Green stuff growing on the sidewalk in the alleyway?

(7-9-15 2N)

This green stuff is very slippery, is a hazard, and cause for litigation if someone slips on it.

The goal is:

          1. KILL the living organisms (bleach)

          2. Wash away the dirt and dead algae/moss/mildew (soap and scrub broom)

          3. Only then can you seal the concrete with whatever you can afford (Thompson's water seal, etc)

        As of 7-9-15, chlorine bleach in small amounts was still a legal substance.
By the time you read this you may have to buy something more environmentally friendly.
(Oxygen Bleach will not kill it. Chlorine bleach/Clorox will.)
Don't waste your money on the stuff Home Depot sells.
I worked there and never
became sleazy enough to sell such a rip-off product(s).

Get a gallon container (or 2) put a half cup (or more) of bleach and detergent in the container and fill with the hottest water you can get. Hell, boil some if you can! Chlorine bleach works best with hot water. 

Put the stuff on however you can. I just slop it all around right from the container/bucket.

Then make sure it is in contact with every square inch of sidewalk. I spread it around with my scrub broom.

LET IT SOAK IN FOR 30 minutes. Do not waste your energy scrubbing too soon.

Test scrub an area to see if it is working.

Scrub and rinse until clean. You are using the scrub broom and water to move all the dead debris out of the ally to prep for the sealing step.

Wait a day or more for it to dry before sealing.

Please seal it. or we just have to scrub again next year. This alley also gets very slick in the winter, so use the proper type of sealer meant to be walked upon. Not one that makes it slick.