Optimization of anaerobic digestion at a municipal wastewater treatment plant in Lagos, Nigeria

Thesis Abstract

Optimizing anaerobic digestion at a municipal wastewater treatment plant in Lagos, Nigeria addresses the dual challenge of meeting treated effluent quality standards and maximizing energy recovery from biogas. Lagos's rapidly expanding urban population places considerable pressure on wastewater infrastructure, resulting in suboptimal anaerobic digestion (AD) performance, elevated sludge management costs, and underutilized biogas potential. The study aims to enhance AD efficiency, increase methane yield, and reduce operational costs at a representative Lagos plant. Specific objectives are to (i) characterize the baseline AD performance across primary, secondary, and tertiary digesters; (ii) identify key environmental and operational drivers of biogas production and volatile solids reduction; (iii) develop and validate a process optimization framework incorporating feedstock characterization, dynamic co-digestion strategies, and phased operational adjustments; (iv) quantify energy and greenhouse gas emission implications of optimized AD; and (v) formulate a practical implementation roadmap for scale-up within Lagos’ municipal network. The research adopts a mixed-methods design anchored in both quantitative and qualitative data. The population comprises three active anaerobic digestion units at the Lagos wastewater treatment facility, operator staff, and process records spanning a 24-month period. A stratified random sample of digester performance data is drawn, comprising 72 monthly data points per digester (total n = 216) for parameters including hydraulic retention time, organic loading rate, temperature, pH, alkalinity, biogas flow, and methane content. Complementary semi-structured interviews with 12 plant operators and managers capture operational constraints and decision-making processes. Data collection instruments include calibrated online probes for pH, temperature, and redox potential, gas flow meters, gas composition analyzers (gas chromatography, GC/TCD), and standardized operator surveys. Validity and reliability are addressed through instrument calibration, pilot-test adaptation of questionnaires, and triangulation across process data, interviewer notes, and archival records. Regression analysis, ANOVA, and multivariate principal component analysis are employed to quantify relationships among digester performance indicators and biogas yield, while time-series forecasting (ARIMA) models project energy output under optimized regimes. A system-dynamics model informs co-digestion scenarios, and life cycle assessment (LCA) evaluates environmental implications of recommended changes. Ethical considerations follow institutional guidelines, with anonymization of employee data and secure handling of operation logs. Anticipated findings indicate that optimized control of feedstock characteristics and dynamic temperature and pH management can elevate methane yield by 12–18% and reduce volatile solids residence time by 10–15%, without compromising effluent compliance. Co-digestion with readily biodegradable organic wastes is expected to enhance digester stability and biogas production, particularly when combined with staged temperature management that aligns with seasonal temperature fluctuations in Lagos. Regression and ANOVA are expected to reveal significant effects of organic loading rate, alkalinity ratio, and digester temperature on methane production (p < 0.05), while the system-dynamics model highlights leverage points for operational interventions and the sensitivity of energy recovery to biogas purity and leakages. The study also anticipates measurable reductions in net energy costs and a lower carbon footprint per cubic meter of treated wastewater, with quantified trade-offs linked to potential increases in sludge generation requiring disposal capacity adjustments. Contribution to knowledge includes (i) a context-specific optimization framework for anaerobic digestion applicable to tropical urban wastewater plants, (ii) empirical evidence on co-digestion strategies and phased process control under Lagos climatic and regulatory conditions, (iii) an integrated methodology combining process analytics, system-dynamics modeling, and LCA to inform decision-making, and (iv) practical guidelines for scale-up within Nigerian municipal networks. The main conclusion posits that targeted optimization of AD through data-driven process control and strategic co-digestion can substantially improve biogas yield and energy self-sufficiency while maintaining effluent quality. Policy and practice recommendations emphasize (a) installation of robust online monitoring coupled with operator training, (b) development of a pre-treatment and co-digestion plan that aligns with waste streams in Lagos, (c) staged digester operation to accommodate seasonal variability, and (d) integration of energy recovery planning into asset management to reduce lifecycle costs and greenhouse gas emissions.