When we initially reworked the house, we didn't make any provision for re-use of grey-water to irrigate the garden. In our defence, this was before Melbourne entered its decade-long drought. I can actually remember the back garden being waterlogged in winter!
As the drought tightened its grip and water restrictions became ever tougher, we started to consider how we might make use of the water that was otherwise going down the drain. We decided to tackle this on two fronts.
The first was to work out how to capture and use the waste-water from the washing machine. As the block slopes down from the house, we could use gravity to feed the water down to the fruit trees. We decided to only water the trees and not the vegetable garden so as to avoid any problems with bacteria in the water. I fitted a valve into the drain from the laundry to allow us to direct the water into a hose or down the drain as normal (we leave it on this setting for winter). The hose runs down the block and is long enough to move around from tree to tree and put into the perforated hose that I have dug into the ground uphill from each tree.
There are two issues with the washing machine in re-using this sort of water. The first is that the water comes out more quickly than the hose can deal with. I solved this by fitting a surge tank below the house. This is a 90 litre plastic storage tub, with the hose draining it from the lowest point. This takes a full cycle from the washing machine. The second issue is the detergents in the water. We had already solved this by shifting to using Aware brand washing powder because of skin allergies in one of our sons. This is low-phosphate, and only uses natural surfactants and fragrances. We haven't noticed any problems with the trees in using this as the only irrigation in summer.
Shower water is a bit more difficult to re-use, largely because the sorts of products used in the shower are less benign than the laundry powder described above. We ended up going for a low-tech solution, and showering with buckets. We only put the buckets under the shower when we aren't rinsing off. From a four minute shower, capped at nine litres per minute, we usually end up with two buckets of water. Times two of us, this is 36 litres of water to put on the front garden. Combined with some careful planting and heavy use of mulch, this is enough to keep a cottage garden going through the worst summer.
The hose I fitted from the washing machine, while larger than a normal garden hose, still gets airlocks. These can be worked around, but it means that the water doesn't drain without assistance. I should have investigated a larger diameter and ribbed hose that would have performed better.
Nothing, really. Other than the hose problem this works pretty well. It was the only thing that saved our fruit trees during the summers when we could only water twice a week and the temperatures were extreme.