Optimizing Smallholder Maize-Soy Biogas for Rural North Vietnam Farm Co.

Thesis Abstract

Smallholder maize-soy biogas systems in rural North Vietnam face suboptimal methane yield, high seed- and labor-costs, and limited integration with crop-larming cycles, constraining both energy access and nutrient recycling in smallholder farms. This study addresses the gap between biogas technology potential and on-farm practical performance, focusing on optimization of feedstock mix, digester operation, and slurry management within the Farm Co. cooperative of Cao Bang Province. The aim is to enhance biogas production, reduce reliance on wood fuels, and improve nutrient recycling for maize and soybean plots while ensuring economic viability for smallholders. Specific objectives are (1) to evaluate the effect of maize-soybean residue ratios and co-feed supplements on methane yield and digester stability; (2) to quantify impacts of temperature, hydraulic retention time, and substrate pre-treatment on biogas performance using a factorial experimental design; (3) to assess farm-level economic returns, including capital, operating costs, and payback period under optimized operating conditions; (4) to analyze agronomic outcomes of anaerobic digestate amendments on maize-soy yield and soil health indicators; and (5) to develop a practical optimization framework for on-farm biogas management aligned with smallholder decision-making. The methodological approach integrates a mixed-methods research design rooted in the Technology Acceptance Model and the Theory of Planned Behavior to understand adoption constraints, alongside a biophysical optimization framework. The population comprises 120 smallholder farmers within Farm Co. and adjacent communes in Cao Bang. A stratified random sample of 60 farms will be selected for quantitative assessment, with 20 purposively chosen for in-depth qualitative inquiry. Data collection instruments include a) biogas performance logs (daily gas production, temperature, pH, TS/VS, retention time), b) feedstock composition records and calorific value measurements, c) economic survey instruments capturing capital costs, maintenance, labor, and opportunity costs, d) soil and crop performance measurements (soil organic matter, NPK, pH, maize and soybean yield), and e) semi-structured interview guides to explore perceived barriers and facilitators to optimization. Validity and reliability will be ensured through pilot testing, triangulation of gas metering with gas meters and gas volume sensors, and repeatability checks for soil and yield assessments. Data analysis will include descriptive statistics, ANOVA to test the effects of feedstock ratios and operating parameters on methane yield, and multiple regression to model relationships between digester performance and influent characteristics. A generalized linear model will assess the influence of digestate application rates on crop yields and soil health indices. Thematic analysis will be applied to interview data to extract farmer-perceived constraints and enablers. An integrated optimization model will be developed using a mixed-integer nonlinear programming (MINLP) approach to identify economically viable operating conditions that maximize net present value over a 5-year horizon while achieving target methane output and crop yields. The study will test hypotheses about the significance of feedstock balance and digester operating stability on methane yields (H1) and about the economic viability of optimized digestate application (H2). Expected findings include (i) identification of an optimal maize-soy residue ratio (e.g., 6040 by weight) and supplementary co-feed that significantly increases methane yield by 15–25% without compromising digestate quality; (ii) determination of critical operating ranges for temperature, pH, and hydraulic retention time that sustain digestion at rural ambient temperatures, with potential recommendations for passive solar heating and insulation; (iii) confirmation that optimized digestate improves maize and soybean yields by 5–12% and enhances soil organic matter and available N, P, and K; (iv) evidence that Farm Co.’s economic indicators improve with a payback period reduced to 3–5 years under optimized management. The study contributes to knowledge by providing a context-specific, empirically validated optimization framework for smallholder biogas systems, integrating biophysical performance with farm economics and agronomic outcomes, and offering a scalable model for similar smallholder contexts in Southeast Asia. The main conclusion is that, under controlled feedstock balance and stable digester operation, maize-soy biogas systems can substantially reduce fossil fuel use, improve soil fertility, and yield favorable financial returns for smallholder farmers, with policy and extension implications emphasizing training in feedstock management and simple digester monitoring. Recommendations include the adoption of standardized feedstock protocols, low-cost temperature regulation and monitoring tools, farmer field schools to disseminate the optimization framework, and scaling strategies tailored to cooperative farm structures to enhance collective benefits while maintaining individual farmer incentives.