Hi

Does anyone understand the quagmire that is SUR1 in the Code for Sustainable Homes?

Apparently for the mandatory part of the Surface water runoff category of the Code, there is an order of priority of measures to comply if the volume of runoff post-construction exceeds that from pre-construction.

The order is:
1. Infiltration (basically soakaways) or rainwater harvesting
2. The 3 bullet points (ie attenuation or 2l/s/ha or the pre-dev site’s annual Qbar)
3. Sewers

So basically if you’re building on clay/soakaways aren’t possible for other reasons, you HAVE to use rainwater harvesting before moving onto the other options.

Aside from being expensive, RWH is dodgy for a number of reasons:
1. Carbon intensive in manufacture / maintenance / installation
2. There’s loads of water in some areas eg Wales / SW England (where I am) already
3. If the tank’s already full then it’s useless as a flood attenuation tool

So surely I’ve got this wrong.

Can anyone clarify?

You can combine harvesting with attenuation. You just need a correctly designed tank. Putting it simply you just need a "slow leak" situated half way up the tank. The top half acts as an attenuator with water leaking away slowly between storms until it's down to the halfway level. The bottom half acts as a RWH.

PS. In some areas the water co won't allow you to discharge surface water into the sewers. So if soakaways don't work well then rainwater harvesting might be your only option. Question is then what to do with the overflow..

PPS: It's common for sewers, including your own, to be laid in a trench back filled with gravel. If you think about it this looks a bit like a very long narrow soakaway :-)

there's some confusion here

RWH systems can be combined to be soakaways but see syd's response. If they are a combined system, part of the the RWH system will act as a soakaway (see CWatters response). However the RWH element can't be considered as part of the attenuation measures (largely because it is likely to be full when it would be needed for attenuation). However, these systems tend to put the soakaway element into the upper sections of the box.

Swales and the other measures considered in either of the (2) SUDS documents that you can use are alternatives.

If the rainwater harvester is to be used for flushing toilets and laundry, then the tank will act much more as an attenuation element.
You could also fit an automatic solar-powered pump in the tank to discharge the excess water to swales or attenuation ponds in periods of heavy rainfall.
We have tested these pumps and although they are very small, they managed to pump 2500 litres/day at 2 metres head all through last winter from November to April.

http://www.wte-ltd.co.uk

<blockquote><cite>Posted By: jon</cite> However the RWH element can't be considered as part of the attenuation measures (largely because it is likely to be full when it would be needed for attenuation).</blockquote>

I've modelled a RWH system for a fairly wet location (>2000 mm/year) based on 10 years data and the RWH has a significant attenuation effect most of the time. In a few instances (less than 5%), when rain fall is significant, the amount of water discharged is significant but none the less still reduced some 20%.

The RWH is sized for domestic use, and it has a remarkable and consistent effect on outflows (to a swale or soakaway).

Basically yes. In the worst case outflow from my scenario the rain fall was 117mm and the RWH was able to reduce the outflow from 14375 to 10674 litres - an attenuation of 25%.

I'm quite sure that if the tank was designed for attenuation, in that it releases collected water over 24-72 hours, that the attenuation would be much greater - all things considered (rain fall, tank size, collection area etc).

Edit: tried my model, based on real data, with a 72 hour release of collected water. Result, no overflows...100% attenuation...keeping in mind that I have a tank larger than the highest daily collected rain fall (17000) obviously a smaller tank would not be able to do this under my conditions for the few extreme events.

All but the most significant event could be stopped with a 4000 litre tank that emptied over a 24 hour period. Thats quite a bit smaller that the 17000 tank. 1 day in 3650, 0.03% ... guess it depends on the local council if you have to cover that case.

Hi Trule

Interesting and agreed that RWH can indeed reduce surface runoff into watercourses.

I started this thread off in the context of the Code though, which requires that if soakaways / other infiltration is not possible to reduce runoff, that RWH MUST be used rather than straight holding tanks.

However, I think we're all agreed that RWH is not a single solution to flooding - it can form part of it, but not the whole thing.

Presently almost the only houses being built to Code 3 are social housing new-build, by companies who need to make savings wherever possible. Therefore it is unlikely that if they have to use RWH, that the tank will be larger than 3-4m3.

Consider 3 consecutive days of rainfall at 25mm/day (not an excessive amount of rainfall), for a 4 person house with a roof area of 80m2. Each person may recycle 50litres/day for toilet & dish/clothes washing machines.

This means after day 1 that there is 1,800litres in the tank.
After day 2 there is 3,600l.
After day 3 there is 5,400l.
The tank is now probably overflowing.

Therefore an attenuation system / holding tank must also be designed to cope with runoff.

A wider environmental question should now be asked as to whether the energy / resources required to make/install/operate/decommission RWH outweighs the benefit that it yields. I would suggest probably not, particularly in rainy areas where water is not scarce eg anywhere outside SE England.

Anyone disagree? Would like to hear the arguements

I'm doing some code 5 at the moment Delboy

RWH completely excluded from attenuation. I'd agree that the code requirement which doesn't take account of the local conditions seems a fairly pointless exercise.

Delboy,

I would check the regs carefully to determine if you must use RWH as an attenuation solution seems reasonable. The regs can be tricky to interpret correctly. Keep in mind that an attenuation tank would release water over time, so it should not fill up if correctly sized.

RWH itself is cheaper than utility water over longer periods of time, but you have to put up the initial capital for it.

Jon - interested you're doing Code 5 at the moment - not many builders willing to go for that. But what do you mean by RWH being totally excluded from attenuation? In that the soakaway / holding tank is being sized as if the RWH didn't exist? Seems the sensible thing to do.

Trule - I've had long and excruciating dialogue with the BRE about whether RWH is def required if infiltration isn't possible, and they have said very clearly (although it is not at all clear in the Code guidance) that this is the case. In the words of Michael Caine, not a lot of people know that, but the BRE is trying (apparently) to get this message out more clearly.

Their philosophy is that in order to be "sustainable", you should firstly reduce the increased amount of water running off a building into watercourses, either by using infiltration or RWH. Therefore if you're building on clay / rock where infiltration isn't possible, you MUST install RWH. Where still you have runoff after RWH, you can then use holding tanks / hydrobrakes.

CWatters - I like your system - makes sense. But I'm confused by your use of the word "soakaway":

<blockquote><cite>Posted By: CWatters</cite>The outlet size is arranged so that the top part of the tank empties out between storm events - just like a soakaway does. Thats it. You can do both and it's not really complicated.</blockquote>

I understood that soakaways allow water to seep slowly back to the earth, whereas holding tanks contain the outlet / hydrobrake which discharges slowly to a watercourse / sewer. But with your system, if the outlet goes to a holding tank does this not mean that it does not act like a soakaway? I'm no expert in this so defer to superior knowledge so if I'm wrong please say so!

More driven by an interested HA rather than the builder Delboy

"But what do you mean by RWH being totally excluded from attenuation? In that the soakaway / holding tank is being sized as if the RWH didn't exist? Seems the sensible thing to do."

Yes, that's it: Means that you can have heavy period of rain (filling up the RWH Tank) followed by an extreme event and the diversion chamber goes straight to the soakaway once the RWH Tank is full. Holding tanks are just that: for holding and reducing the flow (to allow a drainage system to cope evenly downstream). Soakaways can be made to act as holding tanks (not often) but their primary purpose is to send water directly to soil.

Try this diagram..

Good stuff CWatters - thanks for that.

Is the recycled water in the system you describe filtered / treated in any way before being sent back to the houses for flushing / washing? Does that happen in the tank you describe or somewhere else?

I understand you need to keep a lid on the nasties and also the Code says that you need to ensure microbial contamination isn't possible (citing HSC ACoP and/or CIBSE TM13 as guides). I don't know anything about these processes, but they are important, aren't they?

However, I heard a RWH tank needs to overflow from time to time (read this on info from CAT) to avoid water going manky. Just tied myself in knots I fear!

I meant speed it up - only when required-
to avoid ever overflowing... Lesser of 2 evils?