Recommended street sweeping practices for water quality purposes Revision as of 19:28, 4 August 2021 by Mtrojan (talk | contribs) (→Timing and frequency of sweeping)
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This guidance document is intended to instruct the user on best practices associated with street sweeping and provide the user with key information and resources to successfully develop and execute a street sweeping program. This guidance was developed in partnership with MPCA for eventual incorporation into the Minnesota Stormwater Manual.
Topics covered within guidance include the following, which represent subsections herein:
- Street Sweeping Overview
- Street Sweeping Equipment
- Benefits of Street Sweeping
- Effectiveness of Street Sweeping (locations, timing, frequency)
- Managing Street Sweeping Waste
- Cost Considerations
- Training for Street Sweeping Professionals
- Street Sweeping Program Development
- References
There is a wide breadth of data, research, and resources available related to street sweeping, this guidance is intended to aid in the understanding of street sweeping, its benefits, and links to a variety of helpful resources for a municipality seeking to review or develop its own street sweeping program. Note that the most common street sweeping practices, equipment, and technologies vary by location, but the focus of this document is on the United States, with specific relevance to the State of Minnesota to the extent possible.
Contents
Street sweeping overview
Street sweeping (also called street cleaning) refers to removal of sediment, litter, or other accumulated substances on roadways, particularly in urban and suburban areas. Street sweeping does not include removal of large quantities of leaves brought to the street/verge for removal, large debris or bulky items; removal of these items is typically handled by large vacuum leaf collectors or dump trucks, respectively.
Historically, street sweeping was conducted manually by a sanitation worker with a broom or shovel to remove animal waste from horse-drawn vehicles and other detritus on roadways. Mechanical sweepers such as broom systems attached to horse carts came about in the mid-1800s, and in the early 1900s street cleaning wagons sprayed water onto roadways to wash away debris. Motor-driven street sweeping vehicles were patented in the US in 1917.
Modern street sweeping has improved efficiency of debris removal from roadways dramatically. The focus of street sweeping was simple large “cosmetic” debris removal until the 1970s when concerns about water quality arose. In the decades following, improvements in street sweeping technology focused more on the removal and collection of coarse sand particle-sized street dirt, and smaller particles which contribute to instream sediment and nutrient pollution when swept off of or washed into waterways. Even when a street was cleaned of large refuse, the amount of tiny particulate matter that could not be effectively removed manually remained to wash-off into waterways following precipitation. Pollutants in stormwater runoff have long been recognized as contributors to aquatic habitat degradation, nuisance algal growth, low dissolved oxygen and toxicity in receiving water bodies . More recently, there has been a focus on street sweeping to remove the organic matter produced by street trees (leaves, seeds, flowers, etc), which can contribute significant amounts of phosphorus to runoff, especially in the fall during leaf drop . Particulate matter also poses significant air-quality concerns when entrained in the air due to wind.
Street sweeping equipment
The focus of this guidance is on modern mechanized advanced street sweeping technologies. These types of mechanized street sweepers for roadways fall into four categories.
- Mechanical Broom: Rotating cylindrical brooms flick dirt and debris onto a conveyor moving into a hopper for collection. These perform well in picking up heavy material like coarse sand, gravel, and trash, but are less effective in picking up fine particles. These sweepers are abrasive which can lead to the breakdown of larger particles into smaller particles, and they are less effective at penetrating cracks and potholes in pavement. Sometimes these sweepers have onboard water spraying systems to help control dust, although sometimes a separate flush truck is used in tandem with mechanical sweepers.
- Vacuum: An engine-powered fan creates suction to remove dirt and debris up into a hopper. A windrow broom directs detritus into the path of the vacuum nozzle. While the vacuum is better at picking up fine material than mechanical broom sweepers, there are still some difficulty in penetrating cracks and potholes with the windrow broom. Vacuum exhaust can emit dust into the atmosphere.
- Regenerative Air: An engine-powered blower pushes a blast of air across the width of the sweeper truck which penetrates cracks and potholes, then similar to a vacuum sweeper, debris is sucked into a hopper. A windrow broom directs detritus into the path of the air blaster and vacuum. To avoid dust emission problems associated with vacuum sweepers, regenerative air sweepers employ a closed loop cycle of blast air and suction air.
- High-Efficiency / New Technology: These trucks represent new technology that includes a combination of technologies from other sweeper types, incorporating both mechanical and vacuum aspects. New technology includes both electric/hybrid sweepers, as well as high-powered electric, autonomous street sweeper vehicles. Vehicles are smaller in size than traditional street sweeping trucks and use LiDAR-based machine vision technology to operate under all weather conditions.
Effectiveness and costs associated with different mechanized street sweeper types varies and are summarized in other sections of this guidance.
When selecting the type of street sweeper that is best for a given municipality, the factors to consider are listed below and discussed in the adjacent table.
- Understanding where and how frequently sweeping will occur (e.g. urban roadways, parking lots, permeable pavement; quarterly, monthly, seasonally).
- Characterizing the type of sweeping waste most prevalent (e.g. loose or compacted materials, fine or coarse sediment, level of trash debris)
- Taking stock of whether it makes sense to conduct street sweeping in-house with trained municipal staff or contract the work to a private street sweeping company.
- Consider the cost, user-friendliness, and efficiency ratings of equipment.
Key functionality, limitations, and examples of street sweeping equipment. Modified from Kuehl et al, 2008
Link to this table
| Sweeper type | Sub-type | Functionality | Limitations | Hopper capacity (cubic yards) | Dump style | Addresses | ||||
|---|---|---|---|---|---|---|---|---|---|---|
| Water quality | Air quality | Appearance | Safety | Road maintenance | ||||||
| Mechanical | Chain-and-paddle | Effective for wet/matted leaves and digging/ sweeping packed dirt; Able to sweep millings and coarse sand better than belt sweepers (no “inside” areas of buildup); Compared to belt sweepers, less daily build up; Requires less power than regenerative air and vacuum sweepers | Paddles limit debris size to 6” diameter or smaller; Compared to the belt, chain-and paddle needs to be replaced more often; Does not pick up fine materials as well as other sweepers; Particles that do not get picked up are spread across the street surface sometimes making the street look dirty or streaked | 4.5-7.5 | Side Multi-Level Rear Mid-Level |
Small Particles (Poor) Large Particles (Fair) |
X | X | X | |
| Belt | Able to pick up large debris (plastic bottles, cans, branches); Able to pick up wet/matted and large amount of leaves better than other sweepers; Effective at “digging into” and removing packed dirt from roadway; Requires less power than regenerative air and vacuum sweepers | Conveyor must be cleaned daily to prevent buildup of debris; Chip seal aggregate and winter abrasive (sand) can build up inside belt; Does not pick up fine materials as well as other sweepers; Particles that do not get picked up spread across the street surface sometimes making the street look dirty or streaked | 3.5-4.5 | Side Multi-Level Rear Mid-Level |
Small Particles (Poor) Large Particles (Fair) |
X | X | X | ||
| Vacuum | NA | Removes fine sand and silt, but surface must be dry; Best for situations with most debris in gutter; Will vacuum material directly from gutter; Ability to pick up entrained material within cracks under vacuum head; Can have vacuum hose attachment (i.e. for catch basins) | Difficulty picking up wet/matted leaves; Cannot pick up tree brush; Water must be used in the hopper for dust suppression (prevents dust from being blown out via the fan exhaust); Debris is limited to 3-inch diameter or smaller; Requires more power than mechanical broom sweepers; noise may be a consideration; Water should be used or excessive fan wear will occur; More efficient operation on flat pavement surface; Should be used in above freezing temperatures only | 8.0-8.5 | Rear tilt | Small Particles (Fair) Large Particles (Fair) |
X | X | ||
| Regenerative air | NA | Can remove fine sand and silt, but surface must be dry; Ability to pick-up materials entrained within cracks; Can have a larger than average hopper; Can have vacuum hose attachment (i.e. for catch basins); Regenerative head reaches up to eight feet in width | Debris is limited to diameter of air out hose; Difficulty in picking up wet/matted leaves; Particles that do not get picked-up are spread across the street surface sometimes making the street look dirty or streaked; Requires more power than mechanical broom sweepers; noise may be a consideration; Should be used in above freezing temperatures only; More efficient operation on flat pavement surface | 4.0-9.6 | Rear tilt | Small Particles (Good) Large Particles (Fair) |
X | X | X | |
| High Efficiency / Newer Technology | Mechanical/ Vacuum | Removes fine sand and silt; Able to pick up wet, matted vegetation; Able to pick up large debris (plastic bottles, cans, small branches); Wet operation with skirts removed; Can use dry vacuum or water to suppress dust; Year round operation | Broom skirting limits ingestion of large amounts of leaves in the fall; More skirting parts that are prone to wear | 3.5-4.5 | Front Multi-Level Side Mid-Level |
Small Particles (Good) Large Particles (Good) |
X | X | X | X |
| Regenerative air | Removes fine sand and silt; Year round operation | Should be used on flat surface to seal sweeper head; Debris is limited to diameter of vacuum hose; Difficulty in picking up wet, matted vegetation | 4.5-7.3 | Rear tilt | Small Particles (Good) Large Particles (Fair) |
X | X | X |
Benefits of street sweeping
Roadways accumulate debris and material such as sediment, vegetation, vehicle debris/waste, industrial emission particle deposition, and litter. Harmful pollutants which accumulate on roadways, parking lots, and pavement include metals, organics, nutrients, and particulate matter, which street sweeping helps remove. There are a number of benefits associated with street sweeping, the most typical cited being improved appearances, improved roadway safety, and improved environmental quality through both reducing air pollution and water quality pollution. Many key benefits associated with street sweeping have cumulative impacts as well. For example, increased removal of fine particulate matter can reduce the sediment load to downstream BMPs, extending the life of these practices to continue to provide improved water quality further downstream. As the old adage goes, “an ounce of prevention is worth a pound of cure” when it comes to source removal before sediment enters the stormwater system.
Effectiveness of street sweeping
Street sweeping effectiveness is determined by several factors including the type of street sweeper, particle size, land use, tree cover, timing and frequency of sweeping, and whether there is a curb and gutter and parking restrictions,. Effectiveness is generally defined as the efficiency of the sweeper. Efficiency can be represented in a few ways, which tends to vary across studies. Most commonly, the efficiency is represented as the portion of particles/pollutants/debris removed by the sweeper on a mass basis. Note that this measure of efficiency is different from an evaluation of the changes in runoff water quality as a result of sweeping. The Minnesota Stormwater Manual provides comprehensive discussions of phosphorus and total suspended solids concentrations in runoff.
There are two main categories of materials removed by street sweepers – sediments and coarse organics. Sediments include dirt, rocks, and other inorganic components and are typically categorized by size ranging from the smallest particles in the silt and clay category (<0.063 mm) up to gravel (>2 mm). Coarse organics are larger particles of vegetative matter, including leaves, sticks, grass, blossoms, fruits, seeds, etc.
Sweeper type and particle sizes
| Regenerative air street sweeper removal efficiency by particle size in Cambridge, Massachusetts (Sorenson 2013) | ||||
| Land use | Removal efficiency (%) | |||
| Total | Coarse (> 2mm) | Medium (0.125 - <2mm) | Fine <(< 0.125mm) | |
| Commercial | Yes | TN | Medium/high | |
| Multi-family residential | 83.3 | 89.5 | 82.6 | 49.8 |
| Commercial | 78.2 | 92.4 | 79.4 | 48.6 |
| Total Median percent reduction in street solid pollutant yields by season and land use using a regenerative air sweeper (Sorenson 2013) | ||||||||||
| Land use | Phosphorus | Arsenic | Barium | Cadmium | Chromium | Copper | Lead | Nickel | Silver | Zinc |
| Residential - spring | 82 | 70 | 85 | 78 | 55 | 74 | 70 | 69 | 78 | 77 |
| Residential - summer | 99 | 77 | 80 | 84 | 80 | 90 | 80 | 80 | 84 | 76 |
| Residential - fall | 94 | 90 | 93 | 94 | 98 | 97 | 93 | 95 | 94 | 95 |
| Commercial - spring | 62 | 72 | 54 | 69 | 59 | 30 | 73 | 64 | 37 | 70 |
| Residential - summer | 97 | 79 | 84 | 79 | 66 | 31 | 65 | 71 | 37 | 75 |
| Residential - fall | 83 | 79 | 84 | 88 | 93 | 90 | 93 | 91 | 88 | 86 |
| Approximate phosphorus concentrations in sweeper solids by particle size (adapted from Chittenden County RPC et al. 2018). | |
| Particle size | TP (mg-P/kg-dry) |
| > 2mm | 950 |
| 0.125-2mm | 390 |
| 0.063-0.125mm | 450 |
| < 0.063mm | 1400 |
| Total composite | 480 |
Selbig and Bannerman (2007) found that in weekly sweeping of residential areas in Madison, Wisconsin with a regenerative air sweeper, the mean street dirt yield reduction (mass per unit length) was 25%. Similarly, a vacuum assist sweeper removed 30%. In contrast, weekly sweeping with a broom sweeper only removed 5% of street dirt and added to the street dirt yield over a third of the time (Selbig and Bannerman 2007). These efficiency values represent street dirt removal from April through September and do not account for fall leaf drop. Several other studies found regenerative air or vacuum assisted sweeper technology efficiencies to range from 35 to over 90%, but these studies were in controlled settings with pre-applied dirt mixes on a test surface. Selbig and Bannerman (2007) point out that their Madison study represents typical use conditions.
In another sweeping efficiency study, also in Madison, Wisconsin, Horwatich and Bannerman (2009) evaluated changes in street dirt yields as a result of sweeping. Using a vacuum assist sweeper, the median reduction rate was 32%, while the mechanical broom sweeper only reduced street dirt yields by 7%. Horwatich and Bannerman (2009) also found that efficiencies were higher (60-80%) when street dirt yields were higher, and efficiencies were reduced (20-30%) in low yield situations. Weekly and monthly sweeping with the vacuum-assist sweeper resulted in relatively similar median efficiencies of 29 and 32%, respectively. A meta-analysis evaluating sediment and street dirt removal efficiencies of varying land uses and sweeper frequencies showed that overall efficiencies were 47% for mechanical broom sweepers, 63% for vacuum assisted sweepers and 74% for regenerative air sweepers (Tetra Tech 2020).
Sorenson (2013) evaluated regenerative air sweeper efficiencies in Cambridge, Massachusetts. They measured street solid mass before and after sweeping with the change in mass representing the sweeper efficiency. While total efficiency was quite high, efficiency decreased with decreasing particle size as summarized in Table 2. A similar pattern is shown in the pollutant reductions. In some instances, the change due to sweeping was an increase in pollutants associated with the smallest particles (<0.125mm, very fine sand, silt, and clay) but an overall reduction in the pollutant (Table 3). Generally speaking, street sweepers of all types are more effective at removing larger particles and less effective at removing smaller particles, but regenerative air and vacuum assisted sweepers are consistently more effective than mechanical broom sweepers.
Breault et al. (2005) evaluated the removal efficiencies of a mechanical broom sweeper and a vacuum sweeper using a mix of street dirt with a known particle size distribution. They found that mechanical sweepers, in addition to being overall less effective at removing street dirt (20-31% efficiency), they were particularly ineffective at small particle removal (9-13% removal efficiency of particles less than 0.250 mm). In contrast, vacuum sweepers were able to remove 60-92% of street dirt overall and were able to maintain 31-75% efficiency in removal of particles less than 0.250 mm.
It is important to remove the larger particles, including coarse organics, like leaves, for successful sediment and phosphorus removal. Waschbusch et al. (1999) showed that approximately 50% of the total phosphorus and 70%of the sediment in street dirt is in particles greater than 0.25 mm and leaf litter contributes another 30% of the total phosphorus load. However, when sweeping for water quality improvements, it is important to consider the smaller particles, which often have the highest phosphorus concentrations on a mass basis. Preliminary data from the Chittenden County Regional Planning Commission showed that street sweeper solids in the smallest particle fraction (<0.063 mm) had TP concentrations almost three times as high as the larger particles (0.063-2 mm) and nearly twice as high as the largest particles (>2mm), as shown in Table 4. Refer back to Table 1 for a general evaluation of how suited the various types of sweepers are for removal of different particle sizes.
Land use and tree cover
Land use can have any impact on the amount of street dirt and organics generated; however, there is little evidence that land use type alone significantly impacts the amount of dirt and debris removal on a given street. SPU and HEC (2009) found a similar amount of material was collected from both residential and industrial areas, and there was high within-site variability of removal efficiencies during repeated sweeping events. Sorenson (2013) found that sweeping residential and commercial land uses resulted in similar removal efficiencies (refer to Table 2). The specific activities occurring on a given street may be more relevant than the general land use. Janke et al. (2017) noted that factors such as traffic volume, population density, and vegetation are important land use variables controlling stormwater runoff nutrients. When evaluating a street sweeping program, consider specific characteristics of the surrounding landscape. For example, a roadway in an area classified as industrial may have a relatively low level of street dirt accumulation, if the area is primarily storage warehouses; however, if the adjacent area is a gravel and sand supplier, higher street dirt is likely, due to spillage from trucks passing through and generally dustier conditions. Within residential areas, the age of development, level of urbanization, and tree canopy cover can influence street dirt and organics accumulation. Waickowski (2015) found that the total phosphorus load from low-density older residential neighborhoods and downtowns were about twice as high as those from high-density residential neighborhoods and recently developed low-density neighborhoods. The lower rates from the new development and high-density development were attributed to the lack of tree canopy.
Tree cover is an important consideration in determining how effective sweeping can be. Janke et al. (2017) monitored 19 watersheds in the Minneapolis-Saint Paul metropolitan area to evaluate the influences of trees, vegetation and impervious cover on nutrient concentrations and loading in stormwater runoff. The presence of street trees within five feet of the curb was found to be highly correlated to the total phosphorus event mean concentrations in runoff, highlighting the importance of prioritizing leaf litter removal either through street sweeping or dedicated leaf litter collection.
Kalinosky (2015) conducted a two-year study of street sweeping in Prior Lake, Minnesota to evaluate the impacts of street tree canopy cover on the characteristics of swept materials. The study found a strong seasonal correlation between the amount of coarse organic material collected, tree canopy coverage, and seasonal leaf drop. Coarse organic matter was found to be 15% of the total dry weight of swept material, but contributed 36% of the TP and 71% of the TN. The amount of overhead tree canopy was determined to be a significant predictor of recoverable loads of coarse organic matter and nutrients throughout the year (Kalinosky 2015). A similar pattern was identified for nutrient content per curb-mile, as shown in Figure 2 and Figure 3.
Timing and frequency of sweeping
The impact and effectiveness of street sweeping is affected by when and how often streets are swept. Targeting sweeping prior to major storm events and after major tree flower and leaf dropping events can increase the volume of swept debris. Street dirt and debris, including leaves and other vegetation, build up over a period of time and are then washed off, entering the storm drain system. Effective street sweeping relies on the timing of sweeping to capture dirt and debris before it has the opportunity to wash off. This effect is illustrated in Figure 4. If sweeping is too infrequent, the majority of the accumulated materials will be removed via washoff, rather than sweeping. In addition, during smaller rainfall events that do not washoff larger leaf litter particles, leaching from rewetted organic matter can mobilize nutrients into runoff. An appropriate street sweeping frequency will vary based on the frequency of runoff-generating rainfall events and the amount of debris on the street. However, there is a point of diminishing returns when considering street sweeping frequency. If sweeping is conducted too often, dirt and debris will not have accumulated to a point where each individual sweeping pass is collecting a substantial amount of material.
Sutherland and Jelen modeled the total suspended sediment (TSS) reductions by various sweeper technologies. Most of the improved overall removal efficiencies were gained in sweeping at least once a month and up to weekly sweeping (Sutherland and Jelen 1997). Sweeping less frequently than once a month misses a lot of accumulated street dirt, and sweeping more frequently than weekly, while still reducing overall loads, has a much smaller marginal increase in loads captured.
Seasonal timing of street sweeping should also be considered. Sorenson (2013) found that in Cambridge, MA, fall street sweeping, which included fall leaf litter removal, had the maximum phosphorus yields throughout all four seasons, but the end of winter (March) sweeping had the highest median phosphorus yields. Sorenson (2013) also used SLAMM (Source Loading and Management Model) to model street sweeping pollutant removal and found that it consistently underrepresented leaf litter loadings in the fall. Sorenson (2013) suggested that the model may be underrepresenting TP reductions from street sweeping because of this underrepresentation of leaf litter loading. Kalinosky et al. (2014) in their sweeping study of Prior Lake, MN found that the amount of coarse organic material recovered per curb-mile increased as the tree canopy cover over the street increased, as shown in Table 5. The study also showed that in areas of medium and high canopy cover, there were benefits to sweeping more than once a month, with biweekly sweeping picking up more material on a per mile basis than monthly sweeping (Kalinosky et al. 2014).Canopy cover was determined qualitatively by the amount of tree canopy over the street, on average, for the sweeping route. Subsequent to the canopy cover determination, canopy cover was analyzed using a geospatial analysis. Medium canopy cover was calculated at 5.6% coverage and high canopy cover was 13.9% coverage over the street.
| Average coarse organic (dry weight) recovered per sweep by route type, using regenerative air sweeper technology (Kalinosky et al. 2014). | |||
| Sweeping frequency | Low Canopy (lb/curb-mile) | Medium Canopy (lb/curb-mile) | High Canopy (lb/curb-mile) |
| 1x/month | 10.6 | 23.4 | 59.9 |
| 2x/month | 10.7 | 35.3 | 89.2 |
| 8.1 | 33.0 | 49.1 | |
Removing leaf litter soon after it has fallen is important for maximizing the phosphorus removal benefit. Cowen and Lee (1973) showed that the length of time leaves remain in contact with water and the degree to which the leaves are broken down increase phosphorus leaching. Leaching is an important factor in considering stormwater runoff quality. In this context, it is the release of soluble phosphorus from organic matter into stormwater runoff which eventually reaches receiving waters, or downstream BMPs. In a laboratory setting, cut up leaves leached nearly three times as much phosphorus as intact leaves, highlighting the need to collect leaves soon after they drop, to minimize breakdown through natural decay processes and by vehicular and foot traffic, and prior to rainfall events that can mobilize the phosphorus. Hobbie et al. (2013) found that leaf litter in the curb gutter decomposes faster than in natural areas (natural, non-urban forests and prairies) with most species losing 80% of their initial mass after one year but still retaining more than half the nitrogen and phosphorus. After an initial loss period, Hobbie et al. found there were several cycles of phosphorus immobilization and loss, but there was significant variation among species (Hobbie et al. 2013). Kalinosky (2015) found leaching rates of street litter were highest in May and declined throughout the summer.
While traditionally sweeping has been limited to the spring through leaf drop in the fall; it has been shown that winter snowmelt in contributes roughly 50% of the annual nitrogen and phosphorus export off roadways in highly urbanized areas of Saint Paul, MN (Bratt et al. 2017). This was attributed to decomposing leaf litter on roadways that leaches phosphorus throughout the winter as snow melts into runoff. Further Bratt et. al. (2017) estimated that winter leaf litter may contribute up to 40% of the annual total dissolved phosphorus loading. In more suburban areas snowmelt leaching is a much less significant contributor to overall TP loading.
In areas where sweeping is conducted primarily for total solids removal, sweeping throughout the year, especially in the summer, can be just as important as spring and fall sweeping, which may be done for vegetative debris, winter salt and sand, and leaf removal. As shown in Figure 5, with the exception of December and January, the total recoverable dry solids collected during sweeping can remain high throughout the year (Kalinosky et al. 2014). Fine sediment is the primary contributor from February through September, while coarse organics increase in the fall.
While the most appropriate sweeper schedule will depend on local conditions and objectives, an example from Schilling’s (2005) assessment of street sweeping policy options for the Ramsey-Washington Metro Watershed District and recommended street sweeping frequencies based on the land use and special area types is presented below. The recommended sweeping frequencies were informed by the sweeping frequencies reported in an earlier survey of jurisdictions in Minnesota and the US and Canada, more broadly. Part of the findings from that survey indicated that Minnesota jurisdictions tend to street sweep less frequently than other jurisdictions. The resulting recommended frequencies represented a balance of closer alignment with typical frequencies and “a reasonable and defendable approach” (Schilling 2005). The resulting recommendations are summarized in Table 6.