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Wet or dry rainwater harvesting system?

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.

Water Tank Piping Wet or Dry GIF.gif Water Tank Piping Wet or Dry2 GIF.gif

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Comments

EricL

Hi @Steve5

I'm not a big fan of digging and if there was a way to do the project with less digging that's the option I would go with. Looking at your diagram 14 meters is not that far and can be easily covered with 3 Holman 100mm x 6m PVC DWV Pipe. If you put a supporting post every 1.5m you'll need a total of 9 posts and not all of them will need to be at full height, so you'll need even less.

The pipe can be anchored to the top side of the posts using Kinetic 100mm DWV Pipe Saddle Clips. The pipe run as I like to call it can be disguised using plants, shrubs or built in such a way that it blends in with the surroundings. The best part is that should any repairs be needed; It will be easy to access the pipe assembly.

To add to the filtering system of your rain harvesting system, I suggest putting an Everhard Industries Series 600 Stormwater Pit at the start of the run right next to the shed base. This will catch any debris that the first flush might miss.

Let me call on our experienced members @Dave-1and @TedBear for their recommendations.

If you need further assistance, please let me know.

Eric

MikeTNZ

Hi @Steve5,

First off, what is the advantage in having the tank lower that the feed point from the roof of the shed?

To my way of thinking, you should have the tank near the shed to take into account of the height and extra gravity from the water stored in the tank.

There is no need for having the tank lower, because it will overfill, wasting water.

Having the tanks at a higher level also gives you extra head/pressure available, because of simple physics.

The way we do this in New Zealand, is to take the down-pipes straight into the tank(s) at the top from the guttering, if there is more that one tank, you use a 100mm balancing pipe between the tanks, this is also done at the top.

At the bottom of the tank, you have your outlet(s), from there you take the supply lines.

@Steve5,

If you need further help with this, by all means let me know, I'm an electrician that works with water stuff (among other things).

Cheers,

Mike T.

Steve5

https://www.workshop.bunnings.com.au/discussion/comment/32596#Comment_32596

Hi Mike, We didn't have any other place to put the tank. Property is all sloped, so shed is built on fill for level pad.

Steve5

https://www.workshop.bunnings.com.au/discussion/comment/32594#Comment_32594

Hi Eric,

Yeah, using above ground has the advantages of better flow and easier maintenance. But, I also want to have the option of moving the pipe if I need to bring equipment thru so I would have only used 2 support posts.

How would you set up the stormwater pit?

Thanks

TedBear

Hi @Steve5 , I am wondering how you came up with the figure for 32M of pipe holding still water. If the top of the tank is below the bottom of the shed, then the max length of (buried) pipe that will hold water will be that which is below the level of the inlet into the water tank. Since water finds its own level, anything above that inlet level must run down (and end up in the tank). So it would be a max of 14M of pipe if the inlet was at the same level as the shed's floor level.

You could flush out the buried pipe from time to time by inserting a T piece and a valve in the pipe near bottom of the tank. That would drain the pipe and the feed up to the tank when you open it.

If you ran the pipe down into the tank, the pipe would then only hold water up the level of the water that was in the tank at the time, but the risk of that is that if you had any leaks in the pipe then it could syphon water out of your tank.

But maybe there are things that are unclear to me about this plan and situation?

Steve5

https://www.workshop.bunnings.com.au/discussion/comment/32629#Comment_32629

Hi @TedBear,

There will be separate lines to the tank (2 X 14m) plus 2 X 2m up to inlet; 14+14+2+2 = 32

Yes, there will be flush ports at the pipes as they come out of the ground by the tank.

Not sure how to clean any sediments from pipes below these flush ports if pressure isn't enough from water in 14m of pipe.

Plan is to only put pipe up side of tank and above inlet, not into tank; so inlet screen is not penetrated.

There's also a sun blocking insert but not sure if it will limit flow.

All thoughts appreciated

Steve5

https://www.workshop.bunnings.com.au/discussion/comment/78693#Comment_78693

Forgot to add 13m of pipe that is below the foundation that is from the front side of the tank.

These will be 300mm below foundation so will also be holding water

EricL

https://www.workshop.bunnings.com.au/discussion/comment/103569#Comment_103569

Hi @Steve5

Now that I've had a chance to double check the size of your roof, I take it back the pit might not be able to cope with the amount of water flowing from your shed roof. I should have explained further regarding the pipe layout. Instead of running it along the ground, I suggest installing it up in the air at the same level as the shed foundation but with a slight fall. This will prevent the water from gathering too much momentum while travelling towards your water tank.

This will also allow you to pass through the posts and the pipework as it will be at least 2 meters up in the air near the top of the water tank itself.

If you need further assistance, please let me know.

Eric

TedBear

https://www.workshop.bunnings.com.au/discussion/comment/78693#Comment_78693

Hi again @Steve5 Another possible option, depending on your circumstances..

Perhaps you could install a mini tank at the shed to catch the runoff and feed it directly to the main tank using just one run of 100mm pipe. That way you could get an extra filtration point, less unwanted water stored in pipes, less pipes to run and to manage whether above or below ground.

I couldn't find any suitable example of small tanks at Bunnings, but the link below shows a number of options for small tanks for less than $200.

Since the 2 pipe option holds

~ 250 L of water, you'd only need to store up to 125L to buffer that which would have been contained in the second pipe.

https://mazeproducts.com.au/product-category/water-saving/minitanks/

Steve5

https://www.workshop.bunnings.com.au/discussion/comment/103579#Comment_103579

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

https://www.workshop.bunnings.com.au/discussion/comment/118847#Comment_118847

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

https://www.workshop.bunnings.com.au/discussion/comment/32629#Comment_32629

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

https://www.workshop.bunnings.com.au/discussion/comment/118847#Comment_118847

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

https://www.workshop.bunnings.com.au/discussion/comment/127931#Comment_127931

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

https://www.workshop.bunnings.com.au/discussion/comment/133848#Comment_133848

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

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