So...I'm about to do Rainwater Harvesting

[Temp]

We had an IBC on our allotment, it had a metal frame but I doubt I could be buried. The plastic part was more flexible than a watering can.

 

[saveasteading]

3 hours ago, mads said:

 the sustainability aspect of it and reducing the need for potable water

OK. So I specified a rwh for our own office development. Try it on yourself principle before recommending to a client.

I forecast a 10 to 13 year return on capital. There were about 30 occupants, us and tenants which is rather different to a house.

It paid itself in 4 years, because I hadn't thought about the sewage saving ( sewage quantity is based on water supplied).

BUT I haven't included electric supply to the pump, or a replacement pump or future maintenance.

Also, the duplicated pipework cost was a guess. 

Sustainability? How much carbon in manufacture, construction and operation? 

Hence my logic that you don't do it for a house not connected to the sewer.

[Simon R]

On 19/03/2025 at 04:41, mads said:

I'm interested what the Kingspan control system was meant to do? And if the new pump covers it all?

The Kingspan system has a display showing tank level and options to customise auto fill from mains water.

As I mentioned the pump had failed before first use and Kingspan wanted silly money just to send someone out. Our system now is very simple. A Divertron pump which switches on when it senses a pressure change in the system ie switching on a tap. No header tank. I also bought a cheap radio tank sensor which displays tank depth. In practice it’s pretty irrelevant and the tank has never run dry.

[saveasteading]

3 hours ago, Simon R said:

the tank has never run dry.

That is important with long dry summers becoming more the norm. Excuse me if you've said already, but what 

capacity?

was changing the pump  very technical? ie is there justification for a specialist?

 

[mads]

I decided to create my own calculator on how much I could save on water by RWH. Here is the calc (I included a waste water section though not applicable for me). Happy to upload the file for others to use for themselves, if helpful.

 

I have only done it from a water consumption and waste water perspective. I have not factored in electricity costs (as I personally feel this is negligible and most of us will have PV) but have included replacement pump costs.

Edited March 21 by mads

[mads]

17 hours ago, saveasteading said:

That is important with long dry summers becoming more the norm. Excuse me if you've said already, but what 

capacity?

was changing the pump  very technical? ie is there justification for a specialist?

 

 

Kingspan Klagster 4,600ltr system was mentioned. Re: long dry summers, I think this is where making sure you have a high volume tank is important so can store more during the shoulder months when we do have rain.

[saveasteading]

Nice to see your calls. 

From a quick skim.

The garden usage looks high. A) perhaps showing a falsely high cost/ potential cost.. b) you would empty the tank in 3 weeks of no rain.

Water cost sqm should be cm ?

Does waste cost belong here as a cost?  You could show the saving due to lower purchase volume perhaps.

[mads]

22 minutes ago, saveasteading said:

Nice to see your calls. 

From a quick skim.

The garden usage looks high. A) perhaps showing a falsely high cost/ potential cost.. b) you would empty the tank in 3 weeks of no rain.

Water cost sqm should be cm ?

Does waste cost belong here as a cost?  You could show the saving due to lower purchase volume perhaps.

Garden is 1 acre which will be 0.4 acre lawn, 6-8 sqm vegetable patch, medium to large greenhouse and 6m diameter pond to keep topped up, plus plants and hedging so expect fairly high use - in-laws are avid gardeners too.

 

When I sent my request to suppliers, they came back saying I need a 5,000L tank but to be on safe side for droughts then consider a 7,500L one. I was going to get a 10,000L as I also have outbuilding roofs which I haven't factored into the water capture calc.

 

Water cost is actually m3 - had the wrong units described and have corrected in my file. Doesn't change the calcs.

 

Waste cost doesn't apply to me so I would zero it out. I included it in case I share this with anyone and they may need to use it due to connection to mains sewerage.

For me, the ROI is around 30-40 years so one to think about in terms of capital cost outlay. Was useful to do the exercise and hope it helps others.

I had to do a few iterations to get to a point where the ROI time frame balanced out. The replacement pump cost drove it up and I would look to see if 3 years is too frequent.

Edited March 21 by mads

[saveasteading]

In my office example above the calculations said use 5m3. I went for 10 which I think was right as it never ran dry.  Negligible garden use.

I think the ROI on the extra 5m3 would have been very short as it was only the extra tank cost...about £1k?

Well done on the exercise.

[mads]

2 hours ago, saveasteading said:

In my office example above the calculations said use 5m3. I went for 10 which I think was right as it never ran dry.  Negligible garden use.

I think the ROI on the extra 5m3 would have been very short as it was only the extra tank cost...about £1k?

Well done on the exercise.

Thanks. Though I have to say that my exercise presumes the costs stay the same for the entirety. We know that the cost of water supply is going up, with numbers of around 36% over the next 5 years. If I assumed a 3% YOY increase to the fresh water price, that goes up to 360 pence per m3 in 10 years. At that cost, it could shave off around 8-9 years from a ROI timeline. I need to do some financial modelling to see what this might look like over a 10-15 year horizon as I suspect it will be different to the static view I have started with.

[SteamyTea]

1 hour ago, mads said:

We know that the cost of water supply is going up,

Base the price on South West Water.

Have I ever mentioned that we have the most expensive water and waste in the country, which probably makes it one of the most expensive in the world.

 

[mads]

4 minutes ago, SteamyTea said:

Base the price on South West Water.

Have I ever mentioned that we have the most expensive water and waste in the country, which probably makes it one of the most expensive in the world.

 

£348 for the year vs £304 for ST Water on a like-for-like

[mads]

So some modelling shows...
Over 10 years, based on a 5% YOY price increase (I think this is too low imo), the fresh water supply cost for my scenario would be:

2025/26: £304.56 per annum

2034/35: £472.47 per annum

 

The total over the 10 years would be  £3,830.72 vs  £3,045.60 if the costs were to remain static.

 

Looking at just today with the current costs, the ROI timeframe is 26.6 years

Looking over a 10 year horizon with the above increases, the ROI timeframe reduces to 21.1 years
Over a 20 year horizon with still a 5% YOY increase, the ROI timeframe reduces to 17.3 years

[AartWessels]

I moved here from Belgium, where rain water harvesting is mandatory. 

We had a 5000L tank installed. 

Water costs around £2.20/m3 there. 

We used rain water for laundry, toilets and the garden. 

There is however absolutely no way to get an ROI, or at least not in Belgium. A major part of summer, the tank will run dry. In the wet periods, it will be full, and therefore the rain water will overflow to a drain. 

So the theoretical volume of rainfall can not be used, as it cannot be harvested in practice,  and part of the year you will have to fill the tank from mains water to keep toilets and laundry running. 

 

Anyway, that's just my practical experience. 

[mads]

9 minutes ago, AartWessels said:

I moved here from Belgium, where rain water harvesting is mandatory. 

We had a 5000L tank installed. 

Water costs around £2.20/m3 there. 

We used rain water for laundry, toilets and the garden. 

There is however absolutely no way to get an ROI, or at least not in Belgium. A major part of summer, the tank will run dry. In the wet periods, it will be full, and therefore the rain water will overflow to a drain. 

So the theoretical volume of rainfall can not be used, as it cannot be harvested in practice,  and part of the year you will have to fill the tank from mains water to keep toilets and laundry running. 

 

Anyway, that's just my practical experience. 

Thanks for sharing. I'm interested as to why they then mandate it? And could it be different if the tank size was doubled?

[SteamyTea]

15 minutes ago, mads said:

I'm interested as to why they then mandate it?

Probably as a general water management policy, similar to our SUDS.

That raises an interesting point, if SUDS has to be employed, can a rainwater harvesting system replace it.

[AartWessels]

2 hours ago, mads said:

Thanks for sharing. I'm interested as to why they then mandate it? And could it be different if the tank size was doubled?

It's mandated to prevent water problems. Prevent a bit of flooding, etc. And then 'sustainability'. I don't think it's hitting either of these targets. 

 

Doubling the tank size depends on the size of your roof. Still, watering the garden in summer will suck it dry in 2 weeks. 

 

 

[AartWessels]

2 hours ago, SteamyTea said:

Probably as a general water management policy, similar to our SUDS.

That raises an interesting point, if SUDS has to be employed, can a rainwater harvesting system replace it.

Maybe. 

I'm not against storing rainwater, I just think using it in a house is not a case with an ROI. 

 

In our new build we'll store it in a wildlife pond. 

[saveasteading]

6 hours ago, mads said:

presumes the costs stay the same for the entirety.

That's pretty standard. You are laying out funds that could otherwise earn interest. 

2 hours ago, AartWessels said:

Doubling the tank size depends on the size of your roof.

It also depends on catching all the rain in any storm. So for a dual pitched roof there may be more drains and deeper runs.

 

 

4 hours ago, SteamyTea said:

if SUDS has to be employed, can a rainwater harvesting system replace it.

Yes, or part of the proposal at least. But anything differing from the accepted hierarchy can be difficult for a planner to accept, and you get into the need for an expert proposal.

 

I showed planners why a harvester was better than a green roof, for example, which surprised them.

 

 

[AartWessels]

8 hours ago, saveasteading said:

It also depends on catching all the rain in any storm. So for a dual pitched roof there may be more drains and deeper runs.

Although I agree that is factually true, I don't think in real.life it matters that much. With that, I mean, there are so many factors in the actual environmental conditions,that you can't accommodate for all of them, not even when designing. 

KISS. 

An oversized track will hardly ever fill completely,  so the tank must fit to the roof surface. 

Only water from the roof areas should go in. 

Keep your gutters clean. That has a very big impact. 

Clean the inflow filter regularly. Again. A big impact. 

In a big storm, you won't catch it all. Many reasons why not, but mostly because the time of year these usually come (the wet time), when your tanks are filled already anyway. 

 

Now, if you don't intend to use it for the house, but just for the garden, allotments, greenhouse, etc., the story changes. Using a catchment basin, IBC containers, etc. change the ROI dramatically I'd say. 

 

[saveasteading]

46 minutes ago, AartWessels said:

change the ROI dramatically I'd say. 

The cheapest rwh I ever installed was a series of IBCs. 4m3 for £400 plus some plumbing.

It was only for buckets of water for slunging vehicles OR could be set to dribble to slow down flow to drains and ground. Not pretty but the client loved the value.

[AartWessels]

6 minutes ago, saveasteading said:

The cheapest rwh I ever installed was a series of IBCs. 4m3 for £400 plus some plumbing.

It was only for buckets of water for slunging vehicles OR could be set to dribble to slow down flow to drains and ground. Not pretty but the client loved the value.

That's it. Works wonders in a rural environment, and an ROI of around 10 years if you included a pump, or less than 7 years if you don't. 

And maybe even less if it is in a location without any existing infrastructure. 

[Simon R]

On 20/03/2025 at 22:08, saveasteading said:

but what 

capacity?

Our tank was is 4600ltrs. Just the two of us and a small garden. Lots of washing though as we’re both sporty.