Numatic TTB 3226 Owner's Instruction Manual page 18

The primary clean-water tank filter is located under the fillter cap
and should be checked at regular intervals.
Remove basket filter and rinse using clean water if required and
refit.
The secondary clean-water tank filter is located to the rear of the battery compartment and
should be checked at regular intervals.
To remove the filter you first have to release the clear tube attached to the pump, move the
retaining clip located under the pump and remove the pipe from the pump.
Lift the white plug and remove the filter.
Inspect and clean the filter as required.
Ensure tank has been emptied and care is taken when working in the vicinity of batteries.
NOTE: ANY SPILLS SHOULD BE WIPED-UP BEFORE TANK IS LOWERED.
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Clean Water Filters