Why size matters

Particles of less than 250 microns comprise 30% of TSS on most city streets but carry 50% of the stormwater pollutants targeted by the Clean Water Act. The particles are easily transported by rain and wind and can dissolve, the state of most concern to those regulating the aquatic environment.

While mechanical broom continues to be municipalities’ preferred sweeper type, it’s the least effective design for removing this small material. The machines are popular primarily because public works departments are familiar with them and know they handle wet vegetation and large debris, the highly visible material that prompts calls to city hall, well.

In the early 1980s, sweeper effectiveness was studied as part of EPA’s Nationwide Urban Runoff Program (NURP). In 1983, mechanical broom TSS pickup efficiency was 20% to 35%. In some instances, the machines actually left behind more small particles than they removed.

Efficiency has roughly doubled since then, but they still do only a fair job on small particles. Although to the untrained eye the path behind the sweeper looks clean, the main broom’s rotation against the pavement breaks larger particles into smaller ones. Without a vacuum system to pick them up, the machines leave a significant level of small particles in place on the street.

Next page: Removing the toughest pollutants