You should have started off with a interceptor where you have connected to sewer then your manhole.
We are working in a very tight space - there would be no room for an interceptor and a manhole I doubt - its already encroached on where the extension was planned.
In your other post imo you should avoid all those y junctions and fit another manhole and bring all connections into it separately
The diagram with the y-junctions was my initial feeling of how things might have to be connected - however, I don't want to do that anymore since this will be a 'temporary' install until the bathroom is moved & the extension is built. A manhole here would seem maybe excessive (do you mean a small 290mm access chamber though?) - plus it would end up internal when extension is built, unless it is removed at that point - quite a waste of parts/materials for a few months of functioning drainage!
also internal manholes are not a good idea
From what perspective? From a sewer smell perspective (double sealed presumably would negate this assuming proper ventilation via stack) or are you thinking from an aesthetic perspective, or something else?
the interceptor manhole should be vented with a FAI and the other end of your drains vented via soil stack vent, this gives natural air flow through your drainage system.
I have to say, I'm quite amazed at how little problems there are with drain smells here - even when we have had blatant leaks & even open sewer! I am not sure what FAI stands for (i've tried to look it up) something (beginning with 'F') air intake I'm assuming...
Rats can get through the water seal, but not as easily as a open pipe, also the water seal prevents smells the same as your wc and sink waste traps.
So the water seal of a trap is merely a deterrence to rats, the smells should be prevented by traps at base of rainwater bottle gully, sinks, toilets etc, so can't see the point of doubling up there (when as I've mentioned the drains don't seem to smell much here anyway compared to other places I've been)... maybe a rat flap might be a similar level of deterrence since, you weren't around to offer your advise prior to us doing the work...
Google interceptor traps not the type you had to start with.
I have googled interceptor traps *endlessly* (how I worked out that mine is definitely a Buchan patented one, and that the Kernon and Beancliffe type patents are the ones most people think of when they say 'Buchan trap' or 'interceptor trap') - I suspect I have now read everything there is on the internet about them!
Yes I can see there are modern manufacturers of clay ones. However, the majority of individual opinions and modern regulations etc, seem to be that they are obsolete - a hangover from the days of belief in 'miasma', and cause way more problems with clogging than doing anything useful! I'd guess we would have to build maybe a brick manhole to fit the clay interceptor prior to it - there is not enough space really, without being at less than 45° from the base of the already shallow/sinking foundations. If there is genuinely good, proven reasons for installing something that keeps drain unblocking companies in good business, I'd love to find/hear the evidence, because so far you are the only person who has recommended one!