All very good questions.
Firstly, Adelaide has the highest % of water tank ownership of all capital cities - for good reason. Adelaide mains water is c##p and being able to wash your hair in rainwater must have some intrinsic value.
DOWNPIPES. The number will depend on their size but for 90 mm round or 100 X 50 mm rectangular, your house will probably need a minimum of 8 plus another for the pergola. A lot also depends on the roof pitch as there is a multiplier used on the roof plan area to allow for the effect of wind. For a standard 23 degree pitch, the roof plan catchment area between the gutter high points is factored as the area X 1.21 as per AS/NZS 3500.3.4.3(b) table 3.2.
If you have slotted gutters, the cross sectional area's effective carrying capacity will be smaller than an unslotted gutter of the same size. This means that you will need even more downpipes but this is an area of rampant non compliance in Australia so be aware if you are considering slotted gutters.
RAINFALL. Let's work on 549 mm per year (the 1977 - 2012 Adelaide average) on a 330 sq metre roof.
Most people (and many websites) would calculate the roof harvest as 181,170 litres per year. If you check the stats for Adelaide, it will show that over 1/3 of the average number of rain days per year have less than 2 mm per day. When you total this with the initial evaporation that occurs during warmer months on days when rainfall is greater than 2 mm, then you have lost a minimum 8% yield before first flush and other losses. If you also calculate a first flush loss of .5 mm per sq m on each rain day, you will see that yield loss is very real.
An 85% yield is considered very good and nearly impossible to achieve if you also have a decent first flush. An 85% yield would reduce the 181,170 litres back to 153,995 litres. Wet systems complicate the equation as they can hold a lot of water and most need flushing due to their retention of organic matter.
PVC PIPE. These use to contain lead but regulations now state that products in contact with drinking water must comply with AS/NZS 4020. This was not forced on the industry as the industry initiated the change.
The biggest threat to water tank quality comes from roof contaminants like possum droppings, foul wet systems and drawing the worst quality water from a very low outlet valve supplying the pump. If you have a good harvesting system and use a suitable filter, you won't have any problems. Most Adelaide people who drink tank water may not even have a filter fitted; it would be an interesting statistic to know. To access the best quality water, you could fit something similar to this device...
http://www.crystalclearwater.com.au/waterboy/
USAGE. Utilisation will depend on your annual usage and what the regulations allow. I cannot advise on Adelaide regulations. If you did calculations based on harvesting 150,000 litres, this averages at 410 litres per day. If you use a standard wet system that would entail regular flushing, you would not have this yield.
Plumbing to the main toilet and having a rainwater cold tap in the laundry should be the first calculation done. You generally allow for 160 litres per household member per week for the toilet. Plumbing to the HWS saves a large amount of water, is cheap and easy to do but not often considered. You should also not overlook harvesting the washing machine's final rinse for garden watering.
WET SYSTEM. The vertical riser is the big problem with a wet system as it causes the horizontal pipe to retain bed load due to the pipe normally having insufficient velocity to flush debris up the riser but when it does rain heavily and bed load is flushed up the riser, it can block the meshed inlet. Retained organic matter creates anaerobic conditions as it breaks down and stagnation is the next step. Nearly all rainwater tank installations that have a vertical riser are sub standard. As per your concern, wet systems also waste a lot of water when flushed.
Another problem is that the vertical riser discharges above the top of the tank to the
top meshed inlet, reducing the available head. Wet systems also need leaf diverters on the downpipe and fitting these further lowers the available head. I have seen tanks installed where head calculations were taken from the gutter to the height of the pipe above the tank without allowance for leaf diverters and this has caused big issues later.
Full bore flow into the top of the tank also causes significant sediment resuspension through the generation of toroidal vortices that 'cloud burst' on the sediment layer.
A charged pipe can also accommodate the rainfall from several downpipes but the tank's overflow pipe is often not considered. Simply put, if the overflow does not have a capacity that is equal to or greater than the inflow capacity, the tank will overflow during heavy rain when the tank fills. A decent storm would easily do this from your roof area to a 10,000 litre tank and it must be remembered that the 85% yield I quoted is a yearly average; the yield during a storm will be much higher.
Better get on to the positives.
You should eliminate the vertical riser as these are ancient history. Simply have the tank fitted with a large inlet about 100 mm from the bottom of the inlet to the bottom of the tank and within a 75 degree arc from the pump's draw valve. Don't use 90 degree elbows to connect to the inlet (with a flexible coupling), only use 45 degree elbows.
Feeding into the tank from a low point will provide a variable and greater head pressure, oxygenate the anaerobic zone, the pipe will self flush, there will be less 'exposed' charged pipework, less maintenance, greater yield and it is neater and easier to plumb. A DIY sediment trap can also be easily and cheaply installed to divert most of the bed load with minor yield loss and substantially reduce sediment in the tank.
The pipe size and number will depend on the available head.
The provision of a tank overflow safeguard can be done two ways, one I can post and one I cannot as it involves a proprietary product. The first method is ugly. Note that there is also a way to eliminate the need for an overflow altogether (further cost savings) but I also cannot post these details for the reason already given.
A large tank doesn't cost all that much more than a small one. If I designed the system, I would also have another tank at the house for several reasons. It would probably be a tall 2,000 litre round tank.
The cost of plumbing to a toilet can be reduced substantially and I can further advise.
People also get ripped off with pumps. Using larger diameter pipes to reduce friction loss is the answer, not big pumps. Also option the tank with the outlet feeding the pump a minimum 100 mm above the bottom (unless you use a Waterboy).
Is it worthwhile? I think so. Just avoid the rip offs and option the tank well. Also have a 100 mm overflow.
The cost can be a lot less than you may think.
