Application of fluidised-bed reactors for the biological treatment of a nitrogenous industrial effluent

Abstract

The primary aim of the study was to evaluate the suitability of biological fluidised-bed reactors (BFBR's) for the biological treatment of a nitrogenous, saline industrial effluent at varying loading rates as well as optimisation of the process. During this study a synthetic effluent analogous to the condensate stream as generated by SASOL Agri, Secunda, South Africa, containing NH4NO3 concentrations of approximately 1000 mg/I as N was treated . The first objective of this study was to optimise nitrification processes using the BFBR' s, while comparing and evaluating the use of either sand or granular activated carbon (GAC) as support matrices. The BFBR's were fed with the ammonium nitrogen (NH4-N) concentration gradually being increased from 10 mg/I to 1000 mg/I. At NH4-N loading rates below 1 kg NH4-N/m3.d, removal efficiencies in excess of 99% were achieved in both BFBR' s using sand and GAC as support matrices, respectively. However, at loading rates in excess of 1 kg NH4-N/m3.d, further increase in the NH4-N loading rate was limited by an associated decrease in the dissolved oxygen concentration (<1 .5 mg/I). At a loading rate of 2 kg NH4-N/m3.d, the NH4-N removal efficiency dropped to 58% and 48% in the BFBR using sand and GAC as support matrices, respectively. The reduction in the removal efficiencies also resulted in a significant increase in the concentrations of total suspended solids (TSS), volatile suspended solids (VSS), chemical oxygen demand (COD), NH4-N, nitrite nitrogen (NO2-N) and nitrate nitrogen (NO3-N) in the effluent. Upon further increase of the NH4-N loading rate to 4 kg NH4-N/m3.d, the removal efficiency dropped to 28 % and 47% for the BFBR's using sand and GAC as support matrices, respectively. Further increase in the NH4-N loading rate was limited by the complete absence of dissolved oxygen in~the system (<0.1 mg/I). Although both sand and GAC performed well as support matrices, slightly enhanced removal efficiency was obtained using GAC as the support matrix. The next objective was to optimise the denitrification process using the BFBR's wh ile comparing and evaluating the use of either sand or GAC as support matrices. The reactors were fed with a synthetic effluent analogousto the condensate stream as generated by SASOL Agri, Secunda, in which the nitrate-nitrogen (NO3-N) concentration was gradually increased from 1 O to 1000 mg/I. Sodium acetate was used as the carbon source for the heterotrophic denitrifying microorganisms. During the initial start-up phase, the C:N ratios for both reactors was maintained at 1.14. During this period , the NO3-N removal efficiencies dropped from 99% to 60% and 40% for GAC and sand, respectively. The low removal efficiencies were also characterised by a significant accumulation of nitrite in the effluent. The C: N ratio was subsequently increased until low c_oncentrations of nitrite were present in the effluent. The optimal C:N ratios were determined to be 1.6 and 2.0 for the BFBR' s using sand and GAC as support matrices, respectively. In both reactors, NO3-N removal efficiencies in excess of 99% could be achieved at a NO3-N loading rate of 4 kg NO3-N/m3.d. As the NO3-N loading rate was increased to 8 kg NO3-N/m3.d, the NO3-N removal efficiencies dropped significantly to 67% and 23% in the BFBR's using sand and GAC as support matrices, respectively. A further significant drop in the NO3-N removal efficiencies was observed when the NO3-N loading rate was increased to 26 kg NO3-N/m3.d. At this loading rate , NO3-N removal efficiencies of 66% and 17% were achieved in the BFBR's using sand and GAC as support matrices, respectively. In comparison to GAC, significantly higher removal efficiencies, higher N03-N loading rates and more stable reactor operation was achieved in the denitrifying BFBR using sand as the support matrix. Using the information gained from these and other similar studies, the final objective of this study was to achieve the complete treatment of a high strength nitrogenous industrial effluent using dual-stage BFBR's (two nitrification and one denitrification BFBR's) operated in series. The nitrification reactors were fed with a synthetic effluent analogous to an industrial effluent in which the NH4NO3-N concentration ranged from 600 mg/I NH4NO3-N to 1200 mg/I NH4NO3-N . The influent of the denitrification reactor was the effluent which exited from the nitrification reactors in which the NO3-N concentration ranged from 1000 mg/I NO3-N to 2400mg/l NO3-N . During this study NH4-N removal in excess of 99% was achieved in both nitrifying reactors at a loading rate of 1 kg NH4-N/m3.d. As the loading rate was increased to 2.4 kg NH4-N/m3.d, the removal efficiency in the nitrification reactor with sand as the support matrix dropped significantly to 48%. However, in contrast to the results obtained previously this increase in the NH4-N loading rate did not have an effect on the nitrifying reactor with GAC as the support matrix. Removal efficiencies (NH4-N) in excess of 99% could be achieved at this loading rate. Nitrate nitrogen removal efficiency in excess of 99% was achieved in the denitrification reactor at a loading rate of 2.4 kg N03-N/m3.d. An increase in the loading rate to 8.35 kg N03-N/m3.d did not have an effect on N03-N removal and the removal efficiency remained at 99%. The results from these studies subsequently indicated that dual-stage BFBR' s could serve as a suitable technology for the treatment of the synthetic nitrogenous industrial effluent analogous to the condensate effluent generated by SASOL Agri , Secunda, South Africa. With the exception of the dissolved oxygen limitation as experienced during nitrification, no other problems were experienced and high loading rates could be obtained . Based on the results obtained , granular activated carbon was the best support matrix for nitrification whilst sand was the best support matrix for denitrification.