RESULTS AND DISCUSSION
4.1 PERFORMANCE OF SEQUENTIAL ANAEROBIC–ANOXIC–AEROBIC CONTINUOUS MOVING BED REACTOR (CMBR) SYSTEM
4.1.3.3. Effect of influent phenol variation on performance of aerobic CMBR (R3) Average steady state performance of R3 is shown in Tables 4.9 (a) and (b). R3 received a
The effluent from R2 was rich in sulfate through out the study which was generated as a byproduct from oxidation of SCN– in R2 along with ammonia–N (Equation 2.2e). The sulfate concentration in the effluent was 800–920 mg/L and the highest concentration was evolved when influent phenol concentration to R2 was the minimum. However sulfate generation was less in R2 than that of theoretical value showing higher error at low influent phenol study [Table 4.8 (b)]. During the thiocyanate and HRT variation study, error in sulfate generation was observed to increase when influent thiocyanate concentration was high (~288 mg/L) though thiocyanate removal occurred efficiently.
During the phenol variation study, R2 always received higher concentration of thiocyanate (396–400 mg/L) and thiocyanate removal occurred through out the study and might be the reason for higher error in sulfate generation due accumulation as other intermediate sulfur compound. Influent pH to R2 was maintained fixed at 7.5± 0.5, and the effluent pH was always observed to be higher being 8.1–8.4.
4.1.3.3. Effect of influent phenol variation on performance of aerobic CMBR (R3)
respectively. However R3 released nearly 13 mg/L SCN– removing 90% of its influent SCN– at influent SCN– and phenol concentrations of 125 and 452 mg/L, respectively [Table 4.9 (a)]. Both high concentrations of phenol and SCN– might have troubled the reactor in this situation as reported previous literature (Banerjee, 1996; Kim et al. 2007, 2008 a and b). Figure 4.21(a) shows that contribution of R3 in total SCN– removal was only 0.5% when feed phenol was low as R2 almost completely taken care of the influent SCN– releasing only 3 mg/L in effluent. Fractional SCN– removal in R3 increased to 14%
when feed phenol was 2500 mg/L. Thiocyanate removal rate in R3 increased from 0.001–
0.075 g/L.day irrespective of phenol [Figure 4.22].
Table 4.9 (a): Performance of aerobic CMBR (R3) at feed phenol concentration variation
Phenol Thiocyanate NH4+–N NO3––N NO2––N
S0 Se Rem S0 Se Rem S0A Se Rem S0 Se S0 Se
NR
2.25 1 (0) 55 3.3 1 (0)
70 250 43
(2.5)
83 189 250 (16)
0 55 0.08
5 2 (0) 60 9.8 1 (0)
89 324 48
(2.2)
85 173 247 (10)
0 100 0.11
100 1 (0) 99 32 1
(0)
97 352 100
(0)
72 169 185 (4)
0 100 0.08
452 1.3 (0.6)
99 125 13 (0)
90 372 124
(13)
67 111 179 (8)
1.8 95 0.10
S0: Influent (mg/L), Se: Effluent (mg/L), Rem: Removal (%),
A Influent NH4+–N of R3 = {Effluent NH4+–N of R2 + 0.24x (SCN– removed in R3)};
NR: Nitrification rate (g/L.day);
Numbers in parenthesis indicate standard deviation values.
The main role of R3 in the system was as a polishing unit and to achieve nitrification.
NH4+–N loading was 0.167–0.248 g/L.day during the study. NH4+–N removal rate in R3 initially increased from 0.138 to 0.184 g/L.day when influent phenol was 2–5 mg/L and then decrease to 0.158–0.165 g/L.day at higher influent phenol concentration of 100–452 mg/L [Figure 4.22].
Figure 4.22 Performance of R3 at varied COD loading 0.0
0.2 0.4 0.6 0.8
0.0 0.2 0.4 0.6 0.8 1.0
COD (g/L.day)
Removal rate (g/L.day)
COD Thiocyanate Ammonia-N Phenol
R3 released 43–48 mg/L NH4+–N in its effluent with more than 80% removal efficiency when influent phenol, SCN– and NH4+–N concentration were 2–5 mg/L, 3–9 mg/L and 250–324 mg/L, respectively [Table 4.9 (a)]. However at influent NH4+–N concentration of 352 mg/L and phenol concentration of 100 mg/L, the reactor failed to achieve the same performance level and released nearly 100 mg/L NH4+–N in its effluent and removal efficiency dropped to 72%. Also, highest influent NH4+–N concentration of 372 mg/L flowed to R3 when phenol and SCN– concentration was also in their maximum concentration of 452 and 125 mg/L, respectively. In this situation NH4+–N removal achieved was least of 67% and R3 released a high concentration of NH4+–N in its effluent.
Effluent nitrate concentration of R3 continuously decreased from 250 mg/L to 179 mg/L nitrite concentration of 55–100 mg/L was observed with increase in influent phenol.
Phenol compound is known to negatively affect nitrifying bacteria even concentration as
decreased nitrification rate as initial phenol concentration was increased. Particularly, phenol and thiocyanate above 200 mg/L, there was significant inhibition on nitrification, and no nitrification occurred at phenol concentration 500 mg/L. Amor et al. (2005) reported high phenol removal efficiencies, above 99.9% along with ~99.8% ammonia removal with no inhibition by phenol on nitrification in an activated sludge reactor, at low as 5.6 mg/L (Dyreborg and Arvin, 1995; Chen et al. 2008). Kim et al. (2008a) reported
applied ammonium loading 0.14 g NH4+–N/L.day (350 mg NH4+–N/L) and at increased phenol (35–2800 mg/L).
FA concentration was found to increase with increase in influent NH4+–N and ranged from 26–44 mg/L which also have inhibitory affect on nitrifying bacteria and responsible for nitrite accumulation. In present study during higher feed phenol study, influent phenol in R3 was accompanied with higher thiocyanate concentration, FA and nitrite which might have affected the nitrification rate in combination.
Contribution of R3 in removal of total nitrogen (TN) was very poor being 5–8%. It was mainly due to the nitrogen was transferring from one form (NH4+–N) to another (NOx––N) not getting eliminated from the reactor. The NH4+–N concentration was oxidized to either nitrate or nitrite and was available in the reactor. The unaccounted nitrogen fraction in R3 was 17–38% which was higher during lower influent phenol [Table 4.9 (b)]. Nitrification rate in R3 observed was 0.08–0.11 g/L.day [Table 4.9 (a)], which was lower than NH4+–N removal rate. This might be due to some nitrogen loss through denitrification during this condition. Helmer et al. (1999) reported that under low DO concentrations autotrophic ammonia–oxidizers might be the causative agents of nitrogen loss by performing aerobic/anoxic denitrification with nitrite as electron acceptor and ammonia as electron donor. In R3 same phenomena might have occurred with the biofilm towards interior of the sponge cube.
Nearly 451–1400 mg/L residual COD entered R3 and the loading rates were 0.33, 0.44, 0.63 and 0.93 g/L.day. Almost 46–82% removal of COD occurred releasing 230–245 mg/L of effluent COD and removal increased with increase in influent COD concentration. The contribution of R3 in COD removal increased from 5% to 14% of total COD removal when it received increased influent COD [Figure 4.19 (b)]. Figure 4.22 shows that COD removal rate increased linearly from 0.14–0.77 g/L.day with increase in COD loading rate 0.33–0.93 g/L.day. Due to high amount of influent COD, higher amount of heterotrophs made biofilm at the outer site of the sponge limiting oxygen content for the slow growing nitrifiers grown in the interior of the sponge cubes. This also could be another reason for less nitrification at higher influent phenol/COD to R3. From Table 4.9 (b), it can be seen that with increased phenol feed, COD to NH4+–N ratio in the influent increased from 1.80
to 3.68. This high ratio is prone to favor higher growth of heterotrophs in R3 as mentioned earlier (Hankai et al. 1990).
In R3, suspended biomass concentration was fluctuating in 3000–3600 mg/L during the study, whereas attached biomass concentration was observed to increase with increase in influent phenol. The attached to suspended biomass ratio was 2.3 to 2.8 at influent phenol 2–100 mg/L and increased to 3.2 when R3 received 452 mg/L influent phenol. Total biomass concentration in R3 increased from 11479 mg/L to 13070 mg/L during the study with increase in influent phenol to R3 [Table 4.9 (b)].
R3 released 897–930 mg/L of sulfate in its effluent with 3–130 mg/L sulfate generation irrespective of influent phenol. Low sulfate generation than theoretical value (higher negative error of –54 mg/L) was observed when R3 was receiving higher phenol, COD and thiocyanate in influent. This might be due to accumulation of other sulfur compound like polysulfide etc as reported earlier.
Table 4.9 (b): Performance of aerobic CMBR (R3) at feed Phenol concentration variation
Phen ol
COD SO42–
S0 S0 Se Rem
COD : NH4+
–N
TVS pH FA UN
S0 Se Gen Th.
SO42–
Err
2.25 452 245 (0)
46 1.8 11479 8.4
±0.2
26.3 31 920 923 (2.5)
3 4 -1
5 666 230
(0)
65 2.06 12280 8.4
±0.2
33.2 25 888 907 (11)
19 14 5
100 950 245
(8)
74 2.88 12830 8.4
±0.2
40 38 850 897 (32)
47 51 -4
452 1400 241 (26)
82 3.68 13070 8.4
±0.2
44.3 17 800 930 (30)
130 184 -54
S0: Influent (mg/L), Se: Effluent (mg/L), Gen: Generation (mg/L); Err: Error (mg/L);
UN: Unaccounted nitrogen (%); FA: Free Ammonia (mg/L) Th. SO42–: Theoretical sulfate generation (mg/L); TVS in mg/L Numbers in parenthesis indicate standard deviation values.