Research works by PhD students of the College of Science and Technology https://dr.ur.ac.rw/handle/123456789/35 2026-06-13T05:49:30Z 2026-06-13T05:49:30Z LEGRACE, Nyandjo Bamen Hetsron https://dr.ur.ac.rw/handle/123456789/2961 2026-05-27T09:50:37Z 2025-08-01T00:00:00Z

Impacts of external variability and randomness on infectious disease dynamics: A modelling study with applications to COVID-19 and vaccination strategies LEGRACE, Nyandjo Bamen Hetsron Disease dynamics are inherently influenced by the interplay between hosts and parasites. And this interaction may be subject to demographic and environmental variability. As environmental conditions shift and human interventions evolve, understanding these dynamics is crucial for effective disease management. This thesis investigates into the wide area of disease modelling, with a particular emphasis on the impact of external variability and stochasticity on epidemic dynamics within the context of host-parasite. First, we investigate the global stability dynamics and sensitivity of COVID- 19 transmission models considering timely and delayed diagnosis. Through theoretical analysis and numerical simulations, we show that the disease persistence depends on the basic reproduction number, R0. Our results suggest reducing the inflow of new individuals into the country or ensuring early diagnosis will lower the basic reproduction number and thereby limiting secondary infections and preventing multiple epidemic peaks. Next, we study the impact of imperfect vaccines, vaccine trade-offs, and population turnover on infectious disease dynamics. Using a mathematical model, we compute the basic reproduction number, establish global stability conditions of equilibria and perform sensitivity analysis. We derive conditions for the vaccination coverage and efficiency to achieve disease eradication assuming different intensity of the population turnover (weak and strong), vaccine properties (transmission and/or recovery) and trade-off between the latter. We show that the minimum vaccination coverage increases with lower population turnover, decreases with higher vaccine efficiency (transmission or recovery), and is increased/decreased by up to 15% depending on the vaccine trade-off. We then formulate a stochastic model for disease transmission in heterogeneous populations of vaccinated and unvaccinated hosts. We prove the existence of a unique nonnegative weak solution and derive conditions for extinction and persistence in mean. Simulations illustrate how demographic variability can alter vaccination outcomes and long-term disease persistence. Finally, we outline future research on metapopulation models for malaria forecasting, incorporating temperature variability and some vaccination strategies to account for climate-sensitive transmission regarding a specific vaccine. This extension aims to enhance predictive accuracy and support targeted interventions for malaria control in a changing climate. Doctoral Thesis

2025-08-01T00:00:00Z NIYOKWIZERA, Felix https://dr.ur.ac.rw/handle/123456789/2935 2026-05-25T10:22:47Z 2025-05-01T00:00:00Z

Prospects of solar distillation on levelized cost of Water: A comparative analysis with conventional desalination technologies NIYOKWIZERA, Felix Access to clean water is a significant issue in off-grid communities around the world, where typical infrastructure is unfeasible. This thesis reviewed two renewable energy-powered technologies Solar Distillation Systems and Membrane Capacitive Deionization (MCDI) for providing drinkable water in rural places. Both leverage photovoltaic energy, making them suited for places with little electricity. The analysis assesses its economic, technical, environmental, and sustainability aspects, concentrating on the Levelized Cost of Water (LCOW) for a village community consuming 50 m³ daily (18,300 m³ annually). Solar Distillation uses solar energy to purify water by evaporation and condensation, while MCDI employs an electrochemical desalination method. Capital costs, operations and maintenance expenses, and LCOW are compared, with Solar Distillation costing $1.94/m³ and MCDI at $0.886/m³, suggesting MCDI’s cost advantage. The analysis utilizes a 20-year system lifespan and a 12% interest rate, applying capital recovery and sinking fund variables to assess fixed costs and salvage values. Environmental implications and sustainability are studied to determine long-term feasibility. Results indicate MCDI’s economic superiority due to lower LCOW, however Solar Distillation may offer simplicity in maintenance. Environmental factors favor both systems for their renewable energy use, although MCDI’s efficiency decreases its ecological imprint. Recommendations include enhancing Solar Distillation’s architecture to minimize costs and scaling MCDI for greater adoption. This analysis emphasizes MCDI’s promise as a cost-effective, sustainable option for off-grid water supply, with future enhancements needed to enhance accessibility and affordability in underserved places. Master's Dissertation

2025-05-01T00:00:00Z TUYISHIMIRE, Jean de Dieu https://dr.ur.ac.rw/handle/123456789/2932 2026-05-25T10:10:08Z 2025-07-01T00:00:00Z

Assessment of the chemical and biological safety of vermicompost and water produced at Rulindo Faecal Sludge Treatment Plant in Rwanda TUYISHIMIRE, Jean de Dieu Growing global concerns about environmental sustainability and public health have emphasized the need for safe and effective waste management solutions. Key challenges, such as insufficient faecal sludge treatment, inadequate policies and regulations, and weak institutional frameworks, hinder sustainable sanitation in developing countries. In low- and middle-income countries like Rwanda, where sewer networks are often lacking, on-site treatment and reuse of faecal sludge have emerged as critical strategies to address sanitation challenges while promoting resource recovery. Vermicomposting, a biological process that utilizes tiger worms to decompose organic waste, has gained attention as an eco-friendly method for converting faecal sludge into valuable products like compost and treated water. However, the chemical and biological safety of these by-products remains a significant concern, particularly regarding potential contamination of organic matter, nutrients, pathogens, heavy metals, or other harmful substances. The Rulindo Faecal Sludge Treatment Plant in Rwanda uses vermicomposting technology to treat faecal sludge. Its outputs, vermicompost and treated water, show potential for agricultural and non-potable applications, but their safety must be thoroughly assessed to ensure they pose no risks to the environment or human health. There have been fewer research works in sub-Saharan Africa focused on assessing FSTP wastewater, black soldier fly larvae use in FSTPs, faecal sludge valorization, and faecal sludge management technology and ultimate applications. Even as yet, there has been no discussion of chemical and biological safety assessment of vermicompost and water released from FSTPs in Rwanda. Therefore, the aim of this study is to assess the chemical and biological safety of vermicompost and water produced in Rulindo FSTP, offering useful information on their suitability for sustainable reuse. Physical, chemical, and biological parameters were extensively checked. Average results of the physical parameters in FSTP wastewater are as follows: pH = 7.83, turbidity at 13 NTU, total suspended solids (TSS) = 7.5 mg/L, and dissolved oxygen (DO) = 3.74 mg/L. Chemical parameters included chemical oxygen demand (COD) of 198.9 mg/L and removal efficiency of 94.34% and biochemical oxygen demand (BOD) of 38.5 mg/L and removal efficiency of 94.16%. In addition, the total nitrogen (TN) was 305.25 mg/L, nitrates (NO₃⁻) were 225.53 mg/L, total phosphorus (TP) was 8.38 mg/L, potassium (K) was 397.5 mg/L, calcium (Ca) was 12.83 mg/L, magnesium (Mg) was 103.03 mg/L, and sulfur (S) was 58.95 mg/L. Biological analysis of the wastewater indicated high levels of total coliforms, faecal coliforms, Salmonella and Shigella with a removal efficiency of 93.54%, 55%, 91.75% and 98.09% respectively, and the v absence of E. coli (about 100% removal efficiency). Although some physical and chemical parameters of the wastewater comply with Rwanda Utilities Regulatory Agency (RURA) and FAO guidelines, high nutrient levels and microbial contaminants, including total coliforms, faecal coliforms, salmonella, and shigella (exceeding 1,000 CFU/100 mL), raise concerns about its suitability for irrigation. To identify water quality, significant indices such as Sodium Adsorption Ratio (SAR), Kelly's Ratio (KR), Soluble Sodium Percentage (SSP), and Magnesium Adsorption Ratio (MAR) were ascertained. SAR, KR, and SSP values indicate the water as suitable for irrigation. However, the MAR value of 88.92% exceeding the tolerance limit of 50% due to the high content of magnesium suggests otherwise. In addition, the Wilcox diagram that classifies irrigation water in terms of salinity and sodium hazards positions produced effluent in class C4S1, which is not suitable for irrigation due to high salinity. These findings conclude that the wastewater from the Rulindo Faecal Sludge Treatment Plant is not suitable for irrigation unless further technical adjustments are made. On the other hand, the solid vermicompost produced at the Rulindo STP was analyzed using the Gravimetric Ignition method for proximate analysis. The results revealed a moisture content of 58.21%, total solids of 41.79%, volatile organic solids (VOS) of 39.18%, total organic carbon (TOC) of 22.73%, total nitrogen of 1.96%, and a carbon-tonitrogen (C/N) ratio of 12:1. Inductively coupled plasma optical emission spectrometry (ICP-OES) was used for the chemical analysis, identified macronutrients, micronutrients, and heavy metals. Detected macronutrients included phosphorus (P) at 1.747%, K at 0.248%, Ca at 2.203%, Mg at 0.392%, and S at 0.531%. Micronutrients such as iron (Fe) at 2.565% and manganese (Mn) at 0.058% were also identified. Heavy metals detected included lead (Pb) at 0.004%, titanium (Ti) at 0.043%, vanadium (V) at 0.004%, and zinc (Zn) at 0.052%. According to the RURA guidelines for faecal sludge management, FAO guidelines for organic fertilizers, and other national standards, the vermicompost produced at Rulindo FSTP is of good quality for agricultural use and is chemically safe due to the absence of toxic heavy metals. Taken together, the study found that the Rulindo FSTP effectively reduces most contaminants to acceptable levels. However, nutrient concentrations in the treated wastewater remain high, and biological pathogens require further disinfection to meet acceptable standards. To support sustainable waste management practices and contribute to Rwanda’s environmental sustainability goals, additional research studies are recommended to complement this work. Master's Dissertation

2025-07-01T00:00:00Z SEBAZIGA, Ndakize Joseph https://dr.ur.ac.rw/handle/123456789/2930 2026-05-25T09:36:37Z 2025-08-01T00:00:00Z

Trends and Variability of Rainfall, Temperature and associated extremes, and their future projections in Rwanda using regional climate models SEBAZIGA, Ndakize Joseph This study examines observed trends, variability, and future projections of rainfall, temperature, and associated extremes in Rwanda using outputs from the Coordinated Regional Climate Downscaling Experiment-Coordinated Output for Regional Evaluation (CORDEX-CORE) regional climate models (RCMs). Observational trends and variability were analyzed for 19832021 using data from the Rwanda Meteorology Agency, while future projections were examined for 2026-2060 and 2066-2100 under two representative concentration pathways (RCP2.6 and RCP8.5). The performance of CORDEX-CORE RCMs in simulating observed rainfall was evaluated for 1983-2005. The study also assesses the influence of large-scale climate drivers, particularly the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), on Rwanda’s rainfall patterns. For the observational period, trends and magnitudes were assessed using the Modified Mann-Kendall test and Theil-Sen estimator, while temporal variability was analyzed using standard deviation and coefficient of variation. Rainfall characteristics, namely onset day (OD) and cessation day (CD) of the rainy season, were determined using methods based on accumulation thresholds, rainy day counts, dry spell and potential evapotranspiration analysis. Climate extremes were evaluated following the methodology of the Expert Team on Climate Change Detection and Indices (ETCCDI). Results show that the multi-model ensemble (CORDEX-CORE-MME) outperforms individual models in simulating Rwanda’s present climate. During the observational period, maximum temperatures exhibited higher variability than minimum temperatures. However, minimum temperatures exhibited greater warming magnitudes than maximum temperatures. Rainfall showed high spatial and temporal variability, with mostly non-significant trends across regions. Rainfall onset and cessation dates revealed west-east and east-west progression patterns, respectively. Seasonal length (SL) and number of rainy days (RD) were generally longer and more frequent in the southern, western, and northern regions, while central and eastern areas had shorter seasons and fewer rainy days. Climate extreme indices for rainfall and temperature displayed spatially and seasonally heterogeneous trends, with significant increases or decreases depending on the index and emission scenario. Future projections suggest shifts in OD and CD, with earlier onsets and longer seasons more common under RCP2.6 than RCP8.5. Rainy days are projected to increase during the long rains, but exhibit more complex patterns in the short rains, including regional decreases under RCP8.5 by century’s end. Heavy vi (R10mm) and very heavy (R20mm) precipitation days are projected to decline during long rains, while consecutive wet days (CWD) are expected to rise, particularly under RCP2.6 in 2026-2060. For temperature extremes, cold days (Tx10p) are projected to increase during long rains for 20262060 but decline in 2066-2100, while warm days (Tx90p) show the opposite pattern. These findings provide crucial insights to inform climate-resilient planning and policy development across key socio-economic sectors, including agriculture, health, water resources, infrastructure, tourism, and disaster risk reduction, and support the formulation of effective adaptation strategies at local and national levels. Doctoral Thesis

2025-08-01T00:00:00Z