Current guide lines are that a Septic Tank (Fosses Toutes Eaux) should be emptied once the sludge level has reached 50% of the tank capacity. This should be done by an approved company which are normally listed in the Yellow Pages (Pages Jeune) under "Vidangeur". We would recommend Eco Vidange at Ambazac but there is a wide choice and proximity to their depot makes all the difference.
Normally your new installation would resemble image 3. Images 2 & 3 show "what lies beneath". The downhill cover hides the "Pre-filter" which will periodically need cleaning .... typically every year. To do this withdraw the yellow plastic sack and jetwash or spray with a hose. If the sack does not come free then the black plastic bung shown in image 5 can be withdrawn.