Tanks should have 3 layers – fats, oils, and greases float to the top; other waste settles to the bottom; the middle layer of effluent (mostly water) should move to the outlet side of the tank, where it should settle some more, before the effluent goes to the drainfield (leaving the solids – fats, oils, greases, other waste – in the tank).
Eventually the solids need to be pumped out. These are what are called “Flowable Solids”. Think of it like having a bathtub of full of slightly melted milkshake and sucking it up with a shop vac – relatively easy, right?
If the solids get too solid, we can no longer pump it the standard way – think of a bathtub full of a block of clay and sucking it up with a shop vac – it’s not going to work. We can usually still pump it, but its going to be a “special project pump”.
This happens most often in tanks that haven’t been pumped in 10+ years. Basically, as waste keeps being added to the tank, it pushes together and pushes the water out, until the tank is completely clogged and then it backs up to the house. Again, as this is a special project pump, its not something that usually going to be fixed after hours or on weekends as its going to require more supplies and man power.
It occasionally happens if a tank has solids in it and has been left unused for many years – eventually the solids can dry out, but this is less common.