A combination anaerobic digestion scheme for biogas production from dairy effluent—CSTR and ABR, and biogas upgrading

Highlights: • CSTR and ABR in series are able to treat low strength dairy waste water. • The system is shock-load resistant according to high fluctuating dairy effluent. • The CSTR acts as hydrolysis stage and buffer tank ensuring stable operation. • COD removal and biogas production takes places in the ABR. • The produced biogas can be used as CO₂ source for energy storage. - Abstract: Anaerobic digestion of low-strength dairy waste water was used for the production of biogas which is aimed at serving as a concentrated carbon dioxide (CO₂) source for further methanation. Using hydrogen (which can be produced via electrolysis using renewably sourced electricity), the CO₂ fraction of the produced biogas can be used as a mechanism to store surplus electricity by the Sabatier process, which converts the CO₂ fractions to methane (CH₄), i. e. synthetic natural gas. This study investigates the use a combined reactor scheme for the anaerobic digestion of dairy waste water, and the further upgrading of the biogas products from the process. A combination pilot scale process was established with a 90 d start-up time using a 1 m³ continuous stirred tank reactor (CSTR) and a 0.2 m³ baffled reactor (ABR) in series. The system was fed at constant retention time in the ABR of 1.6 d and with varying substrate organic loading rates between 1.25 and 4.50 kg m⁻³ d⁻¹. The average chemical oxygen demand (COD) removal was 82% with a biogas yield of 0.26 m³ kg⁻¹. The use of the derived biogas for the Sabatier process to convert hydrogen into CH₄ showed no disadvantages compared to synthetic gas mixtures. The combination of CSTR and ABR overcame the individual disadvantages of both reactor types. The investigated anaerobic digestion system can be further optimized and adopted to replace conventional waste water treatment systems.