I got through planning for my replacement home without mention of the state of the existing septic tank, however Building Control were quick to point out that a 70odd year old septic tank would need to be replaced.

The problem is bedrock only 2.5 m down! Can anyone suggest a low profile system that could sit on the bedrock with so little soil cover.

I would go non mechanical possibly two small ones like those big flask shaped things.

What's the fall like to the proposed location - ie do you actually have 2.5m? and is that 2.5M flat over the entire area - from bitter experience I can tell you that a seemingly small hump, esp in granite, can translated into one hell'uva pain in the arxe! If you have a failed field, a new solution can also make the water fit for direct discharge onto the land or a stream - saving ££££s.

I too would always opt for unpowered if poss.

Very unlikely that there are filters, more likely a single tank with inlet and outlet elbows or possibly a "Pasteur Tank", i.e with two compartments separated by a dwarf wall.

Building control probably condemned the tank on the basis that after 70 years it is likely to have lost its watertight integrity.

2.5 m is more than enough to bury a septic tank, or better, a treatment system. I don't understand the antipathy towards a continuously powered system, a relatively simple PV set-up would be enough to provide what ever power you required. No one complains about having a fridge or hot-water cylinder permanently powered.

All mechanical and electrical sewage systems break down, mal function and often without the householder knowing about it. Expensive to fix and not nice.

When biological processes work why go for complexity, 70 years and still there is a good track record,

Did the BCO say why the septic tank would need replacement - as said above worth a challenge. If its not demonstrably broken - don't fix it.

Otherwise have a look at Biorock, no power needed unless you don't have enough fall away from the unit and then it needs a pump on the output to lift the output to the outflow.

Posted By: orangemannotWhy do they require to be watertight? if thoughtfully located, I could never understand that requirement.

When I took apart mine (as required I was told by new EU wide regs) I found roots had inhabited the tank and were filling 60-70 on the inlet pipe to a distance of several meters. So pewrhaps it is not so much water getting ut but other things getting in...

Do you have training and experience to back that up? It doesn't tally in anyway with the research that I have done on the subject.

A 2 chamber septic tank, discharging into an adequately sized leach field cleans up the effluent at least as well as package treatment plants and is a lot more forgiving and reliable. Of course, if you don't have room for a decent leach field, then a packaged plant is the easy option to discharge into a watercourse; the makers have generated the numbers to tick the boxes.

If the OP doesn't have room for a good leach field, then the EA will probably insist on a some sort of treatment plant.

Bleach doesn't kill the bacteria in a septic tank, unless you put gallons a day into it.

I went with a passive solution that is basically 2 septic tanks, the first is a 2 chamber septic tank but its outflow goes directly into another identical tank without the 2 chamber insert that is filled with 1000s of fist-sized plastic shapes that have a huge surface area - good to discharge into stream and lands etc. Sadly, I gave details of it to another GBF guy 2 years ago or so but he couldn't source it in the UK - still they exist and things change. It was a good thread too, so worth doing a search.

I too am going down this route, existing cesspit to be replaced, very flat land on clay (not sure what type yet but very wet in winter). I dont fancy a leach field because of the clay although we do have a big plot (and a field next door). I found some three tank systems that said they were licensed to discharge to a surface water course and plan to discharge to a ditch draining the field. I too dont fancy anthing complicated or powered.

Posted By: ArchmocoWith bedrock and in my case boulder clay, a septic tank was not permitted, although neighbour have them. Plot sizes now a days are too small and insufficient area for leach fields.

Yes, in those circumstances a septic tank/ leach field isn't an option; that doesn't rule out such a system if you have the room and suitable soil. A gravity powered system looks greener to me than one that runs a 60W pump 24 hours a day, 365 days a year.

Posted By: ArchmocoRegarding household chemicals my understanding was they have a detrimental effect and in my case with 4 persons;
4nr showers a day
Couple of baths a week
Dishwasher every other day
Washing machine average once a day"........it all adds up.

On a green forum one would expect to be minimising water consumption! Normal household chemicals used in normal amounts aren't detrimental to septic tank performance, after all the are already pretty dilute and they will be diluted many times more in the tank. As the package plants also rely on the action of bacteria, they will be at least as susceptible to excess bleach use as septic tanks.

Posted By: ArchmocoI've surveyed a few older systems in the past, and there have been issues with broken concrete tanks, collapsed clay pipes or in more modern unsuitable flexible perforated pipes. I would have to disagree with reliability of septic against treatment plant, certainly the treatment plants with moving parts might have issues, but one with an air pump with an alarm, not much problem there. I considered the newer Biorock but I had not heard any reports, and it was too expensive.

Well, if you don't maintain things they fail eventually, buildings fall down, trees grow, earth moves, I'd guess that those failed systems had been operating for decades without any maintenance, not something that I would expect to be able to do with a motorised or pumped system.

Back to the OP. I accidentally came across this firm http://uk.tricel.eu/ who make low profile plastic septic tanks (Vento), they are about 1.4M high, so should be able to be installed in 2.5M soil depth. Never heard of them before, so not a recommendation.

As per Triassic original OP. any other suggestions to a specific solution or company. Also is a reed bed considered good enough to replace a third chamber before disposing to water course?

Try the biorock:

http://www.biorock.co.uk/_files/file/uk/folder-biorock-uk-ie.pdf

Posted By: joe90Also is a reed bed considered good enough to replace a third chamber before disposing to water course?

One company that offered reedbeds has stopped offering them as they are no longer a fit and forget option, what with flooding and extended cold weather killing off the reeds and the good bacteria.

When considering all the options I can across this 'Site Assessment and System Selection' flow chart. http://www.ask.klargester.com/downloads/assessing-your-site.pdf page 4

Is this flow chart still valid, do the decisions meet current planning requirements?

If so, It looks like my first option is a septic tank, as I've passed the peculation test, the distance requirements and have the space for a drainage field.

Is it just me, or are manufacturers and suppliers reticent to publish prices? How much do these things actually cost?