Home DIY

You may find success with a low hazard suggestion.

Removing all the biofilm is probably impossible.
Given the pipework is designed to allow volumes of water to discharge,
using strong chemicals, (strongly alkaline NaOH based 'drain cleaner', strongly acidic
'chew through anything' liquids), as you do not have a blockage,
most of the (relatively expensive) chemical will just literally 'go down the drain',
with little contact time.

Cheap table salt/ice-on-path clearing salt can be poured into the u-bend,
dissolving into the water and 'blocking' it.

You then pour on freshly boiled water. The salt will instantly dissolve into the hot water,
but you will have the entire volume of the u-bend full of hot, (hot enough to kill biofilm), very saline, (saline enough to kill biofilm), liquid for (hopefully) enough contact time to kill most of your biofilm.

Salt/hot water is cheap enough to try a couple of times.

Provisos: waste pipe is not designed to, nor capable of withstanding an entire kettle full of freshly boiled water, so have a container of cold water to flush the hot saline away.
Exercise due care & attention when pouring the hot water onto the u-bend of salt. You don't want any splashes getting onto exposed skin/into eyes.