Sludge

A group of fine particles suspended or deposited in the bottom layers of rivers, lakes, harbors, and ocean areas. Civil engineering workers lamented the difficulty of handling the material and the unpleasant odor it caused, and the term was coined from the words "hedo" (sludge) and "mud" (mud) to describe the material's properties. In the late 1960s, Tagonoura Port in Shizuoka Prefecture was filled with paper waste and waste liquid discharged from nearby pulp factories, restricting the passage of large ships through the port, attracting great social concern. This incident was called "hedoro pollution," and the term spread and has come to be used widely in connection with environmental pollution.

Mud is derived from human activities or nature, and is broadly divided into organic and inorganic. The most common problem is organic matter and toxic substances from domestic and industrial wastewater. When organic matter decomposes, it first consumes the dissolved oxygen in the water, and as anaerobic decomposition progresses, toxic gases are produced, and in either case, it hinders the habitat of aquatic life. It also releases nitrogen and phosphorus, which causes the abnormal growth of algae and leads to eutrophication. Harmful substances include heavy metals such as mercury, cadmium, and lead, as well as organic matter such as PCBs, pesticides, and dioxins, which not only pollute water systems, but also bioaccumulate in the food chain, causing contamination of fish and shellfish. Meanwhile, fine particles in river water coagulate and settle when they come into contact with seawater at the river mouth, and in this process, they often turn into mud.

Measures to combat sludge include reducing the inflow of pollutants, introducing purified water, dredging the sludge, etc. Dredged sludge is dumped or disposed of as is, or after solidifying and stabilizing it with the addition of coagulants or cement, but great care must be taken to prevent secondary pollution.

[Takahashi Takao]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

河川・湖沼や、港湾・海域などの底層に浮遊・堆積(たいせき)した微細粒子群のこと。土木工事関係者が、その取り扱いにくさ、異臭などによる不快さを嘆き、「へど」と「泥」から造語した俗語で、そうした性状を有するもの。1960年代後半、静岡県田子ノ浦港が周辺パルプ工場から排出された製紙かすと廃液で埋まり、大型船舶の航行が制限されて社会的に大きな関心をよび、「へどろ公害」とよばれたことから、このことばが広まり、環境汚染と関連して広く使われるようになった。

　へどろは人間活動あるいは自然に由来し、有機性のもの、無機性のものに大別される。生活排水や工場排水に由来する有機物や有害物を多く含むものがしばしば問題とされ、有機物の場合、分解されるとまず水中の溶存酸素が消費され、嫌気性分解が進むと有毒ガスが発生して、どちらの場合も水生生物の生息を妨げる。また窒素やリンを放出して藻類を異常発生させ、富栄養化を引き起こす。有害物には水銀、カドミウム、鉛などの重金属類やPCB、農薬、ダイオキシン類などの有機物があり、水系を汚染するとともに、食物連鎖のなかで生物濃縮されて魚介類の汚染を引き起こす。一方、河川水中の微粒子は河口で海水と接すると凝集沈降するが、この際に、へどろ状を呈することが少なくない。

　へどろ対策には、流入汚濁負荷の軽減、浄化用水の導入、へどろの浚渫(しゅんせつ)などがある。浚渫されたへどろは、そのまま、あるいは凝集剤やセメントなどを添加し固化・安定化して投棄・処分するが、その際、二次汚染の防止に十分な注意が必要である。

［高橋敬雄］

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