Differentiating fouling from ageing for a condition-based diffuser maintenance

Diffuser maintenance such as cleaning and membrane replacement are key to energy-efficient aeration at water resource recovery facilities (WRRFs). In previous research, fouling and ageing effects on diffuser performance have been lumped together by only comparing new and used membranes. But meaningful diffuser maintenance requires that fouling (mitigated by cleaning) can be separately assessed from ageing (calling for membrane replacement). Therefore, we analysed how the standard oxygen transfer efficiency (SOTE) and dynamic wet pressure (DWP) changed due to fouling, separately from the ageing effects, in six diffusers after 1.5 to 15 years in operation at different WRRFs. This required an elaborate cleaning procedure of the used membranes and additional measurements. The results showed that fouling increased DWP with up to 40 mbar, which is in line with previous findings and up to twice as much as new membranes. Ageing, on the other hand, surprisingly led to a reduced DWP in the order of 5-10 mbar. The SOTE loss for the combined fouling and ageing effects (up to 28 %) were on par with previous research. However, fouling was only accountable for less than half of this performance loss (12 % SOTE loss and below), whereas ageing was attributed up to 25 % SOTE loss, which has not been seen before. The extent of performance loss as well as the relative contributions of fouling and ageing varied between WRRFs. Altogether, these new insights emphasize the need to monitor both ageing and fouling to facilitate effective maintenance and calls for more research on how ageing impacts diffuser membranes.