Low-cost digesters and wetlands for decentralized wastewater treatment: Performance and GHG emissions in the...

Decentralized wastewater treatment systems, particularly low-cost options, are crucial for rural communities in impoverished countries, where access to conventional sanitation infrastructure is limited. This study evaluates the performance of decentralized wastewater treatment systems in rural Napo, Ecuador, comparing septic tanks with low-cost tubular digesters paired with constructed wetlands. Monitoring hydraulic retention times (HRT), chemical oxygen demand (COD) reduction, biogas production, and effluent quality revealed that sequential tubular digesters achieved the highest COD removal (87 %) at a 71.7-day HRT, outperforming single-chamber septic tanks (46 % reduction, 7.2-day HRT). Household digesters produced 75.6 L/day of biogas (18.9 L/person/day), insufficient for cooking but emitting 0.33 t CO₂eq/year per system (7.2 % of a car's annual emissions), underscoring the need for flaring. Constructed wetlands reduced COD by 81-82 % and ammonium (NH₄⁺) by 65-84 %, though effluent nitrogen levels exceeded Ecuadorian standards. The digesters combined with subsurface wetlands (≥3 m²/household) effectively stabilize effluent, with plant biodiversity enhancing nutrient removal. However, challenges persist in NH₄⁺ and E. coli elimination, requiring further optimization. Techno-economic analysis revealed trade-offs: sequential digesters offer superior performance but demand skilled installation, while commercial systems are cost-prohibitive for rural households. This study provides evidence to stakeholders and researchers that low-cost hybrid systems (digester + wetland) are a viable, high-performance alternative to traditional septic tanks, balancing treatment cost, complexity, and efficiency, all aspects to be considered in rural implementation.