Septic Tank Ventilation Design

[leighduffy]

Good Evening All,

May I first start by offering my congratulations and thanks for a very informative forum and excellent research tool for all elements of DIY; both professional and novice.
Apologies for the lengthy first post, but I wanted to be as comprehensive as possible to gather information from the experts!

I am having some trouble designing the ventilation of my sanitary/soil system to best work with my newly installed foul waster septic tank/soakaway system on a new-build. To try to clarify the issue I have sketched my current setup, with installed foul drainage pipework in blue, along with details of appliances in each bathroom/kitchen, and additional options for septic tank ventilation in red:
https://www.dropbox.com/s/ic8j1qekg3bi8ex/Septic Tank Ventilation Plan.jpg

My plumber has limited experience with plumbing to a septic tank system; he has used Air Admittance Valves int the majority of his past jobs (onto mains sewer) with no issues but obviously this will not be acceptable in my case. I have a couple of queries as to how we can optimise our installation and I hoped somebody here with past experience / better knowledge of Part H of the Regs may be able to offer some pointers, or raise any issues for discussion.

Here are my thoughts:

• Ventilation in the house to be provided by soil vent pipes into loft/roof from Upstairs WC and and Ensuite to ventilate heads of both long drain runs to Septic Tank. (Kitchen waste to be unvented due to smaller appliances draining to this run.) Can we get away with AAVs here or are open Vents better suited, given the lengths of drain runs from the Septic Tank?
  If Vents are required, can somebody advise if it woud be acceptable to bring SVP into loft space vertically, onto a 90deg elbow and terminate to a grill such as this through wall horizontally, as a more asethetic option? Would this not conform to Regs or affect performance of the vent?
  The Septic Tank is fitted with an Inlet and Outlet Tee, both of which have capacity to vent. The Tank cover/frame has an opening to take one vent to just under the cover, alongside the main inspection neck. I had initially planned to vent the outlet vent through the cover as I had assumed there would be less smell as this 'side' of the tank is clarified water, whilst the inlet side will be where most biological activity will occur.
  I had finally planned to run the Inlet vent horizontally to bottom of garden where a vent stack could be concealed and vent to a safe height without looking unsightly and whilst evacuating any smells a fair distance from seating areas. Would this be effective? Would it be better to vent to a closer location such as the wall in close proximity to the tank? In this case could we get away with an AAV to eliminate smells? I believe this vent will primarily function as an air inlet rather than pressure relief, but if somebody could confirm which of these four possible vent points will likely experience positive pressure, negative pressure or a combination that would be very helpful.

Sorry to throw all of this information at you guys, just tried to empty all options the options out of my head! Hopefully the diagram will clear things up and I really would appreciate hearing if anyone had an opinion of what would be the best option here, particularly if it is an option that has not been considered!

Many Thanks in Advance,

Cheers,

Leigh.

 

[Hugh Jaleak]

I think if you vent at the house end(s) at proposed positions and fit an AAV at the tank you should be ok. Vents (IMO) really need to be taken to above eaves level to allow the air passing over to draw any foul air out of the system, venting horizontally may not provide the 'draw'.

The inlet on the 'onion' type tanks usually dips to allow effluent discharge under the full effluent level, so venting the outlet is unlikely to achieve much. IIRC most of the bacterial activity takes place anaerobically, so this also prevents breaking up of the crust that forms on top of the decomposing effluent. It could be wise though to fit an AAV to the inlet pipe to allow a throughput of fresh air in the household drainage system. The AAV should remain closed under any positive pressure from the tank, allowing any gases to be released via the stack ventilation at the house end.

 

[leighduffy]

Many thanks all for the helpful response Hugh, apologies for the delay in response.

Having spoken with Klargester Technical I am contemplating another solution that they felt would be acceptable: fitting Air Admittance Valves fitted within the loft space with both Septic Tank inlet and outlet chambers vented independently to atmosphere. This is more convenient in my case and Klargester felt sufficient to exhaust any sewer gases whilst the AAVs would prevent syphoning whilst flushing. Would welcome any thoughts on this approach?
I haven't yet had Building Control's thoughts on this suggestion but notice that my neighbour who installed his Septic Tank two years ago doesn't appear to have any Soil Vent Pipes so assumed that my local Building Control must be open to other solutions.
Look forward to hearing your thoughts,
Leigh

 

[Agile]

Building control usually take a lot of interest in these things.

Might be better keeping it that way!

If you really must rattle their cage then there may be an old guy who specialises in them and will give you his advice!

Tony

 

[Hugh Jaleak]

Have to agree with Tony. Whilst Klargester may find it acceptable, it's Building Control who have the ultimate say! Really need to run this past them prior to fitting anything, then work to their requirements.

If they BCO isn't happy, he/she wont sign it off. What he/she will accept will vary between inspectors, let alone Local Authorities!

 

[leighduffy]

Thanks for the input again Guys.
Will run it past BC and report back once I have connfirmation - will be interesting to see towards which approach they sway!
Cheers, Leigh

 

[Diggerjohn]

If you follow Klargesters advice I think that you will have sewage smells coming out somewhere else instead, like shower trays, sinks, etc. A soil vent pipe is a VENTING pipe, not an air addmittance pipe. They were not called 'Stench pipes' for nothing as they let the smells out into the atmosphere.
Do let us know what happens.

 

[leighduffy]

thanks diggerjohn, will be sure to.