Another Rainwater Wet System Thread

I have been waiting for a quiet moment to answer this and I also have some queries.

Firstly, you have supplied excellent information which makes replying much easier and you appear to have a good understanding of best practice which is rare. I dips me lid.

Minimum roof harvest during a 1:20 ARI 5 minute average intensity:

House:
419 (sq m) x 2.58 (mm/min) = 1081 lpm.

Two 100mm (104 mm Internal Diameter) DWV pipes. These pipes hold 8.5 litres of water per metre and a velocity of one m/sec equates to a flow rate of 510 lpm. A 100mm DWV pipe with a 0.5 metre head will flow at one L/sec (510 lpm) with 56 metres of friction loss. I have used a 0.5 m head rather than a 0.6 m head for a couple of reasons including that air in the top of the downpipe lowers the atmospheric pressure in that area plus I don't know the height of the vertical riser above the tank.

It is very difficult to accurately calculate what your friction losses will be through each pipe because each tee and elbow must be calculated as an equivalent pipe length but the downpipes further complicate calculations because these also serve as charge points. Are the pipes already plumbed? If not, then plumbing 45 degree junctions rather than 90 degree is much better.

The pipes are servicing 4 and 6 90mm downpipes each, do you know the roof area that each DP cluster services?

Unfortunately, an approximate volume of water carried through each pipe during differing rain intensities also cannot be accurately estimated even when the two respective roof areas are known due to the effect of wind driven rain. If for example the two roof areas were equal, during wind driven rain, one roof area could have an additional 20% of rain and the other roof area have 20% less rain compared to when there is no wind. Do you know your roof pitch?

Does the tank have a flat roof?

No problems there.

I assume that each 100mm pipe is intended to have a 50mm low restriction inlet. These inlets are multi purpose and a successful carry over from our Supadiverta development program. They work great by prioritising flow of highly oxygenated low density water to the anaerobic zone which sweetens the water plus they also supplement the inflow capacity which you need to do. Water however must pass through mosquito proof mesh before entering the tank and leaf diverters are usually fitted to achieve this but as you have rightly pointed out, you can't sacrifice the limited hydraulic head.

The 'lift out' Mozzie Stoppas only fit a 90mm stormwater pipe and I could never recommend fitting them to a downpipe due to most grit etc being washed from the gutters during heavy rain. Flow through filter mesh in an enclosed flow path is referred to as an entrapment filter surface due to it sacrificing open area as it traps detritus and 955 micron filter mesh already has an open surface area of not much more than 50% which will restrict the flow rate, something you definitely don't want!.

If you plumbed sediment traps in the wet system (see further below) to collect most of the bed load, you could then have the two 100mm DWV pipes 'approach' each other at the tank and have three tees in between. All tees would have their branch socket facing upwards, the two outer branches becoming vertical risers and the middle branch reducing to a 90mm stormwater fitting and flexible coupling that was connected to a 90mm inlet instead of having two 50mm inlets. This would allow a 90mm mozzie stoppa to be used on the 90mm supplementary inlet.

Special attention will also have to be given to the tank's overflow pipe because a standard overflow pipe has no hope of discharging your amount of inflow during heavy rain if the tank was full. Increasing the overflow capacity is an easy matter when you know how but you would still need two 100mm overflow outlets or a single 150mm outlet.

Most are fitted at the top of a tank's vertical riser, acts of stupidity that make them useless and a waste of money. Plumbers and even hydraulic engineers just don't understand that the first flush will only fill with the settled water in the riser while the first flush is still in the downpipes at the other end of the wet system!!!

The drippers also waste a lot of water, most waste about 4-6 litres per hour each.

Diverting the initial dirty roof wash is ideal but it is also best if the first flush can also be used and the wasteful dripper eliminated.

You have 10 downpipes at the house. I would like you to consider having a simple first flush diversion at the downpipes and have one common sub surface pipe harvesting all 10 first flush pipes and have that pipe diverted to a single sealed tank via a 40 or 50mm DWV branch pipe. That tank would need a small vent pipe that was higher at the top meshed vent than the height of the house gutter. Having this arrangement will allow you to divert an exact amount of water to a single tank for later use. The tank below isn't set up as I have described but it shows the general principle.
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You will need a good strong small tank because the water column in the vent tube exerts a reasonable amount of pressure.

Alternatively and because the pipes will already hold plenty of first flush, you can just use connected pipe work with a drain valve at the low point. Depending on the intended use of this water, fitting a sediment trap may also be wise. Capturing the first flush this way will be easier, more efficient and cheaper.

These are cheap and easy to install (I asume you are referring to my design) and they work wonders plus they can be used as an emergency overflow should the tank fill and the overflow pipe doesn't have a discharge capacity equal to or greater than the inflow capacity. Just make sure that they are installed at least +5 metres past the last area of turbulence (tee or elbow) so that the suspended sediment has had time to settle as bed load.

Below are links to two sediment trap mentions by happy Homeone members.
viewtopic.php?p=1146540#p1146540
viewtopic.php?p=1551215#p1551215
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I don't recommend using the pipe reducer anymore as they can accumulate sediment. Using an invert taper is much better.
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Automatic switching devices bleed pressure, do you know what sort of dynamic head you will have? If you know, give the manufacturer a call but it doesn't sound good.

Do you know the long term average rainfall?

You have a large 569 sq m roof and you could do a few calculations of rainfall variables using 0.85 as your expected yield. Using Maitland as an example, their average 932mm annual rainfall calculates as 569 x 932 x 0.85 = 450,760 litres captured annually (which will vary) less first flush which could be retained for use anyway. This is a daily average of 1,235 litres!

Because you are having additional rainwater filtration which would have an easy life, I would be looking at plumbing rainwater to the toilets, laundry and HWS via a dedicated rainwater pipe and only using cold mains water with its own line for basins, kitchen and showers. This would eliminate the need for a switching device.

Your tank will have high quality water delivered to it but if you want to draw the best quality water in the tank to the pump, you can get an irrigation place to make a floating filter inlet hose that draws the best quality water in the tank that is found 200mm below the surface.

A Homeone member recently set up a top class rainwater harvesting systems for a large vegetable garden and he had his own floating inlet filter hose made, you can read about it and see photos in the link below. It's evolution is discussed on pages 3 & 4.
viewtopic.php?f=35&t=97686&hilit=Jomo&start=40

It is paradoxical that most rainwater harvesting systems divert water to the top of the tank and water is taken from the anaerobic zone at the bottom of the tank whereas best practice does the opposite.

Have you considered using a variable speed drive pump or a large pressure tank to save on pump wear and tear? A lot of pump use is to slow flow short duration end fixtures like toilet cisterns and washing machines. Short start/stops are what reduces a pump's life span.

This is indeed unfortunate and unfortunately also common practice. The friction loss through a tee's branch is horrific yet a 100mm DWV 45 degree junction should only cost about $1.00 more than a tee.

NOTE: The DWV tee branch friction loss is not as severe as the figures below because the flow path does have a small bias towards the run.
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This highlights a benefit of having the two pipes converge at the tank as the flow up the risers will be pretty much equalised.

I'm not but with 'some' reservation. Is the top of the riser above the tank?

The 1:20 ARI figures are the minimum qualifying intensity, I always design for at least 20% more flow and even this is the bare bones although most gutters will overflow before this intensity is reached.

A 22.5 degree roof is factored with a 1.21 multiplier allowing for wind driven rain when calculating eaves gutter 1:20 ARI compliance. If you calculate the flow from 60% of the roof with the 1.21 multiplier during a windy minimum qualifying 1:20 ARI (419 x 0.6 x 1.21 x 2.58 = 785 lpm), the required wet system flow rate with the available head is unobtainable. A ball park calculation to remember is that it requires x4 the head to double the flow rate.

A saving grace is that not all of the 60% roof area will be facing the weather.

Ok...second thoughts.

There is available head space where they could be fitted and accessible for maintenance prior to rain but you could not fit them "almost directly under the gutter" due to the gutter pop being inside the downpipe.

My Q. How quickly would they block and how frequent would the maintenance requirement be?

My first thoughts are the number of instances I have heard and know of where a water tank's horizontal meshed overflow outlet has blocked. Those blockages however have been the exclusive domain of organic matter adhering to the overflow's filter mesh whereas a downpipe is vertical. A rider here however is that the water will initially flow down a downpipe's inner wall, not freefall down the core as many believe but I think that mesh on a vertical pipe is more likely to be cleaned of organic matter to a greater degree than mesh in a horizontal pipe is.

I have also heard of incidents where Mozzie Stoppas have blocked when fitted to a tank's direct inflow pipe but ALL of those incidents were on standard wet systems.

Unfortunately, I have never done any testing of these scenarios but my primary concern remains the capture and hold of 1-2 mm accumulated grit particles that are flushed from the gutters during heavy rain more so than a possible but not probable build up of small organic matter on mesh in a vertical pipe. It must be remembered that gutter mesh promotes the accumulation of dirt which reconstitutes due to gutter mesh restricting the normally turbulent cross flushing from front to back during heavier rain.

If you have half round gutters that have superior flushing characteristics, I would say to give the Mozzie Stoppas a go but if you have quad guttering, I could not recommend it but the final decision is obviously yours. Maybe trial the 4 downpipes on the lesser roof area first.

If you look at the blocked 90mm outlet filter mesh in the photo below, you will see that the offending material on outlet overflow mesh is organic material that floats off the water surface.
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Fantastic! No drainage problems there.

What is the distance between the bellmouth crest and the top of the tank?


Unfortunately yes.

If you don't have possums or rats, maybe do away with the first flush and install a floating filter inlet instead...as well as the sediment trap ofc.

Yes, you have the availability of town water to use as a top up if needed and the plumbing will be simpler with no switching devices and backflow prevention valves etc.

I have seen the Scala2 advertised (from memory) for less than $700 which is incredible value for a VSD. Some people have trouble adjusting the pressure though but I believe this is due to not also adjusting the small pressure tank. There were also leakage problems with the earlier units but the problem was redesigned and all units sold were offered to be replaced. You use to be able to get a 4 year warranty but I don't know if that still applies.

There is a guy on the Whirlpool Home forum who posts as paulvk, he seems to have good intimate knowledge about pumps. Maybe start a thread on there asking for a recommendation for a VSD pump.

With the assistance of the low restriction inlet this should be less of an issues the more empty the tank is. So it's really decent rainfall when the tank is already full or close to full isn't it.

I am still concerned about this scenario though. Originally i had planned on having an extra 400mm head available however once the earthworks were completed for both house and tank this disappeared.

There is one more option that has it's pros and cons of course, and that is to re-site the tank further downhill to achieve the required height differential. The tank has not been constructed yet however the pad has been cut and prepared, plus all of the pipework has been run to this location so there would be a decent cost. At this location the block falls at approx 1:18 so I could achieve an extra metre of head by moving it 18m further down the hill. This would mean an extra 18m of trenched pipework, power etc to the new location. Major issue there is that it would then fall into the area zoned as the "reserve" effluent management area. This is the secondary area designated for irrigation via the OSSM system. Not sure how flexible this is to be moved but will talk to the builders about that.

So the big question is what is the ideal height differential from eaves to the top of the bellmouth overflow given all of the other system data stays the same but with a further ~20m of underground 2 x 100mm DWV piping?

I understand what you are saying and agree it has the potential to require fairly regular maintenance. It's definitely quad guttering.


I think I'm still half keen to give these a shot in the downpipes. Even if the tank gets moved as above it sounds like a relatively low cost option to trial and if they don't work then I can either just remove them or install a proper leaf diverter, pending on what height differential I manage to achieve. I did some very rough calcs based on the closest process I could find on the web to try and estimate what the pressure losses through the mesh screen may be and I came out at around 120mm head loss at a flow rate of 2L/s. Absolutely not exact science but means potentially could be less detrimental to the system flow than the head loss suffered from fitting leaf diverters. Certainly the aesthetic would be superior to the leaf diverters if nothing else.

Still waiting for them to get back to me on this one... I asked the same question a week ago.


Yep, thinking this is the way I will go. Especially since all tank water to the house will get filtered through one of these Water Filters also.

Thanks for the recommendation. I'll start dealing with the pump once I sort out the tank haha.

Sounds reasonable, I'll see if I can make that change to the tank. They will be getting sick of me changing my options by now.


Yes, correct. The reserve level will be ~500mm up the tank wall, this is where the pump outlet will be located. The tank salesman was convinced that I could not fit inlet valves within this reserve area of the tank so the extra inlets were optioned at the pump takeoff height also. However I phoned the local RFS and they advised as long as they were only inlets that they did not have an issue wherever they were fitted so I will be getting them positioned 150mm above tank base.



I'm thinking of branching the two 100mm pipes together to help balance flows between the two roof sections and reduce frictional losses due to excessive flow rates. I was only planning on having a single low inlet for the 2 x 100mm pipes from the house and a second one to service the shed stormwater. As you mention above it may be best to make the house inlet a 65mm valve.



This sounds like more surface run pipes to me, as well as an extra tank near the house? Definitely something I'm trying to avoid to be able to get the balance between function and aesthetics. If I connected the 2 x 100mm pipes to a single 150mm pipe run for the extra distance then frictional losses would be minimised. It would mainly be the extra fittings contributing. An 18m length of 150NB pipe flowing at 1081lpm would contribute ~100mmH2O in pressure losses. So effectively I could achieve an extra 1000mm head while sacrificing potentially 2-300mm extra frictional losses. It will cost a bit more upfront but I'm thinking more and more that it may be worth it to save potential long term pain.



Yes it will be above the tank wall height. So height from base of tank to discharge centreline would probably be ~2.3m. This would place it ~400mm below the eaves, something I get more concerned about everytime I think about it.


Yep this i where the problem keeps coming back to haha. Even if I was happy enough for the gutters to overflow during heavier rain events when the water tank is already reasonably full, I still need to find a workable solution to exclude mozzies.

If I leave the tank in it's currently planned location then I can't see that I have any other option but to use the retractable mozzie stoppa style mesh at the downpipes to screen for insects. If they were to be an epic failure due to restriction of flow and buildup of material, which there is every chance they could be haha, then I'm leaving myself with no simple fix aren't I? If I can increase the height differential then at least I have the option to fit traditional leaf diverters if these were to be a failure.

Also speaking of overflowing, in your experience have you been able to identify a rule of thumb style guide to what sort of rain inflows standard quad guttering can handle before they overflow, regardless of the resistance in the downpipes and stormwater system? I imagine this is a possibility as a 115mm gutter cant transfer an infinite amount of water. Would be a good rough figure to have, although I'm sure near impossible to get, as then we would know a ceiling limit to design to.

Thanks again for all your help SaveH2O.