We have a septic tank. We have 3 bathrooms. The bathroom toilet in our children's bathroom has a urine smell coming from it. We've had a plumber come out but the smell is still here. This is the only toilet with a smell - we built this house 9 years ago, the smell started about 6 months ago. Help... thanks

We have a septic tank. We have 3 bathrooms. The bathroom toilet in our children's bathroom has a urine smell coming from it. We've had a plumber come out but the smell is still here. This is the only toilet with a smell - we built this house 9 years ago, the smell started about 6 months ago. help..... thx
There are only two ways for toilets, basins, and showers to pass such a smell :
1 - If someone or some animal urinates there. Clean and disinfect with bleach daily. Use an automatic toilet cleaner (block or dispenser)
2 - If the water seal closing the toilet and drainage pipes is too low. On long term this can happen due to water evaporation, but on short term it has to be caused by water leaking away.
I expect however that your problem is mentioned under point 1...
Advice : ensure that your children always flush after peeing. Also let all children assist with the cleaning and disinfection (of course only if they are old enough to do that : 6 years or older I would say).
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Success!
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With children I would have to go with Credendovidis on #1, A leaky wax seal would leave you with sewer gas. Urine smells different. However if you think the smell may be coming from bacteria growing in the tank and bowl rim in your toilet then try this. Tonight at bedtime shut the water off and drain the tank. Scrub the sides down with bleach and pour 1/2 gallon of bleach down the flush valve, (flapper hole) and let it set all night. Next morning fill the tank and flush it away. Good luck, Tom