ANDOVER — Environmental advocates for the Merrimack River, the technicians who operate the sewage treatment plants on its banks and people who said they just like to swim in it or kayak on it brainstormed Wednesday about what can be done to cut the growing amount of raw sewage that’s dumped into it.

On one day alone this year – Sept. 18 – the six plants that discharge untreated sewage into the Merrimack, during heavy rains to avoid being overwhelmed, dumped a putrid, 120-million-gallon mix of storm water and sewage into the Merrimack. Four of the six plants are in Massachusetts, including one in North Andover that serves Lawrence, Methuen, the Andovers, Dracut, and Salem, New Hampshire. Two plants are in New Hampshire.

In all this year, 650 million gallons of untreated sewage and storm water will get dumped into the river, Rusty Russell, executive director of the Merrimack River Watershed Coalition, said during a meeting about the problem at the Andover public library. The North Andover plant so far has dumped 70 million gallons, the most by far in a decade, Russell said. He expects the dumping will worsen as global warming increases rainfall across the Northeast.

The group reached one consensus: not much can be done without significant state and federal aid.

“Lawrence is one of the biggest problems,” Russell said, referring to the amount of sewage the city sends to the North Andover plant operated by the Greater Lawrence Sanitary District, which he said is about half what the plant treats daily. “It has one of the lowest incomes in the state. It has few businesses. The industry that raised the money to build (the sewage system) are long gone.”

“This isn’t a problem that can be cleared up in a year or two,” he added. “It can’t be. It would bankrupt the communities.”

Treatment plants that still dump into water bodies in the United States have been put under consent decrees with the federal Department of Justice to develop ways to contain the dumping, in exchange for getting waivers from the ban on dumping imposed by the federal Clean Water Act. Some communities that own the plants are separating the pipes that collect sewage from the pipes that collect storm water, which is the most expensive remedy because it involves digging up streets and installing separate collection systems. Others have built massive holding tanks to store the storm water until the storm is over and the water can be released to the plants without overwhelming them, as Lowell has done.

Although the federal government paid 85 percent of the cost of building or upgrading most treatment plants nationwide, including those with combined storm water and sewage collection systems, there is little money going forward to comply with the consent decrees.

“The federal government is telling communities, you need to do this,” Mark Young, the director of water and waste water treatment in Lowell. “But there’s no federal money.”

He said Lowell already has spent $150 million to reduce its discharges – called combined sewer overflows – by 80 percent.

“To get the other 20 percent, we need $1 billion,” he said. “The answer is more federal money.”

Russell recommended other ways to confront the problem, including passing a bill in the Massachusetts legislature that would require sewage treatment plants to issue broad and immediate public warnings when they spill untreated waste into rivers.

Rocky Morrison, director of the Merrimack Clean River Project, also attended the session. The mission of his group is different – removing the tires, cars, shopping carts and floating debris that is dumped into the river – but he said he has the same problem getting it done: funding.

Morrison recently sought up to $400,000 to buy a barge with a conveyor belt that would scoop floating waste from the surface of the river. However, communities along the river told him they didn’t have the money. When he suggested they seek state aid, they said his project “wasn’t a priority.”

Others who attended the session were state Rep. Linda Campbell; Cheri Cousens, director of the Greater Lawrence Sanitary District; and Brian Pena, commissioner of water and sewer operations in Lawrence. 

Dan Graovac, a Newburyport resident with a home on Plum Island, also attended. 

“I’m appalled that three days after a CSO, there aren’t more people here,” he said, referring to the combined sewage overflows that occurred over the weekend. “I’m in the water every day and I see sewage floating by.”