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This document summarizes 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 (Municipal Separate Storm Sewer System) permitting agencies do not require or provide a nutrient or sediment reduction credit for street sweeping, regardless of whether there is an applicable Total Maximum Daily Load (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.
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.
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.
|Chesapeake Bay Watershed (entities subject to Bay TMDL requirements)||
||TN, TP, TSS|
|Florida||Mass load reduction||TN, TP|
|EPA Region 1 (New Hampshire and Massachusetts MS4 permittees)||Mass load reduction with efficiency factors||TN (NH only), TP|
|Massachusetts – stormwater requirements (not linked to MS4 permit)||Reduction efficiency - %||TSS|
|Washington Ecology – MS4 permittees with TMDL||Mass load reduction, effective developed acres reduction||TP, sediment|
|County and City of San Diego and MS4 Co-permittees||Performance-based compliance||Cu, Zn, Pb, TKN, TP|
|Orange County and MS4 Co-permittees||Reduction efficiency - %||Bacteria, sediment|
|City of Santa Cruz, California||Modeled load reduction||Sediment|
|Vermont (Under development)||
|North Carolina (Under development)||Mass load reduction||TN, TP|
Summary of Chesapeake Bay Program street sweeping crediting.
Link to this table
|Pollutant Reductions Associated with Different Street Cleaning Practices (SCP)|
|Practice #||Description||Approx passes per year1||TSS Removal (%)2||TN Removal (%)2||TP Removal (%)2|
|SCP-1||AST- 2 PW||100||21||4||10|
|SCP-2||AST- 1 PW||50||16||3||8|
|SCP-3||AST- 1 P2W||25||11||2||5|
|SCP-4||AST- 1 P4W||10||6||1||3|
|SCP-5||AST- 1 P8W||6||4||0.7||2|
|SCP-6||AST- 1 P12W||4||2||0||1|
|SCP-7||AST- S1 or S2||15||7||1||4|
|SCP-8||AST- S3 or S4||20||10||2||5|
|SCP-10||MBT- 1 PW||50||0.5||0||0|
|SCP-11||MBT- 1 P4W||10||0.1||0||0|
1Depending on the length of the winter shutdown, the number of passes/yr may be 10 to 15% lower than shown.
2% removal represents the proportion of the impervious surface unit area load removed
Table abbreviations: AST: Advanced Sweeping Technology ; MBT: Mechanical Broom Technology ; 2PW: 2 passes per week; 1PW: 1 pass every week; 1P2W: 1 pass every 2 weeks; 1P4W: 1 pass every 4 weeks; 1P8W: 1 pass every 8 weeks; 1P12W: 1 pass every 12 weeks
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.
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.
Efficiency multipliers for street sweeping under qualifying lane miles approach - Chesapeake Bay.
Link to this table
|Technology||TSS (% reduced)||TP (% reduced)||TN (% reduced)|
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.
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.
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).
To receive credit, semi-annual sweeping must be conducted following the spring snowmelt and road sand applications and in the fall after leaf-fall and prior to the onset of snow regardless of timing or frequency of other sweeping activities. As an alternative to the crediting described above, a credible model of the watershed that can reflect street sweeping with continuous simulations reflecting buildup and washoff of P and/or N using long-term rainfall data can be used to determine TN and TP removal credits. This alternative is only applicable to New Hampshire (USEPA 2017a).
Mass DEP provides a TSS Removal Calculation Sheet, a spreadsheet tool that calculates the percent removal for BMPs applied to a project area. There is no initial load calculated, but rather percentage reductions applied to the entire project area, with the starting TSS load assumed to be 100% of the post-construction load, and BMPs reducing the load according to their efficiency values (MassDEP 2008c). Tracking and Reporting: If street sweeping is used as a compliance strategy, it is documented in the Stormwater Report that is required to be submitted to comply with the Stormwater Management Standards to receive a permit under the Wetlands Protection Act Regulations and/or the 401 Water Quality Certification Regulations (MassDEP 2008d).
In addition to compliance with the developed urban area performance standard, MS4 permittees subject to TSS or TP TMDLs are also required to meet WLAs. The baseline loading condition, prior to TMDL reductions, should assume the MS4 20% or 40% TSS control requirement before applying TMDL allocations. Total phosphorus baseline loading is calculated by converting from the TSS baseline using a fixed formula (WI Bureau of Watershed Management 2014). Under the modeling requirements, street cleaning credit can only be claimed if there is a curb and gutter or equivalent barrier. If both mechanical broom and vacuum cleaning occur within two weeks of each other, only the removal efficiency from vacuum cleaning can be claimed. If broom sweeping occurs in the spring and fall, but vacuum sweeping occurs the remainder of the year, a combined efficiency can be calculated as: Model entire street cleaning program with mechanical broom + model just the period of vacuum cleaning – model the vacuum cleaning time period with a mechanical broom = overall combined sweeping efficiency (WI Bureau of Watershed Management 2014). WinSLAMM runoff calculations are based on normal rainfall events and associated pollutant loading. Calculating TP and TSS loading requires entry of rain, runoff coefficients, pollutant probability distribution, particulate solids concentration (i.e., TSS concentration), land use and particle size distribution, etc. BMP efficiency determination for street sweeping requires input of frequency, sweeper technology, street cleaner productivity (street texture, parking density, parking controls), parking density and parking controls.
P8 inputs are precipitation, land use, hydrologic conditions, soil type, impervious connected fraction by land use. P8 uses SLAMM hydrology calculations. To determine street sweeping efficiency impervious area is entered as either swept or not swept; and sweeper efficiency is based on particle inputs and adjusted based on parking controls. Credit only applies to vacuum sweeping and efficiencies are revised down to 5-15% depending on particle size.
Tracking and Reporting: Stormwater Management Planning (SWMP) is required for MS4s subject to a TMDL and should include the estimated pollutant reductions from various practices, including street sweeping. TMDL implementation planning is incorporated into the MS4 permit and requires a compliance schedule and benchmark tracking. TMDL compliance reporting includes the practice type, areas affected, date of implementation, treatment performance efficiency and percent reduction towards the TMDL target reduction (WI Bureau of Watershed Management 2014). General MS4 annual reporting for street sweeping includes the roadway swept, frequency of sweeping, and volume of material collected (Milwaukee County 2017). For water quality management reporting requirements in the MS4 annual report, a pollutant loading analysis is required, but load reductions are reported as a cumulative percent TSS reduction for all relevant practices.
Crediting Method: Reduction efficiency - leaf management Applicability: Wisconsin MS4 permittees subject to an approved TMDL Details: WDNR has approved a 17% total phosphorus annual load reduction from leaf collection efforts. The reduction can be taking from the “no controls” phosphorus load in the selected model. This credit only applies to the medium density residential no alleys (MDRNA) land use; credit cannot be used for other land uses; and credit can only be taken for the portion of the land area that is MDRNA because there is insufficient data to quantify credit on other land uses at this time. Medium density residential is 2-6 units/acre consisting of single-family homes. Medium density with alleys can received credit if the alleys are receiving the same level of leaf collection and street cleaning as streets. The credit assumes light parking density and a curb and gutter system. It is unclear if the alleys must also have a curb and gutter system to qualify for credit. To be eligible for credit the area must have an average of 1 or more mature trees between the sidewalk and curb for every 80 linear feet of curb, or when sidewalk is not present, trees within 15 feet of curb. WDNR has determined that this equates to tree canopy over street pavement of 17% or greater. In addition, the municipality must have an ordinance prohibiting placement of leaves in the street and a policy that residents may place leaves on the terrace in bags or piles for collection. Leaf collection must occur at least 4 times in Oct and Nov. into a fully enclosed vehicle. Within 24 hours of collection, mechanical broom or vacuum street cleaner must remove remaining litter from street (WDNR 2018). The phosphorus reduction credit for the leaf management program may not be taken in addition to phosphorus reductions from other BMPs in the same drainage area because additional evaluation is needed to determine the impact of leaf collection on loading to structural BMPs (WDNR 2018). Tracking and Reporting: Tracking and reporting is not specifically addressed; however, this is a credit related to the development of SWMPs for MS4s subject to TMDLs. As such, it will likely be reported through the same mechanisms described under the Wisconsin street sweeping crediting.
Tracking and Reporting: For the City of Bellingham, enhanced street sweeping is required to be reported in each MS4 annual report as the estimate of the mass of total phosphorus removed and the estimate of the equivalent reduced effective developed acres (Ecology 2019). However, no specific methodology for determining the total phosphorus removed was identified.
Factors that were included in the LSPC optimization for street sweeping included start and end months for sweeping (year-round), typical days between high, medium and low frequency of route sweeping, fraction of land surface available for sweeping, total solids removal efficiency of different sweeper technologies, fraction of sand, silt/clay and gravel in solids, and the concentration of copper, zinc, lead, TKN, total phosphorus and bacteria in collected sediment, as determined by a pilot sweeping monitoring study (AMEC 2015).
Tracking and Reporting: Specific load reductions are not tracked or reported in association with street sweeping, but the annual report identifies whether and how the optimized strategy is being implemented (performance measures) (Wood 2019).
Part of the WQIP Monitoring and Assessment Program is to move to a modeling framework that includes non-structural practices, including street sweeping, to allow for the optimization of BMP opportunities (The County of Orange et al. 2017). A report of findings is expected in 2019 but is not yet available. Tracking and Reporting: The most recent annual report, from 2017-2018, did not address programmatic measures.
Tracking and Reporting: As of the 2017-2018 Annual Report and the 2018 Annual Report for TMDL BMP Implementation, the City of Santa Cruz was not reporting on load reductions but instead on curb miles swept and tons of solids collected (City of Santa Cruz 2018). The annual reports indicated that TELR was being used; however, it was not clear what the resulting sediment load reduction was determined to be.
Vermont is in the process of developing a phosphorus street sweeping credit to address nutrient TMDLs for municipalities that will be regulated under the Vermont Clean Water Act. An interim credit will be developed based on the BATT model from EPA Region 1 and other crediting methods, including the New Hampshire MS4 permit and the Chesapeake Bay Program Expert Panel report (Vermont Agency of Natural Resources 2019). A permanent credit will be developed based on “phosphorus source area delineations of municipal streets using real time materials sampling of sediment and nutrients, forest cover measurements, forest tree species, seasonal deposition of organic materials from trees, street sweeping efficiencies and street cleaning frequencies” (Vermont Agency of Natural Resources 2019). Additional information on the program was not available.
North Carolina Department of Environmental Quality is also in the process of developing a street sweeping and catch basin cleaning credit. The credit is only intended to apply to existing development requiring nutrient reductions to meet applicable nutrient strategies (NCDEQ 2018). It will not be applicable as credit towards nutrient reduction requirements related to new development. The streets must have a curb and gutter system. The street sweeping and catch basin credit is determined by applying conversion factors to the weight of freshly collected material (wet), as determined by direct measurement of the average weight of the contents of a collection vehicle. The conversion factors are 0.023 lb TN/lb of debris and 0.002 lb TP/lb of debris, respectively. These are derived from gross solids measurements in catch basins throughout coastal and Piedmont North Carolina. Dry weight is not used in this method. Custom conversion factors based on laboratory analysis may also be used, but consultation with the NC Department of Environmental Quality, Division of Water Resources is required. The product of the weight of material collected and the conversion factors is the mass of nitrogen and phosphorus removed. To receive credit, the entity will submit to North Carolina Division of Water Resources an estimate of annual weight collected at baseline, weight of material collected in reporting year, difference between baseline and reporting year weights, nutrient credit calculation, type of collection vehicle and the average weigh of contents of a full truck. The credit specifically does not include credit for fall street-side leaf pickup.
A final crediting method was presented to the North Carolina Nutrient Scientific Advisory Board in 2018; however, the Board determined that more work needs to be done to determine whether it is feasible to determine a baseline for this practice (NSAB 2018).