Out of sight out of mind right? Wrong!
Just like changing the oil in your car, Septic systems require de-sludging.
A septic pump-out is needed when the thickness of the floating scum layer and settled sludge layer occupy so much of the tank volume that the free liquid area is "too small" - the "net free area" has become so small that the EFFLUENT RETENTION TIME, the time that wastewater has in the tank to allow for solids and floating scum to separate from effluent being discharged to the absorption trench is too short.
Short septic tank effluent retention time means we're pushing solids into the absorption trench, clogging it up & shortening its remaining life.
The tell tail sign that an overload of solids has flushed into the trench, is it generally breaks out in the weakest point and flows out over the ground.
NSW Health recommend septics to be de-sludged every 3 to 5 years, it can be more or less depending on size of tank and the number of people in the household. The only true way to measure sludge depth is with the use of a core sampler, however this will not accurately sample on one that has been neglected.
The majority of older tanks in the Bega Valley are of 3000L or less and have no internal baffle as above. Newer ones do and they help restrict the movement of solids passing out of the tank somewhat.
Also if you are looking at buying a property on septic, it may be a good idea to have it de-sludged before settlement. I have experienced people who have bought properties having to have this done not long after purchase and also up for the cost of a new absorption trench.