LAWRENCE - John Griffin is out on the Merrimack River almost every day, so he was happy to hear about a $20 million construction project at the Greater Lawrence Sanitary District's sewage treatment plant that will mean less sewage getting into the river.

Griffin, executive director of the Greater Lawrence Community Boating program, has lived near the river all his life. He said anything that will continue to clean up the river is good news for the people who use it.

"It just makes sense," he said. "It's a great river, and you want to keep it nice."

Lawrence is the only community of the five communities that make up the GLSD that has areas along the river where sewage runs when the pipes overflow after heavy rains.

GLSD Executive Director Richard Hogan said the problem comes from an antiquated sewer system in the city, which dates back to the 1800s. It was built as a combined sewer, meaning rainwater drainage and wastewater coming from houses run into the same pipes. In newer systems, the pipes are separate.

During heavy rains, so much water rushes into the pipes that the wastewater treatment plant in North Andover can't handle it, so the overflow is discharged into the Merrimack, posing a potential health problem because of the untreated bacteria in the sewage.

Four discharge points are along the Merrimack River, and one is on the Spicket River near where it meets the Merrimack.

The GLSD was fined $245,000 last November by the Massachusetts Department of Environmental Protection and the U.S. Environmental Protection Agency for dumping an average of 112 million gallons of sewage and wastewater into the Merrimack and Spicket rivers every year since the 1990s.

The upgrade will increase the plant's capacity from 52 million to 135 million gallons per day, so the amount of sewage and wastewater dumped in the river will decrease to 45 million gallons per year, helping the plant meet requirements set by the state and federal government.

Hogan said this is Phase I of a project the district plans to continue.

"This is a good first step," he said. "We are heading in a direction of no combined sewer overflows."

The plant also supports Methuen, Andover, North Andover and Salem, N.H. Sewer improvements come out of the fees sewer users pay.

Lawrence Conservation Commission Chairman Tennis Lilly said combined sewer overflows are one of the three biggest environmental threats to the river, so any effort to clean it up is important.

"Even though the city of Lawrence has upgraded the sewer system, the fact they still dump untreated sewage into the river should concern everybody," Lilly said. "There's no reason to continue to have this in the 21st century."

The river is more than 40 feet deep, Lilly said, so anything dumped in the river gets diluted in the millions of gallons of water flowing. But after major flooding, he said, water problems downstream from the discharge points are visible, such as a tainting of the water.

How long the effects of the discharge last are still unknown, said Christine Tabak, executive director of the Merrimack River Watershed Council. The council has just started regular monitoring of the river as part of a long-term project to study the health of the river.

She said Lawrence is one of five communities that still have combined sewer overflows along the Merrimack River. The others are Haverhill, Lowell, Nashua, N.H., and Manchester, N.H. They are all working to reduce the overflow into the river.

Tabak said wastewater treatments plants need to continue to improve until sewage overflow into the river is stopped completely because the impact on the river can be significant, including loss of wildlife.

"Combined sewer overflows are a problem," she said. "The fact that these folks are working on reducing them is a good thing."

Box:

What the sewage treatment plant improvements mean for the river:

Before

Plant capacity: 52 million gallons per day

Number of combined sewer overflows: 14

Average amount of sewage dumped in river: 112 million gallons per year

After $20 million improvement:

Plant capacity: 135 million gallons per day

Number of combined sewer overflows: 5

Average amount of sewage dumped in river: 45 million gallons per year