We smell a pump-out problem
QAbout four or five days after the fortnightly emptying of our 18-month-old boat’s waste tank (and adding Odourlos to treat it) we start to get unpleasant odour in the cabin where the tank is. We have tried adding a further dose of Odourlos, but it makes little difference. There are no obvious leaks. What can we do?
via the website
AI have a feeling you might be asking this question about 18 months too late, because holding tank breathers need considering at the design stage.
Some domestic toilet cleaning products contain chemicals such as bleach that will destroy the aerobic microbes in the tank (which break down the waste with minimal smells) and leave the tougher anaerobic ones to breed. Likewise, if you have recently changed from using toilet blue which also destroys aerobic bacteria, it will take a fair few pump-outs to get rid of the residual blue – so it may just be a matter of time.
Aerobic bacteria require a plentiful supply of fresh air so holding tanks should have two large bore (one-inch or more) breathers, one towards each end. Some people with two smaller breathers have rigged a brushless computer fan in a metal box in one of the breather hoses, switched so it starts before using the toilet and runs for a fixed period.
Lower quality hose products can leach odours, so look at the material of the pump-out hose and any hose connecting the toilet to the tank to make sure it is of the best sanitary grade (see leesan.com). Personally I would use metal for as much of the pipework as I could, especially if it is impossible to stop effluent sitting in the pipe.
This sounds like a macerator system so no smells should be able to get back out via the toilet. I would suspect effluent is lying in the hose between toilet and tank. If it is adjustable, try increasing flush time/ volume so you stand more chance of flushing it into the tank.