NEWBURYPORT — It seemed like they had mostly disappeared from our local shores, perhaps swallowed up by the vast sea or picked up by beachcombers.
But in the past few weeks, those little white discs that two years ago escaped by the millions from a New Hampshire sewage treatment plant have been showing up again, in large quantities. They are scattered all along Plum Island beach, they are caught in the reeds and detritus along the banks of the Merrimack River, and they’re peeking out from the storm surge debris found in waterfront places like Newburyport’s Cashman Park.
According to officials, this sudden appearance isn’t the result of a new disc spill upriver. This is a lingering phenomenon that will likely visit local waters and the coast whenever there are ocean storm surges or high water events in the river, for months to come if not longer.
In early March 2011, an estimated 4.3 million sewage treatment discs -- about the size and thickness of a silver dollar, with a screen pattern running across them -- flowed into the river when a downpour overwhelmed the Hooksett, N.H. sewage treatment plant. The plant is located on the bank of the Merrimack River near Manchester, and the discs came down the river like a plastic tidal wave.
About 4 million were picked up, according to Geoffrey Brown of Enpro, the Newburyport firm that was hired to clean up the mess. Enpro was employed at the task from March through November 2011, and a number of local environmental groups and volunteers have also conducted cleanups.
But this winter’s storms have dislodged perhaps many thousands of discs that were stuck along the riverbank. The storms tossed ashore others that were floating on the seas.
“We anticipated that this would happen, especially with big storms. I’m not surprised that people are finding them on the beach,” said James Martin, a spokesman for the New Hampshire Division of Environmental Services.
Martin said along the river, discs tend to get “kicked out of pockets” where they have been lingering for months. They get flushed downstream and appear on the riverbank and beaches.
As thorough as the clean-up was, Brown said there are discs lurking all along the river. They are stuck in remote places, like tidal areas, or washed up on top of embankments, or hidden in reedy marshes. They are not always easy to find.
The discs are also on the high seas. Martin should know -- his phone number has been used as a primary way for people to report their disc discoveries.
“I still get calls,” he said. “From really all over the coast, from Rhode Island to northern (coastal) Maine, people reporting that they have found these discs and they want me to know about it.”
Thus far, the furthest reported disc discovery is Nova Scotia, said Brown.
The discs were part of an alternative sewage treatment technology that the Hooksett plant installed. The screens on the disc collect sewage matter and help it break down more efficiently than if it was simply left in a large treatment pool. It’s a lower cost alternative to machinery that is often used in plants. Millions of discs are required in order to create enough surface area for the process to work right.
The Hooksett plant endured a fierce downpour that caused the discs to spill out of the plant in the course of several hours. Since then, the plant has undergone substantial changes meant to prevent another spill, and the town has spent over $1 million on the clean-up, according to published sources.
At first it was feared that the discs were contaminated and posed a health threat. But tests concluded they are no more hazardous than any other piece of plastic detritus that may be found in the ocean or river.
“They’re not hazardous, they’re just beach trash,” Martin said.