Water tanks for the technically challenged

Hi Liliana,

Looking at page 8 on the plans you Emailed, there is a notation at the back of the garage that states: RWH to RWT. This means Rain Water Harvesting to Rain Water Tank. There is also a black line drawn from that point to the tank that undoubtedly is an underground pipe. The diagram however does not show a downpipe but the fourth photo in your second post clearly shows a downpipe. The other downpipe half way between the tank and the garage would, I expect, tee directly into the underground pipe.

There are three downpipes marked DP to RWT. This means that there are possibly four downpipes diverting water to the tank but my guess is that only the three downpipes on the same side of the house as the tank are diverted to the tank. Three downpipes coincides with the builder’s advice and you have the right to know which ones.

The downpipe diverted at the tank appears to be 90 mm PVCu. If the other two downpipes connect as suspected into the bottom of this downpipe to use as a vertical riser, then it is fair to assume that the underground pipe is also 90 mm PVC storm water pipe. This thin walled pipe should not be buried as you have reactive soil but it is allowed.

My main concern is the need for flush maintenance of the underground pipes to prevent stagnation and of course the probable inadequacy of the tank overflow during a storm. The lack of leaf diverters is easily rectified. If the system stays as is, everything that settles on your roof harvest area will wash into the underground pipes.

Inspection openings should be fitted for access but these have negligible flushing effect when they are above the height of the underground pipe. A 90 mm PVC stormwater pipe is measured as an outside diameter, giving an internal volume of 5.8 litres per metre. The recognised minimum horizontal flushing velocity of .6 metres per second is 208 litres per minute. This is more than the two roof areas diverted to the vertical riser would supply during a 1:20 Average Recurrence Interval and of course the vertical riser is VERTICAL, not horizontal. The buried wet system will never flush!

Unfortunately, you need to wait until a storm before knowing how the system and overflow performs. Your overflow pipe, contrary to my earlier post and the advice given to you by the PIC, is probably allowed by the inadequate regulations.

If there are three or four downpipes diverting water to the tank and unless there is some very clever and unique system in place that diverts part of that flow elsewhere during heavy rain, the overflow pipe will not cope unless there is an induced syphonic flow similar to the Tankvac.

The tank overflow pipe needs to divert to a separate stormwater pipe as it cannot use the downpipe at the tank. The plans you Emailed state that the tank must overflow to a legal point of discharge but only one pipe is shown. This was probably done for clarity. There has to be another stormwater pipe that the tank's overflow connects into and it would be good to know its flow path.

Given the size of the overflow pipe, I would be surprised if hydraulic calculations were done when designing the system.

A well designed wet system will be fitted with mosquito proof leaf diverters (aka rain heads) to divert larger organic debris and other pollutants.

First flush diverters need to drain somewhere, either by a dripper or manually. Most also need maintenance/cleaning due to blockages. As it is a ‘sealed’ wet system, there may not be any storm water drain points and this is most probably why first flush diverters were not fitted.

Re siting of tank: burying the tank’s ball valve that supplies the pump is sub standard workmanship but again I don’t think that there would be a regulation preventing it as it is hard to legislate against stupidity. There is also the consideration that the area might be paved in the future and that the valve would need accessing should the pump ever need disconnecting. If the tank is slanting, then this is also sub standard. The problem is that if the tank is re-sited on a slab, it will block much of the window and the pipe work would also need altering.

Given the lack of regulatory protection, one course of action is to have a friendly discussion with the builder about your concerns re the absence of leaf diverters, a flap valve on the wet systems pipe at point of discharge, the absence of inspection outlets needed to access the underground pipes, the way the tank is sited and the unsuitable below ground position of the ball valve. I would also ask for details of the wet system’s underground pipe size and locations in case future maintenance is required.

Perhaps also phone Reece Plumbing and ask for the price of the pump and the mains water switching device that you have been supplied. That will allow you to compare it to the Rainbank and equivalent size Davey pump although I could not see where the model of the Davey pump to be supplied was mentioned in the plans. It may all be futile anyway given your builder’s attitude.

You are faced with certain expenses unless the builder rectifies the botch up. They are...

Re-siting the tank to a slab away from the window.
Fitting three-four leaf diverters.
Replumbing the downpipe nearest the tank.
Replumbing the wet system to the tank's new location.
Fitting IO’s.
Fitting a suitable overflow pipe.

There is also the question about the type of pipe buried given that you are on highly reactive soil and the possible need for future access to that pipe. There is also the inherent flushing problem with a wet system.

A SUGGESTED SOLUTION

To use the current tank...

Have someone lay a slab away from the window and re-site the tank away from the window. Make sure they use some REO to prevent cracking.

The Melro tank is about 2.2 metres high.
http://www.melro.com.au/WaterTanksMelbo ... mould.html

The 2.2 m height plus the height of the slab (perhaps 50-60 mm and also remembering that the tank is now buried approximately 130 mm deep) will have the tank’s height about ½ way up the window! If you then factor fitting leaf diverters and given that the gutters are 2.7 metres high, the available head becomes a problem if diverting up a vertical riser. The leaf diverters would need to be fitted to the fascia and the downpipe diverted under the eave and down the wall. The anticipated head would perhaps be at most a meagre 250 mm. The solution to this and the flushing problem is to use the infeed method employed on this thread: viewtopic.php?f=35&t=60317 This will give a minimum +150 mm head increase. This is the height difference between the bottom of the overflow pipe's outlet to the mid point in the wet system's horizontal pipe after allowing for the rim depth above the tank's top meshed inlet.

I suggest connecting the downpipe at the pump to a 50 mm DWV pipe and connect this pipe via a flexible coupling to a 50 mm inlet fitted approximately 100-150 mm above the bottom of the tank. The additional minimum head this gives would provide a flow capacity well in excess of requirement with a full tank and the cost is not expensive. Eliminating the vertical riser from the flow path also provides for effective flushing during heavy rain.

The flow path for the other two downpipes would be similar but verification of the current flow path and pipe type and size needs to be known before I can suggest further. I currently assume a single pipe is supplied from two downpipes and if so, this would be best connected to a 50 mm DWV pipe for a sufficient flow capacity and flushing with a full tank.

Fitting a syphonic Tankvac system http://www.tankvac.co.nz/ would be one solution to the overflow capacity problem and the Tankvac also cleans the bottom anaerobic zone whenever the tank overflows. Tankvac will not syphon the tank dry as there is a syphon break at the top of the internal overflow pipe. The only query I have is whether your current overflow pipe outlet hole is too big to allow the system to be fitted as the Tankvac uses an 80 mm DWV pipe. This would be no problem however as a second overflow pipe could be connected to the stormwater instead of a Tankvac. I have no commercial association with the Tankvac.

Relocating the tank won’t cost the Earth but it won’t be cheap either. If you follow my suggestion, it will involve fitting 3-4 leaf diverters, the cost of the slab, some plumbing of 50mm pipes and fittings, a new 50 mm inlet valve fitted 100-200 mm above the bottom of the tank and possibly a Tankvac system. Resiting the tank regardless of where it goes is necessary as is the slab and the leaf diverters, the rest simply upgrades the system to overcome the low head and allow the wet system to flush. I think that it is best done at this still early stage and certainly before you pave.

I hope this helps.

EDIT:
If the downpipe at the rear of the garage and the downpipe further along the house towards the tank are the other two diverted wet system downpipes as I strongly suspect, then it is highly unlikely that there will be a separate stormwater drain in this area. As such, first flush diverters are really out of the equation.

For the difference in cost of three first flush diverters, I would buy another suitably sized tank and have it on the far side of the slab next to the other tank. This way, you can have the water diverted into the new tank and have a balance line linking the tanks so that the current tank that will still connect to the pump receives settled water plus you will gain another 2,500 litres of storage. In time, you will find the 2,500 litres of storage capacity you currently have to be frustratingly small.

Hi SaveH2O,

I've just seen your post. I'm going to have to make myself a cup of coffee and have a good read of this! I am overwhelmed by the trouble you have gone to to help me out particularly since you have done it all on your time and without payment. What a contrast with my "sustainable" building designer who won't lift a finger without a $110 prepayment even though she is the one who has got me into this mess in the first place. You are a lifesaver as well as a watersaver and I hope that many other people are helped out by your posts.

I am still trying to work out why they hell they have put a t fitting on the pipe and what that is suppose to do, the water is just going to keep going down the pipe and very little is going to go back into the tank...

The gutter above the tank feeds the aforementioned downpipe while another 3 downpipes connect into the bottom of the same downpipe. The tee is the only escape route and it feeds the tank.

It is a wet system with numerous issues. Unfortunately, woeful and non existent regulations allow such poorly designed systems.

An update on our rainwater tank woes - as we are going to have to wait at least until Christmas now to do something about relocating our rainwater tank we decided to put down some road base in the meantime to cover the mud which is horrendous every time it rains. Because of other issues with the house (the house has not been left with the required 150m clearance to ground level) we have had to do remove some mud to put down the roadbase and found this:





You can actually bury pipes at this depth?





Luckily we didn't get a concreter to do the work or these would have been smashed. They were only under 3 or 4 centimetres of clay.

Hi Liliana,

I still can't believe that they buried the tank's pump draw outlet!

AS/NZS 3500.3:2003 table 7.1 states that...

For a single dwelling, if the (PVC) stormwater pipe is not subject to vehicular loading and there is no paving, the pipe must be buried a minimum (from the finished surface at the top of the pipe) to a depth of 100 mm.

For other than a single dwelling, it is 300 mm.

Hi SaveH20

Thanks for that piece of information. The area is unpaved for the moment as you can see but, if it were to be paved, as the building designer advised us to do in her final report on our build, does that mean that the pipes could be buried higher than 100mm? Though I don't really see where a layer of concrete is going to fit between the pipes and the tank's ball valve!

The standards as per table 7.1 state as per the above plus...

(b) with pavement of brick or unreinforced concrete
50 mm below the underside of the pavement.

One problem I see with what you say is that the pipes were laid a long time ago and there were no plans to lay paving at the same time as the pipes. Also, who is to say that you may not have changed your mind and planted lawn. I would think that the regulations as to the situation at the time of laying the pipes should have been followed.

The second pipe interests me to say the least!

The ball valve must not be buried or concreted over but there are no regulations against this as far as I know. It is hard to legislate against such stupidity.

There was no paving shown on the plans but I can now see why the building designer put in the final report that we needed to pave with fall away from the house. This was also to give the builder an out if the tank falls over since it was never put on a concrete base.

I'm presuming that one pipe is the overflow pipe from the tank and I thought that the second would be the pipe from the downpipes feeding the tank. But now you've made me wonder. Shouldn't all these pipes appear individually on a drainage plan? Doesn't the building surveyor have to inspect the pipes to make sure they are at the correct depth?

The ball valve had to be buried so that the builder could say they had buried the pipes at the right depth. All of this to take advantage of our ignorance.

The BCA regulations require a fall of 1:20 away from the house for the first metre.

The building inspector would have to dig down to see if the pipes are at the correct depth and I don't think that they would be doing that.

One pipe has to originate from the downpipes at the garage end (top of photo) and connect to the other wet system pipe coming from the other downpipes on the other side of the tank. A plain tee would then connect the common pipe to the downpipe / vertical riser at the tank to transfer water to the tank's top meshed inlet. The common pipe's required fall of 1:100 (10 mm per metre) and the height of the common pipe at the tee however makes the validity of this (assumed) installation let's say...interesting!

The above would also mean that the buried pipe nearest the tank is the tank overflow pipe. The overflow pipe however was advised as having being connected to a drain on the neighbours property. To do this, it would have to be plumbed under the parallel pipe but I am wondering why the pipe wasn't plumbed under the other pipe and directly to the fence.

You wouldn't have to remove much dirt to discover the flow paths and this is best known for when it comes time to fix the tank installation problem.

They buried the tank because of regulations regarding the tank's proximity to the window.

Hi SaveH20

You are right of course. They did bury the tank to stay clear of the window.

On the issue of the stormwater pipe draining to the neighbour's property - that is an assumption my husband made when we failed to get an adequate explanation of our drainage from either the builder or the building surveyor.

Any more excavating will have to wait until my husband returns in a few months - I'm not prepared to tackle it on my own with all the mud and having laid road base over everything to try and minimise movement of the slab over winter.

Hi Liliana,

I would be interested to know how high off the bottom of the tank the bottom of the ball valve is.

Burying the ball valve would make no difference to the depth the pipes were buried.

If the wet system/stormwater pipe has the proper slope, why is the pipe so high at the tank?

According to Melro the bottom of the ball valve would be 25mm from the bottom of the tank. Okay, so this is why I called this thread "water tanks for the technically challenged" - so the ball valve is not that blue lever thing you can see in the photo?

The lever you see is part of the ball valve. The lever turns the internal valve to open and close it.

http://sunshoweronline.com.au/pipe-fitt ... valve-15mm

Fitting the pump draw valve 25 mm from the bottom of the tank is way too low as the pump will ingest sediment. A lot of people in the industry have a lot to learn unfortunately. I see very few installs that are not sub standard.

Don't worry about the ball valve's height above the bottom of the tank as the best solution to the rainwater harvesting plumbing will involve using a second settling tank.

I wonder how great a house one could make, if you got an architect with passive house experience, some good ideas, and hired bashworth (anewhouse), Fu Manchu, and saveh20 to help with the pitfalls and the general and specific design. It would be the best house ever!

But then people would complain they are being ripped off by the custom builder as they can build a house from project builder X for $8,000 per square and still expect a quality house.

LOL, true, but I still would like to be able to hire them all for a project where money was, if not no object, then at least not so important = they would have the freedom to design/make something that was mostly decided by "what's best".

One can dream

Well, we paid over $20,000 a square including professional fees for our custom built house and we didn't get quality either. I've seen people on here with volume built houses who have better quality. So if only we had known SaveH20, Bashworth and Fu Manchu at the time we were designing it, plus a few other people on this forum, I wouldn't be feeling so enraged now.

Hi Liliana,

I have just been looking at the earlier photos that you posted and noticed a couple more things with your rain harvesting system.

The pump is sited higher than the tank's pump outlet. This means that the water will drain from the pump when the tank's water level falls below the pump's suction inlet unless the pump has a check valve fitted. Most pump do but some don't and it would pay to check this. When everything is redone, it would be best to have the pump flooded if convenient.

We discussed on another thread that the watertank pump is not connected to the toilet as it is an estate requirement that toilets are connected to the recycled water that is supplied to the estate. Unfortunately, this also means that the "Rainbank" automatic mains water switching device was an unnecessary expense as an automatic or manual mains water switching device is mandatory only when the watertank pump supplies water for sanitary flushing. This is too late for you but maybe someone else reading this will benefit.

Hi SaveH2O,

Thanks for the additional advice. An, yes, hopefully these bits of information will help out a few other people too.