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.