Our methodology for determining household-level wastewater affordability is based on utility rates, OWTS costs, household incomes, and the USEPA benchmarks of community financial capacity that are used for regulatory compliance. The USEPA drinking water affordability threshold (2.5% of MHI) was used to assess wastewater affordability because many sewer utilities base wastewater bills on water consumption, and there are few combined sewers in Alabama, making the 2.0% threshold not directly relevant. To prevent populations affected by equity challenges from becoming neglected in the analysis (
Mack and Wrase 2017;
Roller et al. 2019), this study used census tracts and boundaries for incorporated areas (municipalities), census-designated places (CDPs), and a federally recognized reservation. The 2019 Census Tracts Shapefile for Alabama (
US Census Bureau 2021b) was used to build the GIS map because 2019 data were not affected by data collection challenges that have resulted from the COVID-19 pandemic. Shapefiles were obtained from the Alabama Geographic Information Office database to help define utility service areas (
Alabama Geographic Information Office 2019,
2022).
Wastewater rates data for this study were primarily gathered from the Environmental Finance Center at the University of North Carolina’s Utility Financial Sustainability and Rates Dashboard (referred to in this study as the UNC Dashboard) (
UNC Environmental Finance Center 2019). The sewer rates for Alabama were provided by the Alabama Department of Environmental Management (ADEM) and based on January 2019 data from 222 sewer utilities. Of these, 18 utilities provide only sewer services and 204 utilities provide both water supply and sewer services (
UNC Environmental Finance Center 2019). Gap-filling methods were used to obtain a comprehensive data set for utility providers and bill rates across the state covering 262 utilities (with service populations ranging from 90 to 650,700 people) and 471 municipalities, census-designed places, and a federally recognized reservation.
Estimating Water Use
The USGS Water Use Survey for Alabama estimated that an average of 280.1 L (74 gal.) of water per day is used per capita from public supply for domestic purposes “such as drinking, food preparation, bathing, washing clothes, dishes, and dogs, flushing toilets, and watering lawns and gardens” (
USGS 2015,
2018). This estimate includes both indoor and outdoor water use. The USEPA has estimated that outdoor water usage accounts for 30% of the nation’s daily household water usage (
USEPA 2023c). Other studies have found that higher outdoor water usage is often associated with higher household incomes, larger lot sizes, greater property values, the presence of swimming pools, and landscaped yards (
Balling et al. 2008;
Barnett et al. 2020;
Mini et al. 2014;
Potter et al. 2022). Estimated water usage based on public supply for domestic purposes was chosen to reflect the current state of customers’ water usage used for billing rates. Based on the average household size in Alabama (2.6 persons) and UNC Dashboard rate estimates listed in 1,892.7-L (500-gal.) increments (
US Census Bureau 2023;
UNC Environmental Finance Center 2019), the estimated water use per household is 22,712.5 L (6,000 gal.) per month.
Wastewater Rate Data Collection
The process for collecting the wastewater rate data involved matching utility sewer rate data from the UNC Dashboard with municipalities (incorporated areas) and other data sources to fill in data for unmatched municipalities (Fig.
1). There were five CDPs and one federally recognized reservation added to the analysis when identified within the service area of a sewer utility.
The other data sources used to fill in data were online utility data, the list of national pollutant discharge elimination system (NPDES) permits administered through ADEM, and online real estate listings. The list of NPDES permits was used to determine the inclusion of all utilities that serve large groupings of permanent residences where households are responsible for paying a monthly sewer bill (and not including utilities serving only correction facilities, military bases, hotels, retreat centers, and so on, or only a single neighborhood or subdivision).
OWTS were assigned for municipalities with municipal systems that have treatment plants with low actual daily flow rates, limited-service populations, and real estate listings showing OWTS. If the rate reported on the UNC Dashboard was from 2013 or earlier, the most recent Municipal Wastewater Planning Program (MWPP) was used to confirm or change the assigned rate. Monthly sewer rates in Alabama ranged from $2.00 to $87.27 for 22,712.5 L (6,000 gal.) with an average of $34.06.
In Alabama, most municipal sewer utilities only provide service within municipal boundaries, so for this study, it was assumed that every household in the municipality is serviced by the municipal utility and pays the corresponding rates. Utilities that use pressurized sewer systems, including some private utilities that use septic tank effluent pump or grinder systems, may be more flexible in service areas than conventional gravity systems, so the utility’s rate was assigned to the area it primarily serves.
The residents outside of areas served by sewer utilities were assumed to not be connected to sewer networks and are responsible for managing their own wastewater using OWTS. It was assumed that all residents in Alabama are able to install conventional OWTS [estimated to cost $5,000 (
Cellucci 2022;
HomeAdvisor and Botelho 2022)] except the residents living in the 17 Black Belt counties: Barbour, Bullock, Butler, Choctaw, Crenshaw, Dallas, Greene, Hale, Lowndes, Macon, Marengo, Montgomery, Perry, Pike, Russell, Sumter, and Wilcox (Fig.
S1). In the Black Belt counties, the native soils typically require advanced engineered OWTS; for this analysis we used the most common engineered system, a mound septic system [estimated to cost $20,000 (
Cellucci 2022;
HomeAdvisor and Botelho 2022)].
All OWTS are typically either installed and paid for by the homeowner as a lump sum or, more likely, incorporated into a mortgage loan (at the time of home purchase or mortgage refinance). OWTS are typically designed to operate for 30 years, but may have a longer useful life. The time frame before the system needs repairs or rehabilitation will vary based on a number of factors such as system care and drainfield soil type. This is the first study to analyze the affordability of OWTS, so we focused on the capital cost and the main ongoing cost to pump the system; replacement and rehabilitation costs are not included (except to the extent they are implicitly included in mortgage and rental payments).
To be consistent with typical mortgage and system design life spans, the loan was estimated for a 30-year repayment. The monthly loan payment is dependent upon the loan’s interest rate (Table
1). For this study, the average mortgage interest rate in the US since 1971 (7.75%) was used to estimate the monthly loan payment needed for a financed system (
Freddy Mac 2023). Both conventional and mound septic systems have a tank that must be pumped every 5 years and which is estimated to cost $500 each pumping or $8.33 per month (
USEPA 2022e). The OWTS expenses are represented in the maps as annual loan payment and annual pumping cost (conventional: $44.15 per month; mound: $151.62 per month). The Supplemental Materials offer an analysis using capital cost paid as a lump sum and households only paying pumping cost.