Once considered a useless waste product, oyster shells are gaining a reputation as an environment-friendly wastewater cleanser. An experiment launched five years ago in a town in Miyagi Prefecture, Japan has shown that the shells make very cheap, effective water purifiers. A 450-meter section of a 10-kilometer-long drainage canal running through eastern Hasamacho, about 60 kilometers north of Sendai, is lined with white oyster shells. Through the 40-centimeter-deep water, it can be seen that the shells are covered with dark brown slime. Upstream the water is muddy and smells, but it is clear and odorless after going through the "oyster filter."