Influence of C/N ratio & double helical ribbon (DHR) impeller on microbial granulation in anaerobic co-digestion (AnCo-D) of industrial effluents.

The present study demonstrates the significance of the C/N ratio and double helical ribbon (DHR) impeller in the anaerobic co-digestion (AnCo-D) of sugar refining process (SRP) effluent and molasses-based distillery spent wash (DSW) for improved biogas production. Both SRP & DSW were mixed in different percentages to achieve an optimum C/N ratio. Further biomethane potential analysis of mixed feeds with different C/N ratios was performed. The batch with a C/N ratio of 23 wherein, SRP & DSW blended in 80:20 volumetric percentage resulted in maximum biogas generation of 660 mL/gCOD utilized. This mixed feed was chosen for co-digestion in a continuous stirred tank reactor (CSTR). Another important aspect demonstrated in the present study is the use of DHR impeller for improved reactor hydrodynamics. Optimization of organic loading rates for maximum biogas yield and productivity was performed in CSTR with DHR and rushton turbine (RT) impellers to understand the impact of mixing on substrate utilization, uniform granulation pattern, and biogas yields. The maximum substrate utilization, biogas yield, and productivity of 85%, 82.7%, and 2.5 L/L.day, respectively, were observed at the organic loading rate (OLR) 4gCOD/L.day in CSTR with DHR impeller. Scanning electron microscopy was conducted to evaluate the impact of the C/N ratio and radial-axial mixing imparted by the DHR impeller on microbial granulation pattern. The granule size was found to be 200-800 µm and the average density of granules was in the range of 1.020-1.050 Kg/m³ at an average settling velocity of 0.56-3.00 cm/sec.