What is "screenwash"? Is it windshield washer fluid? What would people use instead of windshield washer fluid? water alone?
Sounds like an opportunity to make fluid that doesn't allow that bacteria to breed. I'm guessing something high in citrus.
Yes it's washer fluid and apparently the washer fluid kills the legionella bacteria which would grow in warm water like the water in your washer tank which gets heated cooled and reheated when you use your car.
I don't have any windshield wiper fluid :O