NOVELTY - Sludge obtained by dredging recovery, and a heat-resistant coagulant are add-mixed. The kneaded material of the sludge and coagulant, is formed into grain or block (1). The neutral or alkalescent porous grain or block having a strength which prevents crushing under pressure in polluted water, is then dry bake-processed to obtain a contact material.    USE - For purifying harbor, lakes and marshes (both claimed), fishery installed in rivers, dams, and other contaminated water areas.    ADVANTAGE - The contact material has a crushing strength which prevents crushing under hydraulic pressure or tidal current. The contact material also carries out effective adhesion and growth of aquatic plants and microorganisms. The sludge is effectively decomposed, and high-quality water is obtained. Purification of contaminated water area is promoted, and the dredging cost is reduced. The need for reservoir relaxation dredging of sludge is eliminated. The processing time is shortened.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) manufacture of contact material, which involves removing an organic substance during dry bake-processing the grain or block; and    (2) purification of harbor, and lakes and marshes, which involves using the contact material for purifying contaminated water area. The block-shaped contact material is supplied to the contaminated water area where sludge is deposited. The upper portion of the contact material is exposed to at least the upper portion of the sludge layer. An aquatic plant and microorganisms are adhered to the contact material. The water is purified while the marine plants and microorganisms decompose the sludge. When the sludge layer becomes thin, the contact material is supplied.    DESCRIPTION OF DRAWING(S) - The figure shows the block diagram of the manufacturing equipment of contact material. (Drawing includes non-English language text).    block (1)    sludge dredger (2)    operation ship (3)    sludge storage layer (4)    mixing tank (5)