Connecting multiple tank overflows into stormwater

I have three water tanks side-by-side (2 x 4000g; 1 x 2000g)…just had a new one delivered. The larger tanks have 2 x 90mm overflows, while the 2000g tank has a single 90mm overflow. I am wanting to connect all these overflows to a single stormwater line, which will run past all the tanks and then out to the garden (rural property, no stormwater system to connect to). <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p>
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My questions are:<o:p></o:p>
o Could I get away with a 90mm stormwater line or should I increase the size to, say, 100mm? I want to avoid any possibility that the overflows might back up in a storm situation (in the past, I had two tanks feeding into a 100mm line & had no backup problems…I found the gutters tend to overflow in a heavy downpour situation anyway, so not all the heavy rain makes it to the tanks).<o:p></o:p>
o Assuming that the 100mm stormwater line is best, what is the best way to connect the 90mm downpipes into the 100mm line? (I’m thinking a 90-100mm connector + a 100mm y-shaped junction to take the 2 x 90mm downpipes to a 100mm t-junction for each tank, so that when the 2 x 90mm downpipes flow together it will be in a 100mm pipe…the problem is that the 100mm fittings seem to be about double the price of 90mm).<o:p></o:p>
o Also, is there a difference between 100mm PVC pipe & 100mm sewer pipe? Is one thicker, are the fittings going to be more expensive?<o:p></o:p>

Mate I would go with 100mm for piece of mind, the 90mm stuff is rubbish.
100mm sewer is PVC, or UPVC to be correct, as is 90mm and any other of the commonly available "plastic" drain pipes.
You can get 100mm stormwater pipe but I have only ever seen it in rural plumbing supply stores.
Yes 100mm sewer grade pipe is more expensive that the 90mm stuff but its a much better job in my opinion and with 90mm pipe its really brittle and easy to break, also the 90mm 90degree bends are a sharp 90 degrees where the 100mm bends have a nice radius, the same for the junctions so if it does block up, you have next to no hope of getting an eel or waterjwet around a 90mm elbow.
To adapt you simply use a 100 x 90 bush or 'donut' as theyre called and they come in two types, one fits inside the 100mm pipe and the other fits inside the collar of a 100mm fitting.
And make sure you use primer on the pipe and fittings.......:2tsup:
Also tanks sometimes have two overflow points so that you can use either end at your convenience and cap the other one off.