Liquid Stream Fundamentals: Screening

Wastewater contains large solids and rags (screenings), abrasive inert material (grit), floating debris, and grease. These constituents can cause operational problems with downstream processes or increase maintenance of downstream equipment, piping, and appurtenances. To minimize problems, preliminary treatment is required for Water Resource Recovery Facilities (WRRFs) to remove, reduce, or...

Screening devices are typically classified into four categories based on screen opening size.

Typically, there are four types of screening media used, which are bars, wedge wire, perforated plate, and mesh as shown in Figure 2.

A mechanically cleaned screen is almost always specified for new facilities of all sizes, especially in combined systems where high quantities of storm water debris and screenings are needed to be handled. However, manually cleaned screens are still an appropriate alternative for smaller plants with few screenings. The method of cleaning the screens, manual or mechanical, are compared in...

The location of screening devices varies at different the treatment facilities. Screens can be located either upstream or downstream of influent pumps, because many common, and particularly larger, raw wastewater pumps are capable of pumping screenings. Screens are typically located upstream from the grit removal system. When fine screens are used, they could be located either upstream or...

The screen retention quantity is a function of wastewater characteristics. Unwashed, uncompacted screenings can contain 10% to 20% dry solids with a bulk density ranging from 600 to 1100 kg/m3 (40 to 70 lb/ft3). Typical performance specifications for washer/compactors are 90% reduction in organic content and an increase to 50% in dry solids.

In mechanically cleaned units, rakes or the screen media move screenings up the screen to above-deck where they discharge to a conveyor, sluicing troughs, washer and compactor, or removable containers. Some dewatering of the screenings typically occurs as they are lifted from the wastewater. When discharging screenings directly from screen to container, sufficient clearance under the discharge...

WEF Municipal Resource Recovery Design Committee