In existing sewer systems of large cities such as Calcutta or Delhi sometimes the sewer systems have not been maintained for well over a 100 years.
The deposits are so large that the whole channel system can be blocked. The hydraulic performance of the whole system will be extremly reduced. Inspection and repair is very restricted or impossible.
Loose deposits like sediments, sand or sewage can be removed by high pressure water jetting. A pump truck (super sucker) takes the rinsed fluid away and removes it safely.
Blockadges can also be caused by sewer collapses or inflooding. Large pieces of rubble or encrusting can only be removed manually.