Survey of street sweeping crediting approaches

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This document addresses Task B Subtask 1 of the project work plan: to prepare a report summarizing a literature review of existing street sweeping credit methods. Research revealed these categories and considerations to be integral in developing a crediting method: scheduling, equipment, debris measurement, and nutrient quantification.

Crediting methods vary by governing agency. Over 70% of state MS4 permitting agencies do not require or provide a nutrient or sediment reduction credit for street sweeping, regardless of whether there is an applicable TMDL. Generally, street sweeping is regarded as a good housekeeping measure and annual miles swept are tracked. Most street sweeping programs set schedules and prioritize streets based on traffic volume. Some programs attempt to measure the amount of debris removed by street sweeping. However, relatively few entities establish documented quantifiable crediting methods using research-based estimates of pollutant load removal. Of the roughly 30% of states and the District of Columbia that do have a street sweeping crediting program, half adhere to the Chesapeake Bay TMDL crediting method. While California does not have consistent crediting programs throughout the state, individual permitted entities do have street sweeping crediting programs. While MS4 permits require varying degrees of quantitative and qualitative documentation on street sweeping activities, this literature review focuses on documenting permitting entities that have an established pollutant reduction crediting methodology. These programs typically require documentation, at a minimum, of the length or area of street swept or a measurement of the materials collected with reductions calculated from established pollutant concentrations or reduction efficiencies or individual load reductions based on permittee-specific data.

Approach

In addition to resources provided by the University of Minnesota, researchers reviewed state and local stormwater/MS4 websites, EPA’s regional MS4 websites and performed general Google searches to identify street sweeping crediting programs. Federal, state and local websites were reviewed to identify the requirements of the relevant MS4 permits, with particular attention paid to TMDL-specific permit requirements. If specific street sweeping requirements were not evident in the state permitting pages, local MS4 permits and annual reports were identified and reviewed to determine the types of street sweeping information reported. The general Google review was conducted using the following search terms or a combination of terms coupled with specific state names: street sweeping, street cleaning, nutrient management, credit, reduction credit, TSS, phosphorus, nutrient removal, nutrient load, stormwater, quantification, qualifying lane miles, mass loading, TMDL and MS4. Once results were located using search terms, researchers reviewed document references to locate additional sources. When a street sweeping crediting program was identified, researchers attempted to locate the following information: description of each crediting method, location and scale of applicability, factors considered, inputs, key assumptions, quantification approach (if applicable), and conditions under which credit cannot be received or is reduced. All street sweeping crediting programs that were identified are included in the following section.

Existing crediting methods

Crediting methods vary widely across programs. Some rely on weight conversions of wet or dry material to an estimated reduction of total nitrogen, total phosphorus, or total suspended sediment. Other programs credit based on lane miles swept. Some programs use advanced modeling and provide a wide array of options based on equipment used, number of times swept, and seasonal breaks. Table 1 provides a summary of the crediting methods identified, followed by a more in-depth discussion of each method. For Minnesota, the goal is to provide a mass-based approach, backed by research-based empirical relationships, for the crediting program. This approach is the most defensible as it involves direct measurement. Examples from Table 1 that use a similar approach include: Chesapeake Bay, Florida, Washington Ecology, and potentially North Carolina (in progress). This overall approach is yet to be discussed with stakeholders and may be modified.

Table

Chesapeake Bay Watershed (Chesapeake Bay Program, U.S. EPA)

• Crediting Method: Reduction efficiency based on acres swept
• Applicability: Chesapeake Bay watershed (NY, PA, DE, DC, MD, VA, WV) jurisdictions seeking Chesapeake Bay TMDL nutrient reduction credits.
• Details: The Chesapeake Bay Program Expert Panel on Street Sweeping developed a crediting method for jurisdictions subject to the Chesapeake Bay TMDL (Schueler et al. 2016). WinSLAMM was used to simulate sediment reduction from nearly 1,000 street cleaning scenarios that varied by the length of winter shutdown, types of streets, sweeper technologies, cleaning frequencies and street parking conditions and controls. WinSLAMM does not explicitly simulate nutrients. Nutrient percent reductions are based on empirical nutrient enrichment ratios of street solids (Schueler et al. 2016).

To simplify the crediting, the modeled scenarios were reduced to 11 different practices, based on the type of street sweeper, and the number of times a street is swept per year. Mechanical broom technology was assigned 0% efficiency for all sweeping regimes and only a negligible TSS reduction efficiency. The TSS percent reduction is applied to the unit area sediment load for impervious cover in the Chesapeake Bay Watershed Model (CBWM) to determine the mass reduced. The nutrient reductions are applied to the unit area nitrogen and phosphorus loads for impervious cover in the CBWM. Table 2 summarizes the crediting for the Chesapeake Bay Program.

The credit is only available for curb and gutter roadways and is reported as curb-lane miles swept. Each mile of curb-lane is equivalent to one impervious acre.

• Tracking and Reporting: To receive credit for the Chesapeake Bay TMDL, annual street cleaning information is reported by practice, as shown in Table 2, and must include the curb-miles or acres-equivalent swept, as well as the type of land use where sweeping occurred (roads, tree canopy over impervious, or roads and tree canopy over impervious). The efficiency is applied to the selected land use. The Expert Panel recommended that MS4 communities report annual street sweeping in annual MS4 reports that are submitted to the state agency. The state agencies are responsible for providing the information on an annual basis to the Chesapeake Bay Program, where it is entered into the Chesapeake Bay watershed model. For verification purposes, localities are required to maintain records that include length of routes swept, frequency, sweeper technology, and parking conditions and controls. In addition, localities are required to collect a single sample from a single route by a single sweeper once a year to characterize the mass and quality of sweeper material collected. The MS4 should measure or estimate the volume of sweeper waste collected, total wet mass, number of curb miles swept on the route and sweeper conditions (date, weather, days since antecedent rainfall, street type, and parking conditions). A sub-sample should also be sent to a laboratory to measure dry weight of sweeper material, particle size distribution, and average carbon, nitrogen and phosphorus content.

Although the reporting mechanism allows the user to indicate that tree canopy over impervious was swept, WinSLAMM did not explicitly account for leaf drop impacts on sediment.

• Additional Information: The Expert Panel also developed a storm drain cleaning credit based on the mass of solids collected. The sediment credit is based on the dry weight of the mass of solids removed and the nutrient reduction is the mass of solids times a nutrient enrichment factor. Enrichment factors for BMP and catch basin sediments and organic matter/leaf litter are based on midpoint data from several referenced studies.

Table 2

In 2011, an earlier Chesapeake Bay Program expert panel developed two street sweeping crediting methods – mass loading and qualifying lane miles. In 2016, the Chesapeake Bay Program expert panel suggested immediately phasing out both methods of calculating street sweeping credits; however, they are briefly summarized here for completeness.

• Crediting Method: Qualifying lane miles
• Details: This method required entities to record the number of lane miles swept annually and convert them to total impervious acres. To receive credit for this practice, an enhanced street sweeping program was required. The eligibility requirements included:
  • Urban streets with high average daily traffic volume,
  • Streets swept 26 times/year, biweekly or concentrated in the spring and fall.

The impervious calculation was 5,280 ft times 10 ft per lane in width divided by 43,560 acre/ft2. Impervious acres were multiplied by pre-sweeping annual nutrient load using the Simple Method unit loads: 2.0 lbs/impervious acre/year TP and 15.4 lbs/impervious acre/year TN. The pre-sweep baseline loads were multiplied by pickup factors based on sweeper technology, as shown in Table 3. The pickup factors represent percent reductions from the baseline loads.

Table 3

Tracking and Reporting: Reporting required either the impervious acres swept or annual dry solids mass collected. Practice verification was not required during the time this credit was used (Schueler 2016).

Crediting Method: Mass loading method Details: This method credited nutrient and sediment reductions based on the annual wet mass of debris in pounds and required determining the hopper capacity of the sweeper, weighing the street solids collected and developing a relationship between street solid mass in tons and hopper capacity. Records were to be kept by each MS4 on the annual mass of street solids collected from qualifying streets (those swept at least 26 times/year). This mass was multiplied by 0.7 to convert to dry mass, then multiplied by 0.0025 pounds of dry weight to calculate total nitrogen, 0.001 pounds of dry weight to calculate total phosphorus, and 0.3 pounds of dry weight to calculate total suspended solids. This correction factor between dry mass solids and TSS is to remove particles larger than 250 microns, which are too large to be considered TSS. The TSS factor is based on particle size data from SPU 2009 and Law et al. 2008 (in Schueler et al 2016) showing that only 20-30% for street sweeping solids are less than 250 microns. Tracking and Reporting: Pounds collected were reported to the Chesapeake Bay Program through the states’ annual submissions; however, the Chesapeake Bay Program did not provide verification procedures.

Florida Department of Environmental Protection

• Crediting Method: Mass loading method
• Applicability: Florida MS4s
• Details: The Florida DEP MS4 Permit Resource Manual indicates that street sweeping tracking needs to occur to evaluate the effectiveness of the stormwater management program, especially if the MS4 discharges to a waterbody with a TMDL or Basin Management Action Plan (DEP 2013). The sweeping program only applies to streets with curb and gutter. The Florida Stormwater Association developed an MS4 Load Reduction Assessment Tool (Assessment Tool) to estimate the nutrient loads removed from street sweeping, catch basin cleaning and BMP cleanout (FSA 2012). The tool requires the weight or volume of solids removed, so street sweeping material must be measured or weighed. The methodology recommends direct measurement of the weight of a full truck for a year to develop reliable average values for the weight or volume of solids collected, and then allows the use of these data to develop an accurate estimate that will allow the jurisdiction to move to “counting trucks” as a way of tracking the amount of solids collected (Bateman 2012).

Solids are converted to dry solids using default moisture content and dry bulk density values. However, the default values are only to be used for the first year of permitting. Each permit group (e.g., Palm Beach County) is required to take monthly samples for moisture content and bulk density for at least one year. At least 24 samples, including replicates, per permit group are needed to develop final statistically valid values for bulk density and moisture content. Monthly sampling results are reported in MS4 annual reports. These values then replace the default moisture content and dry bulk density for each permit group.

Once the mass of dry solids is calculated, the Assessment Tool automatically applies the nutrient enrichment values for TN (563 mg/kg) and TP (361 mg/kg) and determines the pounds of TN and TP that were removed for the collection period. The enrichment values are based on sampling conducted at 14 MS4s throughout Florida on highway, commercial and residential land uses.

• Tracking and Reporting: The FDEP Phase I MS4 Annual Report Form requires the frequency of sweeping, total miles swept, an estimate of the quantity of sweepings collected, and the TN and TP loadings in pounds that were removed as a result of sweeping activities. Each permittee can choose to report in volume or mass (DEP 2004). This is part of the Stormwater Management Program Roadways element of the MS4 annual report.
• Additional Information: Catch basin (679 mg/kg TN, 417 mg/kg TP) and BMP cleanout (899 mg/kg TN and 364 mg/kg TP) nutrient reduction credits are also available with Florida-based TN and TP enrichment factors.

EPA Region 1 - New Hampshire and Massachusetts

Crediting Method: Mass load reduction based on efficiency factors Applicability: New Hampshire and Massachusetts MS4s subject to TMDLs Details: EPA Region 1 is the MS4 permitting authority for both New Hampshire and Massachusetts. Enhanced street sweeping crediting applies to MS4 jurisdictions subject to nutrient-related TMDLs or discharging to water quality limited waterbodies and their tributaries where nitrogen or phosphorus is the cause of impairment. Other MS4 permittees are only required to sweep once a year in the spring and report the miles cleaned and volume or mass removed. MS4 permittees that discharge to waterbodies subject to a lake or pond phosphorus TMDL are required to develop a Lake Phosphorus Control Plan that includes planned non-structural controls to contribute to meeting the waste load allocation; an enhanced street/pavement cleaning program is a non-structural control practice eligible for TN and TP reductions (USEPA 2017a). As part of Good House Keeping and Pollution Prevention, MS4 permittees discharging to a nutrient impaired waterbody must increase street sweeping frequency on municipally owned streets and parking lots to a minimum of two times a year, once in the spring and once in the fall following leaf fall (USEPA 2017b). In New Hampshire both TN and TP crediting is available. Massachusetts is limited to TP crediting for TMDLs, but the formula is the same for both states (USEPA 2017a): P credit (lbs/yr) = Impervious area (acres) x P load export rate for impervious cover and specified land use (lb/ac/yr) x P reduction factor based on type and frequency x annual frequency N Credit (lbs/yr) = Impervious area (acres) x N load export rate for impervious cover and specified land use (lb/ac/yr) x N reduction factor based on type and frequency x annual frequency EPA provides a table of 13 land use categories including the distinction between directly connected impervious and pervious land cover, with individual phosphorus and nitrogen load export rates for each land use category and land cover combination. The P and N reduction factors are based on the frequency of sweeping and the sweeper technology and are unitless reduction multipliers. Credit is available for sweeping between 2 times a year and up to weekly. Credit is provided to mechanical broom, vacuum assisted and high-efficiency regenerative air-vacuum technology at variable rates, as shown in Table 4. The annual frequency is a reduction factor that is applied to represent the portion of the year when sweeping is conducted (e.g., if sweeping only occurs for 9 months of the year) the annual frequency factor is 0.75 (9 months/12 months = 0.75).