The present invention relates to a high-molecular flocculant. BACKGROUND OF THE INVENTION This invention relates to a high-molecular flocculant rendered water-soluble by conversion of cyano groups contained in a high-molecular material, a method for producing the flocculant, and method for efficiently processing water using the flocculant. Description of the Related Art In the field of processing waste water, a flocculant is used as a reagent for causing aggregation and precipitation of micro-sized particles dispersed in waste water. In the waste water, such as industrial waste water, it is a frequent occurrence that mud or high-molecular impurities . are dispersed as colloidal particles. The flocculant serves for aggregating these colloidal particles for purifying the polluted water. Of the wide variety of known flocculants, those having a molecular structure of a long chain and exhibiting high hydrophilicity are high-molecular flocculants which are used extensively because addition of only a limited amount thereof to the colloidal particles leads to significant aggregating effects. The high-molecular flocculants are classified into an anionic type, a cationic type and a nonionic type depending on static charges on dissolution in water. Since a majority of colloidal particles are charged to the positive or negative polarity, extremely large floes are formed to lower the turbidity highly efficiently by properly selecting the high-molecular flocculants depending on the polarity of tpe static charges. In the field of manufacturing industrial products, high-molecular material, containing acrylonitrile as a monomeric unit, are used extensively. Among the high-molecular materials containing acrylonitrile, there are polystyrene based resins, typified fey nitrile |' resins, ABS (acrylonitrile- butadiene- styrene resins), S4N resins (styrene- acrylonitrile resin), AAS resins- (acrylonitrile- acryl-styrene resins) and ACS. resins (acrylonitrile- chlorinated ! polyethylene- styrene resin), acrylic fibers obtained on spinning a i : polymer having acrylonitrile as a main monomeric unit, ^nd NBR ' i (acrylonitrile- butadiene rubber, also termed nitrile rubber); A resin molded product, containing acrylonitrile as monomeric units, is superior in stiffness, dimensional stability and workability | ; and hence is used frequently! as a cover or a casing foif various j. usages, a casing for an electric appliance or a car or as a material for components. The acrylic fibers are lightweight, bulky and is excellent in heat insulating properties, skin touch feeling, weatherability and resiliency, so that they are used extensively for apparel al