Planning board hears engineers weigh monitored treatment plant against large subsurface septic fields for proposed development
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Summary
Engineers and consultants debated three wastewater options — a packaged treatment plant with a constructed vegetated corridor, a geotech‑style subsurface septic field, and a conventional plant — focusing on treatment levels, footprint, monitoring and ecological risk to a nearby stream. The board requested DEC/DOH correspondence and monitoring specifications before deciding.
Warren Longacre, an engineer with MJ Engineering, outlined a subsurface disposal option that would replace a centralized wastewater treatment plant with two 10,500‑gallon septic tanks, a dosing tank and a large absorption field sized according to soil percolation rates. Longacre said design uncertainty near the equestrian center led the team to use a conservative 11–15 minute perc rate rather than 1–5 minutes, which increases the field footprint; he estimated a design maximum day flow of about 18,600 gallons and calculated an absorption area need of roughly 23,500 square feet using a 0.8 gallons/day/square‑foot application rate.
Rodney Morrison, civil engineer for the applicant, described an alternative that pairs a packaged wastewater treatment plant with a constructed, vegetated corridor roughly 190–200 feet above the stream to slow, diffuse and increase infiltration of treated effluent before it reaches the watercourse. Morrison said the corridor is intended to improve outflow characteristics and is not being proposed to replace regulatory discharge requirements; he framed the corridor as an added, site‑specific polishing step to the regulated discharge.
Michael Fishman of Edgewood Environmental Consulting told the board that tertiary wastewater treatment is “generally highly effective, much more effective than septic systems,” and cited a Westchester County example where improved plant discharge corresponded with higher aquatic‑insect indicators. Fishman and other presenters cautioned that subsurface septic effluent percolates into groundwater and, because groundwater commonly migrates toward streams, can be a “blind” pathway that may not be detected until a stream problem appears. Fishman said monitored package plants offer continuous on‑site testing, professional operators and faster detection of problems.
Presenters contrasted land requirements and oversight: Longacre said conventional trench systems would require far greater lineal trenching (on the order of 11,000 lineal feet under one estimate), while premanufactured geotech modules reduce trench length (to roughly 4,000 lineal feet) and overall footprint. He also warned that subsurface modules rely on a sand layer that can accumulate detergents and reduce pore spaces over time and that, once installed, subsurface fields typically have fewer routine testing requirements under current practice unless imposed as a local condition.
On regulatory oversight, presenters said New York State Department of Environmental Conservation (DEC) and county health authorities review and approve plans for projects that exceed 1,000 gallons per day and set effluent limits tied to the receiving stream class. The applicant team committed to providing correspondence from the Dutchess County Department of Health and DEC for the board’s review.
Board members pressed for technical clarity on monitoring and failure response. An unnamed presenter explained that packaged plants typically include SCADA and alarm systems that automatically notify primary and backup operators (and can be programmed to escalate until acknowledged). Participants asked whether alarms could also be routed to town staff; presenters agreed that alarm recipients and reporting could be specified in permit conditions or the special‑use permit. The board also asked whether monitoring schemes — including test wells and groundwater sampling near subsurface fields — could be designed to detect off‑site impacts; engineers said test wells at multiple depths are feasible, but source attribution can be challenging when neighboring systems exist.
The meeting concluded with the board requesting written DOH/DEC correspondence, a monitoring‑and‑alarm specification for any proposed package plant (including which parameters and recipients would receive real‑time alarms), and additional detail on any proposed subsurface monitoring protocol. The applicant and consultants agreed to provide those materials by the agreed deadlines and to return for a focused meeting on SEQR/Part 2 and unresolved issues on Jan. 20/21.
Next steps: the board asked for the county DOH letter, proposed SCADA/alarm details and any additional groundwater testing protocols; those materials were requested for delivery to board staff in advance of the January meeting so the board’s consultants can prepare written comments.
