Rainwater tank issues

Hi,
We are enjoying being in our new home near Lake Macquarie in NSW. There are some issues with our rainwater capture system that we feel they could be improved on.
1) In peak down pour conditions the tank over flows. It is situated under the house in a sub floor area which we use for storage/workshop. Having this area flooded from time to time is not good. I am yet to actually see the overflow in action, but suspect that the design and sizing of the tank overflow just can't cope with the catchment (roof area connected to the tank is approximately 208m2).
2) It is a charged down pipe system. The lowest part of the system is a buried near horizontal pipe connecting the catchment on the south side off the house with that on the north and the tank. There is no way to inspect this section of pipe. Surely this section is at risk of blocking with time?
3) The laundry, toilets and one outside tap uses water from the rainwater system. There is a noticeble staining of the toilet bowls starting to happen and no doubt the whites in the wardrobe probably aren't as white as they once were. Is there a filter recommended to mitigate this? I assume I would fit it in the lines to laundry and toilets.
I have uploaded a pic of the tank (3000L) entry and over flow arrangement. The pump is submersible.
Thank you for any advice.

Surprised there is staining from rainwater. It shouldn’t have any impurities, so they might be coming from the roof or the system. Are you in or out of town? Is the water in the tank clear?

We are adjacent to bush. We have gutter guard installed on all gutters. The water is generally clear with sediment evident on the floor of the tank. The submersible pump is suspended from its pipework, so does not sit in the sediment.

A set up like this is OK for garden watering and toilet flushing and that's about it.

What happens is all the dirt is washed off the roof and into the tank. the submersible pump then is likely to suck the dirt off the bottom.

Steps to improve water quality include:

• Cleaning the water coming off the roof, Check out this website ( http://www.aquatrek.net.au/) from SaveH2O
  
• A pump that takes water from at least 100mm above the tank bottom.
• An overflow that takes water from the bottom of the tank.

Is the section of pipe mentioned above effectively a sump? If so, how tall is the immediate vertical riser along its flow path to the tank?

I am assuming that the wet system is one continuous 100mm DWV pipe.

Are you able to take a photo that shows a lot more of the downpipe that diverts to the tank's top meshed inlet? What I need to know is the flow path of that downpipe, it is very high above the tank and I would like to know whether it also dips down at some point or whether it comes in horizontally.

Is every downpipe diverted to the wet system?

Does the wet system have leaf diverters fitted to the downpipes?

Do you know the make and model of the pump? (I need to know where it draws its water from).

Hi SaveH2O,

The section of pipe I refer to buried along the eastern side of the building is not a sump in my mind as there is no designed access to it. It is a charged connection between the down pipes on the southern wall and the those on the northern wall. Hopefully the pics below will make things clearer.

The charged system comprises 100mm and 90mm down pipes.




There are no leaf diverters fitted to downpipes, although all gutters have gutter guard protection.

The pump is a DiverTron C6.


View of the north wall above. The subfloor area is entered through the open door in the centre of the pic. The charged 100mm pipe that enters the subfloor area is behind the retaining wall just around the corner from this door.
The buried connecting pipe I refer to above runs along the eastern foundation connecting the three downpipes on the northern wall to the three on the southern wall.

I hope this makes things clearer??

Thanks for the photos...they tell a story don't they?

Whoever designed and installed your system was well intentioned but they lacked the smarts. Nearly all of the rainwater harvesting systems I see are sub standard and have issues sooner or later.

I'll explain things in detail rather than just give answers in case you need to tackle the builder.

Many areas north of Sydney have a mandated requirement to use mosquito proof leaf diverters on wet systems to prevent mosquitoes breeding. This mandate is usually found in the health regulations or the local requirements. I don't know what applies in your area but mosquito proof leaf diverters should be mandatory for all wet systems regardless. The ICON Leaf and Debris Controller is a mothhful but is also the best leaf diverter on the market at the present time. If you do install them, do it at a serviceable and accessible height.


The south side pipe forming a sump is a problem because it will store nutrients that provide a smorgasbord for anaerobic bacteria. This need to be regularly drained.

The Divertron submersible pump is a quality item. Most submersible pumps are simply lowered to the tank's floor where they vacuum the tank's sediment layer...most often against the manufacturer's term and conditions of warranty. The Divertron terms and conditions of warranty require a 200mm separation between the pump and the floor, you say that your pump is raised which is good but it also sounds like it is suspended by the pipes. I hope not!

The Divertron pump is a powerful good quality pump. I suspect that the submersible pump was chosen because of noise considerations due to the tank's location.

EDIT 08/01/2024 - Edited out reference to a variable speed drive pressure pump.

You also have an Acquasaver mains water switching device which is an excellent choice that again tells me that the designer was well intentioned.

The tank is a shocker!!!

This style of tank is difficult to desludge but yours also has a really poorly thought out design issue in that the top meshed inlet literally sits above the tank and the overflow pipe is part of the inlet. Many companies are now making tanks with this woeful design feature as a way to maximise the volume of water held in a tank but look at the overflow pipe's location! It has no mitigation and no consideration for best practice!!

The overflow pipe is 100mm DWV (the common use SN4 is 104.2mm internal diameter). A horizontal overflow pipe's discharge rate can be calculated with Torricalli's law which accounts for the height of the water above the pipe's invert (the bottom of the pipe).

AS/NZS 3500.1 Figure 8.2 is a table that shows different flow rates at different heights above a horizontal pipe's invert for various pipe sizes including a 100mm pipe. The flow rate given for a 100mm pipe with a 100mm height above the invert is 4.67 litres per second which is 280.2 litres per minute.....BUT.....water tank overflows are required to be meshed and most mosquito proof wire mesh has an open area that rarely exceeds 55%. The actual flow restriction depends on the mesh design, a flat mesh is much more restrictive than a rounded wire but a substantial restriction still exists with round wire nevertheless.

A vertical overflow intake has a higher inflow rate than a horizontal overflow intake but a horizontal overflow intake can be made more efficient by 'dipping' an internal elbow downwards then upwards inside the tank and fitting a large vertical bell mouth to the top of the pipe.



The overflow pipe and bellmouth in the photo has a much lower side outlet than your tank has and is why I have stated that your overflow pipe has to dip down and then come back up.

The first thing I suggest you do is to look inside the top inlet to confirm that you only have a standard overflow fitted. If so and even if the outlet is unmeshed, the overflow capacity is abjectly inadequate.

Rainwater harvesting regulations are poorly written and often overruled by local councils, other regulatory bodies and even health regulations. It is also difficult to quickly know what is mandated and what isn't and the various rules (often guidelines) and regulations including those by the regulatory bodies in charge can be woefully inadequate and just plain inept.

In New South Wales roof drainage systems are regulated in:
(a) the Environmental Planning and Assessment Act 1979 and the Environmental Planning and Assessment Regulation 2000, which references the Building Code of Australia; and
(b) the Local Government Act 1993 and the Local Government (General) Regulation 2005.

The Lake Macquarie Council has a document titled: Handbook on drainage design guidelines. Dec 2013. This booklet however is a Guide and as such, it has no legal standing unless council has mandated it. It also isn't accurate or up to date, for example, it references AS/NZS 3500.1 2003 instead of AS/NZS 3500.3 2016 for roof drainage design. AS/NZS 3500.1 is for water services.

It also states: Roof water and site drainage is to be sized in accordance with the Building Code of Australia and AS/NZS 3500.1 2003. Even if the correct edition and year was referenced, the Australian Standards are superior to those in the BCA but the BCA, being the primary document which references the AS, has hierarchy.

I mention this because if you become involved with a discussion with your builder and he starts mentioning that the system is built to compliance/Standards, then you need to know what actually applies. The facts are that if a water tank overflows back to the stormwater, then it is part of the stormwater system and the drainage must be designed to cope.

Eaves gutter and downpipe sizes are calculated by referencing the area's 1:20 Average Recurrence Interval (ARI) or else the area's Intensity Frequency Duration (IFD). The Lake Macquarie 1:20 IFD is 181.91mm /hour (the AS referenced 1:20 ARI is 180mm/hr) and this is based on an average rainfall intensity of 3.03mm/min over a 5 minute duration. Your roof catchment also has to include the gutter's area but based on the 208 sq m roof area given in the opening post, a 1:20 IFD would produce an average of 630.6 L/min over a 5 minute duration. This is way, way, way in excess of the tank's overflow capacity.

The heavily concentrated flow of water jetting into the tank through the tank's top meshed inlet even during moderate rainfall will cause significant sediment resuspension when the tank is > half full and is the reason why the cistern water is dirty.

A large amount of dust and other matter settles on a house roof and gutter guard will not deflect this. Sediment and nutrients will support a large bacteria colony until the nutrients become depleted at which stage bacterial die off occurs.

A prime source of nutrients is organic matter but having the leaf guard restricts the process.

The system's designer was negligent not to incorporate a flush valve extension by using a junction instead of an elbow at the wet system's lowest point on the north side but this may not be mandated. This now needs to be done if possible.

It is interesting that the Lake Macquarie Handbook on drainage design guidelines Dec 2013 states at 2.4.2 "The collection system incorporates an effective first flush device for removing roof surface contamination. This should cater for the first 1mm of rainfall." As I said earlier, this handbook is only a Guide (unless it has been mandated) and it refers to the singular . Note that a First Flush diverter is different to a flush valve.

As an aside, the Lake Macquarie average annual rainfall of 780mm on 208 sq m of roof will produce 137,904 L per annum (average of 377.8 L per day) with an 85% yield.

Questions?

Thanks SaveH2O for your very thorough consideration of our issues.

Some observations and questions…..

It does astound me somewhat that there is such an absence of clear guidance and regulation around the design of these systems. Although I understand from your reply that there is a fundamental given that a stormwater system, of which our charged rainwater harvesting system is part of, must be capable of handling the what mother nature may throw at it (at least to a 1:20 ARI). Our system doesn’t.

The most significant contributor to this lack of capability is the poor design of the tank, specifically; the maximum flow achievable from the tank over flow design and position. This is giving us the current acute issue of spills into the subfloor area of the house.

I can see how an inverted bell mouth style overflow would be a much more efficient option than the current horizontal one. I’m not sure how it could be implemented? Would one block off the current opening and create a new one at a lower entry level?

What is preventing the Builder from fixing the problem by tackling the inputs and just directing the south wall downpipes directly to stormwater, i.e. effectively reducing the catchment area by half?

Following rainwater spilling into the tank following a dry spell there is always a hydrogen sulphide type smell in the sub floor area. Is this the organic decomposition products being displaced from the tank?

Yes, the water tank is simply a part of the stormwater system when there is a direct connection between the roof and the stormwater. It cannot be dismissed as being anything else.



The overflows are a serious problem but nothing like they would be if you were to experience a 1:20 or greater ARI/IFD. A few years ago Melbourne had a widespread 1:500 year storm and localised small high intensity storm cells of short duration have become common all over Australia now.

I always advise for a standard rainwater harvesting system to be designed for a 1:20 event because that is the minimum that eaves gutter roof drainage is mandated for BUT to then add a minimum 20% safety margin but that is for an outside tank. Your tank's overflow capacity and mitigation was clearly not given any consideration at all.



I would have to see inside the tank to suggest a suitable bellmouth design but any improvement wouldn't be sufficient to cope with a major storm. The problem with gravity drainage is that weir flow generates a vortex that fills the pipe mostly with air, much like a sink or bath drain. If you double the orifice diameter, you increase the area x4 which floods the smaller pipe to decrease the volume of air..

The style of tank you have would usually prevent you from drilling a lower overflow outlet but why not talk or write to the manufacturer and discuss the problem with them?

Have you checked to see if the overflow outlet is meshed and also to see whether the overflow is simply an opening on the inside? It is hard to imagine a tank being sold with an overflow like that unless there is some sort of efficiency improvement attached inside. If there is, I would be interested to see their attempt.

The overflow should be meshed but this often gets a build up of gunk on it and it is worth giving it a scrub.


The tank obviously connects to stormwater but what stormwater pipes are there?


Water needs to breathe but water retained in a wet system pipe can't do this. Bacterial activity also depletes the water of oxygen, making it anaerobic and this is the "hydrogen sulphide type smell" that you notice. Just imagine that the water in the South Side Sump is cordial, it takes a lot of water to completely flush the cordial and when it rains, the initial very strong cordial mixes with the water in the tank and pollutes it. This is what you smell.

The South Side Sump is a real problem as is the high vertical pipe in the sub floor area.

The 100mm (104.2mm ID) DWV pipe is a strong pipe that has a volume of 8.5 litres per metre. It will have no problem with delivering the volume of diverted water from all downpipes during a 1:20 due to the available hydraulic head. The problem with wet systems though is that they retain water between rain events and your system will retain a lot of water. If it has a flush valve fitted, it will solve one problem by occasionally flushing the probable +600 litres of water from the pipes but that water has to go somewhere. Maybe a capped pipe could go along the top part of one of the terraced gardens and the water exit through holes drilled along its length.

The sediment flushing velocity required for a full flow (flooded) horizontal pipe is a minimum 0.7 metres per second but in practice there is some variance either way for different materials. 0.7 metres per second through a 100mm DWV pipe is 5.95 litres per second or 357 litres per minute.If the 5 downpipes all drain equal roof areas, then the 3 south side downpipes would collect 408 litres per minute during a 1:20 event and so the south side sump will never regularly flush.

https://www.youtube.com/watch?v=c1xX90ZfBj4

Are you able to run a small 40-50mm pipe around the back of the house? I could have a simple solution for you.

Hi SaveH2O,

Thanks for your further advice. All very interesting.



How do you go from the Lake Mac 1:20 IFD of 180mm/hr to the 3.03mm/min intensity over 5 minutes? Why is it not 180/12=15?



There is no mesh on the overflow outlet. Just an open pipe.


The overflow from the tank flows under the house slab to a stormwater sump in the north eastern corner of the block. There is one other inflow to this sump which collects ag drain/landscaping runoff.

I have taken a closer look at the roof catchment area. The updated total is 227m2, with the south roof area having and area of 154m2 and the north 73m2.


Maybe..........Anything is possible. What did you have in mind.

I have had a chat with a salesman at the tank manufacturer. His response to the overflow issue was that I needed to reduce the input and/or buy more tank capacity (to reduce the frequency of overflow events). He did say that the 3000L tank was on the small size for the catchment we are dealing with. He had no knowledge of a means to improve the overflow performance of this tank.

As usual, expert advice form SaveH2O with astonishing attention to detail.
We are indeed fortunate to have a person with so much detailed expertise on this forum.

Sorry...meant south east corner.

I always bring the figure back from a 5 minute rain intensity duration figure to a 1 minute average figure when calculating flow rates.

The 3.03 referred to the 1:20 IFD, the 1:20 ARI is 3.0.


That is about 680 litres per minute during a 1:20 ARI, nearly 2 1/2 times the overflow pipe's capacity and more if the gutter area is not already included.

The overflow as is can cope with a maximum 90 sq m roof catchment but that doesn't include a safety margin.

There is another top inlet at the other end of the tank, it should also have provision for an overflow outlet. Have a look for a blank that has to be cut out. you might be able to buy a hole saw from the tank company. You could have that overflow fitted and the overflow modified inside the tank with a bell mouth and the external overflow pipe run alongside the tank to join the existing overflow pipe.

It could be a challenge to fit a bellmouth on the existing overflow given the incoming water's flow path but having just one will do.

You would still have to address the need for a flush valve on the horizontal wet system pipe coming from the south side.

As a matter of interest, does the inflowing water pass through mesh at the top of the tank or does the pipe descend into the tank? The water should divert to the tank at the end opposite to where the pump is unless the pipe descends to a calming inlet but if it does descend to a calming inlet without having passed through mosquito proof mesh, then that is another area of non compliance.

https://www.youtube.com/watch?v=ULj6Ar2ncLE&t=99s

I am looking for a video that shows the depth and the amount of air that a fast flowing stream of water carries into a body of water after entry but no luck so far.It is a lot more dramatic than what is shown in the above video.

An addendum to my second post.

The National Construction Code (NCC) comprises the Building Code of Australia (BCA) Parts 1 & 2 and the Plumbing Code of Australia (PCA) Part 3.

The NCC is updated bi-annually on May 1 and the current edition is the 2016 version.

The NCC can be downloaded free of charge.

http://www.abcb.gov.au/Resources/Public ... ete-Series

The screen shots below show the PCA:
Page 71 Section D1.
Page 120, STATE AND TERRITORY APPENDICES -VARIATIONS AND ADDITIONS...NSW.

Thanks SaveH2O! You have been an absolute treasure trove of information and ideas. I intend to spend some time tonight consolidating all the input and proposing a pathway forward for your review. It is probably time to give the builder and plumber right of reply.
Just to close off on "some loose threads"..........



The PCA references the AS/NZS3500.3. Is there a way to access this standard easily? I note you generally need to fork out a not insignificant amount of money to buy it online. I would have thought that making access to relevant standards as easy as possible would have a positive impact on reducing build issues developing in the first place.



This idea has merit, although the other overflow outlet is hard up against a wall that would make the plumbing a little complicated. I still can't visualise how the bell mouth would work inside this tank as the overflow outlet is so high in the tank???


Yes!! Not quite sure how at this stage. The easiest way may be to direct the flushed water into the stormwater sump at the south eastern corner of the house.



The incoming flow passes through a mesh at the top of the tank. By the way there is no insect mesh on the overflow outlet that I can see, nor on any of the downpipes (although we do have gutter guard mesh installed). Should there be for compliance? Would this requirement be in the NSW Environmental Planning regulations?


What was your thinking here?

My intention is to draft a note and discuss with my Builder with the intention of defining a solution that complies with the relevant regulations and standards and delivers a workable solution.

Pragmatically, I can see a potential solution being to remove the south side catchment from the rainwater harvesting system and direct all of this runoff directly to stormwater. This would reduce the rainwater harvesting roof area to 73m2. Unfortunately, we would end up using a lot more town supply and it would be very expensive water out of the tank given the investment in tank, pump etc. The buried connecting line running along the eastern side of the building would be blanked off with a flush valve fitted such that it could emptied and flushed occasionally. Not the ideal solution.

Australian Standards are produced by Standards Australia, a non profit independent company. The Standards are sold under license by SAI Global.

Libraries use to stock various Standards but a year or two ago the fee to do so increases significantly and so not many libraries stock them now. You may be able to find a library that stocks the superseded Standard or even find one that carries the current version.

The rainwater harvesting regulations are all over the place yet nowhere. Part 2 of the NCC addresses drainage in 3.1 but rainwater harvesting doesn't get a mention.

The PCA is also void.

AS/NZS 3500.3 has a lot of detail about roof drainage but nothing about water tanks.

AS/NZS 3500.1 Section 14 INSTALLATION OF WATER SUPPLY SYSTEM FROM RAINWATER TANKS has 2 pages about pipe markings, references to Standards for backflow prevention and a couple of other things but nothing about the overflow requirement or much else.

AS/NZS 3500.1 Section 8, WATER STORAGE TANKS States:

"8.1 SCOPE OF SECTION
This section specifies requirements for the storage of water.
NOTE: For connections of rainwater tanks, see Section 14."

What a joke!!!

BUT.....this is followed by:

"8.2.1 General
This section applies to tanks provided for the storage of water for the following purposes:
(a) Sanitary flushing
(b) Drinking water supply
(c) Firefighting
(h) Make up water"

The list goes to (i) but the above designations and the text in Section 8 unquestionably refers to water tanks. (a) above could however mean to refer to a cistern but it is ambiguous as a water tank will often provide water to a cistern for toilet flushing.

The point is that Section 8 includes the previously mentioned figures 8.2 and 8.3 for horizontal and vertical overflow pipe maximum flow rates for different size pipes (they go up to 200mm) at varying water level heights above the pipe's invert. The figures clearly show that your overflow is woefully inadequate and therefore not compliant IMHO.

I would use this as the basis to prove the overflow's massive non compliance but you could also check with the Office of Fair Trading. I could scan and Email the charts if need be.


That would be the plumber's problem.

An internal extended overflow pipe would have to extend downwards and then come up again so that it was vertical. A vertical orifice drains more efficiently than a horizontal one even without a bellmouth., for example, a horizontal and vertical 100mm pipe with a 100mm depth above the invert drain at 4.67 L/sec and 6.6 L/sec respectively. This equates to 280 L/min and 396 L/min respectively.

I have been wondering whether you could have a horizontal overflow pipe extension inside the tank. If the pipe had a section of its 'belly' removed after the first 50mm, the water entering from underneath should make the drainage more efficient because the inflow at the end of the pipe and consequently the usual free surface vortex would be lesser. I haven't tested this though.


I couldn't find a You Tube video showing what is termed Plunging Water Jet Air Entrainment but there would be a massive amount of air entering the tank during heavy rain. A pump should never be installed under a wet system inlet.

The overflow mesh is a mandated regulation but I am not sure where you would find it stated apart from regional health regulations if applicable. You know what they say about a rat up a drain pipe.

I am not in NSW, I don't know the scope of the regulatory framework up there.


The builder needs to rectify the installation and that will most probably entail establishing a storm water system in place of the south side wet system but you will also need an extended horizontal pipe fitted with a flush valve because the tall riser inside the north under house area also creates a sump.

You really need to delete the south side downpipes because of the overflow issue and the low wet system sump unless you can cure the overflow inadequacy and the lack of a flush valve at the low point.The other problem is that if you do, you won't be harvesting as much water as you wanted to.

I developed the Supadiverta which I retained rather than license and it would have been ideal for your situation provided that a 50mm pipe could come around the house below ground from the south side. The Supadiverta usually diverts to a valve fitted about 100mm above the bottom of the tank at the valves lowest point but I don't know whether the tank can be retro fitted with a 50mm valve because of the style of tank.


I doubt very much that the builder will know the regulations. Maybe a draftsperson designed the system. The tank was a terrible choice, I would have had a 2 or 3 tank settling system with the overflow at the infeed tank and the pump on the furthest tank.

EDITED; Late hour typos...changed downpipes to wet system and changed sec to min.

Sorry, probably a silly question......but "IMHO"?

If you would be good enough to scan the info and charts related to horizontal flow above invert, that would be much appreciated.



Yes, I can understand the advantage of flow into an inverted bell mouth vs a horizontal pipe. The top of the bell mouth still needs to be as high as the top of the horizontal pipe exiting the tank. Looking at the arrangement at the top of my tank, I can't see how the bell mouth would fit under the top of the tank.

In my humble opinion.

The return vertical pipe should only come up as high as the overflow's invert as a maximum. A small drain hole would need to be drilled at the pipe's lowest point.

PVC becomes malleable when heated and you could mold a bellmouth of sorts from a fitting (easiest) or the pipe. There are videos on You Tube showing how to do this.

https://www.youtube.com/watch?v=fzdnlBkG2mI

A big possible/most probable problem is with the tank's design and the inflow pipe...the overflow's extended internal orifice obviously cannot be under the wet system flow path. This is why I have been thinking of an internal extended short horizontal pipe or fitting with part of the bottom section cut out (or drilled with holes) but it too can't be impacted by the inflowing water because of the stress on the fitting. The pipe or fitting needn't be long though.

To close this thread off......
In consultation with our builder, it was agreed that he would reduce the roof catchment being diverted to the tank to the northern roof area only (about 1/3 of what it had been). The charged underground system now has a screw cap in the storm water sump that can be removed to allow the system to be flushed. We have had a few torrential winter downpours since this modification and no tank over flows ! Yeehah!! Clearly the cost has been a reduction in the catchment and we will spend more often over the dryer months with an empty tank, but we are happy with the outcome that we have got to overall. Many thanks to SaveH2O for your valuable advice.

My wife and I have just moved into a house with 3 x 3000 litre water tanks in Heathmont VIC, and almost zero knowledge of them. Almost all of this thread goes over my head.

We had a plumber do some other work on a toilet and determined that 2 of the tanks are joined together, and feed the toilets and laundry via a pump and tank to main auto changeover switch, although the pump was unplugged when we got here. There is also a tap that appears to make the tanks feed the entire house, this was open, even though the pump was off. That tap is now off for the moment. Inlet filters on the top of each tank had a couple of cms of sludge plus a ton of leaves and other much. I have cleaned them and we are now using it for toilets and laundry, and looks/smells clean to the eye/nose. Can someone reccomend a tank specialist to come out and assess the entire system and give us some advice on it. We would be interested in if it can be used for showers and/or hot water and issues with that.