RESULTS AND DISCUSSION
4.1 PERFORMANCE OF SEQUENTIAL ANAEROBIC–ANOXIC–AEROBIC CONTINUOUS MOVING BED REACTOR (CMBR) SYSTEM
4.1.4.4 Overall performance of three–stage CMBR at ammonia-N variation
Chung, (2006a). In a suspended growth aerobic reactor maximum SCN– degradation rate was reported as 0.019 g/L.day at influent SCN– of 210 mg/L in presence of phenol and NH4+–N by Vázquez et al. (2006a) which is close to SCN– degradation rate observed in the present investigation. Banerjee (1996) observed maximum SCN– degradation rate of 0.2 g/L.day in a rotating biological contactor in presence of phenol. However in present study R3 received very low amount of SCN–than reported inhibitory value of 200 mg/L in its influent and successfully removed the same (Kim et al. 2008b).
In presence of varied influent NH4+-N, 3–62 mg/L sulfate evolved in R3 as byproduct from SCN– biodegradation along with NH4+–N. During the study, sulfate generation was similar to the theoretical sulfate generation value with low error [Table 4.12 (b)].
system as anoxic reactor was highly efficient and helped to improve the overall performance of the three–stage system.
Figure 4.31 Overall performance of three stage CMBR at varied feed ammonia-N concentration
0 20 40 60 80 100
0 200 400 600 800
Feed Ammonia-N (mg/L) Removal and remaining nitrogen fraction in effluent (%)
Phenol COD Thiocyanate
Ammonia-N TN Unoxidized N
Oxidized N
4.1.5 Summary of CMBR 4.1.5.1 Anaerobic CMBR (R1)
Acclimatization of anaerobic mixed culture with gradual increase to high influent concentration helped R1 to sustain toxic multi–components like phenol, thiocyanate and ammonia–nitrogen. Performance of CMBR was evaluated at varying concentrations of feed thiocyanate, phenol and ammonia–nitrogen and at varied HRT. In all studies, R1 showed removal of phenol and COD along with insignificant removal of thiocyanate and no removal of ammonia–nitrogen. Prolonged acclimatization did not improve thiocyanate and ammonia–nitrogen removal in R1. With introduction of thiocyanate, phenol and COD removals in R1 continuously decreased and specific methanogenic activity was nil when feed thiocyanate was higher than 200 mg/L. Phenol removal was comparatively higher (4–
46%) than that of COD removal (2–33%) suggesting accumulation of intermediates.
Maximum phenol removal rate observed in R1 was 0.399 g/L.day at phenol and SCN–
loading of 1.250 g/L.day and 0.055 g SCN–/L.day, respectively during overall study. In presence of high thiocyanate loading of ~0.270 g/L.day, phenol removal rate in R1 was low of ~0.15 g/L.day even at low phenol loading (0.333–0.833 g/L.day). Decreasing phenol loading at higher HRT also did not help to achieve high phenol removal rate in presence of high thiocyanate loading. With increase in feed phenol concentration, phenol and COD removals in R1 decreased, however the effect was less profound than affect of higher thiocyanate in feed. Phenol and COD removals in R1 decreased from 42% to 5%
and 10% to 6%, respectively (decreased by 88% and 42%, respectively) with increase in phenol concentration 1000 mg/L to 2500 mg/L whereas with increase in influent thiocyanate from 110 to 600 mg/L phenol and COD removal decreased from 32% to 12%
and 33% to 3%, respectively (decreased by 63% and 89%, respectively). Similarly increase in influent NH4+–N loading also exerted toxicity to phenol degraders though the affect was comparatively low to that of thiocyanate (phenol removal decrease by 50%). Therefore toxicity intense is SCN–> phenol> NH4+–N to anaerobic phenol degrading microbes. At higher thiocyanate concentration more than 200 mg/L, biogas generation was absent and all COD removed might get incorporated to biomass. High amount of biomass concentration (8–9 g/L) was retained in sponge cube and higher ratio of attached to suspended biomass (10–8:1) was maintained in anaerobic reactor. No clogging or sludge rising effect was observed through out the study. Higher HRT of 2–3 days was observed as favorable for partial treatment of wastewater in anaerobic CMBR when high influent phenol and thiocyanate concentration are introduced in the reactor.
Anoxic reactor was prepared with mixed culture through acclimatization similar to R1. R2 received effluent of R1 containing SCN–, phenol, COD and NH4+–N and recycle from R3.
The recycle effect (recycle ratio of 1) decreased the toxicity of R1 effluent. As nitrate from recycle in R3 was inadequate based on stoichiometric requirement of COD and thiocyanate oxidation, nitrate was added externally to R2 during the study.
In R2, simultaneous degradation of phenol and thiocyanate occurred. Separate study confirmed that nitrate was essential for degradation of SCN– in anoxic environment and 0.38 g NO3––N was consumed for removal of each g of SCN– and NH4+–N and sulfate 4.1.5.2 Anoxic CMBR (R2)
were end products. R2 was able to handle upto 0.4 g SCN–/L.day without inhibition on removal of SCN–. Maximum SCN– removal rate observed in R2 was 0.265 g/L.day in presence of loadings of 0.277 g phenol/L.day, 0.267 g SCN–/L.day and 0.178 g NH4+– N/L.day. It was observed that phenol loading in R2 above 0.5 g/L.day showed little inhibitory effect on thiocyanate removal rather than thiocyanate itself. The maximum SCN– loading of 0.40 g/L.day did not show any inhibitory affect on phenol degradation and maximum phenol removal rate of 0.94 g/L.day was achieved at phenol loading of 1.61 g/L.day. NH4+–N removal in R2 was due to incorporation of nitrogen into biomass and it increased when phenol, thiocyanate and NH4+–N concentrations were low in influent.
Sulfate balance between experimental and theoretical values showed higher error when thiocyanate removal rate was high, suggesting formation of other intermediate sulfur compounds. COD removal in R2 was 50–85% and maximum removal rate of 2.8 g/L.day was observed at loading of 5.4 g/L.day irrespective of phenol and SCN–. The removal ratio of COD to NO3––N was almost 3–7. In R2, with increase in influent COD, higher amount of COD removal was achieved and also more COD was channeled for synthesis of biomass. COD fraction for biomass ranged from 5% to 60%. With increase in influent thiocyanate suspended biomass concentration in R2 increased while attached biomass concentration decreased. Attached to suspended biomass concentration ratio significantly decreased from 8 to 3 with addition of increasing thiocyanate and remained 2–4 in rest of the studies with high influent thiocyanate. Nitrate–nitrogen removal was incomplete, though nitrite–nitrogen was exhausted completely. Maximum NOx––N removal rate achieved in R2 was 0.61 g/L.day at maximum loading of 0.80 g nitrate/L.day at HRT of 0.75 day during HRT variation study. Sludge rising was regular in R2 but after releasing the entrapped gas the sludge was in normal mode.
Recommended HRT of R2 was 1–1.5 days for removals of 57–61% phenol, 61–70% COD, 57–69% thiocyanate, 67–70% COD and 3–6% NH4+–N from influent concentration of 822–1206 mg/L, 264–400 mg/L, 3815–4053 mg/L and 330–401 mg/L, respectively.