The small black hose is a high pressure water hose, with a nozzle attached to its end. Something like that:
It is used to flush down sand and other sediments
The nozzle usually features some backwards facing jets, so the hose gets pushed inside a pipe. The hose can be pulled back with the windlass.
There's a variety of differenz nozzles, as you can see in the pictures, I think nozzle 2 and 3 have a rotating bit and the chains on nozzle 4 are probably to knock free solid sediments like chalk.
The large hose is a suction tube to either empty the manhole or pump out the sediments flushed down by the pressure hose.
Usually the tanks of those types of vehicles have a bulkhead to separate waste water from fresh water, and a methode to return the sediments inside, while returning the sediment-free waste water to the pipes.