Screening and Shredding | Activated Sludge Process | Wastewater Treatment Plant

SCREENING AND SHREDDING

Pretreatment screening protects downstream equipment from problems caused by rags and other solid objects in wastewater. Fine screens are required for preliminary treatment in several modern plants, particularly those with floating or neutrally buoyant media with screens included in the secondary and tertiary systems (to keep the media out of the receiving water), to remove even small solids that would “blind” the downstream process. Fine screening prior to some processes may necessitate the use of perforated plate-type units, which are not screens at all but perform a similar function.

Sewer screens are available in a variety of styles and opening sizes. Coarse screens remove gross debris from the system, such as leaves, sticks, boards, and other similar objects. These screens typically have 12 to 1 inch wide openings and are made of very sturdy stock, typically steel bars on edge to the flow of water. They are typically 12 inches across and up to 3 inches deep, with 12 to 1 inch spacing between them. Mechanical rakes are commonly used to clean coarse screens. These devices have bars that extend between the screen's bars and move upward, dragging debris over the top of the screen and depositing it in a moveable hopper. The rake then swings out and returns to the bottom of the screen, where it begins a new, slow rise to the top.

Fine screenings are frequently difficult to handle and manage because they are putrescible, decaying before they arrive at the treatment facility, and highly odorous. Fine screens are typically made up of moving belts with openings ranging from 1/8 to 14 inches wide. They have a doctor blade that scrapes debris off the screen and deposits it in a moveable hopper. They also usually have a backwash system that washes fine debris that clogs the screens back into the liquid stream that goes to the primary portion of the treatment plant.

In both cases, the screenings from the moveable hoppers are typically discharged into a washing basin where the organic material is washed off the inorganic material and the organics are returned to the plant inlet. This is done to reduce the organic load on the landfill site where the inorganics are disposed of, as well as the screenings' putrification potential. This is a violation of the fundamental principle of wastewater treatment, which states that "once out, keep it out." However, because the rest of the treatment plant is designed to handle the organic fraction, the fraction returned is already fine enough to be effectively treated in the plant's biological treatment portions, and the negative environmental effects of landfill disposal of organic fractions could be significant. Large organics that cannot be effectively treated in the biological system are removed from the washing process and either burned or disposed of with the inorganic fractions.

Shredders, grinders, and comminuters (all very similar pieces of equipment) crush, chip, or otherwise break up the bigger incoming objects, allowing the primary and secondary treatment plants to efficiently handle them. It is critical to position these devices after the screens to prevent large stones or other non-chippable objects from entering these devices. Most of this equipment uses a lot of torque to crush the incoming objects, and sudden stops due to uncrushable debris can cause serious damage to the equipment and injury to the operators.

However, because this equipment does not actually remove anything, it is not as commonly used as screens. Where they are used, a by-pass channel is required in the event of a shredder or comminuter malfunction or breakdown. In the event that the shredder or comminuter needs to be shut down for repairs, screens are used in the by-pass channel. Prior to primary sedimentation, some plants screen out larger objects, then pass them through a grinder or comminuter before returning them to the waste stream. Most are difficult to treat, even when shredded, and provide no benefit to downstream treatment units.