Never heard of the bio washing powder idea before so thought i would give it a go...
Our holding tank is a dump through type, and normally trouble free...Until the weather warms up, then it develops 'the breath of satan' when used!!
All the vents are working as they should, there is just a rather nasty whiff! (Poor old dog gets the blame usually)
Anyhow, threw in a generous handful of el cheapo bio washing powder at the weekend (cost £1.20) and whilst I can't truthfully say it smells like a spring meadow the nastyness is dramatically reduced. Quite impressive results for a cheap, simple, solution to a whiffy problem.