Suffolk County has approved the first on-site advanced wastewater treatment system for residential use, allowing environmentally conscious homeowners — with $12,500 — to replace their septic systems, county officials said.

The Hydro-Action AN system, made by the Indiana-based company Hydro-Action, reduced nitrogen by 75 percent — to an average 16 milligrams of nitrogen per liter — at five Suffolk homes that had test systems installed, county officials said.

County Executive Steve Bellone announced the approval at a news conference Tuesday at the Dix Hills home of Catherine and Jeffrey Lang, where one of the systems had been installed and monitored by the county over the past year.

Catherine Lang said the system had worked well. “There’s never an issue, never a smell,” she said. There are questions about the green cylinders rising a few feet off the lawn. “What is that? Is that a bunker?” people ask her.

Nitrogen, primarily from 360,000 homes not connected to sewers in the county, has been tied to algae blooms in surface water and has been increasing in groundwater. While the county looks to expand sewers in some places, they will be cost prohibitive in other areas, Bellone said. That’s where on-site septic systems could be used.

Kevin McDonald, with the Nature Conservancy of Long Island, hailed the approval for taking a big picture goal of improving water quality and making it reachable.

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“This is what the beginning of change looks like,” he said.

The systems could also be approved for use at commercial locations, like restaurants, later this year.

Another five types of systems are currently being tested by the county. Some could be approved for use later this year, after the county looks at six months of data where nitrogen is reduced below 19 milligrams per liter, planning director Sarah Lansdale said.

For now, the Hydro-Action system is the only one approved and will be installed by Riverhead-based Wastewater Works, the sole New York distributor for the technology. It is owned by Joseph Densieski of Riverhead, who is a maintenance mechanic at Riverhead’s sewage treatment plant.

Densieski said he has already been in discussions with homeowners looking to replace old systems and builders of new houses. The $12,500 cost for the system and installation could be higher depending on access to property and soil, or lower for new construction. Average cost of maintenance is estimated at $300, while electrical costs are $30 to $40 a year.

Mitchell Pally, chief executive of the Long Island Builders Institute, said interest from developers would depend on the price compared to conventional systems, which cost between $4,000 and $6,500. He said the advanced units could be subsidized by the county.

“Ultimately, a developer is going to pick with a homeowner whether it makes sense,” he said.

Kevin McAllister, founder and president of environmental group Defend H20, said in an interview the approval of the system was a positive step. But he wanted to see the county mandate the use of advanced systems for new construction and use subsidies to help homeowners replace old systems.