I am re-building our greywater system right now, and have just converted a 120L plastic stock trough from a totally failed grey water "filter" into a mini septic tank. So far (one week only) it is miles better than the festering old filter ever was, and I have great faith that it will continue to serve us well. I struggled to find any info on sizing septics though, and conventional wisdom around these parts seems to be to take the design loading rate for the household per day and add 2000L to get the recomended size.... So that makes it massive.

My question for Feidhlim Harty is: How small can you go with a septic in your opinion? Below a certain size does it stop being a fully functional septic tank and just become a still pretty handy settle/float tank?

I really like the idea that you can pair up a compost toilet with a mini septic and be dealing with your stuff in an efficient effective manner with a minimum of materials and earth works. I am going to use another stock trough as a mini wetland to clean the water up a bit and use a syphon to dose a simplified disposal field. The bath will dodge the septic tank and go straight to the wetland to avoid overwhelming it - or it could go to its own little swale type drain, not sure but either way it won't be dumping into the mini septic and wrecking havoc there.

Will take some pics to share tomorrow.

James

Hi James,

Yes, there is a reason for building the tank that size. Volume (litres) = P x 150 +2000, where P is the total maximum population size for the house (Irish EPA Code of Practice).

If the tank is too small, you won't get the residence time for liquid and the suspended solids (even just the fines) will travel down the outlet pipe in greater concentrations than you want. It's not much more expensive to go big enough. That said, if your bath is bypassing it, then perhaps you should rethink the 150 litres/person/day assumption and redesign it somewhat. I know that small grey water filters are much smaller than these figures would suggest, about 50 litres max for a kitchen sink.

If you have a flush toilet - I'd say use the full septic tank size recommendations. If you have a compost toilet, then you can play around with the grey water filter to your heart's content (unless there are NZ guidelines that I've not seen that deal with grey water settlement sizes). Just make sure that wherever the water does flow to in the end, it gets well distributed over a good depth of dry soil. Lots of microbial filtration takes place within this infiltration area. I'm sure there are NZ size recommendations or look at www.wetlandsystems.ie/watertips.html for the Irish EPA Code of Practice guidance on percolation areas if you want more background reading.

Does that help at all?

Hi Feidhlim,

Thanks for the reply, that does help - although its a bit late for a redesign for me, as its pretty much done now I am still very interested in the principles involved. On the sizing front, even a drastic scale back of usage is still looking at a 2000L plus tank and for grey water alone I just can't see the need for that, I am now feeling that our system could have been made more elegant by being smaller still. As it is, it will deliver one 300L (roughly) dose every day and a bit (with an estimate of about 90L per person for a 4 person household) - I would have liked it dose more often than that, but in saying that the extra rest period could be a blessing given that we have pretty wet winters typically. For all the effort given to compost toilet design, it doesn't feel like the grey water has had its share of attention - after all every compost toilet must have a greywater system of some description paired up with it.

I am very interested to know what the 50 litre kitchen sink filters look like? Are they just a mini septic? and while I am in question mode - what is the difference between a big grease trap and a mini septic? anything? I can see a system with a miniseptic for the kitchen sink, bark filters for everything else (really effective at removing hair and body grease and soap) which could be combined and then either dosed to a field or dealt with in their own mulch basins - Art Ludwig style. That seems to me to be an appropriate scale solution.

I will try to attach some pics of what I've been doing today...

There's no difference in principle between the grease trap and mini septic. We just don't use the latter term here.

That looks like a good set-up you have there. One concern I'd raise is that you're still getting grease in your mini-septic outlet basin, as well as the inlet basin. That indicates that there is insufficient surface screening between the inlet and outlet - or that the residence time is too low for proper settlement. This shouldn't be a problem if your grey water system is robust enough to avoid clogging. Otherwise over time you may find it working less and less efficiently.

This is one we had in our previous house. We used an Aquatron unit for faecal separation and this type of grease trap for grey water prior to a 1000l single-chamber septic tank for extra settlement: http://www.fittingsdirect.ie/ourshop/cat_478155-Grease-traps.html The wetland was very robust to sludge inputs, so even though our grease trap had a greasy outlet like yours, it wasn't a problem.

Hi, yeah we don't use the term mini septic here either, I just like it. Wow those grease traps are big bucks for what they are!

I would like to get rid of that grease from the outlet basin. There is a surface screen there attached to the green piece that spans the tank in the middle (it came with the trough as a cover for the ballcock) but I think I will change that around a bit - I rekcon a rolled up piece of insect screen would clog up in no time and make a better surface screen.

thanks for your thoughts Feidhlim, all the best with the book

James.

Hi James, yes, once you get into any proprietary system they tend to add up cost-wise.

Something that might be better than tinkering with your current set-up is to add another basin to the first one so that you get twice the volume. That way you'll also be drawing liquid from low down in tank 1 to tank 2 and the grease will stay at the top of tank 1.

That said. If you do find that a new screen on the existing one works well for you, let me know. I'd be interested to hear how the project works out.

Thanks for the questions and the wellwishing on the book