Improving existing rural water catchment and storage system

Excellent information, it certainly makes answering the Qs very easy.

Your system is actually well set up as far as standard wet systems go and certainly a lot better than most of the rural systems I get to see or asked to troubleshoot. However, there is room for improvement to attain rainwater harvesting best practice.

Having an adequate wet system sediment flushing velocity even if only at times during occasional heavy rain is essential to deliver good quality water as unflushed pipes build up sludge and promote anaerobic conditions.

(ND) 90mm pvc stormwater pipe has an internal diameter of 86.2mm which = a capacity of 5.8 L/metre. A velocity of 1 mps = 348 lpm (34.8 lpm @ 0.1 mps). Currently, one 90mm stormwater pipe harvests a 72 sq m roof (C) while the other pipe harvests a total of 145 sq m from rooves A, B and D. The pipe harvesting the three downpipes will have a chance of occasionally achieving a feasible flushing velocity but not so the pipe harvesting 72 sq m.

A major wet system design flaw I noticed while researching wet systems pre Supadiverta is the use of a same size pipe all along the wet system which means that one and commonly more downpipes furthest away from the tank provide no flushing velocity. This can have serious long term ramifications. Even with the smaller 90mm (86.2mm ID) stormwater pipe used in your system, the downpipe harvesting Roof B (40 sq m) would only collect a tad more than 80 lpm during heavy rain that reached 1:20 Average Recurrence Interval (ARI) status. With a flow rate of 34.8 lpm @ 0.1 mps, 80 lpm is only 0,23 metres per second.

If you were to use a 100mm DWV pipe which has an ID of 104mm and a capacity of 8.5 L/m, a V of 1 mps = 510 lpm or 51 lpm with a velocity of 0.1 mps.

It has to be remembered is that a pipe diverted to a water tank is NOT a stormwater pipe and not applicable to those regulations, in fact, the regulations do not permit stormwater to be collected for rainwater storage. If the tank's overflow pipe connects to a Legal Point Of Discharge (LPOD), the pipework is covered by stormwater legislation.

Because you have fitted leaf diverters, a good size pipe you can use between the roof B dp and roof A dp would be a 50mm DWV pipe. You have a lot of head and the 50mm DWV pipe has an ID of 51.6 mm and a volume of 2.01 L/m.

Because tank 4 accepts the diverted water from vertical risers that discharge above the tank, the wet system pipes remain full to the level of water retained at the top of the risers when it stops raining unless you fit a low restriction inlet valve. You are able to do this because you have fitted mozzie proof leaf diverters to all downpipes.

Re providing water to the house from either tanks 2 + 3 or 1 + 3, just remember that you are halving the velocity through each branch line.

Upgrading to 100mm DWV makes fitting a DIY sediment trap a breeze. The pipe reducer shown in the diagram has now been replaced by an invert taper. You lose minimal water when occasionally flushing the captured bed load, the capture is continuous and the retained wet system water is the last water off the roof which is the cleanest.
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Can you drill a hole in a fiberglass tank? Having a low restriction inlet will retain a lot less water in the wet system between rain events because the water in the riser and the pipes will never be higher than the water level in the tank.

You should always use a flexible UV stabilised hose and a ball valve. I recommend using a poly nut and tail to connect the hose to a poly nipple (a poly nipple has a male thread at both ends). The poly M+F elbow in the photo is also angled and the hose is 50mm.
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Using tank 4 as a settling tank is ideal. Also having floating filter intakes in tank 4 and tank 5 is an excellent idea as you will be drawing the best quality oxygen rich water in the tank. Our new floating filter intake is undergoing final prototyping and will be superior (and cheaper!) to current versions currently sold.

You can make a simple portable syphon/vacuum to clean the bottom of the tanks. I made one with a length of pvc pipe fitted with a pvc pressure valve socket connected to a faucet (female threaded branch) tee. The tee has a short capped.pvc pipe at each end and there are some holes drilled in the tee and slots about 5mm wide cut into the pipes. Slots are more efficient than holes and they are placed at about 135 degrees downwards. The total open areas are about x 2.5 the standing pipe's open area. A poly pipe connects to a faucet tee (pressure fitting at the top as it is easier to prime the suction if you fit an inline valve within arms reach part way down the standing pipe and a plug to the tee's port which needs to be also fitted with a short pvc pipe and a pvc pressure faucet socket. To prime, close the inline valve, remove the air via the tee, open the inline valve and vacuum the tank's floor.

Once you vacuum the tanks, it will take a long time for sludge to accumulate again when using a sediment trap...actually, the sludge won't be sludge, it will be a minimal spread that is much different (finer) to normal sludge.

Questions

Will a tank vac still work if its flow is redirected to tank 6?
The TankVac uses a 800mm long vertical syphonic downpipe section that generates syphonic flow. In short...no. The sketch shows water flowing up hill.

Is a check valve between tank 4 and 5 necessary?
It would have to be spring loaded to work and that would be an issue with gravity flow. Just manually open a ball valve whenever necessary.

Does this all seem like a reasonable plan?
Yes. Well thought out and a good example of rainwater harvesting best practice.

Does anything seem redundant or ridiculous?

It is best practice to have a flap valve on a vertical riser's discharge port.

Do you have copper pipes? Rainwater is naturally acidic but putting some limestones in tanks 4 and 5 would remedy this.

Last diagram, tank 6. You would need a hole at the apex to safeguard against a continuous syphon if one was to form...which would be near impossible. To create a tank's bottom suction effect, you would need the bottom horizontal pipe to be connected to a low outlet and manually open the discharge during times of excess. It won't work as per the sketch.

Correct! This is undoubtably the most important but also the most overlooked, unconsidered or just plain unknown facet of rainwater harvesting wet system design. Wet systems have a bad reputation but l'll go into a bit of detail here because a lot of people who read these threads have wet systems and it is vital to have an understanding of laminar Vs turbulent flow and their respective affects on sediment transportation and how they can be used to advantage

LAMINAR FLOW:
Water flows in boundary layers aka streamlines due to the friction of the pipe's walls resisting the flow. This phenonema causes water to flow fastest through the core and slowest against the pipe's wall. In many instances, the water on a pipe's wall regardless of whether it is at the top, bottom or side will be stationary. Laminar flow boundary layers do not mix!

Negligible particulate transportation but great area to fit a sediment trap.

TURBULENT FLOW:
Turbulence is caused either by a flow disruption or favourable velocity and is chaotic due to there being vortices, eddies and the mixing of boundary layers. Detritus is either suspended or pushed along the pipe unless the flow becomes laminar.

It is an endemic mistaken belief that sediments are transported by all moving waters and accumulate near the vertical riser. When there is laminar flow and insufficient velocity to maintain sediment suspension, the resultant bed load reconstitutes as a few small 'colonies' but once their size becomes high enough to be agitated by the increasingly faster boundary layers above, a small section will break away at the front and quickly travel along the bottom of the pipe to the next colony where the action is repeated. It is like watching a fascinating relay race. In time, the colonies increase in numbers and get bigger until there is one long colony that continues to build.

When organic matter is in the bed load, anaerobic loving bacteria will break down their nutrient feast which decreases the oxygen level. Bacterial die off then becomes part of the sludge which often forms a black tarry resinous substance that sticks to the pipes and is often seen in water tanks.

Large pipes at the head of a wet system are really bad news because the velocity even during heavy rain events is insufficient to generate or maintain an adequate flushing flow rate unless large roof areas in high intensity rain areas are harvested.

The video below unfortunately doesn't explain why increased velocity transitions laminar flow to turbulent plus most of the objects selected also project into the small pipe's higher boundary layers. I would have liked the vertical riser to be extended as a separate demo to show the effect of reinstated laminal flow on the suspended items but it is still worth watching. As you will also see, the disrupted flow through the elbow generates a lot of turbulence.

https://www.youtube.com/watch?v=c1xX90ZfBj4&t=347s

It doesn't have to be 40mm, it just has to generate enough velocity (turbulence) to flush the captured bed load. When flushing, you will see initial clear water then a slug of dirty water. That is when you close the valve. Incorporating the trap into the wet system will result in beter quality water and significantly lower yield loss due to the increased flushing efficiency.

The inlet's size depends on the carrier pipe size but when two pipes merge at a tee, then the tee is upsized and bushes fitted to the other two sockets.

The 90 degree threaded M+F poly elbow connects to the flexible pipe from any direction including from underground. It should be fitted within 75 degrees of the outlet that supplies the pump but not too close because of turbulence unless you are using a best practice floating filter.

You should never use a hard fitting on a tank or pump. The photo below shows what happened to a balance line due to tank distortion as it filled. If it was a pvc pipe, all water would have been lost.
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Yes, that's it but make it smaller.so you can manage it better. I have a 32mm one and the two visible pressure pipes between the tee and the caps are only exposed 100mm each but size and length depends on the tank's floor area. It is interesting that you used slots, they are much more efficient than holes. Also drill a hole near the bottom of each cap to extend the suction range.

When you have the sediment trap fitted, you will have minimal need to vacuum the tank for many years. The link is to one member's post commenting on his sediment trap. viewtopic.php?p=1551215#p1551215

Fitting a flap valve to the vertical riser discharge port prevents mosquitos and other pests having access.
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Are there not ball valves already fitted?

The TankVac initiates the syphon in the 80mm diameter 800mm long external overflow pipe. To do it, you need 'plug' flow to purge the pipe of air and the mechanism needed to do this is fitted inside the external pipe. It isn't rocket science to make but if you use a sediment trap, you will have minimal sediment and a much different type of sediment anyway.

The Blue Mountain 'vacuum' depends on having sufficient velocity up the pipe to transfer sediments and the velocity/flow rate will always be the same as the inflow rate. It is NOT a syphon in itself. The serated bottom edge has a minor cleansing affect in the immediate area but that's all.

Re Tank 4-6, why not retain the status quo

The floating intake filter is on temporary hold because I am about to prototype and test some late changes that I obviously can't detail here. This has delayed the 3D printing of two parts.

I am also negotiating to have a small inflatable buoy manufactured for customers with non potable use to reduce the packaging/post costs. The ones I have found so far are too big and often the wrong shape. Rigid ball floats suitable for use with potable water will also be available.

Our floating intake filter will be a vast improvement and also cheaper than ones currently sold.