How does this rainwater system work

Hi all,

Q. How does rain water get into the tank?
The only place I can see is though the pipe coming out of the ground up into the tank inlet. That must require a whole lot of pressure to push water up through there?

The garage is behind the water tank.
The gutters from the main house is about 5metres higher than the top of the garage roof.

Can anyone spare a moment to give me a brief run down of how this system would work. I've seen a few diagrams on the Internet but they/re not quite the same.

Nevermind .. Rainwater | YourHome - charged system by the looks of it

The installation is substandard with several areas of concern. Do you want to know further?

The Vic Gov link also shows some amateurish installation mistakes that no one should copy.

Yes please.

Yes, you have a wet (charged) system. It is called a wet system because the pipes are always full to the height of the vertical riser that discharges water to the tank's top inlet.

Apart from one possible non compliance issue I see, your rainwater harvesting system is compliant but compliance doesn't mean that it is not substandard. As I review the system, just keep in mind that just about everyone has the same issues that fall well short of best practice. I can however offer remedies.

It looks like you have a 5,000 L tank. Do you know how many downpipes are diverted to the wet system and the approximate roof areas harvested?

WET SYSTEM:
A wet system allows water to be transferred underground to a remote located tank which is good but a wet system also builds up sludge in the underground sump unless flushed. The water retained in the pipe during rain events also has no water/air interface and bacterial activity that breaks down any trapped organic matter also depletes the water of oxygen. In time, this water can become stagnant but I can't see any flush provision at the bottom of the vertical riser that allows the underground sump to be flushed. Flushing is best done during heavy rain provided of course that the water can be flushed away from the tank.

FIRST FLUSH DIVERTER:
The pipe that diverts off the vertical riser near the top is called a first flush diverter. The pipe is 100mm DWV (ID 104mm) which holds 8.5 litres per metre and looks to be about 2 metres long. A first flush diverter captures the first dirty roof wash aka the first flush of polluted water that drains down the downpipe. Nearly all have a continuous drain through a small tube at the bottom of the diverter so that the pipe is empty when it next rains and many of these produce a continuous fine stream of water that drains at 4-6 litres per hour. The first flush diverter is useless when fitted to the top of a vertical riser because the first flush will still be in the downpipes at the other end of the wet system after the first flush diverter has filled with the settled water retained in the riser. Together with the wasteful 'dripper', yield is often as low as 60% in areas like Melbourne metro which has more than 30% of the average number of annual rain days recording falls of less than 2mm. The first flush is totally useless where positioned and is best removed.

MOSQUITO PROOFING
The end of the horizontal discharge pipe at the top of the vertical riser appears to have a flap valve on it. This is best practice as it prevents mosquitoes and other pests from accessing the standing water. Best practice also has mosquito proof leaf diverters fitted to the downpipes to prevent the downpipes becoming a mosquito hatchery, to prevent most organic matter from entering the pipework and also to serve as an air gap should the wet system sump be damaged and collapse.


EDIT: Are leaf diverters fitted to the downpipes? They are mandatory for wet systems in most northern regions but not so further south.

OVERFLOW PIPE:
The overflow pipe drains into a bucket type mosquito proofed meshed rainhead but being so low, it offers little hydraulic head which could be an issue during a heavy rain event. It should be raised. I suspect that there has also been ground movement in the past that has damaged both the rigid vertical riser and the overflow pipe which I suspect is the reason the rainhead is used. If the soil is reactive or otherwise unstable, expansion joint couplings should be fitted to pvc vertical risers and pvc overflow pipes. QLD regulations however do require an air gap to be fitted to the overflow pipe and the arrangement your overflow has does satisfy the air gap requirement but it looks like an afterthought or maybe it is just a very amateurish bit of work.

The overflow pipe is also fitted very high on the tank, this is a new trend to maximise water capacity after many purchasers realised that their tank never held the storage capacity advertised once the overflow pipe was fitted. The problem is that the overflow outlet flows faster when there is more depth above the outlet's invert plus it greatly reduces the tank's mitigation capacity. I am interested to know how much roof area drains to the tank and your area's 1:20 Average Recurrence Interval (ARI) so I can advise on whether the tank's overflow capacity is sufficient and how to further increase efficiency if the current overflow capacity is insufficient.

Overflow outlets are also meshed with round wire mesh that typically have an open area of little more than 50% but because the round wire has a bellmouthing affect, the flow restriction is not as great as a flat wire mesh with the same % open area. The mesh however needs an occasional scrub to prevent it gumming up. To clean it, the pipe either has to be removed or preferably cleaned from inside the tank by reaching through the top inlet once the meshed basket has been removed. Many tank designs or optioned top meshed inlet positions have not accounted for the necessity to access the overflow mesh. I feel that this would apply to the roof and inlet design on your tank.


Removing the flow restrictive outlet mesh and fitting a meshed air gap to the overflow pipe will improve the overflow capacity by 20-25%. The air gap is well designed and priced.

WATER QUALITY:
Water layers in a tank are stratified with the low density fresher and cleaner water at the top and the high density (low oxygen) water at the bottom. The worst area for a pump to draw water from is the very bottom of the tank through a valve that is really a drain valve. The valve that supplies the pump is fitted very low on the tank and could even be level with the sediment layer. Your course of action depends on what the water is used for but if there is a sediment build up, I would be desludging the tank as a minimum.

PUMP:
A flexible hose should always be used rather than a rigid pipe to connect the tank's outlet valve to the pump's suction end and the Blue Stripe poly pipe used satisfies this requirement. Pump manufacturers usually state as a condition of warranty that the pipe/hose used must have the same internal diameter (ID) as the pump's suction port but Blue Stripe poly pipe is measured as an outside diameter (OD). The ID of PN12 Blue Stripe which is commonly used on 25mm suction ported domestic pressure pumps is about 21mm. The PN12 32mm Blue Stripe has an ID of about 27mm which makes it closer to 25mm than what the 25mm Blue Stripe is.

Given the shape of the pump cover, there is a good chance that the Blue Stripe connects to a 90 degree elbow fitted to the pump's suction end and this needs to be checked. The pipe connecting to a pump's suction end should respect a straight length of 5 to (preferably) 8 internal diameters. If a pump draws a high flow velocity through a 90 degree elbow fitted to the suction side, it can cause cavitation.

If the pump cover is exposed to sunlight, it needs to have some holes drilled in it for cross ventilation. Excess heat is a prime cause of capacitor failure.

I hope that this helps understand the system a bit better.

Amazing. You sir you need a medal.
Thanks for the effort replying.

It won't require much work and cost to upgrade the system to be top notch. If you intend doing so, just ask and I will give you some simple information to follow. .

Ok I think I'm across it now. Where would the sump typically be located?

I've been doing some digging at the base of the tank but the pipe looks to go quite deep.
What's the best way to find it? Possible it doesn't have one?


Research suggests I should flush the water out after the rainy season but I can't figure that one out.
I'd need sump access I assume?

I can post a pic of the root Surface area too. It's quite a large area as it covers three terraced levels.

Thanks again

"Sump" is a term I use to best describe the subsurface wet system pipe.

Many wet systems have above ground discharge points but these obviously cannot fully drain the sump plus the head pressure is diminishing during flushing because the hydraulic head (think 'elevation head') is provided by the limited amount of water in the downpipes and the tank's vertical riser.

Some properties can flush out their wet system to a lower point and sometimes the subsurface pipes are plumbed to a tank that is down slope from the house which allows the system to be drained. In the case of a rural property off the water grid, this also wastes a lot of water and flushing a thousand litres or more is common.

Having a wet system that can't be flushed is not good.

HOW DOES WATER FLOW IN A FLOODED PIPE? (read on, this is important to understand)

Water flow in flooded pipes is either turbulent or laminar.

Turbulent flow is mostly caused by velocity or a disruption due to intersecting water from a tee's branch or a change of direction through an elbow etc. Turbulent flow transfers sediments etc as suspended material which is good for flushing.

Laminar flow is settled water that travels fastest through the core while the water on the wall is mostly stationary. This allows most suspended sediments to fall and 'colonise' in 'tails' at the bottom of full flow pipes where it remains mostly stationary until some moving sediment particles attach to the tail of one colony which then sees a similar amount of sediment break away from the front and travel to the next colony where the transfer action is repeated. It is like watching relay runners!

Given time, the colonies will form a complete chain of bed load and if there has been bacterial activity breaking down organic matter, the bacterial die off will combine with the sediments to create a gelatinous black tar like substance that many persons have seen when they have cleaned their tanks.

The pipes at the head of a wet system have very limited flushing capability due to very low velocities even during the most intense rainfall and installing the same large subsurface pipes to service the furthest downpipes as what service the downstream downpipes is a recipe for sludge build up.

HOWEVER, we can capture the bed load by making a very effective DIY sediment trap using off the shelf parts. This is fitted to a non turbulent section of pipe about 4-5 metres away from the last turbulent area and the bed load simply falls down the inverted branch. The branch trap is then reduced in size to increase the flushing velocity and to minimise the amount of water flushed. The pipe reducer denoted in the diagram below is now replaced by an invert taper and the inspection cap is required if the branch size is reduced but no leaf diverters are fitted to the downpipes. A 45 degree junction is best used because the hole is larger than a tee branch.

The two links below are property owners discussing the sediment traps fitted to their wet systems.
https://forum.homeone.com.au/viewtopic.php?p=1146540#p1146540 post 61
https://forum.homeone.com.au/viewtopic.php?p=1551215#p1551215 post 50

Knowing the roof area harvested and your 1:20 Average Recurrence Interval (ARI) is important as it will tell you whether the tank's overflow pipe is capable of managing the inflow rate during heavy rain when the tank is full. This is particularly important given your tank design has practically no mitigation capacity and little water depth above the outlet's invert.

One of the many failings of the rainwater harvesting regulations is the non requirement for tank manufacturers to state the maximum overflow capacity of every tank's overflow outlet.

Can you approximate the length of the subsurface pipe between the tank and the nearest downpipe and is it most probably a straight run?