Introduction to Screens In Wastewater Treatment

Previously, we discussed the main causes of clogged pumps and ways to prevent costly equipment maintenance and replacement. To recap, municipal and industrial wastewater can contain large solids made up of debris, which can damage downstream treatment equipment. One of the ways to remove these solids is to install a wastewater screen. Screens come in a variety of types, shapes, and sizes, but can be broken down into two simple categories: coarse and fine screens. 

Coarse Screens

Coarse screens remove rags, sticks, large solids and other debris ranging from 0.25 to 6 inches. They can be as simple as a trash rack or as complicated as a mechanically cleaned bar screen. Generally speaking, larger treatment facilities will install a mechanically cleaned bar screen whereas small, outdated facilities are likely to use a manually cleaned bar screen. 

Manually cleaned bar screens require little to no mechanical maintenance, but do require routine cleaning by an operator. If you speak with a wastewater operator who works at a facility with a manually cleaned bar screen, I bet he'll tell you that one of his least favorite job duties is cleaning out the bar screen. With that said, the majority of treatment facilities are gravitating towards automated, mechanically cleaned bar screens for the following reasons: 

• They produce a higher screening capture rate
• Provide improved flow conditions through the screen
• Because they automatically remove solids, manual cleaning is not required

Fine Screens 

Where coarse screens use bars or rods to remove solids, fine screens employ wire cloth, wedgewire elements or perforated plates. Fine screens are used to remove particles that may cause maintenance issues for process equipment and/or operational problems to the treatment process. Typically in smaller treatment facilities, fine screens can be used in place of primary clarification.

Fine screen openings typically range from 0.06 to 0.25 inches. The smaller size openings allow the fine screens to remove 20 to 35 percent of suspended solid and BOD. Depending on your specific situation, static wedgewire, rotary drum, or step fine screens may be used. Static wedgewire screens are typically used in industrial wastewater treatment facilities and small municipal plants.

Compared to static wedgewire screens, drum screens can handle a large flow rate in a smaller footprint. Rotary drum screens can either be internally or externally fed. Internally fed drum screens are capable of handling higher flow rates compared to an externally fed drum screen.

Step screens (or stair screens) are comparatively a newer technology. Debris is lifted up the screen by means of fixed and movable plates. The movement of the plate provides a self-cleaning feature for this particular screen. An additional benefit of the step screen is its capability to form a filter mat as the trapped solids build up. This filter mat will enhance the solids removal rate of the screen.