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Title: Advances in the use of aerobic sequencing batch reactors for biological wastewater treatment
Author: Rasheed, Adamu Abubakar
ISNI:       0000 0004 6424 3466
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2017
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The overall aim of this research was to contribute to the optimisation of aerobic wastewater treatment processes in sequencing batch reactors, by investigating the performance of the process with real and model wastewaters in order to achieve the highest possible reduction of influent COD with the minimum reactor volume and oxygen consumption. Six industrial wastewaters from the food and drink companies were treated in lab-scale aerobic sequencing batch reactors (SBRs) inoculated with soil and monitored for COD and total suspended solids (TSS) removal. The results showed high COD removal efficiencies for all the wastewaters, in the range of 64-95 %. Extended aeration tests were carried out on the reactor effluents and the results indicated that the residual soluble COD was not further biodegradable. This indicates that the soluble BOD removal in the reactors was virtually 100 %. The biodegradation efficiency was investigated over two values of the solids retention time (SRT) and the performance of the reactors was essentially unaffected by the SRT (in the range of 7-18 days considered in this study). This means that very good COD removal can be achieved at relatively lower SRT, with potential savings in capital and operating costs. The removal of TSS for the wastewaters was not satisfactory, largely due to the low food to microorganism (F/M) ratios (0.13-0.29 kg COD/kg in the reactors. Three batch tests at different initial substrate to biomass (So/Xo) ratios were carried out on each of the two industrial wastewaters for kinetic characterisation of the wastewaters. Oxygen uptake rate (OUR) was measured during the tests and the OUR profiles were used for the estimation of the kinetic parameters using a mathematical model consisting of substrate hydrolysis, biomass growth and endogenous metabolism. The results showed large variations with regards to the values of the parameters for each wastewater due to day-to-day variability in the biomass's response to substrate utilisation. For the two industrial wastewaters, the hydrolysis rate constant (kh) and half saturation constant for slowly biodegradable substrate (KX) were found to be in the range of 2.21-14.8 kg COD/kg and 0.006-0.45 kg COD/kg biomass respectively. The maximum growth rate (μmax) and the half saturation rate constant for readily biodegradable substrates (KS) ranged between 1.21-7.3 day-1 and 0.004-0.89 kg COD/m3 respectively. The biomass growth yield (YX/S) and the endogenous metabolism coefficient (b) were found to be 0.3-0.57 kg biomass/kg COD and 0.001-0.41 day -1 respectively. The hydraulic retention time (HRT) and SRT were optimised in order to minimise the SBR volume and maximise the organic loading rate (OLR) of the SBR process. Two model wastewaters, glucose and ethanol, were used in the study. An experiment of eleven different SBR runs (HRT in the range of 0.25-4 days and SRT of 1-65.3 days) was carried on the glucose wastewater. Nine different SBR runs were carried out on ethanol wastewater (HRT in the range of 0.5-4 days and SRT of 1-73.6 days). The minimum HRT and SRT values for the successful operation for glucose wastewater treatment were 0.25 days and 3.1 days respectively while the minimum HRT and SRT for ethanol were 0.5 days and 4.9 days respectively. The highest corresponding OLR values from the minimum HRT and SRT which gave satisfactory process performance were 4.28 g COD/ and 4.14 g COD/ for glucose and ethanol wastewaters respectively, which are among the highest OLRs reported in the literature for aerobic conventional dispersed-growth processes. The calculated oxygen consumption and biomass production were found to depend on the SRT as well as the OLR, where in general, oxygen consumption increased while biomass production decreased at higher OLR. Batch tests were also carried out on the two model wastewaters for kinetic characterisation. The kinetic parameters for glucose wastewater were: 1.07-4.79 day -1 for μmax, 0.24-0.45 kg COD/m3 for KS, 0.04-0.1 day-1 and 0.47-0.6 kg biomass/kg COD for b and YX/S respectively. For ethanol wastewater, the kinetic parameters were: 0.99-2.3 day -1 for μmax, 0.001-0.04 kg COD/m3 for KS, 0.05-0.2 day-1 and 0.38-0.51 kg biomass/kg COD for b and YX/S respectively. A new mathematical model and procedure to calculate the periodic steady state of the SBR using a kinetic model of the biological process and values of the kinetic parameters was developed. This new procedure allows the direct calculation of the steady state profiles of biomass and substrate in the SBR without calculating the dynamics of the system from start up to steady state. The numerical accuracy of the procedure was discussed and the model was applied to show the effect of the operating parameters (SRT, HRT, length of the phases and number of cycles) on the steady state performance in terms of biomass and substrate concentrations. It was also shown how the model can be used for various applications like: optimisation of operating parameters for a minimum reactor volume; simulation of the competition between filamentous and floc-forming bacteria for bulking control; and calculation of the minimum volumetric mass-transfer coefficient required to maintain a desired oxygen concentration. In the end, the periodic steady state of the SBR was simulated for the industrial and model wastewaters at various values of the operating conditions (e.g. HRT, SRT, number of cycles) using the developed SBR model with values of kinetic parameters obtained from the various batch tests. The predicted model performance in terms of effluent quality and biomass concentration was compared with experimental results achieved during the treatment of the wastewaters. The simulation gave very good prediction of the extent of substrate removal for all the wastewaters. However, the prediction was not very accurate for biomass concentration. The study indicated that a good model prediction in terms of biomass production is strongly dependent on the values of the kinetic parameters especially b and YX/S.
Supervisor: Not available Sponsor: Tertiary Education Trust Fund (Nigeria)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Water reuse ; Sewage ; Aerated package treatment systems