DIY sediment trap on charged rainwater harvesting system

Thanks to H2O and others for posting such useful and valuable information. I am wondering if anyone has any photos of a DIY sediment trap. I realise H20 posted this from a user that once had a series of photos up but have since disappeared. Specically, I am interested in the set up from the 40mm pipe up to the surface level. Could one just use another piece of 40mm pipe inverted/ 45 degree? flex pipe and any advice on type of valve to flush. release water? Thanks in advance.

The pipe is reduced in size so less water is wasted and there is increased velocity...the increased velocity generates turbulence which cleans the pipe BUT you should only reduce the flush pipe size if the water has passed through filter mesh because you don't want something large (a dead bird for example) washing into the wet system and blocking the smaller passage way. Fitting a 45 degree junction with an inspection cap as shown in the diagram will manage this possible occurence though.

You can fit female DWV fittings that allow you to use poly nipples that are tapered. Being tapered allows different pipes and hoses to be connected. I have a 4 tank Supadiverta system with smaller pvc pressure pipes and my sediment trap on the main tank is reduced to a 19mm poly pipe with an inline valve at the end that drains to a nearby clematis. You just see a slug of black water and then clear water come out.

You can plumb the flush pipe any number of ways including using it as a defacto tap on a riser stake.

The important thing is to fit the trap about 4 m past the last turbulent section of pipe to maximise the amount of settled bed load captured.

The H1 member in the post below commented on his long term use experience.

viewtopic.php?p=1551215#p1551215

Thanks heaps SaveH20. That's a gret help. I'll upload some photos - perhaps useful to some, perhaps an example of what not to do - I hope not!!

In relation to the connection of DVW pipe to tank inlet (bottom of tank) I am now thinking I shoudl use some kind of flexible connection to prevent any movement in the ground/ movement of pipework putting pressure on the inlet and/or inlet connection. I purchased a flexible coupling the other day to connect 50mm DWV to 50mm tank adappter attached to ball valve, however, the label on the flexible coupling says "not suitable for potable water". Do you know is this just becuase they think it will be connected to mains (pressurised water) or is there another reason? Should I look at getting some kind of other potatble water rated coupling?

Thanks again.

Using a flexible coupling with a poly nut and tail at the tank is best practice and BAD practice if you don't. Have the inlet valve a minimum 100mm above the bottom of the tank at the valve;s lowext point.

Water quickly de-energised when it enters a body of water and low density water also flows upwards.

I am confused about you having a 50mm low restriction inlet at the tank. Unless you use our Supadiverta system that diverts excess water to the stormwater once the tank fills, you migh not have enough head pressure after friction losses to keep diverting all of the water into the tank when full. The exception to this is when you retain a vertical riser and do hydraulic calculations to ensure that the system's inflow and overflow copes during heavy rain but thinking about what you have been studying, I guess this is what you intend doing. The method has many advantages.

A hose or pipe has to be certified for use with potable water if the water is intended for potable use. If not intended, the most important thing is for it to be UV stabilised. Rainwater is pure water that efficiently draws other substances like plasticisers. Irrigation stores usually sell flexible potable use hose cut to size.

Are you off the mains water grid?

Thanks again for your advice SaveH20

You have made me realise I have failed to properly consider head pressure loss. I was hoping not to have to rely on a vertical riser but will have to create one now. Ok, so I'll need a tee piece at junction with tank to allow for vertical riser to go to top of tank and into mesh inlet and flexi hose to feed into 50mm inlet at bottom of tank. Thanks for this. I would have stuffed this up.

I have mains water and the tank will just be for the garden but wanting to keep options open for the future to potentially feed into house and use for potable water.

One half of the house will serve 1 x downpipe for wet system (hence reason for this advice) and other half a dry system as gutter and location of tank allow this.

Thanks for your advice regarding flexi hose. I'll source this.

Humanum est errare.

Best practice is to use our low restriction inlet system with a standard wet system provided the water passes through mosquito proof mesh before directly diverting into the tank. The low inlet valve can be any size but obviously hydraulically calculated for the size to be inflow adequate when no vertical riser is used.

If you would like me to calculate the inlet and carrier pipe size you will need and the head required in order to eliminate the riser, could you supply me with the following information thanks.

1. Your region OR your 1:20 Average Recurrence Interval (ARI) if you know it.
2..The roof plan area draining to the single downpipe.
3. The roof slope. (Standard roof slope is 22.5 degrees).
4. The downpipe size.
5. The distance between the downpipe and the tank.
6. Number of 90 degree elbows.

Fitting a low restriction inlet to a standard wet system fitted with leaf diverters will most often also solve the problem of head restricted leaf diverters overflowing...a simple solution. There were obviously no hydraulic calculations made with the system below.

Best Practice advantages of fitting a low restriction inlet to a wet system also fitted with leaf diverters.

1. The low restriction inlet's hydraulic head is variable as it reacts to the tank's water level, NOT to the top of the vertical riser above the tank.
2. Flow is prioritised to the lower inlet. The emptier the tank, the greater the increased flow rate.
3. The oxygen rich water refreshes the anaerobic zone.
4. Water is retained in the wet system to the height of the water in the tank, not to the top of the vertical riser. This exposes less water to ambient and radiated heat.
5. Leaf diverters stop mosquitoes accessing the standing water and also prevent most debris from entering the carrier pipe.

The most effective (in order) use of good quality harvested rainwater is...
1. HWS.
2. Laundry if you cold wash in a top loader.
3. Toilet cistern but you can slash installation costs by gravity feeding to a separate rainwater cistern valve.

If you use a poly nut and tail at the tank's inlet, you will be able to replace the hose at any time should you later need/want to.

Hi SaveH20

I would really appreciate if you could do some calculations for me. Thank you kindly. I should note that some things are pre-determinged, e.g. I have already purchased some materials, 100mm DWV pipe and tank is in situ. However, it is not plumbed or connected yet so can hopefulyl rectify things to get a good result.

1. Region = Castlemaine. VIC. Sorry, wasn't too sure how to work out ARI
2..Roof area = 70m2
3. The roof slope.= 1 x very steep, gable/ pitched roof which then smoothes out to subtle angle before gutter. Tin (colourbond roof).
4. Downpipe size. = was planning on getting DWV 90mm. At the moment 75mm in use and stormwater runs off to paddock.
5. Distance between the downpipe and the tank = 14 - 15 metres.
6. Number of 90 degree elbows I'd need to use = 2.
7. Tank = 16,000 litres. 50mm inlet about 100-200mm from bottom. Tank has three outlets for use. mesh inlet on top of tank = 500mm.
8. Height from verandah post where I plan on installing Rainharverting Orignial left diverter (bottom of diverter) to height at top of 500m mesh inlet on top of tank = 220mm.

FYI - other half of roof, i.e 70 m2 roof section will flow into tank with a dry system. I just need to reorientate the gutter.

Thanks for your time and energy.

Eaves gutter compliance for gutter and downpipe size and the maximum roof areas they can drain references your 1:20 ARI which is most likely 130 mm/hr. This figure is based on an average rain intensity of 2.17 mm/min over a 5 minute duration.

The roof plan area is multiplied by a slope factor for compliance to account for wind driven rain. This calculates a larger roof harvest area that is used for roof drainage compliance calculations. Given the description of your roof, I can only guesstimate a multiplier of 1.3 to arrive at a roof harvest area of 70 sq m x 1.3 = 91 sq m. I mm of rain on 1 sq m = 1 litre. During a heavy downpour classified as a 1:20 ARI and no wind, the roof will yeild 70 sq m x 2.17 mm/min = 151 lpm but if the roof harvest area was facing a wind, then the yield could be 91x 2.17 which is about 198 lpm.

You would use the leaf diverter's reservoir overflow point as the top of the water column, not the bottom of the leaf diverter but you also need to seal where the drain pipe attaches to the bottom of the leaf diverter as most leak. If you look at the photo I pasted, you will notice water streaming out from the bottom.

When calculating the minimum head for the low inlet, you have roughly an extra 100mm or so of head by including the reservoir when full plus the bottom of the tank's overflow pipe will be at least 100 mm below the tank's flat roof level. If we add say 150mm (only 50mm for the overflow outlet) to the 220mm, we arrive at 370mm head but lets say 300mm. I'll just add here that it takes x4 the head to double the flow rate but also remember that a low restriction inlet has a variable head due to the tank's water level constantly changing.

IMPORTANT NOTE: The 1:20 ARI figure is the minimum figure required to qualify a heavy rain event as a 1:20 ARI, For this reason, a safety margin meeds to be factored into the design. In reality, many house gutters overflow during a 1:20 ARI but better roof drainage designed houses won't. I have been told that Castlemaine had three 1:100 ARIs between 2009 and 2017.

Sooooo, what we have is a system that really should be designed to be able to divert about 240 L/min.

You definitely need to upsize the downpipe to 90mm round.

You have already bought 100mm (104 mm ID) DWV pipe. Fitting this pipe normally causes continuous sediment build up because the velocity through this pipe in your wet system is manifestly insufficient to flush the pipe. To explain, the pipe's internal volume is 8.5 litres per metre. Even if you had a (minimum qualifying and no wind) 1:20 ARI, the 151 lpm would only flow with a velocity a tad less than 0.3 metres per second. Water running along the bottom of a sloped pipe is turbulent yet still requires a minimum flushing velocuty of 0.7 metres per second BUT a flooded 100mm DWV pipe will have laminar flow and flow fastest through the core with stationary water on the walls.

Fortunately, you are also fitting a sediment trap. There will be some minor turbulence at the elbow and the best place to fit the trap in this instance would be about 2 m from the downpipe.

Allowing for the equivalent pipe length of the 100mm elbows plus the 15 m length of pipe gives a flow rate with a 300mm head of about 585 lpm if it were to connect to a 100mm valve. 100mm valves however are expensive, you should enquire about a 65mm valve and downsize the 100mm pipe to 65mm a metre or two before the tank. The 65mm DWV pipe holds 3.28 litres per metre and so a 1 metre per second flow rate equates to a mighty 197 litres per minute. 65mm is an ideal size for the inlet.

LATE NOTE:
The tank's overflow requirement is based on the roof plan area which is 140 sq m. This will yield a tad more than 300 lpm (140 x 2.17) during a minimum qualifying 1:20 ARI. Unfortunately and rather bizarrely, tank manufacturers do not state their tank's overflow capacity nor will they or the salesman even know and practically all plumbers don't know how to calculate it either. What size will your overflow outlet be?

As a matter of interest, has your tank's overflow size and roof area harvested been discussed with the tank seller?

Hi Save H20. Your response has really helped my understanding. Thank you.

With the information you provided above is that provided on the assumpton I also use a vertical riser from the bottom feed inlet to meshed inlet on top of tank?

I'll be relying on a 50mm inlet unfortunately, as I don't want to have to cut out a new section in the tank to make it a 65mm or 100mm.

I was not provided any information about the tan koverflow capacity, however, I will check with the manufacturer. The overlow outlet is 90mm. I plan to purchase one of those Rainharvesting air gap adapters to help matters. They did do the claculation of tank szie based on roof area and recommended a 20,000 litre. It became a 16,500 litre tank in the end as the 20,000 size was too tall and provided next to 0 head. With a slighter smaller tank (16,5000) and a little bit of excavation I was able to get the head I have now, albeit not that ideal.

Regards
Staacey

I always keep in mind that other interested homeowners also read these threads and I try to promote a wider understanding rather than simply instruct so they too are able to help others. Docendo discimus.

Combining the 100mm DWV pipe with a shortish 50mm flexible hose will allow you to eliminate the vertical riser. Read on to find out why.

The 50mm low restriction inlet uses less pipe and elbows (less friction loss) plus gains head over a same size vertical riser by virtue of the water level in the tank always being lower that the level of the water discharging at a height above the riser’s top elbow's invert.

You have an unusual situation that I haven't come across before in that you have a single high yield (but currently undersized) downpipe and a supply of 100mm (104mm ID) DWV pipe. Normally it would be unwise to plumb the 100mm pipe because of the lack of sediment flushing velocity even during a high intensity rain event. The saving grace of course is the sediment trap. The smaller 80mm and 65mm DWV pipes and fittings are also usually more expensive than the much more commonly used 100mm pipe and so it's a good result.

Calculating required head when having a single downpipe even when there are two different pipe sizes is quite easy. We are able to get a good idea of friction losses generated by various hydraulic heads through different lengths and sizes of pipe by using the simple friction loss calculator below.
https://www.nationalpump.com.au/calculators/friction-loss-calculator/

Let's design your system to cope with a 250 lpm inflow rate using a 13.5 m length of 100mm DWV pipe joining a 1.5 m long 50mm flexible and UV stabilised hose that connects to a 50mm low restriction inlet.

By entering a 250 lpm design capacity flow rate and the 104mm diameter of the 100mm DWV pipe, we find that the pipe loses 0,24 m of head over a 100 m distance. In other words, this is the required head to generate this flow rate. There are variables such as a wet system's usual sediment build-up but this will not be applicable to your system.

The two 100mm 90 degree elbows are each deemed to have an equivalent friction loss pipe length of 3.7 m. If you use a 13.5 m length of 100mm DWV pipe with two elbows, the total equivalent length of 100mm DWV pipe is 20.9 m. The friction loss is therefore 240 (mm) x 0.209 = 50.16 mm. Let’s say 50mm.

To remove any doubt, we can also run the figures through a flow calculator BUT it must be one that is used for gravity pipe flow. A Hazen-Williams flow calculator satisfies this requirement.

https://www.omnicalculator.com/physics/pipe-flow

4.17 x 60 = 250.2 lpm.

Now for the 1.5 m of 50mm flexible hose. This can be complicated because of the turbulence at the 100mm x 50mm reducing taper between the 100mm pipe and the 50mm hose but we won’t worry too much for now. You will see why later plus the flexible hose will I assume more likely be one meter or so in length..

Friction loss is 84.8 mm per metre or 127.2mm for 1.5 m due to pipe friction losses increasing exponentially with increased velocity but we have to add the 50mm head loss for the 100mm pipe, bringing the total head loss to let’s say 180mm. We do however also have to factor the turbulence sapping friction loss at the taper but if you have measured the head from the bottom of the leaf diverter and not at the bottom of the mesh to arrive at 220mm head, the available head would be +300mm.

I have to stress here that these simple calculations are for a single downpipe, not downpipes in unison that also serve as water tower pressure recharge stations along a carrier pipe.

Pipes have friction losses all along their length even though the water flows along the pipe at the same velocity. This video demonstrates this. https://www.youtube.com/watch?v=_hSL9_eo4n8

Tell me their response because they won't know. This is vital information and it is staggering that manufacturers don't provide the information or even (seemingly) realises it's importance and don't bother asking a plumber. Just crazy.

A great product. Minimises maintenance and increases the outlet's flow capacity. Brilliant.

This demonstrates a common sub standard practice. Knowing the roof area to be harvested should have them also calculate the roof drainage rates at varying rain intensities. Your area's bare minimum qualifying rain intensity for a 1:20 ARI is most likely 2.1666 mm per minute (130 mm/hr) average intensity over a 5 minute duration. This equates to a total roof plan yield of 303 lpm.

A 90mm PVC-u stormwater pipe is measured as an outside diameter, the inside diameter is 86.2mm. The mesh invariably has an impervious coating around the wire mesh outer edge which reduces the flow path to about 84mm.

Meshed '90mm' outlets flow at about 190 lpm wirh minimal mitigation (maybe 15 mm) above the outlet while the unmeshed ones flow at about 235 lpm. Your tank's overflow capacity is less than your bare minimum 1:20 ARI inflow capacity.

I would design the tank’s overflow capacity to be about 370 lpm. You can easily achieve and exceed this by having a second overflow pipe connecting to a 45 degree junction fitted to the outside vertical overflow pipe but above the air gap.


A familiar high mounted overflow pipe with minimal mitigation. Mitigation provides a storage and higher discharge safety buffer during heavy rain. A horizontal drain pipe is very inefficient when compared to a vertical drain pipe.