Browsing by Author "Ekama, George A"
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- ItemOpen AccessA prototype dynamic model for the co-treatment of a high strength simple-organic industrial effluent and coal-mine drainage(2020) Harding, Theodor; Ekama, George A; Ikumi, DavidThis research study's the use of biological sulfate reduction technologies for the treatment of Sasol Secunda's coal-mine drainage (CMD) using Fischer-Tropsch Reaction Water (FTRW) as a cost-efficient carbon source. The research aims to develop a prototype dynamic model that describes this co-treatment of FTRW and CMD in both a continuously stirred tank reactor (CSTR) biological sulfate reduction (BSR) system and a BSR gas-lift (BSR-GL) integrated system. The BSR-GL system recovers elemental sulfur (S0 ) from the H2S produced and stripped from the BSR unit. Furthermore, this study aims to use the prototype model for a quantitative comparison of the CSTR-BSR and BSR-GL systems. Two bench-scale 5-litre CSTR-BSR and a 20-litre BSR-GL system were operated, under varying feed COD concentrations and hydraulic retention times (HRTs), to generate datasets for use in verification and a rudimentary validation of the prototype model. The BSR-GL integrated system includes 1) a 1-litre H2S gas reactive absorption (ABS) unit utilising an aqueous ferric solution for the recovery of elemental sulfur (S0 ) from sulfide and 2) ferrous biological oxidation reactor to regenerate ferric from the ferrous for re-supply to the ABS unit. The datasets generated in the experimental study allowed for the identification, mathematical modelling and reaction verification of 32 components that interact as reactants and products in 23 reactions observed in the two BSR systems. The prototype model is presented in a mass and charge balanced Gujer matrix that includes, i) 5 SRB mediated processes, ii) 2 liquid-gas mass transfer processes, iii) 3 processes describing the ABS and Fe2+ bio-oxidation units, iv) 4 processes describing sulfide and elemental sulfur oxidation and v) the S0 and poly-sulfide aqueous equilibrium and vi) 9 processes describing death regeneration and BPO hydrolysis. This prototype model was implemented in the DHI WEST® software for initial stage simulation trials. The experimental datasets allowed for the first-stage estimation of the best-fit reaction rate equations and the calibration of the kinetic parameters related to the 23 reactions, using MATLAB® curve fitting toolbox. A pre-processor that describe the pH and equilibrium chemistry of the components of the artificially prepared FTRW+CMD feed mixture batches under varying total concentrations have also been developed in this research. This was done to generated influent file to the DHI WEST® simulations that incorporated the dynamics related to the FTRW+CMD feed mixtures. The sulfate utilisation rate (gSO4 -2 .l-1 .d-1 ) of the GL-BSR and CSTR-BSR systems were compared to determine which system had the best sulfate removal. The results were found to be as follows; a. On comparison it was found that the sulfate substrate utilisation rate for the CSTR_BSR system is 39.28% of that of the BSR-GL_N2 system, where both systems were fed at feed mixture of COD of 2500mgCOD/l, where the COD:SO4 2- was 0.7, b. For the same systems fed a feed mixture of COD at 5000mgCOD/l (COD:SO4 2- = 0.7), the sulfate substrate utilisation rate for the CSTR_BSR system was found to be 17.86% less than that of the BSR_GLN2 system. c. Finally, it was also found that the substrate utilisation rate for the CSTR_BSR system was 30.06% less than that of the BSR_GLN2 system at Se of 4gCOD/l, for both systems fed substrate at 5000mgCOD/l. Thus, it can be concluded that the sulfate substrate utilisation rate for the BSR-GL system is higher than that of the CSTR_BSR system, for systems fed COD feed mixtures at 2.5 or 5gCOD/l where both systems have the same effluent substrate concentrations. However, the difference in the comparative substrate utilisation rate is less at higher feed substrate concentrations. This is the influence of substrate inhibition on the active SRB biomass, which increases with higher effluent substrate concentrations. Finally, this research found that the use of gas-lift reactor technologies is superior to CSTR technologies in the treatment of coal-mine drainage utilising biological sulfate reduction (BSR). The CSTR-BSR system, fed sulfate between 1.6 to 14gSO4 2- /l, produced effluent with high dissolved H2S concentrations, on average 285mgS/l and maximum at >600mgS/l. Releasing this effluent to the environment would be hazardous to aquatic and human health and corrosive to infrastructure. As such, the effluent from the CSTR-BSR system requires further treatment to stabilise the water for any use. The BSR-GL technology allows for the conversion of the H2S produced during BSR reactions to form elemental sulfur, which is a resource recovered from this process, thus complying to the circular economy aim of this study.
- ItemOpen AccessAerobic digestion of waste activated sludge from biological nutrient removal activated sludge systems(2007) Mebrahtu, Michael Kidane; Ekama, George A; Wentzel, Mark CWaste activated sludge (WAS) is a biological sludge that contains biodegradable and non-biodegradable volatile suspended solids (VSS) and non-volatile inorgainic suspended solids (ISS). Stabilization for safe disposal of the WAS is a process of paramount importance at wastewater treatment plants (WWTPs). Hence, aerobic digestion of the WAS from biological nutrient removal (BNR) activated sludge (AS) systems was carried out under batch conditions to (1) measure changes in nitrogen and phosphorus concentrations in solid and liquid phases during aerobic batch digestion tests (2) simulate the parent system with Activated Sludge Model No. 2 (ASM-2) in AQUASIM computer program to obtain the initial conditions for batch test simulation (3) simulate the batch aerobic digestion process with ASM-2 and compare with experimental data (4) develop VSS-based and total suspended solids (TSS) (with the addition of ISS to the VSS-based) batch reactor and steady state models for aerobic digestion of nitrification denitrification biological excess phosphorus removel (NDBEPR) WAS based on the individual biomass die-off rates of phosphorus accumulating organisms (PAOs) and ordinary heterotrophic organisms (OHOs), and (5) evaluate the ASM-2 simulation results with steady state aerobic batch digestion model.
- ItemOpen AccessAnaerobic digestion of Fischer-Tropsch reaction water : submerged membrane anaerobic reactor design, performance evaluation & modeling(2008) Van Zyl, Pierrie Jakobus; Ekama, George A; Wentzel, Mark CIncludes abstract. Includes bibliographical references (p. 209-217).
- ItemOpen AccessCharacterization of organics for anaerobic digestion by modelling augmented biological methane potential test results(2015) Botha, Rudi Frances; Ekama, George AThis study aims to establish a dynamic model for the simulation of biochemical methane potential (BMP) test results, as well as to determine the additional measurements required in the BMP assay procedure for the model to estimate the composition of a homogenous biodegradable particulate organic sample. This research would aid the modelling of anaerobic digester behaviour to predict and prevent inhibition and failure. Anaerobic digestion is a mechanism of biological processes which breaks down biodegradable material to biogas (primarily methane and carbon dioxide) and residual sludge in the absence of oxygen. The process was originally used to treat biological wastes, but is now being used to treat various forms of organic waste, such as food and agricultural industry by products, municipal solid waste, etc. Anaerobic digestion is also a net energy-producing process as it forms the energy laden gas, methane (CH4). The biological processes are, however, sensitive to organic characteristics and inhibitory toxins, among others, which can cause inefficient reactor systems. Currently, only expensive or complex monitoring technologies can provide adequate parameter concentrations to determine when unstable digester conditions are imminent.
- ItemOpen AccessChemical phosphorus removal from municipal wastewater by the addition of waste alum sludge to the activated sludge system(1992) Power, Sean P B; Ekama, George AIn many cases, waterworks waste alum sludge is disposed of by discharging it into a stream. In this investigation the disposal of alum sludge to activated sludge systems treating municipal sewage is investigated. The advantage is that it is a better method of alum sludge disposal, and moreover the addition of alum sludge removes phosphorus from the wastewater through chemical precipitation. Two long sludge age (20 days) Modified Ludzack Ettinger (MLE) predenitrification systems receiving unsettled municipal wastewater at a controlled concentration of 500mg COD/l as influent were operated for a period of 305 days, one as an Experimental system and the other as a Control system. The anoxic mass fraction was large (70), to mimic many long sludge age nitrification/denitrification systems in operation in South Africa. Nitrate was added into the anoxic reactors to maintain anoxic conditions so that biological excess phosphorus removal would not take place and interfere with the chemical removal performance.
- ItemOpen AccessComparing the Metcalf and Eddy and UCT steady state guidelines for sizing of biological nutrient removal activated sludge wastewater treatment plants(2020) Murphy, Katelyn; Ekama, George A; Ikumi, DavidThis dissertation aims to provide both a qualitative and quantitative comparison of two steady state activated sludge (AS) design guidelines - the University of Cape Town (UCT) guideline used in South Africa and the Metcalf and Eddy (M&E) guideline used in North America and other parts of the world. It looks at the key similarities and differences between the two steady state AS design guidelines and how, under dynamic conditions, a system that is sized using a particular guideline (i) compares to its steady state results and (ii) performs under these dynamic conditions. In order to achieve the aims and objectives of this dissertation, an AS steady state model was created in a Microsoft Excel spreadsheet for the UCT guideline and M&E guideline respectively, and the models were analysed in terms of the key similarities and differences in the design guidelines in terms of inputs, equations, approaches and assumptions used. The results produced from each model were also analysed by setting the influent wastewater characteristics the same for each guideline and then analysing the results. The systems that were sized using the steady state AS models were then input into an AS system dynamic simulation software program, UCTOLD (which predicts virtually identical results as ASM1), together with a full set of diurnal influent data, to predict the behaviour of the system under steady state and dynamic conditions and compare the steady state predictions to those calculated in the steady state models and assess how the steady state model sized systems perform under dynamic loading conditions. The results of the analyses found that the two guidelines are similar in terms of organic material removal, nitrification and the sizing of the secondary settling tank, but differ significantly in the sizing of the anoxic reactor to achieve a certain nitrate removal. The key findings are: (1) Both UCT and M&E guidelines close the COD and N flux balances within 1%. (2) For organics removal only, at the same SRT, sludge production and oxygen demand are about 5% higher and lower respectively for the M&E guideline than the UCT guideline. When a UCT and M&E sized fully aerobic system is simulated with ASM1, this difference is repeated. The UCT guideline results are closely correlated with the ASM1 results but the M&E results deviate from those of ASM1. These differences arise because the M&E guideline assigns different values to the kinetic, stoichiometric and temperature sensitivity constants. If these constants in the M&E guideline are assigned the same values as the UCT guideline, virtually identical results are obtained. (3) For nitrification under fully aerobic conditions, the M&E guideline calculates a slightly shorter minimum aerobic SRT for nitrification than the UCT guideline. Again, the M&E guideline assigns different values to the nitrification kinetic (μAm20, bA20), stoichiometric (YA, Kn20) and temperature sensitivity constants (θμAm, θbA, θKn) than the UCT guideline. The M&E guideline calculates the minimum sludge age for nitrification, Rsm, using a fixed maximum specific growth rate of nitrifiers at 20oC (μAm20) at 0.90 g/(g.d), and after correcting for temperature, DO concentration in the aerobic reactor and assigning a safety factor (Sf = 1.5), the minimum sludge age for nitrification is slightly shorter than for the UCT guideline for a selected maximum specific growth rate of nitrifiers at 20oC (μAm20) of 0.45 g/(g.d) and assigning Sf = 1.25. In the M&E guideline the mass of nitrifiers is added to the reactor MLSS concentration which increases the MLSS mass in the reactor by about 1-3%. This is not done in the UCT guideline to maintain the COD balance for organics removal. At the same SRT in a fully aerobic system (i.e. aerobic SRT = system SRT), the oxygen demand for nitrification is closely similar in the two guidelines. This is because the calculated concentrations of nitrate produced by nitrification (called nitrification capacity Nc in the UCT guideline) are closely similar – the difference in the sludge production of the two guideline make little difference to the N taken up for sludge production. (4) If fully aerobic nitrifying reactors sized with the M&E and UCT guidelines are simulated with ASM1 at the same SRT, the same differences as with organic removal are observed. Hence the main difference in the sizing for nitrification in fully aerobic reactors in the two guidelines is the shorter aerobic SRT for nitrification in the M&E guideline (as a result of the different nitrification kinetics and safety factors) than in the UCT guideline. (5) Significant differences between the two guidelines emerge when adding an anoxic reactor for denitrification, such as for the anoxic aerobic nitrification - denitrification (ND) Modified Ludzack-Ettinger (MLE) system. This is because (5.1) the nitrifiers are assumed to behave differently under anoxic conditions in the two guidelines and (5.2) the effective specific denitrification rates of the OHO biomass in the anoxic reactor are much higher in the M&E guideline than in the UCT guideline. (6) With regard to difference (5.1), in the UCT guideline, the nitrifiers are assumed to grow only in the aerobic reactor but die in both the anoxic and aerobic reactors. In the M&E guideline, the nitrifiers are assumed to die (and grow) only in the aerobic reactor, i.e. they neither grow nor die in the anoxic reactor. Hence in the M&E guideline, the MLE system is sized based on an aerobic SRT, which excludes the mass of sludge in the anoxic reactor as in (3) above, but in the UCT guideline the MLE system is sized based on a system SRT, which includes the mass of sludge in the anoxic reactor. (7) With regard to difference (5.2), the faster specific denitrification rate determined with the M&E guideline yield much smaller anoxic reactors by at least 50% to achieve the same nitrate removal. (8) The consequence of these two differences is that the system SRT of the MLE system determined with the UCT guideline is considerably longer than that determined with the M&E guideline leading to larger anoxic, aerobic and system reactor volumes. This difference widens as the influent TKN/COD concentration ratio increases, i.e. as the concentration of nitrate to be denitrified increases. (9) When simulating the UCT sized MLE systems (under steady state conditions) with ASM1, very similar reactor MLVSS and MLSS concentration, effluent ammonia and nitrate concentrations and total oxygen demands are obtained with ASM1 and the UCT guideline. This indicates that the denitrification kinetics of the UCT guideline are well aligned with ASM1. This is not the case when simulating with ASM1 M&E guideline sized MLE systems under steady state conditions – while the effluent ammonia concentration compares well, the effluent nitrate concentration is far higher (increases from 6 mgNO3-N/l to above 20 mgNO3-N/l). This indicates that even though the denitrification kinetics of the M&E guideline were derived in part from ASM1 simulations, the denitrification kinetics of the M&E guideline are very poorly aligned with ASM1. (10) When the fmanx,M&E of the denitrification MLE system in (9) is increased to fmanx,UCT of 0.318 (but keeping the SRT = SRTsys,M&E) and simulated with ASM1, the effluent nitrate concentrations reduce from around 20 mgNO3-N/ℓ to around 6 mgNO3-N/ℓ, which is aligned with the UCT guideline ASM1 results. (11) The enhanced biological phosphorus removal (EBPR) parts of the UCT and M&E guidelines were not compared. While the EBPR part of the UCT guideline is complete and accounts for the phosphorus accumulating organisms (PAO) and their polyphosphorus content in the VSS and TSS calculations, as well as the differences in the denitrification kinetics in NDEBPR system compared with ND systems, which aligns the UCT NDEBPR guideline with ASM2, this is not the case in the M&E guideline. Because there is insufficient information in the M&E guideline to execute a complete NDEBPR system design calculation, the EBPR parts of the guidelines could not be compared. (12) The M&E overflow rates can be aligned with the UCT 1DFT to determine very similar SST surface areas. The lower resultant reactor MLSS of the M&E sized systems when simulated with ASM1 means that the SSTs will operate at a lower than designed for MLSS and thus under peak conditions (fq is 2.5 or greater) the SST will operate at a higher than permissible overflow rate. This is because the M&E SST sizing procedure does not include a 1DFT flux rating of 0.80 (as the UCT guideline does), which has the effect of increasing the SST surface area estimated by the 1DFT by 25%.
- ItemOpen AccessComparison of the 1D idealised flux theory and a 2D hydrodynamic model with full scale secondary settling tank performance data(2001) Marais, Pierre; Ekama, George AIn this research, the two dimensional (2D) hydrodynamic model SettlerCAD (Zhou et al., 1998) was applied to full-scale circular secondary settling tanks (SSTs) with the principal aim to 1) establish whether or not it "automatically" reproduces a flux rating <1.0 with respect to the steady state 1D idealized flux theory (1DFT) and; 2) determine what factor influence this flux rating.
- ItemOpen AccessDenitrification kinetics in biological nitrogen and phosphorus removal activated sludge systems(1989) Clayton, John Andrew; Ekama, George AIn order to size the anoxic reactors in nutrient (N and P) removal activated sludge plants, it is essential to know the denitrification kinetics that are operative in such systems. To date, denitrification kinetics have been accurately defined only for systems that remove N alone; little research has been performed on denitrification in N and P removal plants.
- ItemOpen AccessDesign and performance of BNR activated sludge systems with flat sheet membranes for solid-liquid separation(2006) Du Toit, Geoff J G; Ekama, George A; Wentzel,Mark C,Wastewater treatment technologies have developed out of the need to protect receiving water bodies from the increasingly concentrated municipal and industrial waste streams generated through human activity. Of the methods available to clean wastewaters, biological nutrient removal (BNR) activated sludge (AS) is applied throughout South Africa and internationally as it has many advantages, notably it is cheaper to operate, does not introduce salinity into the water and is a simple and robust process. One of the key steps in the BNR AS process is the separation of biomass from the water. This is traditionally achieved by means of secondary settling tanks (SSTs), however recently the use of membranes for solid-liquid separation has gained popularity for the following reasons: Membranes are able to retain all solids and thus are insensitive to the settling characteristics of sludges, • they can be run at high concentrations and hence smaller reactor volumes are required, • membranes can produce a guaranteed high quality effluent free of pathogens and in some cases viruses too. • Additionally smaller reactor volumes and the obviation of SSTs allow a substantial wastewater treatment plant footprint reduction. Hence the combination of membranes in BNR AS is being increasingly applied. where much research has been conducted on the performance of membranes. The majority of the research has focused on the physical membrane performance, investigating the mechanisms of fouling, or on the membrane biological reactor (MBR) performance in removing organic compounds or nitrogen compounds from wastewater. There are however few case studies investigating BNR using membranes despite speculation that the inclusion of membranes may indeed affect the nature of the activated sludge biomass (Witzig et al., 2002).
- ItemOpen AccessDevelopment of a plant-wide steady-state wastewater treatment plant design and analysis program(2015) Wu, William Ying Xin; Ekama, George AModels are used as prognostic and diagnostic tools in order to design, analyse and optimise the biological and physical processes in a wastewater treatment plant (WWTP). This is done in order to save time and money and to improve the understanding of the behaviour of the treatment system. There are two categories of models in wastewater (WW) treatments, steady-state and dynamic models. (i) Dynamic models consist of sophisticated mathematical solvers and are structured for the optimisation of WWTP’s and not for system sizing. (ii) Steady-state models comprise simple and explicit algebraic equations. With these equations, high-level answers are found easily and quickly but with a much lower level of input information. Hence, steady-state models allow for system sizing and are powerful pre-processors for dynamic models. They can generate the overall WWTP scheme, main system defining parameters, and the initial conditions for starting the dynamic simulation. Currently, there is a lack of a plant-wide steady state design (PWSSD) program. Numerous steady-state models for the different unit processes exist; however, they are yet to be integrated and presented in one holistic software package for the plant-wide design (and analysis) of WWTP. The availability of such program will be extremely beneficial to WWTP engineers as it can be used as a standalone tool for the steady state design, system sizing and capacity estimation, or as a pre-processor to generate the plant wide WWTP initial conditions for dynamic simulation. To fill the above-mentioned software gap, a PWSSD program was developed within the Excel/VBA environment. The developed PWSSD program integrates various steady-state wastewater treatment models with an expert-guided user-interface, thereby creating a platform for step-by-step assisted interaction and exploration of the models. This program draws upon a large body of literature regarding the modelling of wastewater treatment processes. The current version of the program (1) caters for commonly used AS configurations (MLE, JHB and UCT) in South Africa. The steady-state AS models are holistically linked to important upstream and downstream biological and non-biological treatment processes.
- ItemOpen AccessThe development of a three phase plant-wide mathematical model for sewage treatment(2011) Ikumi, David S; Ekama, George ATo aid in finding the most cost effective methods for the design and operation of wastewater treatment plants, for minimization of energy consumption and cost while maximizing nutrient recovery and improving effluent quality, the purpose of this project is to develop three phase (aqueous-gas-solid) steady state and dynamic mathematical models for the anaerobic and aerobic digestion of sludge; including waste activated sludge (WAS) produced by biological excess phosphorus removal (BEPR) plants, within a plant-wide setting.
- ItemOpen AccessDevelopments in anaerobic digestion modelling(2019) Ghoor, Tasneem; Ekama, George A; Ikumi, David SAnaerobic digestion (AD) is notorious for being susceptible to failure and is regarded as unstable and sensitive. Thus, to avoid failure, anaerobic digesters are frequently operated far below their optimal level. In order to run a digester closer to capacity, a better understanding of AD failure is required. Under conditions approaching failure, or during startup, intermediate products such as acetate, propionate and hydrogen accumulate. Successful AD modelling during failure requires the AD model to be suitably calibrated. Some AD models have been calibrated to the initial slow rate-limiting hydrolysis step only with the result that these models cannot be used to predict AD failure. Without adequate modelling of AD dynamics, the AD model cannot be used to model digester start-up, digester failure or even upflow anaerobic sludge bed (UASB) reactors (which have temporary failure conditions at the bottom of the bed). This study aims to develop an AD model capable of predicting failure and digester start-up conditions. Development of an improved model was accomplished by means of calibrating the AD model to a UASB reactor dataset wherein temporary failure conditions are present in the bottom of the reactor, evident by the presence of the abovementioned intermediate products. After comparing and contrasting available AD models to identify one for further development, the AD model subset (PWM_SA_AD) of plantwide model South Africa (PWM_SA) was selected because (1) it characterizes the organics’ composition using routine wastewater treatment measurements rather than carbohydrates, lipids and proteins, which is typical of other models, (2) external speciation reduces model stiffness, (3) includes aqueous, gas and solid phases for pH calculation, gas evolution and mineral precipitation and (4) contains the same components as PWM_SA enabling plant-wide modelling without needing component transformers between process units. Before calibration, PMW_SA_AD was rigorously tested for mass balance, stoichiometric and kinetic correctness. Because the UASB reactor undergoes temporary failure observed by the accumulation of AD intermediate substrates in the bottom of the bed, the glucose fed UASB reactor system of Sam-Soon et al. (1989) was modelled to calibrate the Monod kinetic constants of the acidogens, acetoclastic methanogens, acetogens and hydrogenotrophic methanogens. This required coding into WEST® (MikebyDHI, 2016), the platform on which PWM_SA runs, a six-in-series completely-mixed AD system with a solids retention factor for each digester that retained a fraction of the reactor’s solids. Determination of the parameters that required calibration was identified with sensitivity analysis. Due to the complexity of the physical, biological and aqueous interactions, many model simulations were needed to identify the important parameters with the lasso (least absolute shrinkage and selection operator) feature selection method. Not unexpectedly the most important parameters that required calibration were the retention factor for each digester in the series; and the maximum specific growth rates and the half-saturation coefficients for the four AD biomass groups, which were global parameters, i.e. for each biomass group the same values in each digester of the series applies. Stability of the complex six-in-series UASB reactor needed the initial masses in each digester to be reasonably close to the final steady-state masses. Steady-state Microsoft Excel AD spreadsheet models were set-up to calculate these initial masses. Following the calibration procedure wherein the modelled AD intermediate products matched the measurements from the UASB reactor dataset, it was expected that the pH also would be predicted well. However, this was not the case. So, the assumption of equilibrium between the headspace CO2 partial pressure and aqueous phase CO2 concentration was replaced by a rate-controlled CO2 evolution. With this correction, the predicted pH matched well with that observed along the height of the UASB reactor. The calibrated model was then tested to observe how the UASB reactor system fails irrecoverably by gradually decreasing the influent alkalinity from the dataset value of 6000 mg/L as CaCO3. Irrecoverable failure occurred at an influent alkalinity of 4200mg/L as CaCO3 because the specific growth rate of the acetoclastic methanogens, which progressively decreases the further the pH falls below 7, fell below the minimum required to utilise the high acetate concentration. The role of the sensitivity of the acetogens to hydrogen in digester failure was also tested. Counter-intuitively, it was found that this in and of itself did not cause failure but that to a degree, postponed failure because the acetogen inactivity at high hydrogen concentration delayed the acetate load on the acetoclastic methanogens. To verify the acetogen effect on failure, acetogen sensitivity to hydrogen was increased, and the alkalinity was gradually decreased. Irrecoverable failure now occurred at an influent alkalinity of 4000mg/L as CaCO3. The above modes of UASB reactor failure predicted by PWM_SA_AD were compared with ADM1 (Anaerobic Digestion Model No. 1). ADM1 was coded into PWM_SA_AD as an independent subset using the same external speciation routine. Although ADM1 has previously been documented to be incapable of predicting AD failure, this comparison showed that ADM1 predicted the same failure modes as PWM_SA_AD but at a higher influent alkalinity of 5000mg/L as CaCO3. One of the main reasons why ADM1 fails at sooner is that the specific growth rate of the acetoclastic methanogens in ADM1 is slower than in the PWM_SA_AD model calibrated to the UASB reactor data. The UASB reactor system calibrated PWM_SA_AD model was applied to model digester start-up with primary sewage sludge. This was done by adding to the single completelymixed anaerobic digester a percentage of seed and filling the rest of the volume with wastewater treatment plant effluent with 250 mg/L as CaCO3 alkalinity. The percentage of seed was the percentage of the anaerobic digester volume, which was filled with seed sludge containing the same biomass concentrations as those at steady state after start-up is complete and the set digester sludge age is reached. The hydrolysis rate of biodegradable particulate organics (BPO) was modelled with saturation kinetics (also known as Contois (1959) kinetics) with constants obtained from Sötemann et al. (2005b). Three different startup cases were investigated (1) setting the influent pump at the final steady-state flow rate but switching it off and on with either a pH controller or a Ripley ratio controller, (2) increasing the influent flow by a fixed proportion of the final steady-state flow daily (t1/t -1), where t is the start-up duration, (3) same as (2) but adding either a pH controller or a Ripley ratio controller. Two modes failure, resulting in an inability to start up, were found: (1) BPO overload which causes acetoclastic methanogen overload and surprisingly (2) acetoclastic methanogen starvation. BPO overload results in a high acetate concentration and low pH, which slow the acetoclastic methanogens below the tipping point to start up. It is exacerbated by low percentage of seed and during the slow hydrolysis and acidification of BPO, or a setpoint which is not sufficiently conservative. Acetoclastic methanogen starvation is as a result of a too conservative setpoint which prevents flow from entering the digester, thereby depriving the organisms of the substrate. Plotting the specific growth rate to the maximum specific growth rate ratio of the acetoclastic methanogens indicated that under starvation conditions, the ratio is extremely low. The reason for the low ratio is due to the low bulk liquid concentrations on which Monod kinetics depends. The limitations of Monod kinetics are made apparent here because the initial seed amount is significantly below the final steadystate mass. So, for these cases, further investigations are required to identify if saturation kinetics will allow better predictions. Through the development of the model, although the model was capable of predicting failure and start-up in line with the expected principles, it is not possible to find a unique set of kinetic constants, resulting in a degree of freedom with the choice of a maximum specific growth rate of acidogens. This degree of freedom may have been eliminated if sufficient measurements were available. Overall, the investigation provided useful insight into the mode of AD failure and difficulties regarding modelling digester start-up. There is, therefore, the scope for further additions to the study, with a specific focus on the residual COD, sludge bed measurements, gas flow and hydrogen concentration in the bed and modelling the acidogen, acetogen, acetoclastic methanogen and hydrogenotrophic methanogen specific growth rates with saturation kinetics. This will enable greater insight into the failure modes and the effect of hydrogen and growth rate kinetics on the AD system failure.
- ItemOpen AccessThe effect of alternative detergent builders on the nutrient removal activated sludge sewage treatment process(1993) Kaschula, Wendy A; Ekama, George APhosphate is an essential nutrient for photosynthetic plant growth. However if over-supplied in a water body, it leads to excessive algal growth, a condition is termed eutrophication. Eutrophication is not only unsightly, but renders the water less usable. Most of the highly eutrophied water bodies with a high phosphorus load still receive up to 90 of their phosphorus input from effluents discharged by sewage treatment works (Pretorius, 1983). The phosphorus content of domestic sewage originates from two main sources, namely human waste {±60) and detergents {±40) (Heynike and Wiechers, 1986). There is an argument for a ban on phosphate in detergents to reduce the phosphate load on sewage treatment plants and thereby limit phosphate discharges via treated municipal effluent. The removal of phosphate from detergent formulations has been an effective way in many countries of reducing the phosphorous load to receiving waters. In South Africa, detergent manufacturers are anticipating consumer pressure to reformulate their detergent products to eliminate phosphate. The two possible replacements for phosphates are zeolite 4A and high surface area {HSA) calcite. High surface area (SA) calcite is not yet being used as a detergent builder, but is being seriously considered as a potential replacement for phosphate in South African detergent formulations.
- ItemOpen AccessThe effect of fully anoxic conditions and frequency of exposure to anoxic and aerobic conditions on the growth of low F/M filaments in nitrogen remova(1991) Ketley, David Andrew; Ekama, George AFilamentous bulking, caused predominantly by low F/M filaments (Blackbeard et al, 1986, 1988), results in considerable settling problems in full scale nitrogen (N) and nutrient (N & P) removal activated sludge plants in South Africa. The development of specific methods for control of low F/M filaments in these plants would lead to significant savings because higher flows and loads could be treated in existing plants. From the findings of Blackbeard et al (1986, 1988) an extensive research project was undertaken by Gabb et al (1989a) into specific control of low F/M filament bulking. This project investigated the effectiveness of selectors, the proposed method of low F/M filamentous bulking control and found them to be ineffective. Consequently a second comprehensive laboratory research investigation was commenced in 1989. The work presented in this thesis forms a part of this investigation; the experimental investigation was conducted in 3 phases investigating (1) the effect of fully anoxic conditions and low nitrate concentrations during the anoxic phase of an intermittent aeration cycle on low F/M filament growth in continuously fed completely mixed single reactor systems receiving a synthetic sewage feed; (2) the effect of fully anoxic conditions on low F/M filament growth in continuously fed completely mixed single reactor systems receiving real sewage; and (3) the effect of alternating the frequency of exposure of low F/M filaments to anoxic/aerobic conditions (i.e. increasing the length of the aeration cycle but maintaining the aerobic mass fraction) in intermittently aerated continuously fed single completely mixed reactor systems receiving real sewage.
- ItemOpen AccessThe effect of high temperatures (30 degrees Celsius) on biological nutrient removal performance(1999) Mellin, Hannu Kaarlo Olavi; Ekama, George A; Wentzel, Mark CThe main objective of this investigation was to evaluate activated sludge biological nutrient removal (BNR) performance at elevated temperatures for possible application of nitrification denitrification (ND) and ND biological excess phosphorus removal (NDBEPR) systems to municipal wastewater treatment in the equatorial and tropical regions or to combined treatment of municipal and anaerobically (thermophilic) pretreated paper and pulp industry wastewaters in the very cold northern forested regions. To accomplish this objective, a ND Modified Ludzack Ettinger (MLE) system and a NDBEPR University of Cape Town (UCT) system were operated at 30°C and 10 days sludge age for a period of 582 days. During the investigation 41 sewage batches, each lasting about two weeks, of real sewage from the Mitchells Plain municipal wastewater treatment plant (Western Cape, South Africa) were fed to the systems. The two systems were sampled and tested ,almost daily for Chemical Oxygen Demand (COD), Total Kjeldahl Nitrogen (TKN), Free and Saline Ammonia (FSA), nitrate, nitrite, Total Phosphorus, Volatile Settleable Solids (VSS), Total Settleable Solids (TSS), pH, Oxygen Utilization Rate (OUR) and diluted sludge volume index (DSVI) in the influent, anaerobic, anoxic and aerobic reactors and effluent as appropriate. Also, in order to determine the kinetic rates of nitrification, denitrification and readily biodegradable COD (RBCOD) conversion to Volatile Fatty Acids (VF A), aerobic, anoxic and anaerobic batch tests were conducted at 30°C on sludge harvested from the two systems and microscopic examination of the sludges was undertaken every four weeks to identify the filamentous organisms in the systems.
- ItemOpen AccessThe effect of nitrite and nitrate concentrations on low F/M filament bulking in nitrogen removal activated sludge systems(1994) De Villiers, M E; Ekama, George A; Marais, Gerrit van RooyenFilamentous bulking and its associated poor sludge settleability is a considerable problem in South African activated sludge plants, as indicated by the two surveys undertaken on these plants in 1985 and 1988 (Blackbeard et al., 1986, 1988). Amelioration of this problem would enable a greater daily flow and load of wastewater to be treated by these plants. From the surveys it is apparent that bulking in long sludge age activated sludge plants is mainly due to the proliferation of the group of so-called low F/M filaments (as classified by Jenkins et al., 1984). In the course of a 4-year research program investigating specific bulking control methods, Gabb et al. (1989) concluded that the selector effect, which was proposed as a method for controlling low F/M filament bulking, was ineffective. This research also indicated that in fully aerobic systems low F/M bulking was ameliorated, but in intermittently aerated anoxic-aerobic systems low F/M filament bulking was promoted. The research presented in this thesis forms part of a follow-up bulking research program into low F/M bulking which led to the formulation of a bulking hypothesis by Casey et al. (1992b) and focuses on the effect of different anoxic-aerobic conditions and their effect on the concentration of nitrate and nitrite entering the aerobic zone/reactor in single intermittently aerated and multi-reactor nitrification-denitrification systems.
- ItemOpen AccessThe effect of sludge age and aerobic sludge mass fraction on low F/M filament bulking in intermittent aeration nitrogen removal systems(1991) Warburton, Charles Arthur; Ekama, George ASince 1989 a wide ranging program of research has been under way to identify the factors that effect low F/M filament proliferation. Completed work has established an important factor conducive to low F /M filament proliferation - the presence of anoxic and aerobic zones, or, alternating anoxic-aerobic conditions in a system. It was also established that the presence or absence of readily biodegradable COD (RBCOD) or slowly biodegradable COD (SBCOD) were not deciding factors in their proliferation. The research presented in this thesis focuses on the effects of; • sludge age, • magnitude of the aerobic mass fraction, • magnitude of the nitrate concentration during the anoxic period, on the low F /M filaments. The experimental set-up consisted of two intermittently. aerated anoxic-aerobic (20 minute cycles, peak DO 2 to 2,5 mgO/l), single reactor completely mixed continuously fed systems. The experimental investigation was chronologically divided into 3 phases and examined the effect of the following conditions on the low F/M filaments.
- ItemOpen AccessThe effect of the type, size, position and recycle ratio of the anoxic zone on low F/M filament bulking in nitrogen removal activated sludge systems(1992) Hulsman, Andrew Charles; Ekama, George AFilamentous bulking, which causes deterioration in sludge settleability has been shown, in two nation-wide surveys to be a problem of considerable proportions. Poor sludge settleability in the secondary clarifier limits the daily flow and load that can be treated in activated sludge wastewater treatment plants. Controlling sludge settleability to relatively low levels i.e. Diluted Sludge Volume Index (DSVI < 100 rnl/g) by controlling filamentous organism proliferation would allow increased daily flow and loads by up to 100 on existing activated sludge wastewater treatment plants. From the two surveys, Blackbeard et al. (1986,1988) found that mainly six filamentous organisms tended to dominate in activated sludges in N and N&P removal plants i.e. types 0092, 0041, 0675, 1851, 0914 and Microthrix parvicella. Four of these six filaments are classified by Jenkins et al (1984) as low Food/Micro-organism (low F/M) filaments. At the University of Cape Town and in a 4 year research programme (1986 to 1989) Gabb et al (1989a) investigated specific methods to control low F /M filament proliferation. Traditionally the promoted specific bulking control method was inclusion of either an anoxic or aerobic selector at the head of the wastewater treatment plant. Gabb et al (1989a) found that selectors did not control low F/M filament proliferation but that continuous aeration did. They concluded that the presence of anoxic and aerobic zones in a treatment plant was an important factor in low F/M filament proliferation. In 1989 a second research programme was initiated at the University of Cape Town to identify the factors that influence low F/M filament proliferation. Completed research thus far has established inter alia that fully anoxic and fully aerobic conditions successfully control low F/M filament proliferation but that alternating anoxic-aerobic conditions in single reactor intermittent aeration systems promoted proliferation. The research presented in this thesis focuses on the interchange between anoxic and aerobic conditions in nitrogen removal systems and its effect on low F/M filament proliferation.
- ItemOpen AccessThe effect of thermophilic heat treatment on the anaerobic digestibility of primary sludge(1992) Izzett, Hilton; Ekama, George AIn a time when the world is becoming more environmentally conscious, and is looking for simple, efficient, economical, and environmentally friendly solutions to sewage and sludge treatment, the dual digestion system presents itself as an attractive alternative to other sludge treatment systems. The dual digestion system comprises an autoheated thermophilic (55-65°C) aerobic first stage and a mesophilic (37°C) anaerobic second stage. Past research into the dual digestion system has given rise, inter alia, to the following claims (de Villiers, et al, 1991): a) sludge disinfection and stabilisation occur in one process - disinfection .in the thermophilic aerobic first stage and stabilisation in the anaerobic second stage. b) the stability of the anaerobic stage is considerably 'improved by the increase in H2C03 * alkalinity and pH in the aerobic stage. c) in the aerobic stage the sludge is aerobically or thermally pretreated (conditioned) making it more readily digestible under anaerobic conditions, thereby allowing significantly reduced retention times from 25 to 30 days for normal digestion, to 8 to 10 days. Messenger (1991), in a full scale investigation at the Potsdam Wastewater Treatment Plant in Milnerton, Cape Town, verified claims (a) and (b), but was unable to verify claim ( c) above. It was the intention in this investigation to verify this claim at full scale using the existing dual digestion plant at Milnerton. However, after 9 months of starting up the plant, the fiberglass aerobic tank failed structurally along one of its seams. This failure was so extensive that the plant could not be started up again and the research project was terminated. The failure of the aerobic reactor necessitated continuing this thesis investigation in the laboratory.
- ItemOpen AccessExternal nitrification in biological nutrient removal activated sludge systems(1999) Moodley, Rajan; Ekama, George A; Wentzel, Mark CIn conventional nitrification-denitrification biological excess phosphorous removal (NDBEPR) activated sludge systems, such as the UCT system for example, both nitrification and phosphorous uptake (P uptake) occur simultaneously in the, usually large, aerobic reactor. In the UCT system the nitrate load to the anoxic reactor is limited by the a-recycle (i.e. system constraint recycle from the aerobic to the anoxic reactor) and the internal aerobic nitrification performance. The latter process, is mediated by the nitrifiers having a slow growth rate of 0.45/d, governs the sludge age of the biological nutrient removal activated sludge (BNRAS) system and thus results in long (20 - 25 day) sludge ages and large aerobic mass fraction requirements to nitrify completely. However, if stable nitrification could be achieved outside the BNRAS external nitrification (EN) system then nitrification and the suspended solids sludge age become uncoupled allowing greater flexibility into the BNRAS system.