Browsing by Author "Foxon, Katherine Maria."

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Analysis of a pilot-scale anaerobic baffled reactor treating domestic wastewater.
(2009) Foxon, Katherine Maria.; Buckley, Christopher Andrew.
This thesis presents a chemical, microbiological and mathematical analysis of an anaerobic baffled reactor (ABR) treating domestic wastewater. The purpose of this study was to gain an understanding of the mechanisms of treatment of domestic wastewater in an ABR at pilot-scale, and to use this understanding to develop some guidelines for the design of ABR technology for the anaerobic treatment or pre-treatment of domestic wastewater. Previous research has been undertaken on ABR technology, but no detailed studies of the performance of an ABR on domestic wastewater at pilot-scale have been reported. In this thesis, operating data from a 3 000 ℓ pilot-scale ABR are presented and analysed. Two hypotheses were proposed: that (i) the baffled design of the reactor would facilitate phase separation whereby acidogenic and methanogenic processes predominate in different physical locations in the reactor; and (ii) the critical design parameter is the applied hydraulic retention time. The principle findings of this research were: • The pilot-scale ABR functioned as a solids retention device. Particulate material was retained through settling in the first compartment, forming a gel-like matrix. Reduction of solids occurred through anaerobic conversion to CH4 and CO2. • Partial phase separation of acidogenic and methanogenic communities was observed. • The major factor that controlled biomass washout rate and therefore reactor performance was upflow velocity in each compartment. At higher upflow velocities, slow growing microorganisms failed to establish, resulting in increased solids accumulation rates, while at lower upflow velocities, stable digestion proceeded. • Relatively poor treatment rates were obtained due to the low inherent alkalinity of waters in eThekwini municipality resulting in low operating pH values. • Insufficient pathogen reduction was observed indicating that post-treatment of effluent would be required. It was concluded that the benefit of the baffled design was related to the system’s solids retention characteristics and that the critical design parameters for an ABR domestic wastewater treatment unit were compartment upflow velocity and applied hydraulic retention time.
Analysis of an anaerobic baffled reactor treating complex particulate wastewater in an abr-membrane bioreactor unit.
(2010) Bwapwa, Joseph Kapuku.; Foxon, Katherine Maria.; Buckley, Christopher Andrew.
Providing water and proper sanitation to poor communities by 2015 is one of the United Nations targets for this millennium. In South Africa many communities aspire to waterborne sanitation. However, there is a technology gap for decentralized and sustainable waterborne sanitation systems capable of treating domestic wastewater (Foxon et al., 2006). Although domestic wastewater is more commonly treated using aerobic processes, anaerobic processes may be more appropriate for decentralized applications since they do not require aeration. Research is currently being undertaken to understand the behavior of a combined ABR-MBR unit for treating domestic wastewater. In this study, the anaerobic baffled reactor (ABR) was investigated by analyzing physico-chemical and biochemical data from experiments on a laboratory-scale ABR. This anaerobic reactor was treating complex particulate wastewater made up of sludge from the ventilated improved pit latrine toilets (known as VIP sludge). The main focus of this study was to establish the relationship between the increasing organic loading rates and the effluent characteristics (such as chemical oxygen demand: COD and extrapolymeric substances: EPS). The present work was structured in two parts; in the first part the reactor was operated at constant hydraulic retention time (HRT) without controlling feed characteristics. In the second part, the ABR was operated with step increases in organic loading rates. It was logistically not possible to provide a feed of real domestic wastewater to the laboratory-scale equipment. Consequently, a pit latrine sludge diluted with tap water was used to feed the ABR. This feed was found to have different biodegradability characteristics compared to domestic wastewater. However, the results still give insight into the performance of the ABR and into the treatability of VIP sludge. COD removal ranged from 52 to 80 % depending on the inlet COD. Some COD removal was due to solids retention in compartments, while it was estimated that only 28% of COD removal was due to biological degradation. Soluble extrapolymeric substances (proteins and carbohydrates) which are usually a by -product of anaerobic degradation were higher in the feed than in the effluent despite the increasing organic loading rates. However, more than 50 % of soluble extrapolymeric substances from the influent remained in the effluent and were found (in a parallel project) to influence membrane fouling in the membrane section of the experimental set-up (ABR-MBR unit). Parameters such as pH, conductivity, alkalinity, total and volatile solids were also investigated in this study. The pH decreased slightly from the inlet to the outlet during all runs even though the loading rates were increased. Conductivity increased significantly from influent to effluent with the increasing organic loading rates. Large amounts of total solids were retained in the reactor during the treatment process. Low alkalinity production was recorded during the operation of the reactor. In most cases, the data recorded in this study showed a low biological activity taking place while the reactor was working at room temperatures. Overall, up to 80% of removal efficiencies in terms of total COD and solids were recorded with increasing organic loading rates at constant hydraulic retention time. While these results do not allow the prediction of ABR-MBR performance during the treatment of real wastewater, it was concluded that: Most solids retention occurred in the feed tank. Most COD removal occurred as a result of solids retention and digestion. Loading characteristics did not strongly influence effluent EPS, pH or alkalinity, but did influence COD and conductivity. The relatively low biodegradability of the feedstock indicates that anaerobic digestion is not the most appropriate treatment for VIP sludge.
Analysis of nutrient requirements for the anaerobic digestion of Fischer-Tropsch reaction water.
(2013) Mathir, Aarefah.; Foxon, Katherine Maria.; Brouckaert, Christopher John.
Nutrients play an important role in the functioning of microorganisms during anaerobic digestion. The anaerobic treatment of industrial wastewaters, such as Fischer-Tropsch Reaction Water (FTRW), requires the addition of nutrients suitable for micro-organisms (micronutrients) since these wastewaters are devoid of essential metals. However, the dosing of nutrients is only effective if the metals are in a bioavailable form which in turn is dependent on the chemical speciation of the system. This study aimed to investigate and model the influence of precipitation on bioavailability by considering the extent to which precipitation can sequester metals into forms that are not bioavailable and the extent to which this sequestration can describe biological effects in an anaerobic system. Visual MINTEQ and Excel were used to develop a combined mass balance and chemical-equilibrium speciation model that considered the soluble and the precipitate metal phases. The model was compared to two sets of experimental analysis. Experiment A included metal analysis on the sludge and supernatant from glucose and ethanol fed ASBRs while Experiment B included similar analysis on FTRW fed ASBRs while biological parameters were monitored during a micro-metal washout experiment. Precipitation was found to sequester Al, Zn and Fe to a large extent making them non-bioavailable in Experiment A, while sulphide precipitates were predicted to dominate the metal speciation in Experiment B. In Experiment A, the organically bound metals phase was also a significant phase that sequestered metals. Furthermore, the rates of washout of most of the metals (excluding Mg) were over-predicted, which may have been due to the absence of other solid related phases in the model. This may also be attributed to kinetic effects in the system. Although there were reasonable correlations between the model predicted and the experimentally determined concentrations, it is recommended that the model should include the organically bound phase and consider mass transfer effects in the system. After 12 cycles without dosing micro-metals in Experiment B, the biogas production decreased by 43%. A decline in the predicted and determined soluble concentrations of a variety of metals were observed during this time, suggesting that there may be an agreement between predicted metals washout and reduction in anaerobic activity. Since the soluble metal concentrations did not decrease as rapidly as predicted by the model, a lag period between the two parameters was observed. Therefore, although the model provides an improved understanding of metal speciation and bioavailability such that recommendations may be made for prudent micro-metal dosing, further development is required for more accurate representations of the system.
Development of a model of the aerobic membrane bioreactor treating Illovo wastewater.
(2014) Kay, Liam Grant.; Foxon, Katherine Maria.; Brouckaert, Christopher John.
The Membrane Bioreactor (MBR) at Sezela, KwaZulu-Natal treats a process effluent emanating from a sugar industry by-products plant. Depending primarily on the effluent feed rate to the MBR as well as other less significant factors, the MBR tends to operate at a temperature that fluctuates between 40 and 50 °C. As a result of the temperature fluctuations the MBR may operate at either mesophilic or thermophilic temperatures. In an attempt to avoid the operational instability that accompanies the transition between temperature regimes, it would be conceivable to maintain mesophilic operation through either the removal of heat during feed increases or by continuously maintaining a low feed rate; alternatively to maintain thermophilic operation by providing auxiliary heat to the MBR when low feed rates are experienced, or by maintaining a high feed rate, possibly in conjunction with a buffer tank. A solution to the problem was sought through the formulation of a coupled dynamic mass and energy balance model, with an attached speciation routine. Development of a simulation model allowed the prediction of key operating parameters, namely the temperature, pH, substrate concentration, and volatile suspended solids (VSS) concentration. The sources of data used for modelling were laboratory experiments, historical MBR data, and literature data. Kinetic and stoichiometric coefficients of the model were determined from batch respirometric tests on the MBR furfural plant effluent feed and the activated sludge. The final model yielded a dynamic temperature (Root Mean Square Deviation (RMSD) of 1.61 and 1.34 °C) and pH (RMSD of 0.36 and 0.47) prediction over a continuous 69 day interval, where only the furfural plant effluent feed and sludge wasting rates were required as model inputs. The prediction of the substrate concentration and VSS concentration were found to be unreliable. The results of the comparison of mesophilic to thermophilic operation, through the final calibrated model, indicated that thermophilic operation was advantageous, however a rigorous economic analysis is required to substantiate this outcome. Thermophilic operation at 50 °C can handle feed rates 2.2 times higher than mesophilic operation at 40 °C, but may be more susceptible to process upsets.
Dynamic modelling of anaerobic digestion of Fischer-Tropsch reaction water.
(2013) Lees, Crispian McLintock.; Foxon, Katherine Maria.; Brouckaert, Christopher John.
Fischer-Tropsch Reaction Water (FTRW) is a high organic strength wastewater produced as a by-product in Sasol’s Fischer-Tropsch Reactors. Typically it has an organic load of 18000 mgCOD/L and is highly acidic with a pH of approximately 3.8. It is deficient in nutrients (N and P and other micronutrients). This dissertation deals with the biological and physico-chemical model development of a dynamic anaerobic digestion model, and explores two different approaches to representing the physico-chemical processes that complement and interact with the bioprocesses. The performances of the resultant two dynamic models (ADFTRW1 & AD-FTRW2) were compared in order to assess to what extent the more detailed and rigorous ionic speciation modeling in AD-FTRW2 addressed the shortcomings attributed to the simplified physicochemical modeling in AD-FTRW1. The ionic speciation model used in AD-FTRW2 uses a classic equilibrium formulation along the same lines as in the UCTADM2 model for anaerobic digestion of municipal wastewater sludges (Brouckaert et al., 2010), while AD-FTRW1 uses a simplification of the approach developed by Musvoto et al. (2000) in order to represent short chain fatty acid (SCFA) dissociation and the weak acid base chemistry of the inorganic carbon system. A 44 day extract from a 700 day laboratory-scale dataset (Van Zyl et al. 2008) was used as the basis for comparing the models. During this period the membrane bio-reactor was subjected to varying flow and load conditions. To validate the models, the experimentally measured and model predicted process variables of reactor alkalinity, reactor pH, biogas production and effluent SCFA concentration were compared. It was found that AD-FTRW2 provided superior agreement with pH data, but predictions of alkalinity, gas production rate and effluent short-chain fatty acids were not significantly improved in AD-FTRW2 relative to AD-FTRW1. This outcome was hypothesized since pH is strongly dependent on physico-chemical processes such as ionic interactions in solution and gas exchange which were the components to the models (AD-FTRW1 versus AD-FTRW2) which differed most significantly. Alkalinity, which is also highly influenced by physico-chemical model representations showed substantial improvement however statistical analysis could not show this improvement to be significant. The other two variables that were compared, biogas production and effluent SCFA concentration, displayed very similar agreement with experimental data. These variables depend more on mass balance effects and biological kinetics and were therefore not significantly altered by the more rigorous handling of aqueous chemistry in AD-FTRW2. It was concluded that AD-FTRW2 constitutes an improvement in model predictive power over AD-FTRW1 at a small cost in computing time.
Investigation into the effect of stripped gas liquor on the anaerobic digestion of Fischer-Tropsch reaction water.
(2014) Roopan, Renésha.; Foxon, Katherine Maria.
The Fischer-Tropsch reaction technology is utilised in Sasol’s Coal-to-liquid plant to produce liquid fuels from low grade coal. There are several processes on the Coal-to-liquid plant that generate aqueous streams which contain a high organic load and require treatment. The main contributors to the wastewater are the Phenosolvan plant, producing stripped gas liquor (SGL), and the Synthol plant, producing Fischer-Tropsch reaction water (FTRW). Stripped gas liquor contains water, organic acids, ammonia, and potentially toxic phenols. Fischer-Tropsch reaction water contains volatile fatty acids and alcohol. Stripped gas liquor is therefore nitrogen-rich while FTRW is nitrogen-deficient and requires nutrient supplementation for anaerobic treatment. Therefore co-treatment of the two streams could reduce nitrogen supplementation requirements. This study is part of a larger project to determine the feasibility of anaerobically co-digesting FTRW and SGL. This study has looked at the influence of SGL on the methanogenic activity of FTRW-acclimated sludge and involved the development of a method which allows accurate recording of the methanogenic activity in batch assays. Other studies involving the anaerobic digestion of high phenolic wastewaters showed that the phenol had an inhibitory effect on the specific methanogenic activity of the sludge, which was not acclimated to the phenol. The objective of this work was to test the hypotheses that (1) anaerobic sludge acclimated to FTRW will be inhibited by high molecular weight organics in SGL and (2) FTRW-acclimated sludge will not degrade phenolic compounds in SGL. This information will be used for designing process configurations for simultaneous treatment of the two streams with minimum contamination of the effluent stream. The serum bottle was used as a small batch reactor and the biogas production was monitored as an indication of the state of the reaction. The biogas produced was collected and measured by the downward displacement of a sodium hydroxide solution, which absorbed the carbon dioxide and collected only the methane. A concentration of 1 g COD/ℓ FTRW was chosen as the reference test due to the reproducibility of the replicates within each experiment as well as its reproducibility across different batches of sludge. For the first inhibition test, the test units contained an additional 5% SGL (0.05 g COD/ℓ SGL) and an additional 15% SGL (0.15 g COD/ℓ SGL, i.e. 13% of the total COD load) respectively, added to 1 g COD/ℓ FTRW. The 5% SGL test unit showed no inhibition compared to the reference unit. There was a reduction in the specific methanogenic activity of the 15% SGL test units compared to the reference unit. Since the total COD load was not the same in each unit, it cannot be conclusively stated that the SGL was responsible for the reduction in SMA, but this seems a reasonable possibility in the light of results from the reference test selection experiments which showed higher SMA at higher organic loading rates. For the second inhibition test, the test units contained 85% FTRW (0.85 g COD/ℓ FTRW) and 15% SGL (0.15 g COD/ℓ SGL) to make up a total COD load of 1 g COD/ℓ. There was an increase in the specific methanogenic activity of the test unit compared to the reference unit. There was very little change in the phenol concentration. Therefore, it was concluded the addition of SGL potentially reduced the SMA and that this could be an inhibitory effect, but that any inhibition would be a function of the concentration of potentially inhibitory substances in SGL and that these concentrations vary from batch to batch. However, the degree of SMA reduction is fairly low and would not prevent co-digestion of the two streams at the concentrations tested. It has been shown that FTRW anaerobic digestion can proceed adequately in the presence of SGL. There was some evidence that phenolics were degraded but at a much slower rate than COD. The percentage reduction in SMA due to additional SGL at concentrations and SGL:FTRW ratios tested was between 0 and 51%. Ultimately, this work is a first step in the development of a co-digestion model relating organic loading rate, SGL:FTRW feed ratio to methane recovery and extent of biodegradation of phenol for use in the design and optimization of a co-digestion system.
Modelling of ionic interactions with organic components in wastewater.
(2014) Westergreen, Sarah.; Foxon, Katherine Maria.; Brouckaert, Christopher John.
In current biological wastewater treatment models, physico-chemical processes (ionic speciation reactions, gas-liquid exchange, and liquid-solid interactions such as precipitation and adsorption) either are not explicitly considered, or are incorporated as simplified descriptions. This may result in an inaccurate prediction of digester behaviour. Specifically, the ionic behaviour of biomass is not explicitly included in standard models. The objectives of this study were to develop a model component that describes ionic behaviour of biomass, use this to predict the overall solution pH buffering capacity and determine its impact in an anaerobic digester’s operating range (pH 6-8). The study hypothesises that the ionic behaviour of biomass can be described in terms of glycine equivalence; alternatively, it can be described by a model component consisting of functional groups characterised by concentration per unit mass of sludge and pKₐ value for each group, either at equilibrium conditions, or considering kinetic effects. The methodology involved constructing a mass balance / ionic speciation model capable of simulating alkaline and acidimetric experimental titrations with modifications for each hypothesis. Varying concentrations of glycine or suspensions of biomass (particulate organic matter) in background salt solutions were titrated and the model was fitted to the data by changing the parameters associated with the biomass description and, (where appropriate) associated kinetic terms, with associated estimation of parameter uncertainty. A model component, UKZiNe was developed consisting of 4 functional groups; 2 carboxyl groups, 1 phosphate group and 1 amine group. Kinetic effects including carbon dioxide exchange and pH probe lag were explored. The hypothesis that glycine could represent the ionic behavior of biomass was not supported. The alternate hypothesis, considering UKZiNe at equilibrium conditions, required further testing to evaluate the effects of kinetic reactions; the second alternate hypothesis that non-equilibrium effects significantly influence the measured experimental pH value, was supported. All model formulations predicted that the biomass contribution to the overall buffer capacity in the operating region of an anaerobic digester was insignificant. The study implies that the inclusion of an ionic description of biomass does not considerably improve the pH prediction in digester simulations and can be excluded in future model development.
Scientific and management support for ventilated improved pit latrines (VIP) sludge content.
(2014) Bakare, Babatunde Femi.; Foxon, Katherine Maria.; Buckley, Christopher Andrew.
Providing adequate sanitation to all in the form of VIP latrines as proposed by the South African Government Strategic Framework for Water Services does not end with building toilets. All municipalities need to plan for maintenance during the operation and when these toilets reach their capacity. An understanding of the processes occurring in pit latrines will facilitate better management during their lifespan and identifying suitable options for dealing with the accumulated sludge when they eventually reach their capacity. This research aims at providing scientific support for decision making in management of accumulated sludge in ventilated improved pit latrines during their life span and when they reach their capacity under South African conditions. The approach to this research work was divided into two main thrusts: The first was to provide an understanding of the processes in VIP latrines and mechanism of sludge stabilization in pit latrines. The second approach was to provide management and disposal options for pit latrine sludge before and once it has been exhumed in the context of the eThekwini pit latrine emptying programme. Two options were used as case studies, namely: (i) deep row entrenchment of exhumed pit sludge for agroforestry and, (ii) in situ treatment of pit sludge using additives. Three hypotheses were proposed: that (i) significant biological stabilization occurs in a pit latrine with time, such that the disposal/treatment options depend on the inherent ability of the chosen option to accept the load of solids and organic material in the VIP sludge, the residual biodegradability of the VIP sludge, and the health risks, (ii) VIP latrine sludge can be used in deep row entrenchment for agroforestry since the sludge contains nutrients that are available to plants, and that the sludge is sufficiently stable not to cause a negative environmental impact, and (iii) that In situ treatment of VIP latrine sludge using pit additives had no significant effect on the rate of mass loss or volume loss of pit latrines contents. The methodological approach to this research was aimed at addressing the proposed research hypotheses. Thus to test the first hypothesis, two studies were conducted; the first study investigated sludge accumulation rate in pit latrines and the role of digestion processes on sludge accumulation rate in pit latrines. Direct measurement of sludge accumulation rate from selected pit latrines within a community in eThekwini municipality was performed and a laboratory investigation into the effect of moisture content and aerobic/anaerobic conditions on sludge accumulation rate was conducted. The second study investigated the chemical and biological characteristics of pit sludge at different depths within a pit latrine. Research into deep row entrenchment of VIP latrine sludge for agroforestry was conducted to test the second hypothesis. The effect of deep row entrenchment on sludge characteristics and surrounding groundwater at the site was investigated by monitoring changes in sludge characteristics and groundwater quality at the entrenchment site over time. An investigation into the effect of pit latrine additives on pit sludge was conducted to test the third hypothesis. Two sets of trials were conducted; the first was a laboratory trial conducted to investigate the effect of pit latrines additives on collected sludge samples from pit latrine in laboratory scale test units. The rate of mass loss that could arise from the effect of addition of pit additives to sludge in the test unit was determined. The second was a field trial in which pit additives were added to randomly selected pit latrines within a community in Durban and changes in amount of the sludge in the pit was investigated using a laser tape measure and a stereographic imaging technique. The main findings of this research were: • The sludge volume accumulation rate in pit latrines investigated was between 120 ℓ/year and 550 ℓ/year regardless of the number of pit users. The overall average sludge accumulation rate was 282 ± 46 ℓ/year. This converts to a per capita sludge accumulation rate of 56 ℓ/person∙year for an average of 5 number of pit users obtained in this study. Statistical analysis performed indicated that sludge accumulation rate on a per capita basis does not decrease with an increase in number of pit users. • In the laboratory batch experiments, it was observed that by increasing the moisture content the rate of degradation of sludge samples decreases. Over a period of 230 days, mass loss was inversely proportional to total moisture content, and it was found that the mass of solids have been reduced to somewhere between 17 and 64 % of the original sludge mass. This effect was attributed to the exposure of sludge samples in the test units to oxygen, since sludge samples with higher total moisture content in the test units appeared as increased depth of free liquid between sludge sample and air. The calculated mass loss rates observed is expected to be higher than that which will be observed in a pit because the laboratory test had continuous air exposure but pit contents are usually covered over by new materials added to the pit. • Natural stabilization of sludge within the pit does occur if the pit is managed and maintained properly thus providing a long service life for the pit. It was found that the volume of materials have been reduced to between 50 and 75 % of the volume of material added over the 3 years since the pits investigated were last emptied, based on the observed per capita sludge accumulation rate and an estimate of the material added to the pit per person/year. Thus, by comparing the calculated mass reduction in the batch laboratory experiment with the volume reduction in the field investigation of sludge accumulation rate, it can be infered that sludge densification/compaction could play an important role on the stabilization processes in a pit. • The nature of sludge in pit latrines varied significantly within the pit and between different pits. It was observed that below the surface layer in a pit, additional stabilization of sludge does occur and the degree of stabilization within a pit increases with increasing depth from the surface down to the bottom layer of the pit. Sludge samples from the bottom of the pit were well stabilized. • It was also observed from the investigation into deep row entrenchment of pit sludge for agroforestry that further stabilization of pit sludge does occur and as a result of that, nutrients (nitrogen, phosphorus and potassium) locked up in the buried sludge are released as fertilizers. Trees planted near buried VIP sludge showed better growth rate compared to those buried only on soil without VIP latrine sludge and no profound effect on groundwater was observed for the duration in which monitoring was carried out. Further research is needed to develop models for implementing this method cost effectively across a range of conditions. • Neither laboratory trials nor field trials provided any evidence that the use of pit additives have any beneficial effect on VIP latrine sludge. There were no systematic and statistically significant changes in the rate of mass loss on sludge samples in the laboratory test units as well as changes in sludge content of the pit latrines used for the field trials as a result of pit latrine additives. Although, it was observed that there was significant reduction in sludge height in pit latrines in which only water was added compared to those in which additives were added and those in which nothing was added (control) using the infrared distance measure, this effect can probably not be explained completely to be as a result of increasing biodegradation rate caused by higher moisture content, since this explanation would have been observed in the laboratory trials as well as in measurement taking using the stereographic imaging techniques. Instead, flattening of the surface of sludge content in the pit by the addition of water onto the highest part of the pile may play a part in the apparent reduction of sludge height observed. It is therefore concluded from the investigation conducted in this research, that sludge content in pit latrines has naturally undergone significant degradation and that the options for disposal of pit latrine sludge would be limited by the characteristics of the sludge. Therefore disposal options involving biological treatment such as disposal into wastewater treatment plants and anaerobic digestion are not appropriate because the residual biodegradability of VIP latrine sludge obtained was very low (about 30 %) and as such would only result in accumulation of undigested solid; of the options considered in this research, deep row entrenchment of VIP latrine sludge for agroforestry seems to be an appropriate option for the disposal of VIP latrine sludge. There was no evidence to suggest that pit latrine additives have any effect in reducing sludge content in pit latrines.