Our shower drains into the grey water tank a

I've done the maths!

Shower water /is/ normally collected in grey water systems, but piss would be so diluted as to be a negligable contaminant. A shower uses roughly 70 litres of water. An average human bladder voiding is 400ml, so that works out at 0.57% piss in the grey water collected in a single shower. Negligable - but for fruit trees, a fertiliser.