Abstract: Abstract: In order to avoid environmental pollution caused by biomass ash piled up on the open air and digested slurry discharged directly, and explore the utilization way, biomass ash was used as filter material to clarify digested slurry in infiltration. The results show that biomass ash can be used as filter material to clarify digested slurry. Particle size is the major influence factor for the cleaning of digested slurry, and the small particles have the better cleaning effect than large particles due to their specific surface area and spatial structure. Under the same experimental conditions, when particle size is large than 3 mm, infiltration rate is 4.17 mL/s, COD (chemical oxygen demand) of clarified digested slurry is 1 880 mg/L, TS (total solid) content of clarified digested slurry is 1.46 g/L, and the removal rates of COD and TS in digested slurry are 58.71% and 67.69%, respectively. When particle size is less than 0.5 mm, infiltration rate is 0.46 mL/s, COD of clarified digested slurry is 221 mg/L and TS of clarified digested slurry is 0.61 g/L, and the removal rates of COD and TS in digested slurry are 94.07% and 87.16%, respectively. The ratio of height to diameter of clarification column not only affects the flow path of digested slurry, but also affects its clarifying effect. When COD removal ratio is used as evaluating indicator, in order to get better cleaning effect, the height-diameter ratio of clarification column is more than 1.34 when particle size of biomass ash is less than 0.5 mm, is not less than 2.3 when particle size of biomass ash is between 0.5 and 1 mm, is not less than 2.68 when particle size of biomass ash is between 1 and 1.5 mm, is not less than 5.01 when particle size of biomass ash is between 1.5 and 2 mm, and is not less than 10.662 when particle size of biomass ash is greater than 3 mm, the COD values of clarified digested slurry were 236, 276, 305, 315 and 342 mg/L respectively, and the removal rates of COD were 94.82%, 93.94%, 93.3%, 93.08% and 92.49%, respectively. The clarified digested slurry quality is close to agricultural irrigation water requirements. The clarifying capacity of biomass ash in different particle sizes is 9-10 mL/g when particle size of biomass ash is less than 0.5 mm, 10-11 mL/g when particle size of biomass ash is between 0.5 and 1 mm, 8-9 mL/g when particle size of biomass ash is between 1 and 1.5 mm, 4-5 mL/g when particle size of biomass ash is between 1.5 and 2 mm, and 3-4 mL/g when particle size of biomass ash is greater than 3 mm, respectively.