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In this study, a black-shading cylindrical water tank made of high-density polyethylene was locally manufactured as a household digestor for treating cow manure in Bangladesh. Effluent slurry instead of water was reused for manure dilution under manure-to-slurry ratios of 1:2 and 1:1, to assess this small prototype’s production efficiency and feasibility. The specific biogas production at both ratios matched well, by 0.12 m³/kg VS and 0.14 m³/kg VS, respectively, while the former slurry dilution operation outperformed in daily and accumulative biogas production by 16% and 57%, correspondingly, referring to 0.49 Nm³/d on average and 8.55 Nm³ in total, potentially meeting a 2 h household cooking energy requirement. From a nationwide viewpoint, slurry dilution was proven to be a great initiative to conserve water amounting to 50,286,751 m³ for 114,810 households of 6 person-equivalents annually, while cutting chemical costs by USD 32,720,684/yr and trimming annual greenhouse gas emission by 1.8 million tons of CO₂e. This study revealed that a small prototype digestor could be an alternative energy source for cost-effective and eco-friendly household applications.