Its very rare to find a drain that is blocked. Usually the down pipe becomes blocked either on the bend on on the lower section where the pipe ends above the drain cover. If the pipe end and drain cover is accessible clearing the pipe is easy usually just by using a gloved hand but if thats unsuccessful a drain cleaning attachment on a pressure washer lance will do the trick. Dont offer drain clearing but try not to block any! If one is blocked do your best to unblock it but remember your not a drain cleaner!
Disposal of the contents is easy as its biodegradable. You can throw it on a compost heap, take it to the recycling centre and add it to their heap, spread it on the clients gardens (take a rake) with permission or buy trade waste bags and leave it for the dustman.
The omnivac holds 78 ltrs dry or 55 ltrs wet.
The Omnivac system is easy to set up, the job has a little learning curve but if you have mastered wfp this will be no problem for you.
Hope that helps