Size effect of anaerobic granular sludge on biogas production: A micro scale study

• Granules were categorized as large, medium and small based on their size.
• Utilization of Hg-porosimeter for internal structure analysis of granules.
• In situ biogas production in a reactor was studied using a high-speed camera.
• A positive relationship was obtained between granule size and biogas production rate.
• The study could facilitate understanding the anaerobic process within granules.

This study investigated the influence of anaerobic granular sludge size on its bioactivity at COD concentration of 1000, 3000 and 6000 mg/L. Based on size, granules were categorized as large (3–3.5 mm), medium (1.5–2 mm) and small (0.5–1 mm). A positive relationship was obtained between granule size and biogas production rate. For instance, at COD 6000 mg/L, large granules had highest biogas production rate of 0.031 m3/kgVSS/d while medium and small granules had 0.016 and 0.006 m3/kgVSS/d respectively. The results were reaffirmed by applying modified Fick’s law of diffusion. Diffusion rates of substrate for large, medium and small granules were 1.67 × 10−3, 6.1 × 10−4 and 1.8 × 10−4 mg/s respectively at that COD. Large granules were highly bio-active due to their internal structure, i.e. big pore size, high porosity and short diffusion distance as compared to medium and small granules, thus large granules could improve the performance of reactor.