Cesspool manhole cover

[nhearle]

First time posting here so go easy!

The manhole covers on our cesspool appear to be just placed on top of the openings to the pool itself and the inspection chamber, leaving gaps underneath where it sits on stones/pebbles. We needed to get the pool emptied today which is a bit of a worry as it was emptied in June by the vendor of the house, and we've lived here for only 4 months.

One possible explanation is that rain water in really heavy rain is simply collecting around the manholes and when the level is high enough just pouring into the cesspool under the cover. Basically there are huge gaps under the frame of the manhole cover because it is just placed on top of the hole, rather than the frame having been cemented in place.

I was wondering if anyone had any thoughts about securing the frames with some sort of waterproof sealant or whether cementing it would be a better option? I thought about using a waterproof sealant between the plastic cover frame and the waterproof membrane, but not sure what one would be the right one to use.

I hope this is the right place to pot, apologies if not.

 

[john4703]

Is it a septic tank which has an outlet pipe? These contain the sewage for enough time for it to be broken down by bacteria and the harmless outflow goes into a river or into the ground via land-drains. The outflow pipe has a bend or T on the end so the out-flow is from under the surface.

 

[nhearle]

Thanks for your reply - it's just a simple holding tank - one way in, only one way out (lorry). Not a septic tank. We bought the house knowing this - we discovered that the mains sewer had arrived at the end of the lane a couple of months previous, so this is plan B, though we didn't want to have to go to plan B this quickly!

 

[pinenot]

Sounds like the lid has been lifted on pebbles/stones for venting purpose, as methane gas build up is explosive. Could it be that there's a separate vent and it's possibly blocked? ...pinenot

 

[john4703]

Can you build a brick wall round the manhole so the cover is raised above ground level, it would then not fill with rain but could still be loose and allow for ventilation. One or two courses of brick might be enough to stop surface water running in.

 

[nhearle]

That's an idea, will look into it. I'm not sure why the covers were left like that - there is a vent and it seems clear. I think lifting and lowering them just naturally gets stones underneath unless you're careful.
I may look into the brick course idea, or just work out how to cement the frames for the covers down - I assume the actual manhole covers are reasonably watertight? They are actually on a slight bump, so as long as the surface water is able to drain away I guess we should be ok. I'm wondering/hoping that part of the problem is the sheer ammount of water coming down in short spaces of time with a gappy manhole cover. It could work out very expensive if this is a standard few months of sewerage...

 

[b3taman]

If it's just a holding tank then it will need emptying very frequently - try and find out what it's holding capacity was when new, then make a guess as to how much liquid waste your household produces a day. I understand a typical family of four uses between 300 - 500 litres per day, (someone on this forum may be able to confirm this) and then you'll be able to calculate how frequently it should need emptying. If it's requiring emptying much more than expected then it will certainly be a water ingress problem.
I expect the local water authority insisted on a holding tank rather than a septic tank, and this was specified as a planning condition. No one in their right mind would install just a holding tank otherwise.