Old style septic tanks in Thailand are usually concrete style tanks that fill up over time and therefore a sewage truck must attend and suck out the poo, fluids etc. "Modern" systems are a round plastic tank with two chambers and an overflow valve. Microbes are supposed to break down most of the waste thereby reducing the frequency when it needs to be emptied. However, even a modern septic tanks need to be emptied every few years as microbes don't break down everything. Also, if you clean your toilet with germ killing cleaners, you will upset the balance of the microbes and it all starts to stink. Although it has been illegal for over a decade to discharge the sewage overflow into public spaces, public gutters, or the road, people do it anyway. In my case, the house I currently live in, all plumbing was replaced and a new septic tank was installed last year. First, the bathroom flooded because the plumber did not install a vent pipe to help the grey water flow uninhibited onto the road (which is also illegal). After forcing the plumber to rectify his oversight, the bathroom no longer floods. Second, the undersized vent valve of the septic tank pushed odours back up through the pipes. Thirdly, because the plumber couldn't work out how to connect the p-trap under the sink, he merely pushed a hose going from the sink to a blue pipe in the wall and wiped a wee bit of silicon around it. Therefore anything connected to the "new" plumbing had poo odours come back up through the pipes via the new septic tank. I've given up educating Thai tradespeople. They have an excuse for everything. If they don't have a comeback, they just walk off.
