Assessment of aging of anaerobic digester paddle-mixer material: experimental studies and long-term numerical simulation

In this study, experimental tests and numerical simulations (Abaqus) were performed to examine the durability of four impeller materials [steel, polyethylene, polypropylene and glass fiber reinforced polymer (GFRP)] in an anaerobic digester environment. Specimens of these materials were prepared and immersed in a bath containing anaerobic digester liquor while operated at 40 °C for a period of 8 months. Periodically (2, 4, 6 and 8 months) sample specimens were removed from the bath and the tensile strength and elastic modulus were determined. As expected, thermoplastic materials and especially GFRP exceeded higher absorption of moisture than steel, although aging effect on steel was more pronounced due to corrosion, as evidenced by SEM imaging. The results demonstrate that polyethylene was not acceptable as construction material for anaerobic digester paddle mixer. On the contrary steel, GFRP and PP remained highly unaffected with a negligible increase of the maximum stress, 1.6%, 0.9% and 3.0%, respectively.