Chicago covers alleys with special concrete that cleans polluted rain runoff . The city has been working to clean up the water since it was first built in the early 20th century and is now one of only a handful of cities around the world using this type of concrete. Solution avoids having to rebuild city 's massive alley system . Unique concrete also helps to keep city cooler on sunny days .
Chicago covers alleys with special concrete that cleans polluted rain runoff . Microbes in concrete eat pollutants such as fertilizer and oil . Solution avoids having to rebuild city 's massive alley system . Unique concrete also helps to keep city cooler on sunny days .