Derwent hippo

Derwent hippo was the slang term for floating rafts of rotting wood-fibre sludge seen in the Derwent River, Hobart, Australia in the late 1980s.

At this time the Australian Newsprint Mill's liquid waste outfall dumped hundreds of thousands of litres of effluent into the river every year. The waste contained a mix of pine and Eucalypt wood fibres as well as a mix of chemical pollutants. When Derwent hippos were first seen there was an estimated four million cubic metres of sludge in deposits of up to four metres deep, extending from the mill waste pipe to the Bowen Bridge. In October 1988, flood waters dislodged approximately ninety per cent of this waste, which moved downstream and settled in the shallow bays and inlets near the Bridgewater Causeway. The sludge began to decompose and over the summer period of 1988-89 large rafts of greenish to grey-black gelatinous sludge began rising to the surface.

Derwent hippos floated owing to a supportive buildup of gas, primarily hydrogen sulfide (the gas that gives rotten eggs their characteristic smell), which is released as part of the decomposition process. The majority of the rafts beached along a stretch of the river from Austin's Ferry to the Old Beach. By the summer period of 1989-90 the rotting sludge had decayed further and the Derwent hippos did not return. By July 1990, about 900,000 cubic metres of sludge remained and this was washed downstream to a river section near Green Island. By this time the decayed sludge was an ooze rather than a solid and had only a moderately sulphurous smell. The environmental effect of this material was unclear but thought to be damaging to river-life, particularly benthic fauna. The Australian Newsprint Mill factory developed filtering methods to remove wood fibres from the mill waste being dumped into the river and Derwent hippos have not returned.

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