Environmental Information for Sweeping Professionals

City of San Jose Comparison of Sweepers

Part 2

This report was prepared by Woodward-Clyde Consultants for use only by the City of San Jose (however the company provided it as a courtesy to be posted on the American Sweeper Forum website). Nothing contained in this report shall be considered as an endorsement or a lack of endorsement of any of the equipment mentioned in the report.

The City and Woodward-Clyde Consultants specifically disapprove any use of the information contained in this report for any promotional or advertising of any of the equipment mentioned in the report.

3.0 STUDY RESULTS

3.1 ROUTE DIFFERENCES

One of the major assumptions of the study was that differences in pollutant concentrations between routes would not be so great as to mask any differences between sweeper types. This assumption was tested in the pilot study, the results of which were presented in a memo dated June 30, 1994 . ( Appendix A of the complete report). Based on the concentrations of copper observed in the hopper from the old Mobile broom sweepers used in the pilot study, most routes were similar except for route Blue 3. Other routes with a high copper concentration (relative to the average for all routes) included Red 1, Black 2, Blue 2, Blue 1 and Red 2.

To confirm that routes used in the study were generally similar, sediments were collected by hand vacuuming the street with an industrial vacuum. A total of 40 vacuum samples were collected, two from each route as described in Section 2.3 (available with the complete report from Woodward-Clyde Consultants). The data were analyzed for constituents described in Section 2.4 of the complete report. The copper data were compared to determine if any routes were different from the average route.

Figure 3-1 (available with the complete report from Woodward-Clyde Consultants) presents the copper concentration data (on a dry weight basis) for all 40 vacuum samples collected ranked from highest concentration to lowest. Except for routes Blue 3, and one sample each from Yellow 3 and Red 1, the routes selected for study have approximately the same copper concentrations in street dirt. Blue 3 also had the highest concentration observed in the pilot study and Red 1 had the second highest. Yellow 3 had a lower than average copper concentration in the pilot study. Based on the vacuum results the routes would appear to be similar (note, not the same) except for Blue 3, Yellow 3 and Red 1.

One factor that may explain the higher copper concentrations at these stations is the traffic volume. Route Blue 3 was located along a commercial section of Monterey Highway with an Average Daily Traffic (ADT) volume of 32,500 cars per day, 50% greater than the average ADT of 21,427 for the routes in this study. Red 1 was located along an industrial section of Lundy and Brokaw roads. It had a ADT of 29, 641 cars per day, slightly higher than average ADT. Yellow 3 is located along a commercial section of Tully Road and had the highest ADT of any of the routes sampled, 52,000 cars per day. The copper concentration of the second sample collected from Routes Red 1 and Yellow 3 was close to the mean concentration from all samples.

3.2 SWEEPER DIFFERENCES

3.2.1 Parameters Examined

• total mass of sediment collected per curb mile
• mass of copper collected per curb mile.
• concentration of copper in sweeper hopper sediment.

Mass of sediment (street dirt) and copper per-curb-mile was used in the following analysis rather than mass-per- route to correct for differences in route lengths.

The sediment mass, copper concentration, and copper mass were estimated for each of five size fractions of sediment. This was done in order to determine if sweeper differences were due to differences in specific size fractions or due to differences across all size fractions.

3.2.2 Results

The results in three forms are presented below: the total mass of sediment (material < 1 cm) per mile collected by each sweeper, the copper concentration in the hopper sediments for each sweeper, and the mass of copper collected per mile swept by each sweeper. The data used in the analysis are provided in Appendix B of the complete report.

Total Mass/Curb Mile. Figure 3.2 (available with the complete report from Woodward-Clyde Consultants) presents the results of the comparison of the mass of material collected by each type of sweeper per curb mile swept (mass/curb mile). The figure compares the average of the data ranks for each sweeper. Statistical tests indicated the old Mobil broom and Elgin broom collected significantly less material than the new Mobil broom and the two regenerative air sweepers (P < 0.05). Mass of material collected per curb mile by the Elgin broom was not significantly different than that collected by the old Mobil broom. The mass per curb mile collected by the new Mobil broom, Tymco regenerative air and Elgin regenerative air were also not significantly different from one another (P < 0.05).

Conclusions: The Tymco regenerative air, Elgin regenerative air and the new Mobil broom sweepers tested pick up a greater mass of material than the old Mobil broom or the Elgin broom tested.

Copper Concentration. Figure 3.2 (available with the complete report from Woodward-Clyde Consultants) compares the copper concentration of the material collected by each type of sweeper. The points in the figure represent the average rank of the samples for each type of sweeper. The ANOVA indicated the two regenerative air sweepers collected material with significantly higher copper concentrations than the three broom sweepers (P < 0.05). The regenerative air sweepers were not significantly different from each other and the brooms were not significantly different from each other.

Conclusion: Tymco and Elgin regenerative air sweepers pick up sediments with higher copper concentrations than do the broom sweepers tested.

Copper Mass per Curb Mile Figure 3.2 also presents the results of the comparison of the mass of copper collected by each sweeper type per curb mile. The figure compares the average of the data ranks for each sweeper. The ANOVA indicated that the old Mobil broom and Elgin broom collected significantly less copper per curb mile than the new Mobil broom and two regenerative air sweepers (P < 0.05). The mass per curb mile of copper collected by the old Mobil broom was not different than the mass collected by the Elgin broom. Masses of copper collected by the new Mobil broom, Tymco regenerative air, and Elgin regenerative air were not significantly different from one another (P < 0.05).

Conclusion: Tymco and Elgin regenerative air and new Mobil broom pick up more copper mass per curb mile than the old Mobil broom or the Elgin broom.

Table 3-1 (available with the complete report from Woodward-Clyde Consultants) presents a summary of the ranks of the sweepers based on the three measures of performance; total mass of sediment collected per mile, copper mass collected per mile, and copper concentration. The ranks presented in Table 3-1 are all relative to the rank of the old Mobil sweeper (MBO), that is, each rank is divided by the rank of the old Mobil sweeper. (Note that sweepers can have different ranks and not be statistically different from one another). If the ranks resulting from each of the performance criteria are added together, the composite rank provides an overall measure of performance.

Based on the results summarized in Table 3-1, the regenerative air sweepers rate the highest (rank 3), the new Mobil broom rates second (rank 2) and the new Elgin broom has the same rank as the old Mobil broom.

3.3 QUANTIFICATION OF IMPROVEMENT IN COPPER COLLECTION

The nonparametric results presented above only indicate if two groups are significantly different from each other (e.g., the old Mobil broom and the new Mobil broom) but not how different they are. To determine how different the sweepers are from each other, the following parametric analysis was conducted.

In order to conduct the parametric analysis, data which caused the violation of the assumptions of the parametric analysis were eliminated. Specifically, data collected from the four routes with extreme values (Red 1, Red 2, Yellow 2, and Blue 3) were excluded from the parametric analysis and the ANOVA redone using the actual data values rather than the ranks of the data. After excluding the routes with exceptionally high concentrations, the assumptions of the ANOVA test were met (the results were considered valid). However, the ANOVA results do not apply to the highest routes (or other routes which contain high loads of pollutants).

Figure 3-3 (available with the complete report from Woodward-Clyde Consultants) presents the median copper mass per curb mile collected by the different sweeper types using a one-way ANOVA analysis (not route corrected) and a two-way ANOVA analysis which incorporates the effects of routes on sweeper performance.

The one-way ANOVA analysis median copper mass collected by the old Mobile broom sweeper was 0.011 pounds per curb mile. This is comparable to the median mass collected by the Elgin broom (0.007 pounds per curb mile) and the new Mobil broom (0.010 pounds per curb mile). The regenerative air sweepers collected significantly more copper than the old Mobile broom sweeper (Tymco 0.017 pounds per curb mile and Elgin 0.018 pounds per curb mile) or approximately an 80% greater amount of copper than is currently being collected.

Correcting for routes in the two-way ANOVA indicates even greater differences between sweepers. The old Mobile broom tested and the Elgin broom tested collected 0.004 and 0.0058 pounds/mile, respectively. The new Mobil broom tested collected approximately three times more copper/mile than these two, collecting 0.016 pounds/mile. The Tymco and Elgin regenerative air sweepers collected 0.021 and 0.026 pounds/mile, respectively, or about six times more than the old Mobil broom tested.

3.4 EFFECTIVENESS OF PICKING UP DIFFERENT PARTICLE SIZES

Sweepers using different technologies may collect different sizes of particles. In general, smaller particles contain higher concentrations of pollutants due to a higher surface area to volume ratio as compared to larger particles.

• F1 = 1,000 µm - 10,000 µm;
• F2 = 425 µm - 1,000 µm;
• F3 = 170 µm - 425 µm;
• F4 = 75 µm - 170 µm;
• F5 = < 75 µm.

Each fraction was analyzed for total mass and concentration of pollutants. Copper concentration, total mass, and route length were used to calculate the total mass of copper per curb mile collected in each size fraction.

Figure 3-4 shows the mean fraction of copper mass within each of the particle sizes analyzed. A very small percentage (generally less than 5 percent) of the total copper picked up by the sweepers was found in particles less than 170 microns in diameter (sizes F5 and F4). Both Mobil broom sweepers picked up particles with similar size fractions. The largest percentage of copper was in the 170-425 micron (F3) size fraction for all sweepers except the Elgin broom tested which had the largest fraction in the greater than 1,000 micron (F1) size. The Elgin regenerative air was the best at picking up small particles with almost 60% of the copper in the 170 - 425 micron (F3) category. The Elgin broom was the worst at picking up small particles with almost 50% of the copper associated with the largest size fraction.

Figure 3-5 presents a plot of the median particle size in each particle size category versus median concentration for that category for all data collected. The data clearly show that the concentration of copper increases as the particle size decreases.

All of these figures and tables are available in the complete report. Obtain this directly from Woodward-Clyde Consultants.

4.0 CONCLUSIONS

The objective of this study was to evaluate four different sweepers and the existing sweepers used by the city of San Jose and provide recommendations to the City on which sweeper is the most effective at removing copper from the street surface.

Five sweepers types were tested in the Sweeper Evaluation Study, Tymco regenerative air, Elgin regenerative air, Elgin broom, new Mobil broom and an old Mobil broom. Each sweeper swept eight randomly selected routes between 4 and 5 miles long. Samples of sediment collected by the sweepers were analyzed for metals, petroleum hydrocarbons and organics. The mass of sediment collected by each sweeper was also calculated. The effectiveness of each sweeper in removing copper from the street surface was then evaluated. Because each sweeper did not sweep the same route, route effects on sweeper effectiveness was incorporated into the analysis using a two-way ANOVA. The following conclusions were derived from the results.

1. The Tymco and Elgin regenerative air sweepers tested and the new Mobil broom sweeper tested were significantly better at removing copper from the street surface than the old Mobil and Elgin broom sweepers tested.

2. Route effects were important to include in the analysis. This indicates that the amount of copper on the route appeared to be an important factor in determining the effectiveness of the sweeper. That is, a less effective sweeper will remove more copper from a dirty street than a more effective sweeper will remove from a less dirty street. In other words, the dirtier the street, the less important is the choice of sweeper.

3. The Elgin broom and the old Mobil broom sweepers collect about the same amount of copper per mile, (0.005 to 0.01 pounds/mile). The new Mobil broom sweeper collects more copper mass per mile, about 0.01 to 0.016 pounds per mile. The regenerative air sweepers collect the greatest mass of copper per mile, or about 0.017 to 0.026 pounds per mile.

The above results indicate that the Elgin broom and old Mobil broom sweepers tested collected less copper from street surfaces than the new Mobile broom and Tymco and Elgin regenerative air sweepers tested. No significant differences were seen between the old Mobil broom and the Elgin broom or between the regenerative air sweepers and the new Mobil broom sweeper. Under the conditions in this study (dry streets, two-week sweeping frequency, two weeks with no rain) either the regenerative air sweepers or the new Mobil broom sweeper performed best in picking up copper from street surfaces.

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