5.3 Biosolids Land Application
Category 52
5.3.1 Introduction
Category 52 is an area source that accounts for methane (and/or TOG) emissions from application of biosolids to land farming practices in the San Francisco Bay Area (SFBA).
Land farming or biosolids land application is a process in which waste (biosolids), primarily from dried sewage sludge originating from waste water lagoons, is deposited and spread on rural and open lands. Greenhouse gas emissions (GHGs), namely methane (CH4) and nitrous oxide (N2O), along with total organic gases (TOGs) are emitted in this process.
5.3.2 Methodology
Category 52 is considered an area source category as emissions from this category are diffused and occurring from the entire acreage of land area over which biosolids have been applied. 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.
(a) Activity Data / Throughput
The annual activity (throughput) for this category is the amount of biosolids produced in the San Francisco Bay Area (SFBA) that was applied on land as a waste management option. Data for year 2007 was obtained from a Bay Area Clean Water Agencies (BACWA) March 2009 report136. The amount of biosolids generated in the SFBA in 2007 was estimated to be 158,000 dry tons. Approximately 19% of that amount, or 30,020 dry tons, was “land applied”. The Association of Bay Area Governments’ (ABAG’s) total SFBA population growth profile137 was used to obtain the base year 2015 throughput value of 32,976 dry tons/year.
(b) County Distribution / Fractions
The county distribution of the emissions is proportional to the percentage county-wise distribution of produced solids in the SFBA as stated in Figure 3 of the BACWA report1. Over 40% of the produced biosolids end up in the Santa Clara county.
(c) Emission Factors
Emission factors for CH4 and TOG have varied and evolved over time as different agencies use a variety of emission factors owing to paucity of data and limited field measurments. For this base year inventory, the current TOG emission factor for biosolids (6.46 lbs/tons) is developed from ARB’s statewide emissions inventory methodology for composting facilities138, which includes factors developed for co-composting of biosolids and manure. The CH4 emission factor (3.92 lbs/tons) is derived from ARB’s white paper on greenhouse gas emission reductions achieved in diverting organics from landfills to composting facilities139.
(d) Control Factors
Presently, no specific District regulations or controls have been instituted to reduce emissions originating from application of biosolids land application in the SFBA. The practice of using biosolids originating from wastewater treatment plants as soil amendment and fertilizer is practice as a solution to diverting organics from going into landfills and subsequent landfill emissions, hence there is no impetus to install emission controls over this practice.
(e) Speciation
The ROG / TOG ratio is derived from the CH4 and TOG emission factors and is calculated to be 0.39.
5.3.3 Changes in Methodology
The one significant change in the methodology to estimate emissions from this category is that newer and updated emission factors for CH4 and TOG have been adopted. These emission factors are in sync with those being used by ARB at the state-level to estimate emissions from similar practices and waste management.
The title of this chapter (and thus the name of the category) has also been amended from ‘Land Farming’ to ‘Biosolids Land Application’ to be more specific as to the scope of emissions being estimated from this category.
5.3.4 Emissions
A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.The TOG emissions from this category are estimated to be ~100 tons/year making it a minor source in the current base year TOG emissions inventory.
5.3.5 Trends
This category uses a range of base years to determine historical and future projected emissions. For historical emissions, 2011 is used as a base year for projections. For future emissions, 2015 is used as a base year for projections. TOG emissions from this category depend on the volume of biosolids being produced in SFBA and being diverted to land application as soil amendments.
(a) Historical Emissions / History
The historical growth of emissions (year 1990-2010) for this category is based on the ‘land farming’ profile which is based on increase in biosolids produced at waste water treatment facilities (POTWs), which in turn is proportional to the SFBA’s population growth.
(b) Future Projections / Growth
Sewage sludge which is generated at wastewater lagoons at POTWs is ultimately dried and applied as biosolids for soil amendment. The future projection of throughput (biosolids produced/applied) for this category and subsequently emissions, scale with volume of wastewater going through SFBA’s POTWs, which in turn is represented by the DST_Wastewater_Treatment profile developed for District’s 2015 GHG Inventory140.
5.3.6 Uncertainties
The main source of uncertainty in this category lies with the use of an emission factor that represents an average of those derived from experiments primarily conducted in California’s San Joaquin Valley3,4. The composition of the incoming waste effluent and the resulting biosolids varies greatly from region to region, so there is no guarantee that these emission factors are representative of those emitted in the biosolids production and application process in SFBA.
5.3.7 Contact
Author: Abhinav Guha
Reviewers: Tan M. Dinh and Yuan Du
Last Update: November 06, 2023
5.3.8 References & Footnotes
BACWA. 2009. Bay Area Biosolids Management: Challenges, Opportunities, and Policies. Prepared by: M. Cubed, Sep 2009. https://bacwa.org/wp-content/uploads/2010/03/Bay-Area-Biosolids-Management_Challenges_Opportunities_Policies.pdf↩︎
ABAG. 2020. Plan Bay Area 2040. Association of Bay Area Government. http://2040.planbayarea.org/reports↩︎
CARB. 2015. ARB Emissions Inventory Methodology for Composting Facilities. March 2015. https://ww3.arb.ca.gov/ei/areasrc/composting_emissions_inventory_methodology_final_combined.pdf↩︎
CARB. 2017. Method for Estimating Greenhouse Gas Emission Reductions from Diversion of Organic Waste from Landfills to Compost Facilities. May 2017. https://ww2.arb.ca.gov/sites/default/files/classic/cc/waste/cerffinal.pdf↩︎
BAAQMD. 2017. Greenhouse Gas Emission Estimates and Draft Forecasts. available on request.↩︎