate of 1.25 g/L.day. In R1 removal of phenol decreased from 48% to 12.6% (almost 73% decrease) with increase in feed SCN– concentration from 0–600 mg/L [Table 4.1 (a)]. For removal of COD, the effect was similar in nature but more profound. During feed SCN– variation study, average feed COD were 7400–7980 mg/L (Feed COD was principally contributed from phenol with 1mg phenol contributed 2.38 mg COD and 1 mg SCN– contributed 1.11 mg COD).
COD removal decreased form 50% to 3.7% (more than 92% decrease) with increase in feed SCN– from 0–600 mg/L. In Figure 4.2, it can be seen that both phenol and COD removal rates showed declining trends (phenol removal rate 0.158–0.61 g/L.day; COD removal rate 0.148–1.85 g/L.day) with increase in loading of SCN– in R1. These results show that SCN– inhibited degradations of phenol and COD in anaerobic environment.
Previous literatures clearly reported inhibitory effect of SCN– on degradation of phenol in aerobic environment (Banerjee, 1996). However, no literature report is available on the effect of SCN–, on degradation of phenol in anaerobic environment. Table 4.1 (a) shows that at all levels of feed SCN–, removal of NH4+–N in R1 was almost nil. In anaerobic condition NH4+–N removal might occur due to incorporation of nitrogen in biomass.
Figure 4.2 Effect of thiocyanate loading on performance of R1 0.000
0.005 0.010 0.015 0.020
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 Thiocyanate loading (g/L.day)
Thiocyanate removal rate (g/L.day)
0.0 0.4 0.8 1.2 1.6 2.0
Phenol/ COD removal rate (g/L.day)
Thiocyanate Phenol COD
In Figure 4.3 fractional removals of feed SCN–, feed phenol and feed COD by R1 are shown. It can be seen that the removal of total feed SCN– by R1 was very low at all levels of feed SCN–. However, R1 had significant role in fractional removals of feed phenol and COD only when feed SCN– was low. This decreased with increase in feed SCN–
concentration. In R1, pH decreased from 7.5 to 6.8–6.9 and effluent VFA (volatile fatty acid) concentration observed was 175–379 mg/L as acetic acid.
0 10 20 30 40 50
110 200 450 600
Feed thiocyanate (mg/L)
Fraction Removal (%)
R1 R2 R3
(a)
0 10 20 30 40 50 60
0 110 200 450 600
Feed thiocyanate (mg/L)
Fraction Removal (%)
R1 R2 R3
(b)
Figure 4.3 Fractional removal of (a) Thiocyanate (b) Phenol and (c) COD in individual reactor at
varied feed thiocyanate concentration 0
10 20 30 40 50 60
0 110 200 450 600
Feed thiocyanate (mg/L)
Fraction Removal (%)
R1 R2 R3
(c)
Table 4.1 (a): Average performance of anaerobic CMBR (R1) at feed SCN– concentration variation
SCN– Phenol COD NH4+–N
Feed Se Rem S0 Se Rem S0 Se Rem S0 Se
0 – – 1280
(76)
48.8 7400 3700 (65)
50.0 480
(15) 110 105
(0)
4.5 1701
(8.9)
31.9 7500 5000 (11)
33.0 500
(17.5) 200 190
(5.7)
5.0 1858
(33)
25.7 7700 5900 (56)
23.4 505
(4.2) 450 415.3
(6.6)
7.7 2137
(51)
14.5 7850 6309 (292)
19.6 503
(27.9) 600 572
(0)
4.7
2500
2184 (37)
12.6 7980 7685 (28)
3.7
500
500 (0) S0: Influent (mg/L), Se: Effluent (mg/L), Rem: Removal (%);
Numbers in parenthesis indicate standard deviation values
Table 4.1 (b) shows that biogas generation in R1 decreased with increase in feed SCN–, and no gas produced at feed SCN– of 450 mg/L and above. Specific methanogenic activity (SMA) of R1 biomass was studied at varying concentration of feed SCN–. In absence of SCN–, SMA activity in R1 sludge was 0.349 gCH4–COD/gVSS.day and decreased to 0.1019 gCH4–COD/gVSS.day and 0.036 gCH4–COD/gVSS.day when feed SCN– concentrations of 110 mg/L and 200 mg/L, respectively was added to biomass taken from R1. No SMA was observed when feed SCN– was 450 mg/L and above, suggesting SCN– inhibited methanogenic activity at feed SCN– ≥ 450 mg/L. Fang et al. (1996, 2006) reported SMA of 0.19 and 0.24 gCH4–COD/gVSS.day during degradation of phenol at temperatures of 26 and 37ºC, respectively. Tay et al. (2001) reported SMA of 0.17 g CH4– COD/gVSS.day at initial phenol of 2000 mg/L using phenol as the sole carbon source.
SMA observed in the present study in absence of SCN– in feed at initial phenol of 2500 mg/L was higher than these reported values. As no SMA was achieved at high SCN- concentration, biomass yield was calculated and found in ranged of 0.3-0.7 g VSS/g COD
Table 4.1 (b): Average performance of anaerobic reactor (R1) at feed SCN– concentration variation
SCN– pH VFA
Feed (mg/L)
Biogas (mL/day)
SMA (g CH4–COD/
g VSS. day)
Se
TVS (mg/L)
Se
0 344 0.349 6.7 12600 379 (14)
110 156 0.102 6.8 10200 175 (4)
200 56 0.036 6.9 11000 288 (45)
450 0 ND 6.8 10750 194 (40)
600 0 ND 6.7 10500 285 (31)
Se: Effluent (mg/L), ND: not detectable; VFA: Volatile fatty acid as acetic acid (mg/L) Numbers in parenthesis indicate standard deviation values
Total volatile solids (TVS) in R1 was almost 12600 mg/L in absence of thiocyanate and decreased little to 10,000–11,000 mg/L with addition of SCN– then remained stable at 10700– 10500 mg/L at maximum feed SCN–. The ratio of attached biomass to suspended biomass in R1 was 7.7 in absence of SCN–. This ratio increased to 13.5 with addition of 110 mg/L thiocyanate as biomass in suspension decreased from 1400 mg/L to 700 mg/L (decreased by almost 50%) and then this ratio decreased to 10.6–10.2 at feed thiocyanate more than 200 mg/L. This indicates higher amount of biomass was in attached condition to sponge cube than the suspended biomass concentration through out the study.