Theses and dissertations (Engineering and Built Environment)

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Comparative study of anammox-mediated nitrogen removal in three reactor configurations
(2021-05-27) Kosgey, Kiprotich Eric; Pillai, Sheena Kumari Kuttan; Kiambi, Sammy Lewis; Bux, Faizal; Chandran, Kartik
Anaerobic ammonium oxidation (ANAMMOX) is an efficient and cost-effective process developed for biological nitrogen removal from wastewater. However, widespread application of the ANAMMOX process for wastewater treatment remains constrained due to the slow growth of ANAMMOX bacteria, propensity for out-competition by fast growing microbes, and its sensitivity to environmental and operational conditions. Consequently, understanding the influence of mixing conditions in different reactor configurations on this process is paramount in its improvement. This study focused on the comparative analysis of ANAMMOX-mediated nitrogen removal in a hybrid up-flow anaerobic sludge blanket reactor (H-UASB), moving bed biofilm reactor (MBBR) and a gas-lift reactor (GLR). The study involved experimental study of nitrogen removal, bacterial population dynamics and physical properties of the bacterial biomass within the reactors, as well as the description of process performance and the growth of nitrifying and ANAMMOX bacteria in the reactors using a calibrated mechanistic model. All the reactors were operated for 535 days using the same synthetic feed under anaerobic conditions. K1-type carrier materials were added to each reactor for biofilm development. The concentrations of ammonium (NH4 + ), nitrite (NO2 - ) and nitrate (NO3 - ) in the effluent from the reactors were determined colorimetrically. Among the three reactors, MBBR displayed the highest nitrogen removal efficiency (NRE) during the study (66±36%), and contained the lowest concentration of free ammonia (FA) (19±22 mg-N/L) and free nitrous acid (FNA) (0.001±0.001 mg-N/L). In comparison, the NRE and the concentrations of FA and FNA in H-UASB during the study were 63±28%, 91±41 mg-N/L and 0.006±0.004 mgN/L, respectively, while in the GLR, they were 54±39%, 28±29 mg-N/L and 0.002±0.002 mg-N/L, respectively. Based on the ratios of NO2 - consumed to NH4 + consumed, and the ratios of NO3 - produced to NH4 + consumed, the start-up of ANAMMOX process was faster in the MBBR (144 days) compared to H-UASB (193 days) and GLR (272 days). MBBR also displayed less fluctuations in the NREs and nitrogen removal rates (NRRs) during the study compared to H-UASB and GLR. The microbial communities in the suspended biomass in the reactors were characterised using high-throughput sequencing on an Illumina MiSeq platform on days 125, 192, 260, 309 and 535, while the microbial communities in the biofilms were only characterised on day 535 (last day) due to slow biofilm development. Gradual increases in the relative abundance of ANAMMOX bacteria were observed in the suspended biomass in all the reactors between days 125 and 309, which corroborated the observed increases in the NREs. The relative abundance of ANAMMOX bacteria remained consistently higher in H-UASB during the study than in MBBR and GLR. On the contrary, the highest relative abundance of ammonia oxidising bacteria (AOB) was observed in the suspended biomass in the MBBR on day 125 at approximately 38%, while the highest relative abundance of nitrite oxidising bacteria (NOB) and complete ammonia oxidising (COMAMMOX) bacteria was recorded in the suspended biomass in the MBBR at approximately 30% and 5%, respectively. In all the reactors, the relative abundance of AOB in the biofilms and the suspended biomass was comparable on day 535. In addition, on day 535, higher relative abundance of NOB was observed in the biofilms in both GLR and H-UASB at approximately 7% compared to the suspended biomass, while their abundance in the suspended biomass in the MBBR was comparable to that recorded in the biofilms. Furthermore, in both H-UASB and MBBR, higher relative abundance of ANAMMOX bacteria was observed in the suspended biomass compared to the biofilms on day 535, while comparable abundance was observed in the GLR. The highest total microbial diversity (Shannon and Simpson indices) and evenness (Pielou’s Evenness) was observed in the suspended biomass in the MBBR. Granulation of the suspended biomass was observed in both GLR and H-UASB, while the suspended biomass in the MBBR was flocculent. In the MBBR, the colour of the biomass had turned brown on day 125, while the biomass in H-UASB and GLR on this day was tawny and dark-tawny, respectively. However, on day 309, the biomass in all the reactors had turned red, corroborating the highest relative abundance of ANAMMOX bacteria observed during the study. Faster attachment of biomass on the carrier materials in MBBR was observed in the course of study compared to H-UASB and GLR. On the last day, the concentrations of the biomass on the carrier materials in the MBBR was also higher (12 mg/carrier) in the MBBR than in the H-UASB (8 mg/carrier) and GLR (10 mg/carrier). Activated sludge model 1 (ASM 1), which was modified by separating the activities of Nitrospira spp. from those of Nitrobacter spp. as well as by adding both ANAMMOX and COMAMMOX bacterial activities, was used to describe process performance in the reactors. The modified ASM 1 was able to predict the trends in the effluent concentrations of NH4 + , NO2 - and NO3 - in all the reactors. In addition, the correlation of the actual relative abundance of nitrifying and ANAMMOX bacteria, with the model-predicted relative abundance, was positive. The model also indicated higher heterotrophic activities in both GLR and MBBR compared to H-UASB, an indication that continuous mixing in MBBR and alternation of plug-flow conditions with internal gas circulation in GLR favoured heterotrophic bacterial growth. However, the model was limited in predicting the fluctuations in bacterial abundance and the fluctuations in the effluent concentrations of NH4 + , NO2 - and NO3 - in the reactors. The obtained results indicate that better-mixed conditions in the MBBR led to comparable relative abundance of nitrifying bacteria between the biofilms and the suspended biomass, while plug-flow conditions in the H-UASB favoured ANAMMOX bacterial growth in the suspended biomass and the nitrifying bacterial growth in the biofilms. The alternation of internal gas circulation with plug-flow conditions in the GLR also favoured the growth of nitrifying bacteria in the biofilms. Overall, nitrogen removal in H-UASB was likely dominated by ANAMMOX process, while nitrogen removal in MBBR and GLR was as a result of combined ANAMMOX and sequential nitrification-denitrification processes. The novelty of this study stem from the impact of mixing conditions on process performance and microbial ecology of ANAMMOX-mediated systems.
Enhanced biohydrogen production from carbohydrate rich wastewater through anaerobic fermentation
(2020-11-30) Mutsvene, Boldwin; Chetty, Maggie; Pillai, S. K. K.; Bux, Faizal
In recent times,“the world has faced serious problems emanating from the use of fossil fuels which are detrimental to the environment at large. On the other hand, due to the industrial boom, many industries produce wastewater that is harmful to the environment hence, carbohydrate-rich industrial wastewater can be advantageously used to reduce impact on the environment. If subjected to anaerobic fermentation, organic wastewater has the potential to produce renewable energy sources that have less impact on the environment, including biohydrogen, which has little or no carbon footprint. While reducing the impact of the problems caused by the disposal of wastewater to the environment, the biological methods also offer a solution to the detrimental effects of fossil fuels and their after use effects. The study was mainly based on environmental protection and clean, renewable alternative energy production by generating biohydrogen from organic industrial wastewater as a substrate. Anaerobic digestion has been extensively studied, but dark fermentation, which is an emerging technology within anaerobic digestion that involves the production of hydrogen from carbohydrate-rich substrates, has less information documented regarding this technology. This technology is crucial in the because it forecasts beyond fossil fuel usage and is accompanied with long-term economic expansion and energy security as there are many reservations about fossil fuel reserves and their high risk of exploitation.” Biohydrogen potential tests (BHP) were performed on five different wastewater streams (yeast, alcohols, brewery, sugar, and dairy industries) to determine the stream with the best hydrogen potential. Rigorous characterisation of various wastewater streams was conducted; the main parameters of interest were COD, BOD, VS, TS, pH, among others. The BHP tests were conducted in triplicates in 600 mL Schott bottles charged independently with various wastewater streams and inoculated by the seed sludge from a local wastewater treatment plant at the different substrate to biomass ratios. The highest hydrogen composition was recorded with the brewery wastewater, which had 40.1% H2 in the off-gas as analysed by the gas chromatograph; and the minimum was found in alcohol wastewater, 21.4%. The Kepner-Tregor decision-making tool was conducted to determine the most suitable stream for the scaled-up reactor. A conclusion to use the brewery wastewater in the scaled-up Anaerobic Baffled Reactor (ABR) was reached. Four 10 L Anaerobic Baffled Reactors were used as the scaled-up reactors to optimise operating conditions for the production of biohydrogen using the brewery wastewater. Design-Expert software, under response surface methodology, was used to produce the matrix of combinations of the experimental runs by varying temperature (32-38℃), batch time (4-16 h), and pH (3.5-7.5); in total 20 runs were formulated.” The highest hydrogen production rate of 18.16 mL/h and the hydrogen yield of 30.98 mmol/gCOD were observed at temperature, batch time, and pH of 35℃, 4-10 h, and 5, respectively. The optimum operating conditions were determined to be a temperature of 36℃, batch time of 10.2 h, and a pH of 5.6. A predictive model, quadratic polynomial in nature, was developed after an intensive analysis of variance, a regression coefficient between predicted and actual hydrogen production rates was found to be 0.92. A system was run on optimum conditions to validate the developed mathematical model. The maximum hydrogen potential rate (HPR) determined in this study was 6.11% higher than the predicted value. The validation runs were also performed as control experiments for comparison between a system with nanoparticles and a system without nanoparticles with regards to the HPR. 25.37% H2 and 21.85% H2 were determined for with magnetite nanoparticle system and a system without nanoparticles, respectively. The experiments with nanoparticles garnered 44% higher HPR (23.41 mL/h) than a system without nanoparticles.
Evaluating the removal of emerging contaminants from the eThekwini Municipality REMIX Water Treatment Plant
(2024-05) Manyepa, Prince; Bux, Faizal; Seyam, Mahommed; Banoo, Ismail
The eThekwini Municipalities Department of Water and Sanitation (EWS) has initiated feasibility studies to determine whether it is financially and environmentally viable to implement direct potable water reuse (DPR) projects, and one of them is the REMIX Water Treatment Plant (RWTP) which is located within the Port of Durban and abstracts wastewater and sea water for treatment and potential future re-use. However, a review of the extant literature has highlighted that wastewater and seawater are primary sources and "sinks" for various contaminants of emerging concern (CEC). Emerging contaminants (ECs) can be endocrine-disrupting chemicals or cancercausing agents in humans and animals if they are constantly present in drinking water. This study evaluated the efficiency of the RWTP for the removal of different classes of pharmaceutical compounds by measuring the feed water and effluent of each treatment unit along the RWTP. The Quantitative structure-activity relationship (QSAR) model and OPBT criteria were used to screen these compounds for persistence, bioaccumulation, and toxicity (PBT) behaviour in the water matrix. This was done to produce a priority list that allowed effective monitoring of each treatment unit for observed PBT compounds that should not be present in reclaimed water intended for human consumption. The QSAR is a suitable alternative to the costly and labourintensive in vivo screening experiments in the water matrix. It works in tandem with the new animal rights regulations, is safer than laboratory experiments, and also saves time. The study found that 4 out of 20 compounds were identified as potential PBT compounds by consensus agreement in both methodologies. The goal of this study was to assess the removal of ECs prioritised using the QSARINS model and OPBT criteria by carrying out a human risk analysis for reclaimed water proposed for drinking purposes within the City of Durban. This informed decision-makers, plant managers, and operators on what to constantly monitor or add to the treatment plant for the safe production of drinking water. Excellent removal rates of ECs were observed in the membrane biological reactor (MBR) and the reverse osmosis systems (ROs). The removal rates in MBR and ROs ranged from 38% to 100% and 96%, respectively. Excellent removal rates for heavy metals and nutrients across the treatment technology were also achieved in the final product water. The calculated risk/hazard quotients (RQ) for all ECs and heavy metals were also conducted in the reclaimed REMIX water. An RQ/HQ > 1 meant a high risk of ECs or heavy metals, and <1 meant the risk was negligible. Except for some anomalies caused by ion suppression or matrix effects during the analysis, the majority of the ECs in the reclaimed water RQ were found to be less than 1. Identification of chemical, biological, and physical hazards using HACCP system principles led to the identification of critical control points for the technology. Five critical control points were examined, and techniques for successful RWTP monitoring were proposed based on the study findings.
Sulphur-driven anammox systems for nitrogen removal from wastewater
(2023-09) Hassan, Magray Owaes; Seyam, Mohammed; Gani, Khalid Muzamil; Pillai, Sheena Kumari Kuttan; Bux, Faizal
This thesis explored sulphur-driven Anammox systems as a potential solution for removing nitrogen from wastewater. It focused on using sulphur compounds as electron donors to drive the Anammox process, aiming to reduce energy requirements and costs compared to conventional methods. The thesis provided an overview of existing nitrogen removal techniques in wastewater treatment plants, highlighting the limitations of conventional methods and the potential of Anammox as an alternative. The theoretical framework of sulphur-driven Anammox systems was discussed, emphasizing their advantages in terms of energy efficiency, carbon footprint reduction, and cost-effectiveness. The study examined the principles and significance of the Anammox process in wastewater treatment, where anaerobic microorganisms convert ammonium and nitrite into nitrogen gas, thereby eliminating nitrogen pollution. By harnessing the power of sulphur-driven Anammox processes, this study aimed to address these environmental challenges and develop innovative and sustainable wastewater treatment technologies. The research investigated the potential of sulphur compounds, such as sulphide (S2-), sulphate (SO4 2-) and elemental sulphur (S0), as electron donors in Anammox systems. These compounds serve as alternative sources of reducing equivalents, enabling the Anammox bacteria to carry out the conversion process efficiently. The performance and microbial dynamics of sulphur-driven Anammox systems were extensively analyzed. Various parameters, such as nitrogen removal efficiency, sulphur compound dosage, pH and temperature, are evaluated to optimize system performance. The study also investigated the microbial community composition and metabolic pathways involved in sulphur-driven Anammox, shedding light on the key micro-organisms and their functional roles. The combination of partial nitrification (PN) and anaerobic ammonium oxidation was investigated as a promising technology for nitrogen removal from wastewater. Strategies such as intermittent aeration, pH shocks and sulfide addition were studied for PN start-up and suppression of nitrite-oxidizing bacteria (NOB). Intermittent aeration with low dissolved oxygen (<5 mg/L) suppressed NOB activity, resulting in a 93% nitrite accumulation rate (NAR). Low pH (5) reduced both ammonia oxidising bacteria (AOB) and NOB activity while raising it to pH 7.5 increased AOB activity (84% NAR) but kept NOB suppressed. Adding sulfide (up to 25mg/L) without pH control raised NAR from 63% to 85%. These factors affect nitrite accumulation in our system. The study provided insights into establishing PN in a sequential batch system and highlighted the sensitivity of nitrite oxidation to sulfide. The incorporation of sulphur into carbon and nitrogen removal processes in a wastewater treatment plant was explored in this study. A dosage of 15 mgS/L of sodium salt of sulphide combined with 2-3 mg/L dissolved oxygen, established PN effectively in synthetic and real wastewater. PN was established when pH naturally rose due to sulfide hydrolysis, suppressing NOB activity. Long-term operation achieved a nitrite accumulation ratio of 70 ± 19%, with ammonia and nitrite concentrations of 19 ± 4 mgN/L and 18 ± 4 mgN/L, respectively. NOB communities diminished during stable PN but returned when sulfide dosing stopped, indicating the need for continuous sulfide dosing.The study emphasized the continuous adoption of this strategy for sustained PN and its potential application in nitrogen removal from domestic wastewater. This study explores the intriguing sensitivity of Anammox bacteria to sulfide, revealing captivating insights into their unique response to this compound. The short-term negative effect of sulfide on Anammox performance was observed but quickly recovered under low sulfide stress. Reducing or removing sulfide in the influent is suggested to accelerate the recovery of Anammox performance. The stoichiometric ratio was identified as an indicator of Anammox performance and consortium development. The study highlighted the importance of environmental conditions and sulfide concentration control in optimizing Anammox processes and nitrogen removal in wastewater treatment systems. The study extensively examines the performance of the sulfide-driven Anammox (SPDA) process, specifically focusing on the impact of various hydraulic retention times (HRTs) and external sulfide dosing. The objective was to understand how variations in HRTs and the addition of external sulfide influence the efficiency and effectiveness of the SPDA process in removing nitrogen from wastewater. To gain a deeper understanding of the microbial communities involved in the SPDA process, next-generation high-throughput sequencing techniques were employed. These advanced sequencing methods allow for a comprehensive analysis of the structural and functional dynamics of the microbial communities present in the SPDA system. By analyzing the genetic material of these micro-organisms, it becomes possible to identify and quantify their abundance, diversity and potential functional roles in the nitrogen removal process. The findings from this study are expected to provide valuable insights into optimizing the performance of the SPDA process. By examining the effects of different HRTs and external sulfide dosing, researchers can determine the most favorable conditions for achieving efficient nitrogen removal. Additionally, by evaluating the microbial communities' dynamics, the study aimed to uncover the relationships between specific microorganisms and their contributions to the SPDA process. Ultimately, the results obtained from this investigation can contribute to the development of improved strategies for nitrogen removal from wastewater using the SPDA process. This knowledge can inform the design and operation of wastewater treatment systems, leading to enhanced performance, reduced energy consumption and increased cost-effectiveness in the treatment of nitrogen-rich wastewater.
Thermal conversion of algal biomass and its derivatives to fuels and petrochemicals
(2021-04) Mustapha, Sherif Ishola; Isa, Yusuf Makarﬁ; Bux, Faizal
Thermal conversion processes have gained increased attention since they can be applied to whole microalgae (not lipids alone) resulting in higher biofuel yield with potential for production of other high-value products. The major challenges of microalgal thermal conversion are the high level of nitrogen and oxygen content present in the product stream, as well as high acidity which makes the bio-oil unstable and unfit for use as transportation fuels directly. Transportation fuels are expected to be low in oxygen and acid content for stability and also have low nitrogen content to meet environmental emission standards for combustion. Nutrient stress as a tool for enhancement of yields and quality of bio-oils produced from thermal conversion of microalgae has not received sufficient attention. This study investigated the conversion of Scenedesmus obliquus microalgae via three different thermal conversion processes which include pyrolysis, hydrothermal liquefaction and hydrothermal gasification. Scenedesmus obliquus microalgae were grown under nutrient stressed and unstressed conditions. To better understand the effect of nutrient stressing on the process, pyrolysis experiments were conducted on unstressed S. obliquus microalgae biomass (N3), nutrient- stressed S. obliquus microalgae biomass (N1) and its residual algae biomass after lipid extraction (R-N1) at different temperatures (400 °C to 700 °C) and the results compared. Detailed biomass characterization which includes proximate analysis, ultimate analysis, biochemical analysis, Fourier-transform infrared spectroscopy (FTIR) analysis, and thermogravimetric analysis (TGA/DSC) were carried out on the microalgae biomass (N1, R-N1 and N3) to provide useful information about the combustion behaviour of the biomass during pyrolysis. The biomass characterization results indicated that nutrient-stressed condition altered the microalgae biomass composition and empirical formula for N1, R- N1, and N3 microalgae biomass were CH2.00N0.07O0.71, CH2.36N0.08O0.75, and CH2.35N0.14O0.71, respectively. The maximum bio-oil yield for N1 (46.37 wt%) and R-N1 (34.85 wt%) were obtained at 500 °C, while the highest yield of bio-oil for N3 (41.94 wt%) was obtained at 600 °C. Also, the proportion of nitrogen compounds in N3 bio-oil (47.4 %) was significantly higher than that obtained in the nutrient stressed microalgae biomass (N1) bio-oil (5.92%) at pyrolysis temperature of 500 °C. Thus, nutrient stressed approach is considered more promising to produce a higher yield and good-quality pyrolytic bio-oil from microalgae biomass. A predictive model was developed based on artificial neural network (ANN) and can serve as a framework for the prediction of bio-oil yield from the pyrolysis of microalgae biomass. Finding better heterogeneous catalysts that can enhance the quality of microalgal bio-oils to meet transportation fuels standards is seen as a major advance toward developing efficient and sustainable thermal conversion processes. In this study, pyrolysis of nutrient- stressed Scenedesmus obliquus microalgae over various supported metal M/Fe3O4-HZSM- 5 catalysts (M = Zr, W, Co and Mo) was investigated. The synthesized catalysts were characterized by X-ray diffraction spectroscopy (XRD), thermogravimetric analysis (TGA), high-resolution scanning electron microscopy and energy dispersive spectroscopy (HRSEM/EDS). The catalyst: biomass ratio and temperature influence on pyrolysis product yield was also investigated. Between these, Co/Fe3O4-HZSM-5 catalyst showed better activity in enhancing the bio-oil quality and yield; it had the lowest nitrogen content (4.77 wt%) and highest bio-oil yield (17.73 wt %) as well as highest HHV (40.78 MJ/kg) which is almost similar to that of crude petroleum. The results showed that all the supported metal catalysts during pyrolysis promote aromatization and acid ketonization of bio-oils. The total amounts of acids present in pyrolytic bio-oil significantly decreased from 26.68% (non-catalytic) to between 0.58 – 9.68% (catalytic). Also, production of 2-pentanone was observed to increase from ~10% (non-catalytic) to 27.36 – 53.90% (catalytic). In terms of energy recovery, Co/Fe3O4-HZSM-5 had about 40% energy recovery, which was the highest while the least performing catalyst was W/Fe3O4-HZSM-5 with 24.18% energy recovery in bio-oil. Overall, Co/Fe3O4-HZSM-5 was the most effective catalyst in enhancing the quality of pyrolytic bio-oil produced from nutrient stressed Scenedesmus obliquus microalgae with properties close to that of petroleum crude. Hydrothermal liquefaction (HTL) of nutrient-stressed microalgae (Scenedesmus obliquus) (N1) with and without the use of Zr/HZSM-5 catalyst was investigated under temperature conditions ranging from 250 – 350 °C. The Zr/HZSM-5 catalyst was synthesized using wet impregnation technique and characterization was conducted on the synthesized catalyst for its crystalline nature, morphology and thermal stability using X- ray diffractometer (XRD), High-resolution scanning electron microscopy (HRSEM) and thermogravimetric analysis/differential scanning calorimetry (TGA/DSC). The HTL experiments were also conducted on the unstressed microalgae (N3) for comparison. Under the stressed condition, the protein content of the microalgae was reduced from 42.35% to 22.08% while the carbohydrate and lipid contents were increased from 25.36% to 42.55% and 17.16% to 21.62% respectively. The maximum HTL bio-oil yield of 52.80 wt% and 24.27 wt% were found for N1 and N3 respectively at 350 °C with addition of Zr/HZSM-5 catalyst. Higher denitrogenation and deoxygenation was achieved with N1 compared to N3. At high temperature of 350 °C, the most abundant fatty acid in N1 was found to be cis- vaccenic acid (omega-7- fatty acid), and this could be explored for possibility of extracting products of great value from the bio-oil for applications other than biofuels. Mainly, the use of Zr/HZSM-5 catalyst on nutrient-stressed S. obliquus microalgae resulted in enhanced bio-oil yield and characteristics which compared well with petroleum crude. The potential of using whole algae, lipid and residual algae of S. obliquus microalgae as feedstocks for production of high-quality hydrogen and methane-rich gas via hydrothermal gasification technique was also examined. The effect of operating parameters such as temperature, pressure and biomass concentration on the yield and composition of gaseous products using whole algae, lipid, and lipid extracted algae (LEA) as feedstocks was examined. The results showed that reaction pressure had minimal impact while temperature, biomass concentration and feedstock composition had significant effects on the composition of gaseous products. It was also found that low temperature (400 oC) and biomass concentration of 40 wt% favoured the production of methane-rich gas. In contrast, high temperature (700 oC) and low biomass concentration (10 wt%) favoured hydrogen- rich gas production in all the three feedstock considered. The highest mole fraction achieved for CH4 was 53.45 mole%, 61.70 mole% and 52.20 mole% which corresponded to CH4 yield of 31.14 mmol/g, 56.90 mmol/g and 30.15 mmol/g for whole algae, lipid and LEA respectively. For H2 rich gas production, the highest mole fraction achieved were 55.77 mole%, 52.29 mole% and 55.34 mole% which corresponded to H2 yield of 75.44 mmol/g, 105.51 mmol/g and 73.49 mmol/g for whole algae, lipid and LEA respectively. The ranking order for the yield and lower heating value (LHV) of the product gas from the HTG process was lipid > whole algae > LEA. This study has shown that hydrogen-rich and methane-rich gas can be produced from the hydrothermal gasification of microalgae as a function of the reaction conditions and feedstock composition. Also, the suitability of nutrient stressed approach and use of catalysts to enhance the quality of bio-oil produced from thermal conversion of microalgae biomass was established.