what is the fastest way to clean algae off patios, no chemicals please

Well Molly, one suggestion I came up with is sharp sand sprinkled on your paving washed off with rainwater after brushing with a stiff brush.

very stiff brushing with hot water, not necessarily fast but will burn lots of calories.

A high pressure washer, nothing else comes close! I know you said no pw suggestions but others might. I do my patios annually and its great to see the slimy stuff disappearing before my eyes. Then all the 'gunge' collected can be used on the compost heap. I also use mine for cleaning out my greenhouse in spring - care needed when spraying the glass mind.

I know you say no chemicals, but have you heard of Algon? It is completely safe to use around animals and ponds apparently. I don't like chemicals but having read about, I do use Algon, to great sucess. Worth you looking at the bottle,