High-pressure cleaning often results in strong air-flows. This interaction produces a high and low pressure jump at the nozzle., i.e. there is negative pressure in front of the nozzle (nozzle head) and positive pressure behind the nozzle (jet area). When the cleaning nozzle passes side branches (laterals), seal water can be blown out or sucked out from private drain traps (e.g. of toilets or washbasins) hence causing odour nuisances.

The pressure variations depend on the nozzle type and size, the water pressure, the pipe diameter, the type and degree of soiling, the filling level in the sewer, and the operating principle of the ventilation/aeration system in the private lateral. Cleaning nozzles with a low jet angle as well as a large volumetric flow and flushing pressure produce large pressure fluctuations [Aigne2002].