This page describes methods for sampling street sweeping material including determination of wet mass, dry mass, dry basis moisture content, and organic matter. The most accurate way to account for phosphorus reduction from street sweeping would be to measure phosphorus in the sweeping load directly. However that is both technically complicated and prohibitively expensive on the scale of municipal sweeping events. Therefore, alternative field and laboratory measurements may be conducted to approximate the phosphorus load captured by street sweeping.
The standard operating procedures (SOPs) described on this page were developed by University of Minnesota researchers as part of a study to develop a street sweeping credit methodology (see Acknowledgements for street sweeping credit method). Stormwater practitioners and permittees choosing to use the street sweeping credit to estimate phosphorus load reductions from street sweeping should apply the following SOPs. Sweeping load sampling options are listed below, generally ordered by level of method complexity:
A Credit Calculator was developed to allow practitioners a range of options for calculating phosphorus reductions from street sweeping. The bare minimum sweeping load measurements required for application of the Credit Calculator is curb miles swept, however the more measurements taken (e.g. wet mass and/or laboratory analyses), the more accurate the results will be. The Credit Calculator is conservative in nature; therefore, it is possible that additional laboratory measurements would result in greater estimates of phosphorus load removed.
A decision tree which maps out the actions and data collection activities to employ in using the phosphorus credit calculator are shown in the adjacent figure. Additional information on the credit calculator tool and how it is used may be found in the tool itself and the accompanying user guide.
This section describes field methods for calculating phosphorus reductions from street sweeping.
The simplest measurement that can be used for input into the Credit Calculator is the length of curb miles swept. One curb mile is equivalent to a sweeper-width pass along one side of a street. If both sides of a one-mile long stretch of roadway are swept (1 road mile), that distance is equal to 2 curb miles. To calculate the phosphorus reduction from miles swept, simply input the miles swept into Option 3 of the calculator, as shown in the adjacent image.
If using the wet mass method, after the street sweeping event has occurred, the wet mass of the entire sweeping load should be measured. This is typically conducted using either of the following methods:
1. Some street sweeping vehicles are equipped with an internal scale which informs the driver of the weight of material in the hopper. This weight is considered the wet mass.
2. Calculating the difference in mass between the empty street sweeper vehicle before and after the collection event.
The sweeping load wet mass may be entered directly into the Credit Calculator, or additional measurements can be made to determine dry mass by measuring dry basis moisture content, and/or additionally measuring organic matter content.
Laboratory-based methods for calculating phosphorus reductions include dry mass measurement and measurement of organic matter. Each of these requires load sampling. These are discussed below.
Although we were unable to find a video specific to subsampling from a sweeper pile, the following videos provide general information about collecting subsamples.
After completing a street sweeping event, subsampling of the street sweeping load should occur within 24 hours of sweeping load collection. If the sweeping load is dumped and stored outdoors, perform subsampling before any precipitation occurs or ensure the pile is covered and not subject to impacts of precipitation (e.g. overland flow from paved areas). When performing the subsampling of the sweeping load, prepare the materials and complete the methods listed below. Note that the total load wet mass should be measured prior to subsampling for laboratory analyses.
1. After the sweeper load wet mass has been determined, dump the sweeper load onto a tarp for subsampling.
2. It is important to collect a single representative sample from the sweeping load. Visually assess the sweeping load pile, examining how much of the load is composed of soil and plant debris. If sediment comprises a significant portion of the debris, it may settle at or near the bottom of the pile. This should be considered when determining where to collect subsamples.
3. Before sample collection, label a gallon-sized storage bag with sample identification information (e.g., street sweeping route, date).
4. Wearing protective gloves, use a trowel to scoop at least five (5) small amounts of sample into the gallon bag. Walk around the pile, scooping from various locations. Make sure to collect a sample that accurately reflects the composition of the sweeper pile (e.g., in proportion to observed fine sediments and coarse materials). Scrape away the outer layer of the pile, which may be dry, to collect samples from within the pile with a more representative dry basis moisture content, avoiding large pieces of trash and woody debris. Small pieces of trash are not separated from the sample (anything less than 1 inch in size).
5. Collect sufficient sample to fill the gallon bag approximately three-fourths (¾) full.
6. Seal the gallon-sized storage bag well.
7. Samples should be stored in a refrigerator until dry basis moisture content determination. If dry basis moisture content cannot be determined within one (1) day, the sample must be frozen.
The dry mass of the sweeping load may be estimated within the street sweeping Credit Calculator using measured load wet mass and an assumed seasonal average dry basis moisture content, or dry mass may be determined based on additional laboratory measurements. The dry mass may be calculated using laboratory analyses to determine dry basis moisture content. In order to complete laboratory analyses of subsamples of the street sweeper load, first a representative subsample must be collected from the representative load sample collected following the Representative Load Sampling Procedures described above.
Note: If the representative load sample is submitted to a commercial laboratory for analysis, provide these SOPs to the lab and ensure they provide the metrics needed for the tool (wet mass, dry basis moisture content (not wet basis percent moisture), and/or organic matter content).
Calculate the sweeping load dry mass by multiplying the sweeping load wet mass by the ratio of dry mass to wet mass of the subsample that that was oven-dried.
The wet and dry masses of the subsample may also be used to calculate the dry basis moisture content, which is assumed to be representative of the entire sweeping load. The dry basis moisture content, as a percent, is given by
Because of the strong relationship between percent organic matter (OM) and total phosphorus observed in street sweeping, determination of OM can be used to refine the calculation of phosphorus removed from a given sweeping event. OM concentration of sweepings can be calculated using the Loss-on-Ignition (LOI) method, whereby a sample is combusted in a muffle furnace and the percentage of material combusted is assumed proportional to the percentage OM and/or organic carbon in the sample. It is recommended that the following methodology for determining OM content be completed and analyzed on three (3) separate subsamples to obtain an average percent OM value that is representative of the sweeping load.
Percent organic matter is given by