Influence of Operating Parameters on the Collection Efficiency and Size Distribution of Street Dust during Street Scrubbing

This study investigated the influence of operating parameters on the efficiency of collection of street dust using a street scrubber. Street scrubbing tests were performed in a full-scale, street scrubbing testing field, and on roads. The full-scale testing field was 40 m long and 1.95 m wide, specifically designed and constructed for this study. It was operated semi-automatically. The operating parameters investigated included nozzle type, street dust load, scrubbing speed, water injection loading, water injection pressure, distance above the ground, and water injection angle. Two types of nozzle, flat fan and hollow cone, were selected in the field tests. Four levels of street dust loading (level A: 0.39±0.28 g/m, level B: 2.98±1.34 g/m, level C: 8.02±2.08 g/m, level D: 17.15±4.77 g/m) were used. The experimental results showed that, during scrubbing, the efficiency of collection of street dust decreased as street dust loading, scrubbing speed and distance above the ground increased, but increased with water injection loading and pressure. The determined optimal operating parameters were a scrubbing speed of less than 15 km/hr, a water injection loading of 0.8 L/m, a water injection pressure of 2.0 kg/m, distance above the ground of 30 cm and a water injection angle of 45. Additionally, the efficiency of collection of fine particles was higher than that of coarse particles. A multiple regression model was developed to predict the collection efficiency of street dust, based on experimental results obtained from street scrubbing field tests. The results suggested that street scrubbing should be able to reduce the fugitive emissions of street dust from paved roads.


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The emission factor for fugitive particulate matter from a paved road is highly correlated with the silt content (dp < 75 µm) of the street dust and the mean mass of the vehicles that travel on the roads (Watson et al., 1999;Hesketh et al., 1982).Kuhns (2001) concluded that the emission factor and average size of particulate matter varied with the speed of the vehicles.Jeng et al. (1998) also reported that the concentration of ambient particulate matter varied with the silt content of street dust, scrubbing frequency, traffic flow and wind speed.However, some researchers have claimed that only a slight correlation exists between the emission of fugitive dust and the silt content of street dust (Kantamaneni et al., 1996;Zimmer et al., 1992).Silt content generally accounts for about 4-22% of street dust and its collection efficiency ranged from 20.4% to 70.4% at an average of 41.9% (Jeng et al., 1998).
Removing debris from roadways has been suggested as a method for controlling the fugitive emission of PM 10 (USEPA, 1984), although this has not been practically implemented in the U.S.A. (Chow et al., 1990;Fitz, 1998).Fitz and Bumiller (2000) reported that street scrubbing and sweeping are the Best Available Control Measures (BACM) for reducing ambient particulate matter in areas that do not comply with the National Ambient Air Quality Standards (NAAQS) for PM 10 .Street scrubbing and sweeping are generally performed to remove street dust, particularly silt, to reduce the re-entrainment of street dust into the ambient atmosphere that would otherwise be caused by traveling vehicles and wind.
Street scrubbing is more efficient than street sweeping for removing street dust and reducing the amount of ambient particulate matter.Previous research has demonstrated that street scrubbing can reduce the ambient concentration of total suspended particles (TSP) and PM 10 by approximately 3.2-12.6%and 8.6-30.0%,respectively (Chang et al., 2000;Tainan EPB, 1997)

Size Distribution of Street Dust
Street dust sampled on the floor was screened using a laboratory test sieve (Octagon,Model 200) to determine its size distribution.The test sieve consisted of eight plates (8" (ID) x 2" (H)) with pore size ranges of <45, 45-75, 75-106, 106-150, 150-212, 212-300, 300-425, 425-850, >850 µm, respectively (U.S.Standard Wire Mesh Series, ASTM E11:87).The test sieve was operated for more than 17 minutes per run to achieve an error of less than 3% between the results of two consecutive weighings.The efficiency of collection for each range of sizes was then determined by comparing the mass of the street dust deposited on each plate before and after street scrubbing.

Street-Scrubbing Simulation Tests
A full-scale, street-scrubbing testing field was specifically designed for this study.The testing field was 40 m long and 1.95 m wide, with a gradient of about 2%.Designed to move on rails, a street-scrubbing simulation apparatus was operated by changing scrubbing speed, water injection loading, water injection pressure, distance above the ground and angle of water injection.The field tests were performed to investigate the influence of operating parameters on the collection efficiency of street dust during street scrubbing.
The surface of the testing field was swept using a vacuum cleaner before the tests were conducted.
Scrubbing was usually performed for more than 17 minutes to ensure that the street had been completely cleaned.Street dust collected from 37 streets in metropolitan Kaohsiung was prepared as testing material, being dried at 103-105 o C in an oven for two hours.The dust was mixed to simulate area loadings and size distributions of streets with various levels of cleanliness; the mixed dust was then spread and deposited uniformly on the surface of the testing field for the street scrubbing test.
Street dust sampled in three separate zones with areas of 6.0 m 2 (1.5 m (W) x 4.0 m (L)) in the testing field before and after street scrubbing were used to determine the efficiency of collection, as follows.
where η represents the collection efficiency of the street dust (%); W i is the street dust loading before scrubbing (kg/m 2 ); W f is the street dust loading after scrubbing (kg/m 2 ).

Operating Parameters of Street Scrubbing
The influence of major operating parameters on the collection efficiency of street dust was investigated to obtain the optimal operating conditions of street scrubbing.The operating parameters investigated herein included scrubbing speed, water injection loading, water injection pressure, distance above the ground and water injection angle.

Area Loading and Characteristics of Street Dust
In this study, a wide range of area loadings of street dust was observed in metropolitan Kaohsiung.
The area loading of street dust measured on 37 major streets ranged from 0.08 to 24.66 g/m 2 ,       Water injection pressures of 1.70, 1.85, and 2.00 kg/cm 2 were also tested.As plotted in Fig. 5, injecting water at a higher pressure enhanced the collection efficiency of street dust.For example, for a street with cleanliness level D, increasing the water injection pressure from 1.70 kg/cm 2 to 1.85 and to 2.00 kg/cm 2 enhanced the collection efficiency of the street dusts from 77.0% to 78.5 and 82.0%, respectively.Operating street scrubbers with a water pressure of 1.70 and 1.85 kg/cm 2 achieved a minimum collection efficiency of 79.1%

Influence of Operating Parameters on Collection Efficiency of Street Dust
for street cleanliness levels A-C.However, increasing the water injection pressure to 2.00

Collection Efficiency and Particle Size
Researchers have reported that finer dust is more easily emitted to the atmosphere by wind and/or disturbances due to traveling vehicles (Fitz, 1998;Chow et al., 1992).Thus, understanding the collection efficiency of street dust as a function on the road.Thus, the collection efficiency of particles with diameters of less than 10 and 2.5 µm (PM 10 and PM 2.5 ) can be reasonably assumed to be even higher than, or as high as, that of PM 45 .

Modeling Street Scrubbing
Dimensional analysis was applied to cluster the operating parameters that significantly impact the collection efficiency of street dust during street scrubbing.First, a dimensionless analysis of operating parameters was performed using Buckingham's π Theorem.Three dimensionless parameters were extracted from the experimental results of six operating parameters (W, V, q, P, H, and θ); they were Pq/V 2 W, H/q, and sin2θ.A where η is the collection efficiency of street dust

Conclusions
This study examined the influence of operating . However, the influence of operating parameters on the efficiency of collection of street dust during street scrubbing and sweeping remains unknown.Under the auspices of the Air Pollution Abatement Fund, the Taiwan Environmental Protection Administration (TEPA) and local governments have undertaken a consecutive street scrubbing and sweeping project for the past several years.Local governments routinely clean roads using street sweepers and scrubbers, primarily for aesthetic and safety reasons, rather than merely to meet regulations.The efficiency of this project in reducing ambient particulate matter is controversial, being questioned by researchers and the general public since experimental data to support its effectiveness has been lacking.Therefore, the objectives of this study were to evaluate the collection efficiency of street dust and to investigate the influence of operating parameters on the collection efficiency and particle size distribution of street dust during street scrubbing.
Street dust was sampled in situ using a vacuum cleaner (SANYO, Model SC-6L) in streets, from clean to dirty, characterized by cleanliness levels A to D. Each sampling run exceeded 17 minutes to ensure at least 98% efficient collection of street dust.The mean area loading of street dust was determined by sampling street in three separate zones with areas of 12.5 m 2 (2.5 m (W) x 5.0 m (L)).In this study, the street dust sampling protocol was used on 37 streets in metropolitan Kaohsiung.Street dust, collected by filter bags with 2.0 µm micropores, was temporarily stored in a tagged Tedlar bag, and then transported to the central laboratory at National Sun Yat-sen University for size distribution analysis by weighing.Before weighing, the street dust was dried at 103-105 o C for at least two hours in an oven to prevent the adsorption of moisture, which could affect the measured weights of the dust samples.
Two most commonly used nozzles -flat fan and hollow cone -on the street scrubbers, were selected for the simulation field tests.The street scrubbers traveled on the streets at 10, 15 and 20 km/hr.The amounts of water on the streets were 0.4, 0.8, and 1.2 L/m 2 .The static pressures at which water was delivered onto the surface of the streets were 1.70, 1.85, and 2.00 kg/cm 2 .Water was atomized using nozzles at approximately 20, 30 and 40 cm above the ground, and at angles of 10 o , 40 o , and 70 o .The water injection angle was defined as the angle between the direction in which the water is sprayed and the horizontal.

Figure 2 .
Figure 1.Variation of street dust loading with time after street scrubbing.

Figure 3 .
Figure 3.Comparison of collection efficiency of street dusts between flat fan and hollow cone nozzles.

Figure 6 .
Figure 6.Variation of collection efficiency of street dust with distance of nozzle above the ground.

Figure 7 .Figure 8 .
Figure 7. Variation of collection efficiency of street dust with water injection angle.

Figure 9 .Figure 10 .Figure 11 .
Figure 9. Size-resolved collection efficiency of street dusts obtained from the in-situ tests.
multiple regression model was successfully developed, based on the aforementioned three dimensionless parameters, to simulate the collection efficiency of street dust.

Figure 13 .
Figure 13.Comparison of model-predicted with field-measured collection efficiencies as functions of water injection pressure.

Figure 14 .Figure 15 .
Figure 14.Comparison of model-predicted with field-measured collection efficiencies as functions of water injection height above the ground.

Table 1 .
Operating parameters tested in the street scrubbing simulation field.
Table 1 describes and presents the ranges of operating parameters.C BWater injection angle is defined as the angle between water spraying trajectory and horizontal line.

Table 2 .
Classifying street cleanliness levels and area loading of street dust.

Table 3 .
Size distribution and area loading of street dust for various street cleanliness levels.
dirtiest street (Level D) accumulated street dust approximately 26 times faster than the cleanest street (Level A).The frequency of street scrubbing for cleanliness levels A, B, C and D were at least once per week, once per three days, once per two days and daily, respectively, to maintain street dust loading of less than 20 g/m 2 (Table2).D was comparable to the street dust loading in other major cities in the world.

Table 3
presents the size distribution of street dust sampled from streets at various cleanliness levels.In thiis study, the size of street dust was bimodally distributed.The results indicate that