We have a septic tank. The first few years were fine and then we seemed to have a run of bad luck when it completely blocked. We had a few minor problems, down to our teenage daughter putting facial wipes down the loo - dont do it!
When it completely blocked we got dyno-rod out and they were absolutely useless and costly. We thought it needed major work and had to contact our insurers - then my husband bought some rods himself and cleared it in minutes.
We get ours emptied about once every 2-3 years, partly because of our bad experiences of late. It is costly at around £180.
Our neighbour used to get the farmer do empty hers, spraying crap in our garden at the same time. I dont know whether this is legal now, but could be wrong. He was prosecuted for spreading it on the adjoining fields!
Someone has mentioned looking at a house with a septic tank offsite. Be very cautious about this. My friend had this situation and within a few months of moving in was asked to remove it from the land in question. I also had a shared septic tank which was also a nightmare. No-one wanted to take responsibility and there were continual issues and problems.
Our (wealthy) neighbours could not live with the odour of the tank and had a new state of the art one installed. When she told me it cost £36,000 I could not believe my ears. Our other neighbours had years of problems, partly because they never bothered to get it emptied!
Septic tanks, oil fired central heating, coal fires - the smell of country living!