The Natural Philosopher wrote:
The biodisc is completely free of whiff. I only emptied it because one
day it DID smell and I took the top off..and the pulley had fallen off
the motor. .the man emptied it. washed it, refitted the pulley and had a
nice cup of coffee and a chat. and it cost me IIRC £75quid..

Hmmm. I think I'll google "biodisc". Our septic tank sits to one side
in the front garden between the house and road. This is probably
because we're built into the side of a hill and the soakaway goes under
the road and then into the field opposite and towards the stream at the
bottom of the valley. For some reason the previous owners erected an
expensive glasshouse over it. I can only think this was to disguise the
large concrete block that does protrude about 18" above ground level OR
it may have been meant to capture the stink that emanates from around
the edges of the iron lid! Being in the greenhouse in summer is
painful! (In winter there's hardly any smell, probably because there's
little "action" happening below because of the lower temperature.) I
can't locate a vent anywhere, so the stink must be coming out round the
sides of the lid. One job for the future is to get a vent-pipe fitted,
if possible. It could be difficult because I imagine it could be tough
cutting a round hole down through the concrete. But I have found online
that you can buy filters (charcoal-based, I think) that are designed to
stop the stink from septic tank vents. I'ld be interested to hear if
anybody would recommend these.

Eddy.