10.10 Municipal Sanitary Sewers

Category 2543

10.10.1 Introduction

Category 2543 includes emissions from volatilization of organic compounds from municipal sanitary sewers.

Municipal sewers are collection systems that transport waste materials from each point of origin across the entire region up to the entry point into the sewage treatment plants. Emissions from sewage treatment plants, commonly known as Publicly Owned Treatment Works (POTWs), are not included in this emissions source category (they are covered in Chapter 5.1). Category 2543 is a Total Organic Gases-only (TOG) category.

Following a report released in 1988, the California Air Resources Board recognized the importance and difficulty of quantifying VOC emissions from sewers across urban environments465. In 1995, the U.S. Environmental Protection Agency (EPA) funded research to estimate emission of volatile organic hazardous air pollutants (VOHAPs) from municipal sewer systems466. The methodology to determine emission factors for this category are based on the EPA report.

10.10.2 Methodology

Category 2543 is considered an area source category since it covers facilities / emission sources that are not directly permitted by the District, and hence not systematically cataloged. Emissions for area source categories are determined using the formula:

Current Year Emissions = Base Year Emission X Growth Profile, and,

Base Year Emission = Throughput X Control Factor X Emission Factor

where,

  • throughput or activity data for applicable base year(s) is determined using a top-down approach (e.g. state-, national-level data);
  • emission factor is derived from general literature, specific literature and reports, and/or source testing results provided by Air District staff;
  • control factor (if applicable) is determined by District and state rules and regulations in effect;
  • and, historical backcasting and forecasting of emissions is based on growth profiles as outlined in the Trends section of this chapter

More details on throughput, county distribution, emission factors and controls is provided in the following subsections.

The parameters to determine VOHAP emissions from sewers, representing a large subset of total VOC emissions, are laid out in more detail in the VOHAP study467 arising from the above-mentioned USEPA report2.

(a) Activity Data / Throughput

Emissions are based on the volume of wastewater that is processed by the collective municipal POTW system across all counties. The throughput is influent sewage, expressed in units of Thousand Gallons / Year. Since all POTWs in the Bay Area are considered point sources and the throughput data is reported as part of permitting requirements, the annual volume of wastewater processed can be directly calculated by summing up the individual annual throughput of all POTWs.

(b) County Distribution / Fractions

The county totals (and hence the fractions) are estimated based on the geographical location of each POTW and the annual volume of influent wastewater being treated in base year 2015.

(c) Emission Factors

Emission factor for the Bay Area sewers is derived from the VOHAP study3 by dividing the total estimated emissions (2057 tons/year) of a representative city with 5 million inhabitants by the annual volume of wastewater treated across all POTWs in the city (1443 million gallons/day). The modeled emissions include estimates from two models - CORAL+ and BACT/LAER-IWW/MRE models3 and the latter model is assumed to be more representative. This leads to an effective VOHAP emission factor of 7.8 lbs per million gallons of influent wastewater.

(d) Control Factors

Category 2543 is considered a diffused area source where emissions are happening upstream of the municipal sewage treatment plants, and where there are no active emission control mechanisms in place. Hence these emissions are considered uncontrolled.

(e) Speciation

All VOHAPs are considered reactive and hence emissions under this category are classified entirely as Reactive Organic Gases (ROGs). The ROG / TOG ratio is 1.

10.10.3 Changes in Methodology

Previously, in the base year 2011 inventory, these emissions were classified as sewer loading emissions in industrialized areas under Category 761. The former category also included waste material generated from portable or semi-permanent toilets that was discharged into the sanitary sewer system. The emission factor was based on total industrial sewer loading activity.

Category 761 only included a portion of the organic emissions emitted from Bay Area’s municipal sewer system. The scope of emissions covered in Category 761 in the previous inventory was inconsistent with CARB’s original intent1 to estimate and document organic emissions from the entire municipal sewer system infrastructure. As such Category 761 was found to be inadequate and has been subsequently discontinued. Following that, a new category has been defined (Category 2543) that now accounts for fugitive emissions from sewer systems across the entire geographical domain of Bay Area.

10.10.4 Emissions

A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.

The ROG (= TOG) emissions from this category for base year 2015 equal 754 tons annually for the entire Bay Area. Based on wastewater treated across different counties, Santa Clara county accounts for 25% of these emissions followed by Alameda county (>20%).

10.10.6 Uncertainties

The main uncertainties in this calculation approach is in the derivation of the emissions factor from one study for one U.S. city. While the city in the USEPA and VOHAP study2,3 is similar to Bay Area in terms of population, the geographical area that the city covered is several times smaller than Bay Area indicating that the city in this study was densely populated and the total length of municipal sewer lines across the city is significantly lesser than that across Bay Area. There is a fair argument that fugitive loss emissions will be higher in an urban area where the the sewer line infrastructure is more widespread. Similarly, the age of the sewer line infrastructure is an equally important determinant and we do not have information available to compare this metric.

Finally, the above studies measured VOHAPs only and not all VOCs. The total VOC emissions are likely to be higher and the emission factor calculated here can be considered an under-estimate.

10.10.7 Contact

Author: Abhinav Guha

Reviewers: Tan M. Dinh and Yuan Du

Last Update: November 06, 2023

10.10.8 References & Footnotes

  1. CARB. 1988. Evaluation of Emissions from Selected Uninventoried Sources in the State of California. Final Report ARB Contract # A5-147-32R, Dickson, R.J., Oliver, W.R., and S. Tate, 1988.↩︎

  2. USEPA. 1995. Estimation of Hazardous Air Pollutant Emissions from Municipal Sewer Systems. Final Report EPA/600/A-94/022, Jones, D.L., Burklin, C., and J.W. Jones, 1995.↩︎

  3. Jones et al. 1996. Models to Estimate Volatile Organic Hazardous Air Pollutant Emissions from Municipal Sewer Systems. Jones, D.L., Burklin, C., Seaman, J.C, Jones, J.W., and R.C. Corsi. Journal of Air and Waste Management Association, Vol 46: 657-666, 1996. https://cfpub.epa.gov/si/si_public_record_report.cfm?Lab=NRMRL&dirEntryId=128574↩︎

  4. ABAG. 2019. Plan Bay Area 2040. http://2040.planbayarea.org/reports↩︎