Optimization of continuous two-stage production of biogitan from wastewater of the confectionery industry

Two-stage anaerobic fermentation is a promising method for producing a mixture of H₂ and CH₄ (biogas) from various organic waste materials. However, scaling up this process requires optimization of the conditions to maximize the specific yield and rate of gas production, as well as to ensure an optimal ratio of H₂ and CH₄ in the biogas.  In this work, the hydraulic retention time (HRT) and organic load rate (OLR) were varied in the mesophilic acidogenic (RH) and thermophilic methanogenic (R) reactors in order to optimize the continuous two-stage production of biogitan from the wastewater of the confectionery industry. The specific yield of H₂ (87,8 ± 20,3 ml/g COD) and the rate  of H₂ formation (2352 ± 376 ml/(l∙day)) were maximum at an OLR in RH equal to 26,8 g COD/(l∙day). The specific yield of CH₄ (286,3 ± 29,1 ml/g COD) was maximum at HRT and OLR in R equal to 2 days and 2,05 g COD/(l·day), respectively, and the rate of CH₄ formation (964,6 ± 162 ml/(l·day)) was maximum at HRT and OLR in Requal  to 2 days and 4,6 g COD/(l·day), respectively. The optimal composition of biogitan (10% H₂, 50,4% CH₄ , 39,6% CO₂ ) was obtained when RH was operated at an OLR of 12,8 g BOD/(l·day), and R was operated at an HRT of 3 days and an OLR of 1,37 g BOD/(l·day), respectively. In the optimal operating modes, the H₂-producing microbial community RH was dominated by Caproiciproducens and Clostridium sensu stricto 12, while R was dominated by syntrophic bacteria Cloacimonadaceae W5, Lentimicrobium, Anaerolinea, and hydrogenotrophic methanogens Methanothermobacter. These results contribute to the further improvement of two-stage anaerobic fermentation for the production of biogas from concentrated wastewater.

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