In recent years, water pollution and eutrophication in enclosed seas have become serious problems, such as red tide. We have designed a sea water purification system that apply the selfpurification capability of nature. The outline of this system is as follows: part of a polluted sea area is separated with a rubble mound. Due to the ebb and flow of tides, waves, and so on, the polluted sea water passes through the mound. The water is then purified by way of physicochemical and biological processes, and the enclosed body of purified and calm water can be used for a wide range of purposes; as a beach, tidal marsh, fish or shellfish nursery etc.
In order to verify the water purification efficiency of the rubble mound, we constructed an onsite model facility in a coastal area of Mikawa Bay which water depth was 2〜4m. The mound was composed of stones with diameters of 20〜30cm, and its surface was covered with large stones the diameter of which was about 80cm. The width of crest of it was 5m, its height was 5m, its slopes were 1/1.3 and 1/1.5, and its length was 6m. Steel sheet piles were driven at the shore-side of the mound. The enclosed body of sea water was therefore made with the mound and the steel sheet piles. Approximately 40 days after construction of the facility (July 7, 1993), the water purification effects gradually became evident. The monthly average values of suspended solid concentrations of water enclosed with the facility were 1-4mg/l, while those of water outside were 2〜9mg/l. The enclosed water has 40-50% less concentrations of suspended solid and chlorophyll, and is much clearer than the water outside.
In addition, we made an experiment of a substitute rubble material. A large amount of waste concrete is produced in the restoration and reconstruction of the buildings and public works damaged during the Hyogo-ken Nanbu Earthquake. It is available to reuse waste concrete as a substitute rubble material. When waste concrete is unloaded into sea directly, however, it is supposed that alkaline components' elution occurs from waste concrete and pH of sea is higher. Then, the pre-treatment method of waste concrete was tested in the laboratory. As a result, the amount of alkaline components' elution was much decreased by more than 2 weeks stocking in normal condition after rinsing out the attached powder elements.
Above mentioned, it is clear that this system has the sea water purification property and the waste concrete can be reused as a substitute rubble material.
