Sewers, Septic Tanks, and Water Quality

The following tutorial describes a simple modeling methodology for analyzing the costs associated with sewering to mitigate surface water nitrification. This tutorial specifically focuses on the village of Sayville, on the south shore of Long Island, NY.

Literature Review

The Nitrogen Cycle

Nitrogen is a chemical element (atomic number 7) that is essential to both plant and animal life. It is found in amino acids that make up proteins, in the nucleic acids that encode life's hereditary material passed between generations, and in a wide variety of other organic and inorganic compounts.

Although nitrogen is one of the most plentiful elements on earth, and constitutes 78% of the earth's atmosphere by volume, most of that nitrogen is nitrogen gas (N2) that must be fixed into organic nitrogen and converted by bacteria into ammonia (NH4), which is used directly by plants, which are eaten by animals including humans. After plants and animals are finished with the nitrogen, their wastes and remains are then broken down by bacteria (nitrification) into nitrites (NO2) and nitrates (NO 3), and then converted back into atmospheric nitrogen gas by additional bacteria (denitrification) to start the nitrogen cycle all over again (Bernhard 2010).

The Nitrogen Cycle ( Maraviglia 2019)


The availability of organic nitrogen is a major limitation on how fast and large plants can grow, which historically limited the amount of food that could be grown in an area. However, in 1909, German chemist Fritz Haber developed a chemical process for synthesizing ammonia from atmospheric hydrogen using heat and hydrogen gas in the presence of a catalyst. The process was purchased by the chemical company BASF and improved by staff chemist Carl Bosch, leading to the large-scale production of industrial ammonia fertilizer through what is commonly called the Haber-Bosch process. Smil (1999) estimated that almost three-quarters of the world's population owes their existence to the Haber-Bosch process.

The downside to Haber-Bosch has been a dramatic increase in the amount of ammonia, nitrates, and nitrites in the environment that need to be denitrified back into atmospheric hydrogen.

Major sources of nitrogen in surface waters have their roots in Haber-Bosch, including fertilizer run-off from agricultural operations and residential lawn care, and from human wastes.

Septic Tanks

During the three decades following the Second World War, Suffolk County, NY (eastern Long Island) was dramatically transformed from an area dotted with farms into an expansive landscape of single-family homes. In the haste to build quickly, cheaply, and profitably, cities and developers did not invest in expensive centralized sewage treatment systems, but instead equipped new houses with individual septic tanks or cesspools. As of 2015, around 74% of Suffolk County homes, housing almost one million people, relied on septic tanks to treat their sewage (Suffolk County 2015, ES-2).

After home sewage flows into septic tanks, the solids in the sewage (feces, vegetable matter, toilet paper, soil, etc.) sink to the bottom of the tank. This septage (sludge) needs to be periodically cleaned out and transported to landfills or commercial sewage treatment plants, which can then turn the high-nitrogen and high-phosphorous septage into fertilizer (Clark 2008).

Bacteria break down some of the organic wastes dissolved in the water, notably nitrifying the nitrogen into nitrates and nitrites. The resulting partially-treated sewage is then pumped into a series of underground drain pipes behind the home (a drain field). This nitrogen-rich water seeps into the underground water table and is either pumped back up as drinking water, or flows slowly underground into local rivers, ponds, lakes, and streams.

Diagram of A Septic Tank ( Tilley et al 2014)

Environmental Effects

When septic tanks are used in low-density rural areas, they generally do not present any meaningful environmental threat to the people that live in those areas, unless they are placed too close to water wells. However, in a dense, heavily-populated area like Long Island, the sheer volume of additional nitrogen prompts a comparable response from the ecosystem.

When ammonia, nitrates, and nitrites flow into surface waters, a variety of organisms (notably forms of algae) feed on that nitrogen and the dissolved oxygen in the water. This leads to an absence of oxygen in the water to support other forms of life (hypoxia), killing fish and shellfish.

Blooms of different kinds of algae cause brown tides and red tides, which not only kill shellfish, but necessitate beach closures, and can result in poisoning to humans who eat shellfish from affected waterways.

Nitrification also contributes to the death of ecosystems in salt marshes, which compromises the protective effect of those marshes for human residents from severe ocean storms.

Most of Long Island's drinking water comes from underground aquifers that have increasingly high nitrate rates. High levels of nitrates can be especially detrimental to infants (blue baby syndrome), small children, and pregnant women (Beaudet et al 2014). Nitrates in drinking water, even levels under regulatory limits, may be associated with colorectal cancer, thyroid disease, and neural tube defects (Ward et al. 2018).

Secondary economic effects of nitrogen pollution include reduced tourism and reduced home values. Tourism produces $4.7 billion of revenues annually on Long Island. The cost of installing septic tanks that meet new environmental requirements also constrains the growth of businesses and limits the number of new jobs they can create (Suffolk County 2015, ES-3).

Septic Tank Problems

The sheer volume of nitrogen waste from the 360,000 of septic tanks in the county (Suffolk County 2015, ES-6) is a major environmental issue. While septic tank systems that reduce nitrogen effluent to much lower levels are available, they can cost homeowners $15,000 to $40,000, as much as eight times the $5,000 cost of a conventional septic system (Foderaro 2017; Suffolk County 2015, 8-17). This represents a major barrier to homeowners already burdened by the county's already high mortgage costs and taxes.

Septic tanks in areas with high water tables are especially problematic. Septic systems that are flooded or submerged do not function as designed, and also contaminate groundwater with pathogens, pharmaceuticals, and personal-care products (Suffolk County 2015, 8-27 - 8-29).

Sewage Treatment Plants

The technical solution to this issue is the creation of centralized, professionally-managed sewage treatment systems that can more effectivly handle the volumes and types of wastes produced by Suffolk County residents. All major cities in the United States utilize this approach.

Simplified process flow diagram for a typical large-scale treatment plant (Wikipedia 2006)

The primary challenge with Suffolk County homes and businesses is the cost of building sewers to connect all those parcels to sewage treatment plants. Additional costs include expansion or new construction of treatment plants, connecting homes to sewer lines, and removing septic tanks and cesspools.

With the challenge of cost comes the barriers of politics. Foderaro (2017) noted that the scandal-plagued construction of the Southwest Sewer District covering parts of the towns of Islip and Babylon in the early 1980s was "mired in corruption, delays and cost overruns," that it "effectively killed sewering here for decades."

The Suffolk County (2015, 8-122) Comprehensive Water Resources Management Plan calculated that a proposed $35 million sewer project for 167 properties in Sayville would cost typical homeowners around $6,000 per year and commercial property owners around $16,000 per year to build, operate, and maintain the system. This means the cost of to each household would represent around 5.6% of the community's median household income of $106,000 (USCB 2018).

The report's general conclusion was that "...these projects would become economically feasible for residential property owners only if significant grant funding was provided or some other type of established funding stream was created to fund these and future sewer extension projects" (Suffolk County 2015, 8-122). This then raises an ideological question about why taxpayers from other communities should be compelled to subsidize the lifestyles of comparatively affluent Suffolk County residents.

The costs to construct a sewer system vary widely based on the geology of the community, the distance between homes, distance to new or existing treatment plants and effluent discharge areas, prevailing labor and materials costs, etc. Some cost estimates from the Suffolk County report and other sources are detailed below.

Project Parcels Acres Construction Cost Avg. Per Parcel Cost
Patchogue River 648 248 $15.5 MM $24,000
Carlls River 2,797 1,000 $139 MM $50,000
Connetquot River 500 353 $27.2 MM $54,000
Forge River (Phases I and II) 2,893 869 $170.3 MM $59,000
Deer Park, North Babylon, West Babylon, Wyandanch, Wheatley Heights, and West Islip 18,000 9,152 $2 B $111,000
Southampton Village 151 102 (62) $29.3 MM $194,000
Sayville 167 90 (71) $35.3 MM $211,000
Bellport 131 56 $39 MM $300,000
Flanders / Riverside 89 178 (85) $33 MM $371,000
Lake Ronkonkoma Hub 54 68 (58) $31 MM $574,000

Using the estimates in the table above, the cost of a sewer system closely correlates with the acres served by the system. Fitting a simple linear model results in the following rough estimate for the cost of a sewer system:

14700000 + (acres * 216000) = cost

lm(formula = Cost ~ Acres, data = data)

      Min        1Q    Median        3Q       Max 
-91719926 -29114149  -3094091   6331804 181076330 

            Estimate Std. Error t value Pr(>|t|)    
(Intercept) 14651524   26250095   0.558    0.592    
Acres         216068       8964  24.105 9.36e-09 ***
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Residual standard error: 75570000 on 8 degrees of freedom
Multiple R-squared:  0.9864,    Adjusted R-squared:  0.9847 
F-statistic:   581 on 1 and 8 DF,  p-value: 9.355e-09

Study Area

This tutorial uses the Sayville CDP as an example study area.

Sayville is a hamlet in the middle of the South Shore of Long Island in Islip township. The first formal European resident of Sayville was John Edwards, who built his home in the area in 1761. The residents of the area chose the name Sayville for their post office in 1838. The area became a tourist destination after the South Side Railroad arrived in 1868 and ferry service began to Fire Island.

According to the 2017 American Community Survey 5-year estimates (USCB 2019):

Greene Street in Sayville


Study Area Map

This example uses the boundaries for the Village of Sayville as defined in the US Census Bureau's 2017 Cartographic Boundary Files - Shapefile of census designated places (CDP). The CDP fille has been uploaded to the ArcGIS Online as the TIGER 2017 New York State Places layer in the FSC organization.

  1. Add a layer of census-designated places (CDPs)
  2. Filter the layer to show only the outline of the desired community, in this case Sayville
  3. Change Style to a red outline and no fill so the area stands out over the base map
  4. Save the map under a meaningful name, and Share it to get a link for submission

Affected Waters

Section 303(d) of the Clean Water Act requires that states and territories develop standards for water quality in their jurisdiction, and water bodies that fail to meet those standards are added to a national list of impaired waters. standards for all waters in their (ESRI 2018).

ESRI makes a Living Atlas layer of USA Polluted Waters with features representing waterways from the 303(d) list.

In this example, the polluted water adjacent to Sayville is the Great South Bay, which has been affected by nitrogen, algal growth, pathogens, and PCBs. While the PCBs are associated with industrial activity and electrical utility equipment, the remainder may be attributable to septic tanks.

  1. Add -> Browse Living Atlas Layers the USA Polluted Waters layer
  2. Click on layers features adjacent to your community for more information
  3. View the Waterbody History Report to find information on current and past impairments detected in this waterbody

Parcel Map

One of the primary functions of municipal government is the maintenance of the cadastre containing information about land ownership. Most municipalities in the USA maintain that information in geographic information systems, and some municipalities make that information available to the public as web maps, geospatial data files, and/or web services.

Suffolk County makes their parcel data available as a web feature service at the URL: However, with over half a million parcels, this data will not display fully or cleanly in a web app like ArcGIS Online.

For this example, the Suffolk County parcel data has been uploaded to the FSC ArcGIS Online organization as the Minn 2019 Suffolk County NY Parcels layer and joined with the CDP layer to add a CDP column that can be used as a filter to limit the number of displayed features to a manageable level. The visibility range for the layer has also been set so that it will not display when the map is zoomed out too far to make the features visible.

  1. Add the layer of parcels
  2. Filter the parcels to the specific CDP - in this case Sayville
  3. If your parcel layer has an attribute indicating zoning or land use type, Style the layer to show those categories (Types -> Unique Symbols)
  4. Add a set of filters to include only residential, commercial, and institutional parcels, in this case land use codes one through six. Zoning codes can be complex and the exact codes (when available) will likely be different in different counties
  5. Save As your map under a meaningful new name, and Share it to get a link for submission

Unsewered Parcel Map

This tutorial focuses on parcels that do not have access to existing municipal or private sewer services. These are the parcels that likely have septic tanks that contribute to nitrification of groundwater and surrounding surface water.

Suffolk County makes a web feature service of sewer districts in the county available at the URL: This data has also been uploaded as the layer Minn 2019 Suffolk County, NY Sewer Districts in the FSC ArcGIS Online organization.

  1. Add the layer of existing sewer districts
  2. On the layer of parcels, select Perform Analysis -> Find Locations -> Find Existing Locations to find the parcels that are not within an existing sewer district. This will take a few minutes to complete
  3. Add expression and create a query to find all parcels that does not intersect with the different sewer districts
  4. Give a meaningful Result layer name including your name to distinguish your layer from those created by other people in your organization
  5. Style the layer in an aesthetically-pleasing single color that contrasts with the sewer district layer
  6. Save As your map under a meaningful new name, and Share it to get a link for submission

High Water Table Parcels

Although septic tanks in a densely-populated area present numerous drawbacks, the most direct threat to surface waters are the tanks in areas where the water table is close to the surface. These are areas where the water level will often be high enough to compromise the operation of septic tanks, or where nitrogenous effluent can reach the water table quickly and affect local surface water bodies.

While large-scale data on depth to water table is not available for Suffolk County, and such levels fluctuate based on weather patterns, elevation data is available, and areas with low elevation tend to have a water table that is close to the surface.

Since ArcGIS Online does not work with raster GIS data, it cannot use the readily available digital elevation model (DEM) data available from the USGS. However, for the purposes of this exercise, the Minn 2019 Long Island Elevation from the FSC organization is a layer of elevation polygon contours that can be used for identifying low-lying parcels.

  1. Add the elevation contour polygons layer to the unsewered parcels map
  2. Filter the contours to show just the areas where elevation 10 feet or less. These are areas that likely have a high water table
  3. On the layer of parcels, select Perform Analysis -> Find Locations -> Find Existing Locations to find the unsewered parcels that are within the high-water-table area
  4. Add expression and add a query to find all parcels that intersects with the 10-foot elevation contour
  5. Give a meaningful Result layer name including your name to distinguish your layer from those created by other people in your organization
  6. Style the layer in an aesthetically-pleasing single color that contrasts with the sewer district layer
  7. Save As your map under a meaningful new name, and Share it to get a link for submission


If your parcels layers contain an area field (Acerage in this Suffolk County example), you can Show Table, click the header above the area field, and select Statistics to get a total area for the unsewered parcels.

In this example in the Sayville CDP, the analysis found:


No-Build Option

Sayville is on the northern coast of Great South Bay, which is a part of the South Shore Estuary between Long Island and Fire Island, a barrier island to the south along the Atlantic Ocean. Browne River runs along the east side of the CDP and Greenes Creek along the west side, with both emptying into the bay.

The two most significant pollutants in the estuary are coliform bacteria and nutrients, including nitrites from septic tanks. Nutrients contribute to algal blooms and hypoxia, which then reduce aquatic life. The water along the coast south of Sayville is well-mixed, reducing issues with oxygen depletion that are more serious in areas like those surrounding the Forge River. (Suffolk County 2015, 5-37; 6-39).

The no-build option would not by itself doom the estuary to continued water quality decline, but Sayville is one of a number of South Shore Estuary communities who can either contribute to or help mitigate the problem.

The no-build option would leave in place growth constraints on local businesses caused by wastewater disposal constraints (CDM Smith 2014, 1-8; 6-8).

Advanced Septic System Option

For the advanced septic systems option, multiply the number of parcels in each of your three areas by $20,000 to get an overall cost estimate.

The total cost for installing advanced septic systems in all unsewered parcels would be:

4,952 parcels * $20,000 per parcel ~= $99 MM

The total cost for installing advanced septic systems only in unsewered parcels in high-water-table areas would be:

3,153 parcels * $20,000 per parcel ~= $63 MM

Centralized Sewage Treatment Option

The cost of a sewer system can vary widely depending on the specific physical and economic conditions of the area where construction will occur, and a detailed engineering analysis is needed to make trustworthy estimates. However, a rough model based on the cost estimates listed above is:

14700000 + (acres * 216000) = cost

The total capital cost for constructing a centralized sewage treatment system to cover all unsewered parcels would be:

14700000 + (2115 acres * 216000) ~= $472 MM

The average per-parcel cost would be:

$472 MM / 4952 parcels ~= $95,000 per parcel

The total capital cost for constructing a centralized sewage treatment system to cover only unsewered parcels in high-water-table areas would be:

14700000 + (1376 acres * 216000) ~= $312 MM

The average per-parcel cost would be:

$312 MM / 3153 parcels ~= $99,000 per parcel


Clearly, the exceptionally high cost of building centralized sewage treatment would be economically and politically impossible for local residents to fund entirely on their own, and significant federal or state aid would be needed. The advanced septic tank option is much more practical, but might not be justified given the limited contribution that the community makes to poor water quality in the estuary. This may explain why the most recent study (CDM Smith 2014) only examined sewering commercial areas that were growth-constrained by the limitations of on-site wastewater management.


Beaudet, N., A. Otter, C. Karr, A. Perkins, and S. Sathyanarayana. 2014. "Nitrates, blue baby syndrome, and drinking water: A factsheet for families."

Bernhard, Annie. 2010. "The Nitrogen Cycle: Processes, Players, and Human Impact." Nature Education Knowledge 3(10):25.

CDM Smith. 2014. "Suffolk County Sewer District Capacity Study, CP 8189."

Clark, Josh. 2008. "How often are septic tanks emptied, and where do the contents go?"

ESRI. 2018. "USA Polluted Waters." Accessed 15 July 2019.

Foderaro, Lisa. 2017. "'Dead Rivers, Closed Beaches:' A Water Crisis on Long Island." New York Times 8 May.

Smil, Vaclav. 1999. "Detonator of the population explosion." Nature 400, 29 July.

Suffolk County. 2015. "Suffolk County Comprehensive Water Resources Management Plan."

United States Census Bureau. 2019. "2013-2017 American Community Survey 5-Year Estimates."

Ward, Mary H., Rena R. Jones, Jean D. Brender, Theo M. de Kok, Peter J. Weyer, Bernard T. Nolan, Cristina M. Villanueva, and Simone G. van Breda. "Drinking Water Nitrate and Human Health: An Updated Review." International Journal of Environmental Research and Public Health 15 (7), 1557. Accessed 7 July 2019.