Very smelly soil system with septic tank.......Been told it's impossible to fix!

[Cornish monkey]

Hi all,
I'm come to you begging for advice, the tourist site I work for has had a long problem with a very smelly soil system. They've been told by many plumbers its impossible to fix, I've been asked to see 'what I can do'.
Its a hotel complex site with a septic tank, 4000 litre. it has quite a few soil stack vents around the site (all checked and are clear) and many many manholes, with pipes coming from different parts of the site joining into a main run that takes it all to the septic tank.
Theres a few problems,
1- pretty much all of the man holes really smell of sewer gas, even with the lid closed. So much that you can smell it at some distance from each hole (they're the heavy cast iron type)
2- The area around the tank smells badly, even at some distance from it

but the odd thing is that it's not all the time, very occasionally there's no smell at all, but other times it could make your eyes water, it doesn't seem to be linked to the wind as it can smell on a still day as well as a stormy one.
I'm sure it's a pressure issue but not sure how to start to tackle it.

Any ideas? All I can think of is exterior sealed manholes (if they exist)

 

[joni os]

A properly functioning Septic Tank should not smell. You do not say type, is it onion or dual chamber? In each case, important that anaerobic and aerobic digestion process occur. Capacity important, as small capacity and large throughput gives insufficient time. If manholes are backing up it's a sign that weeper drains need extending. Ensure that direct vent to tank is not obstructed, there is soil vent at end of all long runs at height necessary and manholes covers, if source of smell, are set in grease.
Bleach and other cleaning chemical are detrimental to bacterial action so over use can be part of the problem. 4000 litres would be fine for pair of semi's but a holiday caravan could well equate to a single dwelling and a second septic tank may be required.

 

[Last Plumber]

A properly functioning Septic Tank should not smell. You do not say type, is it onion or dual chamber? In each case, important that anaerobic and aerobic digestion process occur. Capacity important, as small capacity and large throughput gives insufficient time. If manholes are backing up it's a sign that weeper drains need extending. Ensure that direct vent to tank is not obstructed, there is soil vent at end of all long runs at height necessary and manholes covers, if source of smell, are set in grease.
Bleach and other cleaning chemical are detrimental to bacterial action so over use can be part of the problem. 4000 litres would be fine for pair of semi's but a holiday caravan could well equate to a single dwelling and a second septic tank may be required.

Absolutely bang on.

 

[chris watkins]

A properly functioning Septic Tank should not smell. You do not say type, is it onion or dual chamber? In each case, important that anaerobic and aerobic digestion process occur. Capacity important, as small capacity and large throughput gives insufficient time. If manholes are backing up it's a sign that weeper drains need extending. Ensure that direct vent to tank is not obstructed, there is soil vent at end of all long runs at height necessary and manholes covers, if source of smell, are set in grease.
Bleach and other cleaning chemical are detrimental to bacterial action so over use can be part of the problem. 4000 litres would be fine for pair of semi's but a holiday caravan could well equate to a single dwelling and a second septic tank may be required.

Spot on

I guess that that may be the problem the plumbers were asked to fix the problem (cheap solution required) when answer is it is way to small & need to be replaced which is going to be costly.

 

[Last Plumber]

Bleach and detergents are a big prob as stated above. Most people don't have the process explained properly and treat it like a main sewer