Wet or dry rainwater harvesting system?

[Steve5]

I’m having a rainwater harvesting setup installed on a shed so I’m looking at the pros/cons of burying pipes versus routing them above ground.

The shed is 13m X 15m with 100mm down pipes on the ends.

The water tank is located down the hill from the shed so the top of the tank is below the level of the shed foundation.

The tank is about 14m from the shed

The wet system is easier to work around and looks better with the pipes hidden but has lower flow since water pressure is needed to push water up to the tank inlet.

An online calculator indicates 32m of 100mm pipe will hold 251 liters of water before any gets pushed into the tank.

Not sure how to calculate the wet versus dry system flow rates.

One online forum indicated the water trapped in the pipe between rain events will become stale due to lack of oxygen.  Anaerobic.

Sediment will collect in the buried pipe even with first flush and leaf eaters on down pipes so needs flush ports.

The dry system has higher flow but has exposed pipes that need support when routed thru the air. 

Pipes from store are all curved a bit so challenging to build and support a straight 14m run to the tank.

On line forum suggested there will be maintenance issues with long above pipe runs supported on posts.

Any thoughts or experience much appreciated.

[Steve5]

Hi Ted,

Adding a mini tank is interesting.

I have been thinking about joining the front and back 100mm pipes to only have one 100mm pipe run to the tank.

Originally, I wanted 2 100mm pipes going to the tank especially if a buried pipe wet system.

But, 100mm of flow seems like it should handle most rain events.

Gutters will probably overflow anyway with major down pour with one or two 100mm pipes going to the tank.

I've read with major rain the down pipes still aren't full when the gutter overflows, something to do with an air gap in the middle of the down pipe.

Any thoughts?

Thanks

[Dave-1]

Afternoon @Steve5 

Whoops, somehow missed this question

Sounds like it will be a fun project.

Just to play catch up, I liek the above ground option for a tank, water pressure wise, non digging a hole and ease of fixing things if something goes wrong all factor in.

I have replaced my old 90mm stormwater pipes with 100mm pipes. My gutters still overflow due to leaves and amount of rain falling at times. The stormwater pipes dont seem to be anywhere near capacity if that helps. I havnt heard of bubbles in downpipes, just leaves. 

Thinking about the last question from you, my only concern and its not really a concern but if the one pipe gets clogged then you loose all the water in that instance compared to pipes from eiother side at least you will catch half?

Dave

[MikeTNZ]

Hi @TedBear,

The way I've always done tank fills is through the top side of the tank and the outlet (suction side) is always through the lowest point in the tank, to use the most capacity of what you have.

Any other way with fill points, you end up with water back-feeding into the fill inlet, unless there is a non-return valve before the tank.

However, a lot of non-return valves won't open, because of the spring pressure in the valve itself, unless under pump pressure because of the head of water in behind the valve plug.

Just a thought....

Cheers,

Mike T.

[EricL]

Hi @Steve5 

I would probably recommend staying with the two-pipe configuration it will provide a better transfer rate than a unified pipe that will get blocked up and overflow. In regards to the wet system, I don't recommend it as bacteria will tend to breed inside the pipe especially if there has been no rainfall for quite a while. 

Eric

[TedBear]

Hi Eric,

the reason I suggested the upper holding tank combined with a single feed pipe to the main lower tank was to halve the construction issues that were being described. I agree that the dry system would be better as it wouldn't have the issue of holding water, but the delivery capacity of a single pipe shouldn't be a problem since the water is flowing into a tank and isn't needed for instant use. 

The upper tank would hold at least the quantity of water that would normally be flowing through the second pipe until it can be delivered below, so I can see logical no reason that holding water in a small tank would cause overflow or blockages any more than if both pipes reached capacity during delivery. All the water would flow into the lower tank providing that the bottom of the upper tank is higher than the inlet of the lower tank.  The way I picture it, an overflow pipe would be added to the upper tank, which would direct any overflow down the hill away from the shed base.

[EricL]

Hi @TedBear 

My apologies, I should have explained further. I was referring to the possible configuration of joining just the two downpipes into one. I wasn't referring to the upper holding tank (which by the way I think is a great idea). I was under the impression that the pipes were going to be joined together and then run straight all the way to the main tank below. Steve5 had made a query earlier regarding joining the down pipes together.

If a primary tank were to be placed at the top, I totally agree that a single line would be possible. It will technically act like a giant first flush system since all the debris would land in this first tank.       

Eric