Protecting & Restoring Long Island's Peconic Bays
This BMP considers the same type of I/A OWTS systems to treat wastewater, scaled up to handle multiple households in high density housing areas. For this application either the gravity sewer system would be deep or wastewater conveyance pumps for each household would be needed. In either case, the lack of flow equalization would require the cluster treatment system to be sized for peak flows. The wastewater cluster system would also need to provide full primary, secondary, and tertiary treatment and solids management. For this effort, cost analysis is based on a 15,000 gallons per day (gpd: 30 to 35 households) wastewater flow rate. At this scale, numerous treatment options are available that can be custom-engineered and approved for each case. The treatment options may include those already approved as active I/A OWTS systems for which each vendor would need to advise on the technology’s scalability.
Cluster system location is limited to areas where the highest recorded groundwater level is at least 1 foot below the ground surface elevation and cannot be located in steep slope areas (greater than 15%), erosional areas, meadow mat, bog, silt, clay, or other impervious soil areas, areas subject to flooding or improper drainage, limited access areas, and areas expected to create a public health risk (Suffolk County 2017b). In addition, cluster systems must comply with SCDHS setback requirements, including distances to habitable and non-habitable structures and to the property line. The distances are reduced with the installation of odor control systems (Suffolk County 2017b).
The following permits are required for installation of approved I/A OWTS:
The extent of nitrogen removal varies among the different systems, but all approved systems must reduce nitrogen to be below 19 mg/L based on SPDES permit limits.
The influent source for septic systems is septic tank effluent, with a concentration of 65 mg/l (CDM 2020). Effluent concentrations are estimated as the average of the most recent 12-month rolling average results of the individual I/A OWTS systems (12.9 mg/L) and the cumulative bi-monthly sampling results (13.2 mg/L), respectively (Suffolk County 2020b), resulting in an average nitrogen reduction of 80%.
Cluster I/A OWTS Nitrogen Effluent Concentrations, Nitrogen Reduction, and Annual Removal Rate
| Technology | Effluent Nitrogen Concentration (mg/L) | Nitrogen Reduction (%) | Annual Nitrogen Removal (lb/yr) |
|---|---|---|---|
| Cluster I/A OWTS | 12.9–13.2 | 80 | 2,360–2,375 |
I/A OWTS vendor costs (Suffolk County 2021) are scaled for a 15,000 gpd system and estimated at $650,000. Cost assume the existing septic tank will be left in place. The basic size assumes a small neighborhood with each residence having 100 feet of frontage, resulting in 2,400 feet of pipes to connect to the cluster system. Lower range costs include installation of a low-pressure collection system ($200,000) with grinder pump stations at each residence ($3,000 each) for areas where groundwater table is shallow or topography limits gravity-fed sewerage, for a total lower-bound capital cost of $952,000. The higher range costs include a gravity-fed system ($500,000) and a 50-gallons per minute (gpm) pump station ($25,000), for a total capital cost of $1,175,000. O&M costs include $15,000 per year salary for servicing and operations, $1,500 for maintenance and consumables, $7,500 for repairs and replacements, and 15 kWh power. Electric power cost set at $0.22 per kWh (Suffolk County 2020c). Total annualized costs represent capital costs annualized with a 5% discount rate and O&M costs averaged over 20 years with a 2% inflation rate applied..
Cluster I/A OWTS Capital, O&M, Total Annualized Costs, and Costs per Pound of Nitrogen Removed
| BMP Type | Capital Cost | Average Annual O&M Costs | Annualized Total Cost | Cost per Pound N Removed |
|---|---|---|---|---|
| I/A OWTS – Cluster | $952,000–$1,175,000 | $64,276 | $140,667–$158,561 | $59 – $67 |
