An underground anaerobic digester with permissible temperature fluctuations: A parametric study

Abstract

Many digesters are now abandoned or idle due to high thermal control requirements. Providing a suitable temperature range as well as low temperature fluctuations is essential for the efficient operation of anaerobic digesters. The present study aims to design an un-heated un-insulated cylindrical underground cow manure digester so that it works under permissible thermal conditions during winter. The investigation is carried out by using an in-house code based on the finite difference method and is conducted for Tehran, Iran. The results show that a proper adjustment of design parameters (including burial depth, diameter-to-height ratio and substrate initial temperature) leads to permissible thermal conditions during the whole winter without the need for any external heat source (i.e., only through heat exchange with the surrounding soil). It is observed that for a digester volume of πm3, burial depths of shallower than 5 m and diameter-to-height ratios of lower than 1/2 can lead to undesirable thermal conditions (that is, local temperatures outside the psychrophilic range of 10 °C-20 °C, or temperature fluctuations above 1°C/day). The results also reveal that undesirable temperature fluctuations, which mostly occur during the first day of operation, can be efficiently eliminated through the proper setting of the substrate initial temperature.