16Internal gas-lift loop reactor (IGLR) was used as an anaerobic digester and its 17 hydrodynamics were studied using Computer Automated Radioactive Particle Tracking 18 (CARPT) Technique. This paper deals with the experimental study on a laboratory-scale digester. 19An anaerobic digester is a three-phase system consisting of gas, liquid, and solids; however solid-20 liquid slurry was treated as a single phase due to smaller size and lower density of solids. The 21 effect of various geometric and operating variables on the hydrodynamics was studied. The 22