UKC

I have just moved into a house that has no mains sewage but runs on a septic tank that is "owned" by the 9 houses in the close. At times I get a strange smell in both bathrooms, I suspect it happens when the pump kicks in as it is not present all the time.  It's not a sewage smell like you would expect but more a musty, rotting vegetable smell.  It's not particularly strong but mildly unpleasant none the less.

Does anyone have any idea what this may be and how to cure it? Can I prevent or at least minimise it or is it just something that is inevitable and has to be tolerated? The couple of neighbours I've spoken to do not experience it but perhaps their noses are not as sensitive as mine. I have tried all manner of disinfectants and sprays. My wife cannot smell it at all.

Check that all the drains are clear. Check that the tanks don’t need emptying…..

What is the pump you're referring to? Does it pump to the drainage field, or is it some sort of aeration system?

You shouldn't get any smells in the bathroom, the stench pipe should vent any smells, and the water traps in the toilet and sink/bath etc. pipework should prevent any odour entering the bathroom.

I'd try buckets of hot water poured down the sink, bath, shower and toilet to flush the pipework and clear any residues. Maybe something smelling in the shower trap if you have one? Might also be worth rodding the drains to check they're clear and not backed up.

We stayed in  basic place in the Auvergne and experienced some less than pleasant odours. After discussing it later with the owner it was suggested that although we were using the shower and toilet regularly, we never flushed the bidet and the U bend was drying out permitting smells to backtrack. 

From the title I thought this thread was going to be discussing Americans? What's with the sewage chat?

It doesn't matter that you have a septic tank or foulwater to a public sewer... you should not have a bad smell in the bathroom. If you have a bad smell then something from the foul side is somehow getting back to the bathroom. There are several possibilities, and with a recently bought property: dodgy DIY by the previous owner (or cheapo job by owners friend) is always a prime suspect. It's not caused by a septic tank systems (unless it is right outside the window).

* Has the bog been removed and re-installed without a new seal being fitted on the waste pipe? If you remove the toilet, do something floor tile related or whatever, and just put it back again using all the old seals etc then you can get bad smells (due to very slight poor seal) in the bathroom. Diagnose this by gaffer taping up that joint area and seeing if it gets any better (this is just diagnostic step not a silution!)

* Do any of the traps (like U/P bend in bathroom basin, toilet U bend etc) occasionally drop to very low levels - meaning they might not stink-seal any more and smell can get back in to the bathroom from the waste? if so... the suction needs to be eliminated by looking at soil stack to make sure all OK and/or adding an air admittance valve somewhere. Diagnose this by checking the water levels in the loo/sink trap when the smell appears

* Is there a leak, or general bad DIY C*ck up, anywhere. Unfortunately you'll know if there is bad DIY. I've seen plaster skimmed over wallpaper and shocking (literally) electrics- anything is possible

I should have mentioned the property is only one year old

Sadly some new build properties look like bad DIY, so the things to check are still about the same. See if it has an NHBC guarantee. Get a plumber to check and if it is expensive - pursue the original builder, but get the bathroom checked becuase there is some kind of plumbing fault there and it can only be something that connects to waste like loo, basin, bath, shower, or a leaky waste pipe, failing air admittance valve, problem soil stack. There is only a finite list of possibilities. Got to be one of those just listed.

I have a drain in the bathroom for floor washing/any floods and it dries out regularly. I pour cooking oil in to stop the evaporation.

We fairly frequently have nasty niffs coming up through the bath u-bend as we rarely use it (prefer a shower).  Had never heard of putting oil down it, I thought putting oil into drains was not recommended.

I usually run the tap for a minute when we get a bad odour and that sorts it out for a while.

If you are connected to a public sewer don't do this. It cools in the sewer, hardens and causes blockages.

Next time you smell it, give each drain (toilet, shower etc) a sniff (or get your least favourite family member to do it) and find out which one is causing the problem.

Once identified you need to work out why smells are coming up through it.

Does it even have a odor trap? 

Is the odor trap full of water (should be).

All it would take is an U bend to be installed at a funny angle and there'll be an air gap.

What pump is it - mascerator from source to external drain, rising main from outside to Septic tank or tank effluent?

 I'd look to see if you can install your own Septic system. You'll save yourself a whole world of hassle and expense in the long run. 

Cooking fat is the issue, often caused by rinsing off greasy plates with hot water.

A small quantity of oil will cause no problems,  it's just a skin to stop evaporation.

FWIW, we had a similar problem with a little used bathroom, remembering to refill the traps every couple of months sorted it.

I am not connected to the mains sewerage and half a teaspoon of olive oil has and will have no effect.

I'm a bit confused by this, as it seems the development is only a year old. The OP hasn't clarified this as yet.

I would have thought any new build would be on a packaged treatment plant rather than septic tank, especially if it's handling 9 properties.

Must admit I'm not enthusiastic about shared systems. I have a septic tank, but it only serves my house and it and the drainage field are on my land. It certainly does away with things like the Muppet at No 3 showing nappies, bleach and chip pan fat into it.

However it's an existing tank and drainage field so I've a pretty free hand on maintenance and drainage field replacement.

I'm not sure you'd even get consent for a septic tank, even for discharge to ground rather than watercourse, from the EA these days. It also depends if the OP has the land for a drainage field.

To be clear, the house is less than one year old.  The road and septic tank are owned by the company that comprises the 9 houses in the close who have joint responsibility for both the road and the septic tank.  I have been in touch with the management team that handles this but I was in the meantime wondering if there was anything simple I could do. It's both the bathroom and the en-suite so some of the proposed causes are unlikely. I've only been in for 3 weeks so I'm still a lttle unsure about how these things work both in legal and practical terms.

I will try to isolate the smell but it is not there all the time so this may be difficult.

I can only think of two possible sources of the smell and it can really only be one source if it is intermittent, that is air getting past the water traps in your sink/toilet/shower. My first port of call would be to check that all the sanitary fittings have water traps that are fitted and work properly.

If they are all present and in working order it seems almost certain that the pump is creating an air pressure in the sewage pipes which is, in turn, forcing air up through your water traps. If it's only a very faint smell then probably not very much... I think you will have to start looking at where your house sits in the system, where the pump is, what the relative heights of the house, tank and pump are... find out if this is a pump to pump sewage uphill or a pump to aeriate the tank contents or a pump to push effluent into the drainage field or some other pump that does something else...

As per henwardian's post, could you tell us a bit more about how this 'septic tank' is designed, particularly what the pump does.

It probably is an odour trap problem, as it's unlikely you'll get any worse smell issues in a bathroom from a septic tank as you would with a public sewer connection.

I don't know if it's a pump.  All I know is that there is a slight humming sound that comes from the tank area.  I assumed it was some kind of pump. I don't know how it all works which is why I asked the question. The fact that the smell is intermittent made me think it was something to do with the process of the whole system, perhaps when a particular cycle of activity kicks in. In fact this morning I have not experienced the smell at all.

Many modern systems inject air into the sewage at regular intervals, sometimes the pump is in the tank, on mine it's external. I had problems with gas getting past the trap in a utility room sink as the tank airvent was too small, improved the vent and since then nothing.

The humming sound will be for the air injector.

It won't be a Septic tank it will be a package treatment plant - basically a small scale sewage works. Most likely it discharges to water course. As has been stated its hard to get consent for a new Septic tank and soakaway these days. 

How far are you from public sewers - there is a chance you aren't on any treatment system and it is a private sewage pumping station and rising main....   A private set up to pump to public sewer - but normal only 100 - 200m max. 

Most like odour trap issue. 

Rather than air being forced up (which can happen), the problem in drains is more commonly suction which lowers the water level in traps meaning they no longer work as stink traps. When you flush the bog or empty a washing machine or whatever a sudden pulse of water flows and before you can remember how to spell "Bernoulli" you risk suction. The property should have a soil stack but in addition to it localised suction can be avoided by a correctly placed air admittance valve. If the bathrooms are used frequently (not drying out over weeks) then I suspect suction,  especially if more than one bathroom affected. Definitely a plumber needs to rule that out 

Could it be coming in through the window from a vent pipe? That's what my out-laws suffer from when the wind is in a certain direction.

I hate to sound like a stuck record - but have you actually had the covers up and had a look?  If there are nine other houses feeding it, who's responsible for emptying it?  Has one of the houses been feeding it wet wipes?  As it's new, have the pipes been laid properly - maybe a displaced joint (with attendant snagged wet wipes)?  Is the capacity sufficient? How many houses are upstream from you - do you get foul smells if there's a lot of activity?

Find the covers, get them all up and have a look to eliminate that.  Start with the basics. 

This is a bit like a toothache.  As soon as you contact the dentist the pain disappears but in this case it's the septic tank experts. I was resigned to having to sniff all the drains and kept checking the rooms.  Today was the first day since moving in that there was no smell.

Had the house been empty for a while? The water traps could have evaporated, allowing odours to enter. Now you're using them they could be topped up again.