Seen a house which has a septic tank which, according to vendors, is self draining (doesn't need emptying). Does this sound odd, I thought septic tanks always needed emptying? Is the tank likely to be very old (the vendors don't know when it was put in)? How much would replacing it cost?
A septic tank is a mini sewage works and treats the raw sewage itself and leaks out cleanish water at the end of the process. You do need to get it emtied of solid stuff every few years depending on its size and how many people are using it. You have to be careful not to put too much bleach and strong chemicals down the loo as this wipes out the bacteria in the tank and the sewage will not get treated - again depends on you and your lifestyle.
The modern equivalent is something like a Klargester package sewage treatment plant. Google it for details & prices.
(For info, cess pits are the other way of doing it - these are sealed tanks and they basically just fill up with sewage and need emptying them when they are full. There is no treatment - just storage)