Studies on Nitrification-Denitrification and Sludge Granulation Processes in Sequencing Batch Airlift Reactor for the Removal of Nitrogen from Tannery Wastewater
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2020-12
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Addis Ababa University
Abstract
Tannery wastewater is characterized by high organic carbon and nitrogenous compounds due to the raw hides/skins and chemicals used in making leather. The present study employed sequencing batch airlift reactor (SBAR) system to develop the simultaneous removal of nitrogen and organic carbon from synthetic tannery wastewater. The reactor (working volume of 5 L) was run for 250 d at different operational conditions by automated control of the SBR cycles, pH, DO and temperature and constant 50% volume exchange ratio. The performance of the reactor was evaluated in terms of TCOD, sCOD, TKN, NH4-N, NO3-N, NO2-N, DO and pH profile. The reactor was initially run at SRT of 7 d, MLSS of 3.5-4 g/L, pH in the range of 7.25-7.3 and DO of 2 mg/L for 129 days. The startup of nitrification process took about 30 days. During this time, the reactor showed almost complete organic carbon removal and unstable partial nitrification to nitrite with very less conversion to nitrate. Later the reactor operation was changed to different operating conditions: constant SRT of 20 d, resulting in MLSS of 10-11 g/L, DO of 1 mg/L and pH in the range of 7.25-7.3 and different cycle times of 18, 12, 10 and 8 h. The reactor was run for a minimum of two weeks for each cycle times. The result showed stable nitrification and denitrification with 97% removal in terms of COD and a 94% removal in terms of ammonical nitrogen. The total nitrogen removal was observed to increase as the cycle time increases. The total nitrogen removal efficiencies for 18, 12, 10 and 8 h cycle times were, 69, 58, 53 and 43 % respectively. The COD, DO and nitrogen species profiles showed that the removal of nitrogen was observed during the feeding and initial phase of the aeration due to denitrification. It was also observed that the remaining nitrogen in the effluent was more in the form of nitrate at higher cycle times and in the form of nitrite for lower cycle times. Kinetics study also showed that maximum removal of COD and ammonical nitrogen in 8 h cycle time under controlled condition. In addition, cycle time control to change the characteristics of the effluent from complete nitrification to partial nitrification which is one of the important steps to link the reactor with anammox reactor system where in the presence of nitrite, ammonical nitrogen is anaerobically oxidized to nitrogen gas by anammox bacteria. Whole metagenome-16S rRNA sequencing of the bacteria population diversity of initial seed and reactor sludge after 200 d showed that the dominant bacteria were proteobacteria and the reactor sludge showed higher nitrifies and anammox population compared to seed sludge.
Further experiments were conducted to change the flocculent sludge to granular sludge and improve the overall efficiency of the reactor. A COD loading rate of 6.3 kg/m3.d and nitrogen loading rate of 0.425kg/m3.d were first applied to the reactor for 105 days by using simulated tannery wastewater. The flocculent sludge was transformed to granular sludge when the operating settling time was reduced sequentially. The granules formed were having a final MLSS of 12.3 g/L, biomass density of 6.1 gVSS/L granule, settling velocity of 30-40 m/h, SVI of 16-20 ml/g TS and size distribution in the range 1.8-3.2 mm. Evaluation of the performance of the reactor showed a very good quality effluent with average COD, NH4-N and TN removal of 97.7±1.5 (%), 87.1±11.8 and 74.43±11.7 (%) respectively. The effect of settling time on the existence and relative abundance of selected bacteria gene involved in nitrogen cycle is analyzed by PCR and qPCR. The result confirmed the existence of AOB-amoA, nirK and PLA46amx820 gene. The qPCR result also confirmed that the relative abundance of AOB-amoA and PLA46amx820 was higher in the reactor compared to the control.
Biological transformation of nitrogen results in generation of sludge which needs sustainable solution. Anaerobic digestion of these sludge yield energy and reduce the sludge volume significantly. The digestion of excess sludge from the SBAR reactor in this particular study yield biogas yield of 193.2 ml/g VSS and the digester reached steady state in about 30 days. However, the supernatant after anaerobic digestion contains significantly high concentration of ammonia (i.e., 1097 mg/l) which again need sustainable solution. Partial nitrification (PN) of the supernatant after anaerobic digestion is a sustainable alternative to remove the nitrogen either through partial nitrification-denitrification route or through partial nitrification-anammox route. Though the PN-denitrification route may result in generation of unwanted N2O gas. Therefore, the PN-anammox route is more sustainable. For the PN-anammox route, the NH4-N in the wastewater need to be partially nitrified to NO2-N in the ratio of 1:1. As a result, the SBAR reactor was operated at SRT of 7 days, DO of 1 mg/L, pH of 8, at ambient temperature and cycle time of 8 h to start up the PN process and the reactor was started up in 30 days with the required NO2-N: NH4-N ratio for anammox reactor feed. Moreover, the effect of various operational factors on the response variable (NO2N: NH4-N) was studied by considering seven operational factors (i.e. pH, DO, temperature, Cycle time, C/N, MLSS and aeration strategy (intermittent and continuous)). Fractional factorial design (Placket-Burman) was used to study the effect of the seven operational factors. The study showed that the individual factors considered for the study are not significant but the interactions between the factors are more significant. The results from experimental runs showed that it is possible to reach a stable partial nitrification with high pH (7.60), low C/N (0.5), high cycle time (10 h), low DO concentration (1 mg/L), low MLSS/MLVSS (3500 mg/L), high temperature (32ºC) and intermittent aeration.
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Keywords
Tannery wastewater, Nitrogen, SBAR, Cycle time, Aerobic granules, PN