RESULTS AND DISCUSSION
4.1 PERFORMANCE OF SEQUENTIAL ANAEROBIC–ANOXIC–AEROBIC CONTINUOUS MOVING BED REACTOR (CMBR) SYSTEM
4.1.1.3 Performance of aerobic CMBR (R3) at varied influent thiocyanate
Table 4 2 (c): Performance of 5L anoxic batch reactor at varying NO3––N concentration
NO3––N COD Phenol SCN– NH4+–
N
Sulfate
S0 Se Rem S0 Se Rem Se Rem Se Rem Se
TVS (mg/L)
Se Th SO42–
Err
50 6 (2.1) 88.0 700 242.0 (15)
65.43 79.0 (1.6)
60.5 13.00 (1.2)
67.5 12.0 (0)
5211 (440)
44.0 (2)
44.5 –0.5
75 6 (3) 92.0 710 152.0 (30)
78.59 8.8 (1.7)
95.6 9.35 (3)
76.6 13.0 (10)
7202 (555)
45.0 (2)
50.5 –4.5
100 8 (0.9) 92.0 720 138.0 (22)
80.83 0.6 (0) 99.7 6.35 (1.3)
84.1 14.0 (0)
8220 (548)
50.0 (3)
55.5 –4.5
200 38 (2) 81.0 720 135.0 (17)
81.25 1.3 (0) 99.3 1.90 (1)
95.2 12.0 (0)
9432 (600)
58.0 (2.3)
62.8 –3.2
350 127.0 (3)
63.7 720 120.0 (22)
83.33 0.8 (0) 99.6 1.65 (0)
95.4 12.0 (0)
9440 (600)
60.0 (2.5)
63.2 –3.2
0 0 NA 700 550.0
(20)
21.43 120.0 (1.1)
40.0 38.00 (1)
5.0 11.0 (0)
3930 (580)
– – –
S0: Influent (mg/L); Se: Effluent (mg/L); Rem: Removal (%) Err: Error (mg/L);
Influent phenol, SCN–and NH4+–N were constant at 200, 40 and 10 mg/L, respectively.
Th SO42–: Theoretical sulfate generation (mg/L); [1.65* (SCN– removed (mg/L)]
Vázquez et al. (2006b) reported 90% removal of SCN– while treating coke wastewater in an activated sludge process at SCN– loading of 0.02 g/L.day at HRT of 9.83 days in presence of 177 mg/L of phenol. The performance of R3 was higher than these reported values.
Figure 4.6 (a) Effect of thiocyanate loading on SCN- removal rate in R3
y = 0.9693x + 6E-05 R2 = 1
0 0.04 0.08 0.12 0.16 0.2
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14
SCN- loading (g/L.day) SCN- removal rate (g/L.day)
In the three–stage system anoxic CMBR showed less SCN– removal efficiency (44–70%) than aerobic CMBR (almost 97%) at all concentrations of feed SCN–. R3 contributed 13–
24% of total SCN– removal during the study [Figure 4.3 (a)]. However, upstream anoxic reactor was responsible to bring down SCN– concentration considerably from feed. This facilitated SCN– removal in R3 as it was able to overcome the toxicity of feed SCN– when present at high concentrations. In R3, SCN– was found to oxidized and converted to sulfate as suggested by previous researchers shown in equation 2.2d (Hung and Pavlostathis, 1997).
In R3 sulfate balance was carried out [Table 4.3 (a)] considering stoichiometric evolution of sulfate (1.65 mg/mg of SCN– degradation) (equation 4.3b). Actual sulfate generation was much less than theoretical sulfate values and higher error was observed when feed SCN– was maximum at 600 mg/L. Similar to anoxic environment generation of other intermediate sulfur compound such as thiosulfate, polysulfides, element sulfur etc. were probably occurred in R3 as reported in literatures (Janssen et al. 1997; Krishnakumar et al.
2005).
Influent phenol loading in R3 was 0.344 to 0.47 g/L.day. Phenol removal in R3 was more than 99% [Table 4.3 (a)] and it seemed that SCN–concentration up to 122 mg/L did not
inhibit phenol degradation. Influent COD to R3 was 620–1318 mg/L with loading of 0.62–
1.318 g COD /L.day. COD removal efficiencies in R3 were within the range of 50–79%
and it increased with increase in COD loading. In R3, maximum phenol and COD removal rate 0.467 and 1.04 g/L.day, respectively was observed at maximum phenol and COD loading in presence of maximum SCN– loading [Figure 4.6 (b)].
Table 4.3 (a): Performance of aerobic CMBR (R3) at feed SCN– concentration variation
SCN– Phenol Sulfate
Feed SCN
–
S0 Se Rem S0 Se Rem S0 Se Gen Th.
SO42–
Err
TVS#
0 – – – 0 – – – – – – – 10120
(158) 110 30
(3.1) 0.8 (0.2)
97.3 344 (81.7)
1.1 (0.4)
99.9 140 190 (7)
50 48.2 1.8 9845
(140) 200 45
(5.7) 1.6 (1.1)
96.5 410 (67.9)
1.4 (0.5)
99.7 210 272 (16.2)
62 72.2 –10.2 11912 (133) 450 61
(8.5) 1.4 (0.8)
97.6 450 (73.3)
1.0 (0.4)
99.8 590 705 (7.8)
115 98.3 16.7 12408 (175) 600 122
(12) 3.8 (0.6)
96.9 468 (12.5)
1.5 (0.6)
99.7 600 730 (26)
130 195.0 –65 12667 (251) S0: Influent (mg/L), Se: Effluent (mg/L), Rem: Removal (%), Err: Error (mg/L).
Numbers in parenthesis indicate standard deviation values.
Gen: Sulfate generated (mg/L) = (Influent SO42–– Effluent SO42–);
Th. SO42–: Theoretical sulfate generated (1.65x SCN– removed) mg/L.
#TVS: Biomass as Total volatile solids [in sponge + suspension (mg/L)]
Biomass concentration in R3 during feed SCN– variation study was observed to increase from 9845 mg/L to 12667 mg/L when higher amount of COD and NH4+–N was entering R3. The attached biomass to suspended concentration ratio was 8.8–7.7 when R3 was receiving 30–60 mg/L thiocyanate in influent. The ratio found to drastically decrease to 3.5 with increase in influent SCN– to 122 mg/L in R3. Higher detachment of biomass to
suspended liquor might have occurred in high SCN– concentration as total biomass was not affected and observed to increase.
Figure 4.6 (b) COD and NH4
+-N removal rate in R3 at varied loading at thiocyanate variation study
0.0 0.2 0.4 0.6 0.8 1.0 1.2
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Phenol/ COD loading (g/L.day) Phenol/ COD removal rate (g/L.day)
0.00 0.05 0.10 0.15 0.20 0.25 0.30
0 0.1 NH4 0.2 0.3 0.4
+-N loading (g/L.day)
NH4+ -N removal rate (g/L.day)
COD Phenol Ammonia-N
Influent NH4+–N concentration to R3 was comprised of NH4+–N released from R2 and NH4+–N generated in R3 from degradation of SCN– (0.24 g NH4+–N from one gram of SCN– removed) and ranged from 220 to 358 mg/L [Table 4.3 (b)]. In absence of thiocyanate in feed, R3 showed maximum NH4+–N removal efficiency of 90%. Ammonia–
N removal efficiency was 80% (influent 267 mg/L) when influent phenol and SCN– to R3 were 344 and 30 mg/L. Table 4.3(b) shows that NH4+–N removal in R3 was not affected when influent phenol and SCN– increased to 450 and 61 mg/L, respectively at influent NH4+–N of 270 mg/L. However, NH4+–N removal decreased to 72% when influent phenol and SCN– were 468 and 122 mg/L, respectively at influent NH4+–N of 358 mg/L.
Inhibition of phenol on NH4+–N removal is reported by various authors and can starts from concentration as low as 5 mg/L (Neufeld and Valiknac, 1979; Jeong et al. 2006b; Kim et al. 2007; Eiroa et al. 2008). Kim et al. (2008b) reported inhibition of phenol and thiocyanate on NH4+–N removal at 200 mg/L each and substrate inhibition occurs at 350 mg/L NH4+–N and thiocyanate inhibition was mainly due to the NH4+–N generation from thiocyanate degradation resulting higher NH4+–N concentration. NH4+–N removal rate in R3 increased from 0.198 to 0.260 g/L.day with increase in increase NH +–N loading from
0.220 to 0.358 g/L.day as shown in Figure 4.6 (b). Jeong and Chung (2006b) observed maximum ammonia removal rate of 0.28 g/L.day in a fluidized bed aerobic reactor using wastewater containing COD, phenol, SCN–, CN– and NH4+–N concentration of 2200–
2400, 625, 360–390, 17–19 and 220–250 mg/L, respectively. Vázquez et al. (2006b) observed NH4+–N removal efficiency of 65% from influent NH4+–N of 1095 mg/L along with phenol concentration of 280 mg/L in absence of thiocyanate in aerobic suspended growth reactor operated at 4 days HRT. The present NH4+–N removal rate was comparable with literature values.
Table 4.3 (b): Performance of aerobic CMBR (R3) at feed SCN– concentration variation
NOx–N SCN
–
COD NH4+–N
NO3––N NO2––N S0 S0 Se Rem S0A Se Rem S0 Se S0 Se
NR CO D/
NH4
+–N
FA UN
0 620
(21)
310 (8.6)
50 220 22.0 90.0 316 (6.7)
455 (8)
0 0 0.14 2.81 11.9 24
30 950
(5.8)
364 (6.67)
61.6 267 (7.6)
50.7 81.0 47 (2.3)
235 (5)
0 4 0.19 3.56 23.4 0.5
45 947
(42.6) 354 (8.6)
62.6 267 (7.5)
54.3 80.4 45 (1.3)
255 (6)
0 7 0.22 3.41 24.4 0
61 915
(0.3)
342 (0.7)
62.6 270 (4.2)
55.0 80.6 120 (11)
247 (13)
0 58 0.19 3.22 30.3 11
122 1318 (28.5)
275 (9.3)
79.1 358 (12)
98.0 72.0 129.0 (13)
225 (9)
0 100 0.20 3.68 40.7 9
S0: Influent (mg/L), Se: Effluent (mg/L), Rem: Removal (%)
A Influent NH4+–N of R3 = {Effluent NH4+–N of R2 + 0.24 x (SCN– removed in R3)}.
NR: Nitrification rate (g/L.day); FA: Free ammonia (mg/L); UN: Unaccounted nitrogen (%);
Numbers in parenthesis indicate standard deviation values.
In R3, NH4+–N is oxidized to NO2––N and NO3––N. Nitrification rate in R3 was calculated based on generation of NO3––N and NO2––N and reactor HRT. Nitrification rate in R3 was
0.14–0.22 g/L.day and it decreased when feed SCN– was increased above 200 mg/L. In R3, accumulation of nitrite at high influent thiocyanate was observed [Table 4.3 (b)]. The main factors that enhance nitrite build–up include temperature 35–40°C (Peng and Zhu, 2006), hydraulic retention time (HRT) of 12 h or less than that (Yamamoto et al. 2006), DO concentration between 0.5– 1.5 mg/L (Bae et al. 2001; Ruiz et al. 2006) and FA concentration around 5 mg/L or above (Abeling and Seyfried, 1992; Aslan and Dahab, 2008). As high pH was observed in R3, free ammonia (FA) concentration was calculated using equation 4.6 (Anthonisien, 1976, Vadivelu et al. 2007). The FA concentration increased from 12 mg/L to 40 mg/L during the study with increase feed SCN–, influent and effluent ammonia in R3 at pH 8.1–8.4 (considering pH of R3 effluent).
Free ammonia (mg/L) =
+ pH
4
pH
[NH -N]x10
exp[6334/(273+T)]+10 (Eq. 4.6)
Negative effect of nitrite on NH4+–N removal in aerobic reactor are also reported in previous literatures (Philips and Verstraete, 2001; Fux et al. 2006). Philips and Verstraete, (2001) observed activated sludges regularly exposed to NO2––N concentrations exceeding 50 mg/L showed 40% inhibition on nitrification rates. In present study, temperature of the reactor room was controlled within 30±2 oC, HRT and dissolved oxygen was maintained 1 day and 3–4 mg/L which was high enough than reported value and not responsible for nitrite buildup. However the FA concentration in this case was higher and might be the cause of nitrite accumulation. Higher accumulation of nitrite 58–100 mg/L occurred when FA concentration calculated was 30–40 mg/L. Inhibitory effect of FA to nitrite oxidizing bacteria and ammonia oxidizing bacteria reported to occur at FA concentration of 0.1–4.0 mg/L and 10–150 mg/L, respectively (Anthonisen, 1976; Bae et al. 2001; Liu et al. 2001).
The accumulated nitrite in combination with higher concentration of phenol and thiocyanate affected NH4+–N removal in R3.
A balance was made on amount of NH4+–N removed in R3 and amount of NH4+–N oxidized to NO2––N and NO3––N and calculated as unaccounted nitrogen (UN). The unaccounted nitrogen fraction in R3 was 0.5–24%. This could be probably due to incorporation of nitrogen in biomass, suggesting that NH4+–N consumption for synthesis of heterotrophic biomass. Influent COD to NH4+–N ratio in R3 was 2.8 to 3.68 suggesting
opportunities for the growth of heterotrophs (Hankai et al. 1990; van Neil et al. 1993;
Cheng and Chen, 1994).
Clogging in aerobic reactor was absent during the experiment. However sometime the sponge cubes get stuck to the out let discontinuing the release of effluent and make the reactor over flow. The sponge cubes were then removed manually and normal reactor operation was achieved again.
4.1.1.4 Overall performance of sequential CMBR system at varied thiocyanate