Expanding treatment capacity at Orange County's sewer plant and enlarging the pipe that feeds it to handle looming population growth could cost property owners in the service area $200 million, a sharp price increase in just two years.

That's the latest estimate from Delaware Engineering, the consulting firm that has been working with the county since 2015 to plan what would be the Harriman plant's third expansion since it was built in the 1970s. The facility treats wastewater from more than 20,000 homes and businesses in eight villages and towns and discharges it into the Ramapo River. It is permitted to take in up to 6 million gallons a day.

Increasing its capacity to 9 million gallons a day was expected to cost $54 million when an advisory panel endorsed the project in 2020. But consultants now estimate that price would nearly triple to $156 million if construction starts in 2025, following two years of designing the project and getting permits.

Costly options:Growth and new treatment rules force Orange County to weigh expensive sewage treatment upgrades

Another approach:Orange considers piping wastewater from Harriman plant to the Hudson River

Sewer study:County starts quest in 2015 to increase sewer service in southeastern Orange

And they're suggesting the county do another costly upgrade at the same time: install a much wider version of the giant sewer main that carries wastewater to the plant. The $44 million cost estimate for that pipe replacement in 2025 would bring the entire bill to $200 million.

Even at that price, the proposed expansion is expected to meet growing demand for only a limited time. The consultants anticipate so much population growth in the service area that they have outlined two further expansions that eventually would allow the plant to treat up to 19 million gallons a day, more than triple its current capacity.

The plant treats sewage from the villages of Kiryas Joel, Monroe, Woodbury, Harriman, South Blooming Grove and Chester; and parts of the towns of Monroe and Chester. Flows fluctuate widely, surging during rainy periods. Monthly averages ranged from 4.3 million gallons per day to 6.9 million gallons in the first six months of this year, according to federal data. June's average was 4.4 million gallons, or 73% of the plant's limit.

Delaware Engineering updated its plan this month to add development forecasts for the service area and the amount of additional wastewater those new homes and businesses could generate. They estimated the total impact within the next 20 years would be another roughly 11 million gallons of sewage, on top of what the sewer system now handles.

Much of that anticipated growth was in Kiryas Joel, whose officials told Delaware Engineering they expected 2,958 more homes in the next five years and 9,949 altogether in 20 years. Those counts include already-occupied homes in Veyoel Moshe Gardens, a 1,600-unit condo complex under construction off Nininger Road and County Route 105.

South Blooming Grove could get 1,800 new homes in the next five years and 9,600 in all over 20 years, the village's engineer, Al Fusco Jr., told the consultants. But he provided no information to support his estimates and couldn't be reached on Monday to explain them. The 600-home Clovewood proposal, which is nearing final approval, is the only major housing project under public review in South Blooming Grove and it's not expected to have any impact on the county sewer plant, since its developers plan to build their own treatment facility.

Delaware Engineering representatives have been discussing those projections and cost estimates with an advisory panel of county lawmakers and officials at monthly meetings, most recently on Aug. 17.

The consultants recommend the county install a replacement sewer main large enough to eliminate current backups and handle all foreseeable growth, so it would never have to be replaced again. The 60-inch-diameter pipe could carry up to 20 million gallons a day in wastewater.

The big uncertainty still hanging over the expansion plans after several years is the fate of tougher treatment standards the state Department of Environmental Conservation wants to impose on the county plant.

The county is seeking a waiver from the DEC to avoid having to remove elevated levels of total dissolved solids and chlorides from the wastewater as ordered. The equipment and recurring costs needed to meet that standard - without any increase in sewer service − is almost prohibitively expensive: $116 million, plus $25 million a year to haul away brine and $250,000 a year for electricity, in the latest estimate from Delaware Engineering.

The consultants pitched two alternatives if the DEC denies the waiver or grants it but forbids expanding the Harriman plant's capacity. One is building a second plant to treat up to 3 million gallons a day from South Blooming Grove and the village and town of Chester, diverting their flow away from the Harriman plant. That would cost an estimated $151 million in 2025, not including the expense of other upgrades the Harriman plant needs.

The other option is closing the Harriman plant and piping its wastewater nearly 16 miles north to a new treatment facility that discharges into the Hudson River or the nearby Moodna Creek. Consultants estimate that project would cost $340 million in 2025.

Chris McKenna covers government and politics for the Times Herald-Record and USA Today Network. Reach him at cmckenna@th-record.com