Water tank and pump configuration

aaronmurray1987 · Jul 18, 2018 11:27 am

Good morning all.

Long time reader first time poster.

I am after some advice on configuring water tanks for a build in victoria. We have a half acre building envelope that slopes down left to right.

I am going to have a large shed. 190m2 with a large 90k water tank in front of it. This will be situated on the higher ground. About 7 or 8m away will be the house which will be 130m2. This will be downhill from the larger tank so i was going to have 1 or 2 smaller tanks (2k tanks) situated next to the house.

My first unknown is is it better to have the smaller tanks on the lowest section of the house so all home downpipes can flow into the tank. 1 or 2 pipes on the other side of the house will have to go around the house.

Now with transferring water is it best to have a transfer pump operated by floats at the smaller tanks transferring to the bigger tank. Or is it better to have a solonoid valve gravity feeding water from the 90k tank to the smaller tanks.

Which ever way i go i will have a variable speed pump distributing the water into the house.

Any tips greatly appreciated.
Cheers

SaveH2O · Jul 18, 2018 4:08 pm

Will you also be on mains water? I am assuming not.

I am also assuming 3 or (possibly) 4 downpipes at the house.

aaronmurray1987 · Jul 18, 2018 8:19 pm

SaveH2O

Will you also be on mains water? I am assuming not.

I am also assuming 3 or (possibly) 4 downpipes at the house.

No mains water just tank water.

Yes 3 downpipes total. 1 on either side of the house (2 total) with a butterfly roof and box gutter down the middle. Then 1 downpipe on an alfresco roof.

SaveH2O · Jul 19, 2018 8:27 pm

Would you consider having one of the two tanks at the house larger than 2,000L?

I don't know your average annual rainfall but your catchment area totals 320 sq m. This will give you a yield in Victoria of about 85%. For every 100mm of rainfall, you would collect approximately 320 (sq m) x 100 (mm) x 0.85 (yield) = 27,200 litres or about 74.5 litres per day.

For rainwater harvesting best practice, I would not pump water to the 90k tank and I would use one of the two planned 2,000 L tanks as a second settling tank that captured most of the water from the house. Let's make number 1 the 90k tank, number 2 the settling tank and number 3 the tank that supplies the pump.

I am assuming that tanks 2 & 3 are on bases that are the same height.

I would also use leaf diverters to prevent solid bird droppings and other muck washing into the tanks. The only leaf diverter that I recommend is the ICON Leaf and Debris Controller, it is $38 at my local Big Green Shed. Beware of other leaf diverters that do not fully drain or shed leaves which causes splash and yield loss.

TANK 1
This would obviously be supplied by a dry pipe infeed but do not have the top meshed inlet above the outlet that supplies water to the bottom 2,000 L tank. This may sound obvious but I see this mistake all the time.

If you intend fitting an overflow pipe to this tank, then also have the top meshed inlet fitted within arms reach of the overflow outlet so that you can access the mesh for cleaning.

You only need a small pipe connected between the larger tank and the bottom settling tank. This will retain less water and turn the water over faster plus the faster flow will purge the pipe of any possible (but not probable) debris. To draw the best quality water, the outlet valve should be about 300mm above the bottom of the tank.

You would also most probably want a larger outlet valve fitted close to the bottom of the tank for other purposes. Are you in a bush fire zone?

Rainwater is naturally acid. To raise pH, you can put a heap of limestone chips in a nylon bag and lower it to the bottom of the tank. Just tether a buoy to a line for access if ever required.

Also cut out an oversized piece of shadecloth and place it over the tank's top meshed inlet to make cleaning the mesh easier and to also restrict sunlight ingress.

TANK 2 SETTLING TANK
This would collect from two downpipes and I assume that the box gutter will harvest a larger roof area that either of the other two downpipes. Yes/No? If Yes, it would supply the settling tank.

The purpose of a settling tank is to supply the tank supplying the pump with good quality decanted water.

I would have a valve fitted 200 mm above the bottom of the tank which would cut in at 450mm and fill the tank to about 500mm. This valve would receive water from the 90k tank.

An outlet valve would also be fitted about 200 mm above the bottom of the tank and this would supply tank 3.

Because this tank would receive water from 2 downpipes, it must have enough mitigation to store water from a reasonable amount of rainfall. If the tank was 2 m tall, its mitigation would be approximately 1,500 L.

TANK 3
The inlet valve from the settling tank would be about 200 mm high, the cut in would be 400mm and the cut out would be about 450mm.

This tank would be supplied by one downpipe and the valve's top up levels would ensure that minimal water from the settling tank entered when it was raining.

The 400mm cut in also ensures that the pump would not be drawing a free surface vortex.

A variable speed pump is ideal for off the grid use and well worth the additional cost. Highly recommended.

I haven't mentioned first flush diverters. If you choose to have these, just be aware that having one with a dripper will result in a lot of yield loss, you need to have one that is manually drained which also enables you to use the captured water.

I hope that this helps.

aaronmurray1987 · Jul 19, 2018 10:00 pm

Thank you for the reply. Greatly appreciated.

Yes we can have greater then 2000L tanks at the house. Probably up around 5k will be fine too.

"This would obviously be supplied by a dry pipe infeed but do not have the top meshed inlet above the outlet that supplies water to the bottom 2,000 L tank. This may sound obvious but I see this mistake all the time."

I am a little confused by this. Are you saying have the inlet from the downpipes to the large tank lower then the outlet??

Yes in bushfire zone. Will have cfa outlet on 1 or 2 of the tanks.

The Box gutter will collect roughly 90m2 of the 130m2 roof so yes tank two will catch most rain water from the house.

I am thinking that you are saying to have solenoid valves between tanks 1-2 (located closer to tank 2) and also another solonoid valve located in between tanks 2 and 3? Is this correct? Do you have recommendations for a solenoid valve and/or floats?

Thank you again for all your help.

SaveH2O · Jul 19, 2018 11:09 pm

aaronmurray1987

Yes we can have greater then 2000L tanks at the house. Probably up around 5k will be fine too.

The settling tank will need to be larger to accommodate the inflow, if not, you are restricted to either pumping to the 90k tank or else overflowing somewhere. Much easier to have a larger settling tank with good mitigation.

aaronmurray1987

"This would obviously be supplied by a dry pipe infeed but do not have the top meshed inlet above the outlet that supplies water to the bottom 2,000 L tank. This may sound obvious but I see this mistake all the time."

I am a little confused by this. Are you saying have the inlet from the downpipes to the large tank lower then the outlet??

No. What I am saying is that if the tank's top meshed inlet (where the water falls in) is fitted above the bottom outlet valve that will supply water to the number 2 tank, the water falling in this area will stir up any sediment when the tank's water level is low. It is better to have water falling through a tank's top meshed inlet that is well away from that area.

Actually, the water level doesn't have to be all that low for it to cause sediment resuspension due to the generation of impacting toroidal vortices. I can post some information about this subject if you like, it is quite fascinating.

With regards to the outlet valve, a lot of tanks have the valve that supplies the pump fitted very low on a tank which then draws the worst quality water and any suspended sediment from the tank. It is a common yet obvious fault.

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aaronmurray1987

I am thinking that you are saying to have solenoid valves between tanks 1-2 (located closer to tank 2) and also another solonoid valve located in between tanks 2 and 3? Is this correct? Do you have recommendations for a solenoid valve and/or floats?

Topaz had a nice small valve that operated with low pressure, they might still make it.

There are different valves, the one fitted to tank 3 just needs to supply a water level that is no less than 200 mm above the valve that supplies the pump and the tank 2 settling tank level should be a bit higher than tank 3. I prefer a set cut in and cut out height because it gives a good continuous flow through the pipe but that may perhaps be a bit pedantic. The required mitigation capacity of tanks 2 & 3 will determine the cut out levels to a large degree.

EDIT: Most solenoids need a minimum 20 kPa pressure to operate but maybe there are some new ones that don't. I think that a manual valve is the way to go but you need one that operates on very low pressure. The critical one is tank 3.