When New York is hit by heavy rain, some human waste is run into the river untreated as "combined sewer overflows, " Waldman said.

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Referring to the 22, 000 discharge pipes, the charity said the figure only referred to pipes known as combined sewer overflows and emergency overflows.

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Second, the issue of combined sewer overflows is one of the most difficult water-quality issues faced by cities with old infrastructure like New York City.

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"The MCS recognises that Combined Sewer Overflows (CSO) are necessary to avoid sewage flooding roads or people's houses, and that diffuse pollution is an entrenched and difficult problem, " Mr Bell said.

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The findings indicate that wastewater treatment plants are effective at removing caffeine from the water, but that high rainfall and combined sewer overflows flush the caffeine out to sea.

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Like many cities, Cincinnati has a combined sewer system that is easily overwhelmed in wet weather, when untreated stormwater mixes with wastewater and overflows into water bodies or backs up into homes.

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