Cleaning concrete walk and driveway, any tips?

These areas receive a lot of shade from a large water oak. I don't have mildew but dark areas on concrete. I have tried bleach but it takes a lot and is not something I like to add to the environment, I have tried an electric power washer, but not really doing the job and we are on water restrictions here in Florida. Has anyone found either an over the counter product or a homemade solution that works.
q cleaning concrete walk and driveway
q cleaning concrete walk and driveway
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