Depending on the septic system you have, you may want to top up your Effective Microorganisms (EM-1) which stabilizes the biological balance in your septic tank and reduces foul odors by promoting the growth of beneficial microbes. However, in real life, because Thai plumbers generally have zero training, and I'm guessing that you don't have a p-traps, vent pipes or proper seals installed around your pipes, welcome to the never-ending stink. So, depending how the wind blows, the stench comes back through the pipe as there's no way to stop it. Also, even a modern septic tank needs to be emptied every now and again. Thailand has indeed a plumbing standard which has adopted pretty much the International Plumbing Standard - but no 'plumber' here knows anything about that. I'm currently renovating my third house in Thailand and I had to show the builder and his plumber multiple images of a p-traps and vent pipes and explained to them multiple times what they do. I even had to act and do pantomime to explain what happens if pipes don't have an adequate fall and that vent pipes help push the load along. P-traps in the sink and shower are required to stop the stink coming back up. Although I knew that the plumber wasn't qualified, when they asked whether I also wanted them to install a p-trap on the toilet bowl, I explained to them that every toilet already has a p-trap built-in. 😒
