I took this photo behind my place; there was this group of workers for about an hour.
They have a small hose, from the black spool, they were pushing in and out of the manhole and a large hose that didn't seem to move at all.
The truck's engine was quite noisy, so I'm assuming they have a large pump. I'm curious what kind of truck it is and what they could be doing with it.
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.
The hose probably has a head on it similar to the one pictured below.
Figure 1. A Jetter Depot twister nozzle.
The head is attached to a pressurised hose (coming from the right of the photo). Notice that there are multiple jets pointing backwards and these are thrusting the head to the left causing it to advance through the pipe. Meanwhile a smaller (in thrust) jet is shot out the front to cut through the blockage in the pipe.
I had occasion to call in a drain cleaning service for my father's sewers which had become blocked - possibly through collapse. I was very impressed with this. The operator fed the very flexible hose into the sewer, powered it up and the head pulled the hose in and with gentle shaking of the hose it rapidly progressed up to the blockage. The forward jet dislodged the obstruction and the reverse jets assist in breaking it up and flushing it quickly down the pipe. On pulling the hose back further flushing took place.
Presumably the big hose will collect the sludge from the sewer or sump for disposal.
It seems they are city or municipality maintenance crew.
This truck could be a tanker that has two reservoirs. Water, and collected sewage.
They have a variety of tools that can be attached to the hose and inserted into the sewer line through clean-out access.
They clean the sewer lines which are clogged and remove the sewage if needed from septic tanks. The have root and hard sludge breaking drill tips and expandable bladders to temporarily seal the downstream line and contain the mess.
