Rainwater tank 101 for a new build?

Hi cjh,

You have posted valuable information that makes it easy to give an overview.

If we allow for an average annual rainfall of 1,100 mm to compensate for your slightly drier build area, your 211 sq m roof harvest area with an 85% yield will give 1,100 x 211 x .85 = 197,285 litres per annum or an average of 540 litres per day. The additional garage/studio 81.6 sq m roof area will give an additional 76,300 litres per annum when using the above formula. Figures vary for each situation but they give an indication of what a good system will achieve.

Harvesting every downpipe will necessitate having a wet system. Depending on the tank’s location, you may be able to also use a dry system from one or more downpipes. Dry systems have traditionally delivered better quality water than wet systems.

Your metal roof will have greater yield and roof wash efficiency than a tiled roof.

You are fortunate that you have a consistent rainfall pattern (and a clean climate).

Being in a semi rural area, will that entail the necessity to use harvested water for outside use? If so, have you done any calculations? The answer could determine your first flush set up.

You are reliant on harvested water for potable use and the first objective should be to deliver the best quality water to the tank. The second objective should be to deliver even better quality water to the pump. Doing this will also give any additional filtration system an easy life.

In addition to leaf diverters and sediment traps, your high annual harvesting potential may allow you the luxury of using FFDs. Efficient FFDs are the first line of defence but the following must be considered.

FIRST FLUSH DIVERTERS:
Harvesting every drop may be a priority but when you have regular rainfall, a first flush diverter (FFD) with a dripper can decimate yield. If, let's say, you use a small 90 mm PVC stormwater pipe as the collection chamber, the pipe's internal volume will be 5.84 litres per metre. Note that 90 mm uPVC stormwater pipe is measured as an outside diameter.

I see many FFDs dripping with a steady stream but even if it‘s dripping at one drip (not apothecaries' measure) per second, that is approximately .56 litres per hour. If the pipe is 1.5 metres long, it will hold 8.76 litres, hardly enough to wash the roof area draining to a downpipe yet it will still take over 15 and a ½ hours to drain after it stops raining. If you calculate your number of rain days and the duration from start to finish PLUS the additional pipe drain time, our example FFD could easily be dripping for an equivalent 6 months of the year! This is about 2,450 litres for each FFD dripping at only 1 drip per second and if you use a more suitable 100 mm PVC pipe that has 45% more volume, the yield loss is even greater.

I assume that the house will have 6 downpipes. With 6 FFDs operating with the above 90 mm (DIN) PVC pipe scenario, your annual losses would be 14,700 litres per annum but a typical faster dripping FFD will lose much more water than our example and harvesting yields of less than 60% are common.

The video I have linked below is one in a series on rainwater harvesting and it gives some good basic advice. FFDs are discussed but have a look at the FFD dripper from 3:16 on. How much water would be lost per annum? Not the worst that I have seen though. The problem is that most people are oblivious to the yield losses.

http://www.youtube.com/watch?v=N0S5zTcz0FY

The first rule with FFDs is...NEVER EVER fit a standard first flush diverter to the top of a wet system vertical riser! Wet system pipes retain water between rainfall and when it rains, the water that drains into the FFD is the water retained in the riser whereas the first flush is still in the downpipe at the other end of the wet system pipe. In Australia, most new homes have FFDs plumbed like this and while it is actually compliant to do so, it is very much sub standard and displays of a lack of rainwater harvesting best practice knowledge.



FFDs are best fitted with manual drain valves. This allows the first flush to be collected and used elsewhere but leaf diverters need to be used to divert larger debris due to the size of the drain outlets. The drain pipes can also be connected to other FFDs and a single drain tap used to divert the flush to trees, gardens or a separate smaller tank etc. Fitting a manual drain valve also ensures that a pre determined amount is diverted, reducing yield losses.

LEAF DIVERTERS:
Leaf diverters are a compulsory fitting to wet systems in many parts of Australia and in my opinion, should be a compulsory fitting to every wet system downpipe.

The problem with wet systems has always been the vertical riser that transfers the flow from the underground pipe to the tank’s top meshed inlet. Simply put, the velocity of the water travelling up a vertical riser is insufficient to transfer a lot of the debris washed down the downpipe. Apart from a gradual build up of sludge, when organic matter breaks down by bacterial action in an enclosed wet system pipe, oxygen is drawn from the water. The water then becomes anaerobic and this can lead to stagnation, particularly during dry periods. Leaf diverters prevent larger debris from entering the downpipe.

There are different types of leaf diverters. Ones that have a coarse (usually 6-8 mm) outer mesh and a mosquito proof inner mesh allow solid bird droppings and other sizable matter to drop through the outer mesh to the inner mesh. What doesn’t wash or pass through the finer mesh will gradually block it and the problem is that most debris is washed from the roof during heavier rain. Regular maintenance is required.

Some newer leaf diverters now have the fine mesh on the outside but these can have trouble shredding debris. They do however improve the water quality.

I have designed a pre production revolutionary high yield and maintenance free leaf diverter but it could be a while before I get around to tooling up unfortunately.

DIY SEDIMENT TRAPS:
These are easy to fit and are very effective but it is vital that they are fitted close to the house but in a non turbulent section of pipe. You wouldn’t for example fit one immediately after an elbow. The sediment trap only collects slow moving bed load, not suspended solids.

LOW RESTRICTION INLET:
When you have a wet system, you need to plumb a smaller pipe off the bottom of the vertical riser. This is done by using a tee and pipe reducers or invert tapers. The pipe is then connected via a flexible coupling to an inlet valve fitted about 100-150 mm above the bottom of the tank at the inlet’s lowest point and within 75 degrees of the pump’s draw outlet.

Whereas the wet system head is measured to the top of the vertical riser, the low restriction pipe’s head is measured to the level of water in the tank. This means that the smaller pipe will flow with higher velocity and effectively vacuum any remaining fine sediment from the riser’s base. It also means that all of the water will flow through the smaller pipe unless the flow through the wet system exceeds the smaller pipe’s flow capacity. At that point, the excess water will flow up the riser.

There are several advantages with fitting a low restriction smaller pipe.
1. The wet system pipes will only retain water to the level of the water in the tank, not to the level at the top of the riser.
2. The wet system pipe is flushed every time it rains.
3. The water retained in the wet system will be the cleanest water that fell at the end of the rain period.
4. Water quality is substantially improved.
5. Not needing to flush the pipes to waste improves yield.
6. The low restriction pipe improves maximum flow rates. This can be very important when leaf diverters are fitted because leaf diverters reduce the available head. Additional flow rates are requited through the appropriate inlet valve size selection.

Note that the term “low restriction inlet” references the smaller pipe’s flow path. You need to use 45 degree elbows rather than 90 degree elbows to reduce turbulence and Pondflex UV stabilised hose or similar should be used as a reasonable length of flexible coupling.

http://www.clearpond.com.au/go/our-prod ... uctListing

The thread “Need advice on our rainwater collection and storage system” posted by jnk40 (John) utilises leaf diverters, sediment traps and a first flush diverter on the shed. 50 mm low restriction inlet pipes also flush the wet system pipes to a dedicated flush/outdoors tank. The system has since been referenced by many persons both in Australia and overseas. John has also been thoughtful of others by posting a one year on report about the systems (and some mentions of his neighbour's systems).
viewtopic.php?f=35&t=60317

For what it’s worth, my harvesting system uses Supadivertas (it has a 750 micron primary filter and an optional FFD) and a sediment trap is fitted to the low restriction flow path. I tested my household tap water a few days ago with a TDS (Total Dissolved Solids) meter and the reading was 29 parts per million (ppm). This is very low when compared to utility supplied potable water in other cities around the world. Melbourne water is recognised as being amongst the world’s best, in fact, most of the catchments are protected forest areas.
http://melbournewater.com.au/whatwedo/s ... ments.aspx

I then tested water from my main water tank’s low restriction wet system pipe and recorded 32 ppm. A couple of days later, I repeated the test and again recorded 32 ppm. This afternoon, I tested the water supplied to the pump and the readings were 28 ppm. You should also get similar if not better results.

Note that TDS meters are conductivity meters that only detect mobile charged ions, they do not detect neutral (uncharged) compounds. Most substances detected are salts and suspended fine organic matter.

Your post mentioned possibly using Supadivertas but you should use the system as described. The Supadiverta is more for suburban use.

Having a header tank supply the toilets will substantially reduce the number of pump start/stops. Pumps and pressure tanks are discussed in the thread linked below...
viewtopic.php?f=35&t=72054

Regular cistern valve seals are designed for mains water pressure, not gravity feed. You should use Fluidmaster 400UK063 cistern valves and fit the optional very low pressure gravity fed valve seal (242LP071).

EDITED: previous link became redundant. http://www.bunnings.com.au/fluidmaster- ... e_p4926949

Also have a read of the next thread for some more clues about friction loss...
viewtopic.php?f=35&t=61588

You can have a header tank and a pressure tank (pt). A pt also absorbs water hammer, great for the modern high pressure washing machine solenoids. For most situations, a 35 litre pt (drawdown is about 13 litres with a 20/40 psi cut in - cut out pressure switch setting) should be considered a minimum size but a larger tank with a higher pressure setting is best to minimise shower pressure fluctuations when off the water grid.

I have seen the NZ 10 min – 10 year ARI mentioned in AS/NZS 3500.3 but I haven’t had anything to do with NZ regulations first hand. I compared your 10 year/10 min figures with my own area and it appears that you would only require a single 100 mm DWV pipe from the house if one downpipe was diverted as a dry system pipe but the head is critical. I have linked a simple Hazen-Williams flow rate calculator below for you to play around with. Just change the roughness coefficient to 150 for uPVC pipe. The internal diameter of 100 mm SN6 DWV pipe is 104mm.

http://www.calctool.org/CALC/eng/civil/hazen-williams_g

To do it accurately, you need to also calculate the friction losses through all of the fittings, elbows for example, but it seems that you will have a short run and 100 mm DWV runs hard with not a lot of head. Make sure that you calculate the head to about 50 mm above the bottom of the riser above the inlet and not to the tank’s inlet mesh (a common mistake).

When calculating the additional flow through a low restriction during a storm, make sure that you only use the available head when the tank is full. You might be surprised at the flow through a 40 or 50 mm inlet with just a few hundred mm of head.

TANKS:
The first thing to do is work out you average daily use. As you will be reliant on harvested water, you should have a reserve storage of at least 6 weeks to get you through a dry period but many persons would suggest longer. I don’t know the NZ prices but in Australia, 22,500 litre poly tanks are popular and consequently reasonably cheap. If you are in a fire prone area, you need to check if you are subject to a minimum onsite fire fighting reserve and metal tanks. Do you have fire prone areas in NZ?

Don’t have the outlet valve that supplies the tank fitted very low to the bottom of the tank, for a large tank, having it 80 mm above the bottom of the tank at the valve’s lowest point is ok as it looks like you will have a system that will have minimal sediment and good quality water.

If you have several tanks and I am thinking that you should have a minimum 50,000 litres storage capacity, then you can have a settling system. This is when you divert the water into one tank that then supplies settled (decanted) water to another tank that also supplies the pump. Don’t try to divert the water equally to all tanks, having a settling system has a lot of benefits.

Rainwater is naturally acidic. If you have a settling system, you can put nylon bags full of limestone chips in the first tank to ‘sweeten’ it. Acidic water also reacts with copper pipe.

If you have a submersible pump, place it in a largish but shallow terracotta saucer to prevent it from vacuuming the bottom of the tank. You would be surprised at the huge number of submersible pumps that installer in Australia simply plonk on the tank’s floor. I personally prefer to have pressure pumps fitted outside the tank.

Another huge mistake installers make is when they fit the pump’s suction hose to an elbow fitted directly to the pump’s suction side. This can cause cavitation and destroy the pump. You should respect 8 diameters of the suction hose as a straight length immediately before the pump.

The TankVac is a full syphonic delayed overflow system that was invented in NZ by Warren Agnew. It uses an 80 mm diameter 900 mm length of DWV pipe on the outside of the tank that flows into a larger pipe to break the vacuum. The TankVac vacuums the tank’s floor (there is a vacuum break that prevents the TankVac from syphoning the tank dry) and this also improves water quality. You will only need the one TankVac to cope with a major downpour if the tank is full.

http://www.tankvac.co.nz/faq

Re gutters, the ½ round design is the most efficient. Most downpipes are plumbed for aesthetics, the most efficient location is not found at the end of walls but this is where most Australian houses have them plumbed...i.e., away from the greater roof area. You need round PVC downpipes when you have a wet system. Make sure they have a good slope.

If you fit a simple two way diverter to the outside laundry pipe, you can also divert the washing machine’s final rinse during summer. Use eco friendly products.

Thanks Stewie,

The overview should hopefully be of widespread benefit. cjh has obviously learned from some of the other threads before posting which is great.

I found a study (2004) on residential rainwater usage in the Auckland region that outlines the following:

Average household water use per person
Kitchen 10% 17l
Bathroom 25% 43l
Laundry 20% 35l
Toilets 25% 43l
Garden 20% 35l

Total 173l

They also talk about the effect of water saving reductions implemented.

Standard water use facilities: 11l toilet, standard shower heads, automatic washing machine, standard taps - 200l per person per day

Standard water reduction features: dual flush 11/5.5 toilet, shower head flow restrictors, aerator taps, water conserving washing machine - 180l per person per day

Enhanced water saving features: dual flush 11/5.5 toilet, shower head flow restrictors, aerator taps, water conserving washing machine, Fixed orifice flow control valves (FOFC) on all water outlets - 140l per person per day

Full water saving features: dual flush 6/3 toilet, shower head flow restrictors, aerator taps, water conserving washing machine, Fixed orifice flow control valves (FOFC) on all water outlets - 90l per person per day

It would seem given the amount of rainwater that would need go through the first flush diverters to be truly effective that if I could save that and reuse it for flushing toilets and garden use it would have a substantial impact on meeting the needs for the kitchen, bathrooms and laundry.

I was working on the house having 150mm half round gutters and four 100mm downpipes, two on each side placed approx. 5m in from each end so each downpipe would be servicing one quarter of the roof area each and another two, one each side of the garage/studio.

6 weeks of 529l per day is 22,218l for the reserve. Yes, I know that is not necessarily 6 weeks of actual consumption but the way I see it, it has a healthy buffer built in. A delivery from the local spring is $240 for 10,000l or 2.4c/l if things start to get desperate.

I suggest having more than a 22,000 L reserve, the tank won't always be full at the start of a very dry period and your traditional driest months are also your hottest months. Average annual consumption figures vary considerably throughout the year.

Will you have air con? If so, will it be evaporative?

Don't forget that there needs to be a head over the outlet so that the pump doesn't draw air via a free surface vortex. If the pump's pressure switch detects low pressure, it will turn the pump off. For this reason, pumped tanks have a level of unusable water and this is why many tanks have the pump draw outlet fitted close to the bottom the tank. The problem then is that a very low fitted outlet will draw the worst quality water and sediment. Not good when the water is for potable use.

Squat tanks have a greater % of unusable water than a tall tank.

When water is delivered, it stirs up the sediment layer. If paying $24 for a 1,000 litres, I would rather put the money towards a second tank. Just check the prices. 5,000 litre poly tanks are popular in Australia and they are reasonably cheap as a result. Just check the NZ situation.

If you have two tanks of different size, use the larger tank as the settling tank. Also try to have the larger tank's base a little higher than the second tank. Because water would gravity feed from the settling tank, the water will drain down to the bottom of the outlet.

If you initially order one tank, make sure that it is optioned to accommodate a future second tank and decide beforehand which one will become the settling tank.



Just check to make sure that the 1/2 round can take the 100 mm downpipe.

Are you planning to have fruit trees etc?

Do you know the approximate height of the gutter's sole above ground level? You need this figure before working out the head losses from FFDs and leaf diverters. You then decide on the tank's height based on the adjusted available head figures. Make sure you allow for the tank's base etc.

Did you have a play with the flow calculator? Friction losses through the various fittings are expressed as additional lengths of pipe. I can supply the respective figures if you can't find them by Googling.

The link below is a report by Stan Abbott and Brian Caughley from the Roof Water Harvesting Centre, Massey University, New Zealand titled "Roof Collected Rainwater Consumption and Health." Massey University is a world leading centre on rainwater harvesting research.

The report centres on a 5 year microbiological study of harvested rainwater samples from 560 private dwellings in NZ. Needless to say, little attention is given to best practice when installing most tanks yet a quality installation is easy to do and any additional costs are minimal.

http://www.pwwa.ws/pdfs/Stan_Abbott_RWH ... andout.pdf

EDIT: Deleted two obsolete links.

The table below shows imperial measurements and your elbows will be long sweep. Wet systems are somewhat complicated when trying to do accurate calculations because the tees divert water to a common pipe at different points along its length. Just keep it simple and be conservative.

A smaller low restriction pipe should also be flow calculated by the dynamic head at the invert from the larger pipe, not the head at the downpipe.

http://www.engineeringtoolbox.com/pvc-p ... d_801.html

People do bury poly tanks but I'm not that keen on doing it.

You lose head when you fit a leaf diverter but they really are a necessity when you have a wet system. The head loss depends on the height of the fascia, the width of the eaves if applicable plus the diverter itself. The head loss is often as much as 450-500 mm. Without eaves, it could be as little as 300 mm. It would be good if you could approximate this loss for your house design.

The height of the tank isn't important, you just need to know the height of the rim above the tank's top meshed inlet because while the low restriction flow path will supplement the flow rate, you still need a vertical riser. The bottom of the riser's vertical pipe above the meshed inlet should preferably have at least a 300 mm head.

The gutter's sole is let's say for now 3.0 m high. If the leaf diverter loses let's say 400 mm head, a 300 mm head at the top of the riser means that the rim around the top meshed inlet can be no higher than 2.3 m above the ground. You could make up maybe 50 mm because of the smaller branch line and maybe you won't lose quite the initial 400 mm head but the 2.3 m height is probably fairly close.

I often sink a poly tank down a little (usually 80 mm) and sit it on a compacted sand base as it is neat but I like to have the outlets accessible. You could have two different size tanks and divert to the smaller tank. The larger tank would then be a reserve storage that you might have to pump into once a year to top up above its balanced level. Prior to that, the tanks would be balanced when needed. You would most probably later divert water from the studio to this tank.

OPTIONING THE MAIN TANK:

1 x 50 mm inlet valve fitted 100 mm above the bottom of the tank at the valves lowest point.

1 x 25 mm outlet valve to supply the pump. This is fitted 80-100 mm above the bottom of the tank at the valves lowest point.

The two above valves would be fitted within a 75 degree arc of each other.

1 x 25 mm balance valve fitted about 80 mm above the bottom of the tank at the valves lowest point. Note that if the pump connects to a valve on each tank, that flow path can also be used as a balance line. In this case, a separate balance line would not be needed.

1 x 25 mm drain valve fitted close to the bottom of the tank.

Just be aware that tanks generally have a thick molded bottom section and thinner walls. You shouldn't fit valves etc to the wall unless you have spoken to the manufacturer first.

Any overflow outlet should also be fitted within arms length of the top meshed inlet. After it rains, you always get some fine organic matter floating on the water surface but this falls to the bottom in time. This fine matter however does adhere to the overflow outlet's fine mesh and it is not uncommon for the overflow to eventually block. It is always wise to option the tank so that this mesh can be accessed for cleaning.

Don't cover over the top meshed inlet. There are some products now sold that do this but healthy water needs to breathe. The justification used to cover the top inlet is to prevent sunlight making algae grow but a system collecting good quality water will have few nutrients. One of the side benefits of having a low restriction inlet is that the incoming water oxygenates the anaerobic zone.


No. The lesser amount of sediment will be different to that found in standard systems and the incoming water rapidly dissipates. I also advise people not to fit the inlet feed valve and the pump draw valve more than 75 degrees apart.


That would cut down the pump start ups to fill the cisterns to about 1 every 2 days. It's good that they make them, you won't have to worry about trays, float valves etc because they would know all this. A pump's start up energy demand is about 3-4 times its run demand and apart from saving the pump's wear and tear, you will also save energy.


The ICON Leaf and Debris Controller is the best leaf diverter on offer at the moment as it has a continuous steep sloped outer filter and a large internal mosquito proof mesh filter. It would be difficult making your own. I have been trying to find the time to take mine further but I am still behind on other projects. I haven't even had time to consider a trade mark.

EDIT: Our leaf diverter project is currently in limbo.

You are in the fortunate position of having an excess harvest bar a minor dry period over the summer months. This gives you a few options but aesthetics may play a part. Before I suggest some possibilities, you need to decide on where the water is going to go...

1. When the tank overflows.
2. When the sediment trap is drained.
3. When the FFDs are drained.

Sounds good. The 80 mm allows the valves to be above the surface.

They often quote the mesh height and not the rim height. You need to make sure as to what they have quoted.

The riser above the top inlet is often raised above the rim by a bracket and this can add another 20-60 mm or so. There are a couple of little traps like this that catch people out and they end up with an insufficient flow rate during heavy rain.

The photo below shows this situation on the Gympie installation. Fortunately, the Gympie installation was already plumbed with 50 mm low restriction inlets to compensate for the head loss resulting from using leaf diverters. Also note the meshed flap valves fitted to the end of the pipes. These allow the pipe to breathe but stop pests from entering.
viewtopic.php?p=1146540#p1146540




Not my area sorry but is there a maximum flow rate that you can divert to the grey water tank? If you have a TankVac, it will be overflowing water at about 540 litres per minute. This flow rate is most often a lot faster than the flow rate entering the tank. The syphonic action is delayed during the prime and once operating in syphonic mode, it will drain the tank to a level a little under the bottom of the overflow pipe (where the vacuum break is).

If you intend dropping the downpipe vertically, you would have the pop draining directly onto a leaf diverter fitted to the fascia and this will reduce head loss. This is what was done at the Gympie installation. The fitting between the leaf diverter and the downpipe MUST be sealed to minimise head loss and doing this reduced the Gympie leaf diverter's head loss to little more than 200 mm.



A FFD is usually plumbed by fitting a tee (with a PVC sleeve) under the leaf diverter. The pipe directly under the leaf diverter will be the FFD. Just be aware that if you use 100 mm downpipes, some fittings vary in design to the 90 mm storm water fittings.

If you are able, you can couple all of the FFDs to a smaller shared pipe in a trench and plumb that smaller pipe away from the house. An inline tap at the end allows you to to drain the FFDs when needed. You can also plumb the pipe to a small sealed tank fitted with a tall mosquito proof vent tube (it has to be higher than the gutter) to increase the first flush capacity.

I once played around with the idea of having a linked FFD system that had a solenoid valve connected to an irrigation timer. The idea was to have the valve open for a set time every 4 or 5 days but the immediate problem that I encountered was the fact that the valves needed a 2 metre head to shut. There were some that didn't require the operating head pressure but they were expensive.

I was sent details about some different valves a few weeks ago that look promising but I am not taking on any more projects at the moment but if you have an interest in this area, I can PM you some details.

The valve also needs an open flow path, many valves didn't have this. They must also be suitable for dirty water.

If I installed a backflow prevention valve in the line to the tank could I then lower the installed height of the leaf diverters and FFD's to be near the bottom of the downpipes?