The underground chipping will be done when sewage flow is lightest - about 4 to 9 a.m. - company spokesman George Bochanski said.
That doesn't mean a problem with morning showers or a no-flush order, said Water Department spokeswoman Joanne Dahme, who said the sewer has plenty of extra capacity.
The gummy layer - which will be shipped to a hazardous waste incinerator
because it contains toxic phenol, chloroform and 1,2-dichloroethane - extends for only 275 feet inside the sewer. But it was enough to stall the DDT- vacuuming project last year.
That part of the project is now scheduled to start in July, when divers will go after the sandy DDT-tainted sediment, which lies in 1 1/2 miles of sewer an average 2 1/2 feet thick.
Although the DDT doesn't test at hazardous levels, it has tainted all the sludge from the Northeast sewage plant, dooming it to the landfill and limiting the city's ability to market compost. DDT is listed by the Environmental Protection Agency as suspected of causing cancer.
Total cost to the company to get the stuff out of the sewer is now estimated at close to $16 million, Bochanski said. Rohm and Haas also has spent millions to landfill the sludge.
From about July to December, sediment will be hauled to the surface. It, too, will go a landfill.
