Septic Tank

[indiansign]

I have just bought a house with an Aeroclere CLP6 system installed. I don't really know how this works, or whether I should be having it emptied. The problem is that every now and then -always a Saturday for some reason - there is an almighty pong of you-know-what in the house. Two rooms are effected, one of which definitely has waste pipes running through it, and the other most likely does as well. My question is, why is this happening, and how can I stop it?