Screening is a primary treatment in a wastewater treatment process. Screenings are the material retained on bar racks and screens. The smaller the screen opening, the greater will be the quantity of collected screenings (see figure 2). In table 3.2 typical data on characteristics and quantities of screenings removed from urban wastewater with fine bar (mechanically cleaned). In table 3.1 some typical information about operations bar racks are reported.

Figure 2: Example for a mechanical bar screen (Source: Entwässerungsbetriebe Mainz)

Tab. 3.1 Typical design information for manually and mechanically cleaned bar racks

Tab 3.2 Typical information on the characteristics and quantities of screenings removed from urban wastewater with fine bar (mechanically cleaned)

The quantity of collected screenings varies depending on the type of the screen and, in particular, on the type of sewer system and wastewater characteristics. Their efficiency depend on the spacing between the screen bars and is named as follows:

• Fine screening: spacing < 10 mm

• Medium screening: spacing 10 - 40 mm

• Coarse screening: spacing > 40 mm

Usually a fine screening is preceded by a medium or course screening for protection.

For coarse screens, the quantity of removed screenings ranges between 0.005 e 0.05 m³ per 1000 m³ of treated wastewater. These values, in case of combined sewers, could be much higher during storm events. For fine screens the quantity of removed screenings can reach value up to 0.3 – 0.5 m³ per 1000 m³ of treated wastewater.

The collected screenings characteristics are very different; generally the misture content ranges between 70 al 90 % and specific weight between 700 – 900 kg m^-3. In case of urban wastewater putrescible matter is contained within the screenings therefore they must be handled and disposed quite quickly.

The headloss increases when the screen collects material, and cleening operation are needed. The approaching velocity should be higher than 0.6 m s^-1, in order to avoid sand deposition or other suspended material. In table 3.1.3 a typical screen operation sheet is reported; the reporting frequency depends on the WWTP size; details on routine inspections, lubrication and adjustment, performed by the operator, should be reported as well.

Tab 3.1.3 Screen operation sheet