Retrofit a custom rainwater system

10-10-20 · 10-10-20

Hello all,

Like many out there, I'm assembling rainwater system and I'm going to retro-fit it into my house to feed my two (or maybe three) cisterns. I have a plan which seems feasible, but there are a few loose ends which I need to tie up. Perhaps some good folks here can provide pointers!

I'm keen to build the system myself and not buy as a kit.
I'm lucky to have two coffin-tanks already fitted in the attic, both interlinked, and supplying the house. Again, I'm fortunate that the cisterns are on separate piping to the showers and sinks as they reach the tanks, so isolating a tank for rainwater and another for fresh won't be a problem.

I'm going to fit a number of interlinked barrels along an unused passage on the side of the house, the nearest downpipe is well placed for this.

Here are my design considerations and quandaries:

I need to pump the water from the external tanks up 3.2m into the attic space and then 12m horizontally to the tanks.
I need to protect this tank pump from running dry.
I wish to have fresh-water replenishment as a backup in times of drought.
I want to have replenishment direct into the attic tanks (physical separation of rain and fresh must be maintained).
I wish to have leak detection around the rain-water tank with an electronic shut-off of the incoming water feeds.

Q1: I can use a submerged pump, but would a externally mounted pump with a feed taken off the bottom of the tank be better/easier to maintain? Frost/freezing isn't an issue as the area is sheltered. What about sediment and organic matter clogging impellers and filters?

Q2: In terms of water management, I can make this system modular or fully automated and managed: Modular meaning that the tank pump is protected from running dry by a float switch and that fresh-water replenishment happens only by measuring the level in the attic tank. Automated and managed meaning that I fit some sort of a controller which monitors the tank levels and automatically switches the fresh-water replenishment when it senses that the external tank level has dropped. Any thoughts on this matter? For simplicity, I see benefit in the modular approach but if there is a controller which fits this design brief, let me know.

Q3: How is fresh-water replenishment usually achieved? By using some sort of a contorted ball-float of some sort or float switch and solenoid?

Q4: Leak detection around the tank is critical (in my humble opinion) as I really don't want to ruin the house from the top down when something fails.

Some leak detection systems are big-bucks. What options do I have?

Q5: I could design a microcontroller (or RaspberryPi!!!) to act as a leak detector and overall management system, but it needs to be fail-safe to a point and there are a number of unknowns around slapping mini-computers into situations where they aren't designed for. But if needs must a RaspberryPi will go in. Any thoughts or even good systems out there which are easily customisable?

Q6: Isn't design related, but I want to know if I risk having a "green-flush", smelly or off-colour flush if I don't pre-treat the water in some way? Is there a sanitation or dosing system to keep bacteria or algae at bay? UV could be an option I suppose...

Thanks for any comments!!

North West · 09-11-2014 11:43pm

Hi 10-10-20
Did you do a roof calculation to estimate what water you can collect ?
If so how many habitants in house ? Just on using toilets a single person will flush 4 times daily, which is equivalent to 32 Litres of water per person per day and 224 Litres per week and this equates to 11,648 Litres per year. What else would you like to use water for seeing you are building a system ? you will have to size your tank to suit your needs in accordance with BS 8515.
Why I ask is to start at the beginning and build from there. You have your attic sorted, apart from a back up from mains to your coffin tank, it looks like you are now building from tanks to attic. I run my own rainwater harvesting business in the north west so I will be able to discuss with you all needs doing. What kind of budget do you wish to spend, sorry about being a bit personal, it's like everything a budget will let you do the system well without cutting corners.
NW

North West · 09-11-2014 11:56pm

Hi 10-10-20
Looking for a bit of information on your system then I can help you. I run rainwater harvesting services here in the north west.
1.How many occupants in house ?
2.You want to flush 3 wc's
3. What else would you use the harvested rainwater for ?
4. Did you do a water calculation to see how much water you can collect from your roof ?
5.The system you are building will have to be done in accordance with BS8515.

Let me know and I will help you set up your system
NW

10-10-20 · 10-11-2014 11:31pm

Hi NorthWest,

Thanks for the info on the BS, I wasn't aware that there was a standard.
I found this link which was handy: http://rainsavers.ie/data/documents/rainsavers=20guide=20to=20bs=208515.pdf

So from this I reviewed my plan and came up with the following notes:

External Tank:
Tank design to prevent stagnation.
- Mine plan doesn't as I had planned on linking a bunch of water butts together. That's sure to stagnate as I won't create a flow. I'll reconsider 679l bund as my side passage space is limited. (http://www.tanks.ie/cPath/1_66_69/rainew-rainwater-harvesters.html)
Calmed inlet
- If my tank runs low, I'll get sediment being whipped up and into the pump.
Run-dry protection on pump
- I can add that to my list of needs.
Non-return valve on pump
- And this.

External Filter:
Filter organic matter and sediment
- I have a plan for a custom designed filter and primary sediment tank. I've never been happy with the off-the-shelf down-pipe filters, so I'll try to go one better with an home-made inverted Y-junction and SS-mesh. I plan on running the filtered water into a ~10l sediment tank with overflow.

UV treatment not needed.
- Ok, great, but I can add it in if I feel the need.

Internal tank:
Air-gap seperation should be fitted to the replenishment system at the point where both systems meet.
- Yes that was planned and I can achieve that easily.
Overflow protection instead of leak detection
- Well, that's where I was too focused on getting a load of electronics into the system rather than simplifying it.

A suitably sized run-off pipe to the soffit would be easier and more sensible to fit.
Overflow capacity should match the max flow capacity of the pump (at the given height from the pump).
- I'll use 3/4" wavin with a reasonable pitch.

Control Unit:
Required for system operation.
- I'm yet to consider a control unit. Open to suggestions.

In terms of sizing, it looks like this:

2x 9l cisterns.
Occupants: 4x
9l * 4 flushes per occupant * 4 occupants = 144l per day.
1008l per week.
52,416l per year (€130 p/y at a rate of €2.50 per m^3)

I have a third cistern, but I don't think that I can separate the water pipe from the attic feed, but never say never...

Roof sizing:
This is a big unknown. I have a gutter problem at the moment which is in need some work, and if I plan this correctly I may be able to realign some of the existing guttering to double my feed to the storage. I'll review what I have over the weekend and make a decision.

So my biggest remaining questions are about the control unit and the fresh-water replenishment. How is this normally achieved? A ball-cock or a float switch?

Thanks for any feedback.

North West · 11-11-2014 2:19am

Hi !0-10-20. Unable to open link to tanks .ie. What size and dimensions will your tank be ?.
1. On downpipe put in a guttermate (google)
2. Put in a first flush system (google)
3. Use an external pump with a 20Ltr pressure vessel and control switch. You will have to keep pump dry. The idea is your pump will not be on/off every time you need water, The pump will empty pressure vessel first and will then refill vessel. Give your local merchant all the facts and he will size proper pump for you.
4. You will need to use 2 filters after pump. one x 5micron for sedement that still maybe in system and the next is a carbon/odour filter. Then water will pass to your harvesting tank.
5. Check out tanks.ie for top up controller which will be connected from your mains water tank to your harvesting tank, so you will never be without water.
6 If your using the calming piece you can get a filter that will sit in top of tank and do away with the guttermate.

Looking at rainfall stats where you live you rainfall average is between 800 and 1000mm of rain a year. So the formula to estimate rainfall on your roof is:

FORMULA TO DETERMINE THE AVERAGE RAINWATER YIELD ( ARY ) IS;

Rainfall (mm) x Building Footprint Area (m2) x 0.9 0r 0.8 (Tile Roof or Slate/Stone Roof) = ARY
ARY x 0.05 = 5% ARY

FORMULA TO DETERMINE THE AVERAGE RAINWATER DEMAND ( ARD ) IS;

No. of Residents x 16,000 = (1) Area of Garden (m2) to be watered (if any) x 60 = (2)
(1) +(2) = (3)
(3) x 0.05 = ARD

You are not allowed to install a rain harvesting tank that is larger than the smaller of these two figures as the water will become stagnant and a breeding ground for bacteria, including Legionnaires Disease.

The BS 8515 2009 states that the capacity of the rain water harvesting storage tank must be the LEAST of either:

If you have more queries let me know.

NW

Retrofit a custom rainwater system

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