29.2 EPA-developed estimates
The calculations for estimating the emissions from POTWs involve multiplying the wastewater flow rate by emissions factors for VOCs, NH3, and 53 HAPs.
29.2.1 Activity Data
The activity data for this source category is the wastewater flow rate. The EPA Clean Watersheds Needs Survey provides flow rate by facility and estimates the national POTW flow rate in 2012 for all facilities as 32,822 million gallons per day (MMGD) [ref 2]. The nationwide flow rate includes Puerto Rico and the US Virgin Islands. To estimate flow rates in the current inventory year, facility-level daily flow rates in 2012 are multiplied by the ratio of current year population to 2012 population in the county where the facility resides [ref 3]. County-level annual wastewater flow rates are calculated by summing the daily flow rates for all POTWs within the county and multiplying by 365 days in a year.
\[\begin{equation} FR_{c,y} = \sum^{n}_{f=1}{FR_{f,2012}} \times 365 \times \frac{P_{c,y}}{P_{c,2012}} \tag{29.1} \end{equation}\]
Where:
\(FR_{c,y}\) = The annual wastewater flow rate of county c in inventory year y
\(FR_{f,2012}\) = The daily wastewater flow rate at facility f in 2012
\(P_{c,y}\) = Total population in county c in inventory year y
\(P_{c,2012}\) =Total population in county c in 2012
29.2.2 Allocation Procedure
For a given county, county-level wastewater flow rates are calculated by summing the flow rates for all POTWs within the county.
29.2.3 Emission Factors
The ammonia emissions factor was obtained from an EPA report [ref 4] and the VOC emissions factor was based on a TriTAC study [ref 5]. Emissions factors for HAPs were derived using 1996 area source emissions estimates that were provided by Bob Lucas [ref 6] and the 1996 nationwide flow rate [ref 7]. These HAP emissions factors were then multiplied by the 2008 to 2002 VOC emissions factor ratio (0.85/9.9) to obtain the final HAP emissions factors applied.
29.2.5 Emissions
Emissions are estimated by multiplying an emissions factor by the county flow rate. A conversion factor was used to convert pounds to tons.
\[\begin{equation} E_{p,c} = FR_{c,y} \times EF_{p} \times \frac{\text{1 ton}}{\text{2,000 lbs}} \tag{29.2} \end{equation}\]
Where:
\(E_{p,c}\) = Nonpoint emissions of pollutant p in county c, in tons
\(FR_{c,y}\) =Flow rate of county c and inventory year y, in MMGY
\(EF_{p}\) = Emissions factor for pollutant p, in lbs. per MMGAL
29.2.6 Point Source Subtraction
The county-level flow rates include all facilities reported as POTWs in the EPA Clean Watersheds Needs Survey. In some cases, SLT agencies might include facilities under their point source inventory reporting. In these cases, SLT agencies have two options for submitting state-level point source data to EPA for point source subtraction:
- Option A: County-level flow rates associated with POTWs reported as point sources; or
- Option B: County-level emissions of VOC and NH3 for POTWs reported as point sources.
29.2.7 Sample Calculations
Table 29.1 lists sample calculations to determine the benzene emissions for nonpoint source POTWs. The values in these equations are demonstrating program logic and are not representative of any specific NEI year or county.
| Eq. # | Equation | Values | Result |
|---|---|---|---|
| 1 | \(FR_{c,y} = \sum^{n}_{f=1}{FR_{f,2012}} \times 365 \times \frac{P_{c,y}}{P_{c,2012}}\) | \(2.866\text{ MMGD} \times 365\text{ days} \times \frac{55,504\text{ people}}{54,927\text{ people}}\) | 1,057.07 MMGY |
| 2 | \(E_{p,c} = FR_{c,y} \times EF_{p} \times \frac{1\text{ ton}}{2,000\text{ lbs}}\) | \(1,057.07\text{ MMGY} \times 0.00673\text{ lb per MMG} \times \frac{1\text{ ton}}{2,000\text{ lbs}}\) | 0.003557 tons benzene per year |