Algae left unchecked spreads rapidly in dark, damp conditions. It can quickly cover roofs or exterior walls where the surface does not get much exposure to sunlight. Algae on a concrete wall is more of a cosmetic problem than a structural one -- the plant has no way of actually penetrating the surface and causing a structure harm. In fact, some people choose to leave algae on concrete walls for the unique look it provides. However, if you want to remove the algae from a concrete wall, expect it to return if you don't expose the area to more light.
Put on protective clothing. Wear a long-sleeved shirt and trousers, long plastic gloves and safety goggles.
Add 1 gallon water, 1 quart oxygen bleach and 1/4 cup trisodium phosphate to a hand sprayer capable of holding more than 1 gallon of fluid. Tighten the lid on the hand sprayer and move the container to mix the solution inside.
Spray the concrete wall with the solution in the hand sprayer. Cover all visible algae completely. Keep the concrete wet for 15 to 20 minutes, spraying more solution on the wall as needed.
Scrub the algae-covered concrete with a nylon brush after the allotted time passes. Start at the top of the wall and work your way toward the bottom.
Rinse the wall after scrubbing. Begin at the top and work your way to the bottom once more. Use a garden hose to complete the job quickly.
Inspect the wall for any remaining algae. Repeat the process, if necessary.
Algae can return if the damp conditions causing its growth are not corrected.
Avoid coming into contact with the bleach solution.