I have a sanivite that is used for the washing machine and it appears not to be discharging fully as waste seems to run back, the discharge is vertical for 2.5mts I also noticed that the air vent cover is missing could this be a problem
Try replacing the non return valve on the discharge pipe its fitted inside the discharge rubber elbow without this the machine will pulse on an off and you can hear and feel the water going up and down inside the discharge pipe
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix waste disposal units.