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.

[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]

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]

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]

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]

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]

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]

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/