Is there anyone in India who is willing to develop a multi-stage Saferain unit with me?
Imagine a 5000 sq metre factory in New Delhi, (or Shanghai, or wherever).

The air is polluted, the roof is polluted, and, yet, I say that potable. or near so, can be delivered in a rain event. The following is a cut and paste. This is how it's

Nobody has yet designed a system that can filter the solubles and insolubles from a large roof that has 150mm downpipes. Big roofs in polluted industrial areas.

The ideal setup should

• have a minimum pressure drop
• deliver water of acceptable purity such that only a light touch of  UV,O3 or chlorine is necessary to clean up to food grade standard and
• require minimal or no maintenance.

Referring to the attachment which is a single stage 100mm retrofit unit, where it says the "joining pipe", I propose to add a second and a third  and a fourth Saferain (and more, if necessary) in descending series.In pairs. I propose to change the incoming pipe from 100mm to 150mm. So, the first unit is the weakest and gets rid of some of the insolubles. This unit will also slam prematurely shut in the event of a sudden cloudburst. The second, having a higher setting, will take longer to close. And so on. Each stage is alternately left handed and right handed such that each pair returns to the central axis.
I will soon have a drawing of the setup.

I envisage the first discharge to go straight to drain. The second and third and fourth will go to a  tank for toilet flushing and other non-potable uses. Each stage will progressively deliver purer water. And, since once a unit closes, the incoming rainwater only has to go around a 45 deg bend, there will be minimal back pressure. ALL of the incoming water is used except for that first bit.

Compare this setup with Wisy. This, as you should know, does not get rid of solubles. It wastes 10% continuously. It must have a high back pressure since the incoming water changes direction into a swirl. And it costs $900 for a 150mm inline unit. Don't know whether it requires maintenance.
We would need a even number of 6 inch junctions/ Saferain components so as not to have asymmetrical instability.
I am only contemplating 150mm downpipes. The 100mm Saferains are attached to the lower part of the 150mm junction  by a 150/100 reducing bush.
Since the exit diameter of the first flush base of Saferain is only 37mm, the maximum flow rate that it can handle is 35-40 litre/min. Above this, backup will occur. Assuming that the first Saferain does not close in time. This will not matter since this backup will go into the second, lower, unit.
Each unit will have to be designed differently. I have some thoughts on that. The first would have a lower setting but would be designed to withstand the full force of a sudden cloudburst. Since, once a unit is closed, the incoming rain goes around the 45 deg bend and the water inside the unit is relatively static. From past, limited observations, turbulence does not seem to be a factor, either.

The above multi stage unit has never been attempted.
You could have as many stages as necessary.
Obviously, there many questions you would want to ask.

• Durability
  • Saferain has been going since 1994. . I have sold 5000+ units. 500 in the US. I have sold in Brazil (to a university!), Slovenia, Ireland, Malta, Israel etc. I could go on. Each environment has had its particular challenges. All have lasted except for a few due to unique circumstances. It is easily refurbished. I have learned a lot about materials. If you google saferain, you can see the various references. The reference in therainwell is worth looking at. Saferain is durable.One customer recently wanted to know from Pioneer Tanks in Adelaide from whom he purchased it whether it was still covered by warranty after 12 years as it had failed, finally.
• Possibility
  •  There are many parameters to consider but, I believe this could be achieved. It is intensity and duration that determine the pollution removal from a roof. One unit may provide the initial intensity cutoff but it will not provide the necessary duration. 4 or more will in a cascade.

The big advantage is that there is little pressure drop.