A lifespan of 20 years was considered for all materials, except for plastic, which was reduced to 5 years in accordance with manu-facturers'specifications and Nzila et al. [21]. In this way, the capital cost of the tubular digester included 4 times the geomembrane and greenhouse plastic. The initial investment cost would then be 706 $(341 $ for the geomembrane and greenhouse plastic, plus 365 $ forthe rest of materials), which represents 36% of the fixed dome digester investment cost. However, the tubular model would require an investment of another 341 $ every 5 years.