Fine Screens

Fine screens are used over a wide range of applications. Applications that typically use fine screens are pre-treatment in conjunction with a coarse bar screen, primary treatment in lieu of primary clarifiers, and pre-treatment at combined sewer overflows. When clogging of trickling filters presents the potential for a problem, it is common to use fine screens upstream of the trickling filters to remove solids from the primary effluent.

Fine screens that are used for pre-treatment applications include, band screens, static screens, rotary drum screens, and step screens. These screens are usually classified as fine screens by the space opening which is typically between 2 to 6 mm or (0.1 to 0.25 inches [metric system conversion]).

Installation of fine screens will be limited to treatment plants where head loss is not at a premium. Although there is greater capture of solids through a fine screen, you will pay for it with a greater head loss through the screens.

More and more fine screens are replacing primary sedimentation at wastewater treatment plants. As the limits on effluent quality become more stringent, treatment plants are incorporating more advance treatment technologies to meet these increasing limits. Some of the technologies require very good removal of solids and stingy material at the headworks upstream of advance treatment processes. Fine screens are effective at BOD and TSS removal, typical removal rates through fine screens are as follows: BOD is between 20-45% and TSS is between 25-50%, removal rates for both depends on the type of screen.

As previously mentioned, one drawback to the fine screen is the head loss. Head loss through fine screens range between 0.8 to 1.4 m or (2.5 to 4.5 ft), which is very significant especially if you don't have a lot of "head to play with".

In existing treatment plants, head may be limited and there may not be much flexibility, which is why trying to install these screens as a retrofit may present a challenge. That is not to say an existing treatment plant can not receive an upgrade to the fine screens, but special attention must be paid to the hydraulics so that the operation of the plant is not compromised.

The fine screens that are used in pre-treatment and primary treatment are:

Band Screens - Effective for fine screening applications that have high flows.

Static Wedgewire Screens - Typically installed in smaller treatment plants.

Rotary Drum Screens - Effective for applications that require big solids separation and small energy usage.

Step Screens - Cost effective solids separation.

Fine screens are also used in combined sewer overflow where floatable and various other solid materials are to be removed before being discharged to an outfall. There are two main types of fine screens for this application and they are: horizontal reciprocating screens and tangential flow screens. The horizontal reciprocating screen is sturdy with a weir-mounted screen arrangement that consists of stainless-steel bars that span the length of the screen. The bars run parallel to the flow and are installed in continuous runs without intermediate supports as to mitigate solid accumulation.

When the water level in the channel rises the waste stream will begin to pass through the openings between the bars. Debris will start to collect on the face of the screen. Due to the solid build on the screen the water level in the channel will begin to rise. Once the water reaches a predetermined elevation an automatic rake assembly will become activated and will remove trapped solids from the screen. The automatic rake will travel back and forth across the screen, dislodging trapped debris. Screenings are carried to one end of the screen and will either be dumped back into the waste stream heading toward the treatment plant or a collection pit where it will remain for future disposal.

The tangential flow screen utilizes the natural motion of the water to screen and collect particles. This device successful removes solids from the waste stream with a fine-mesh screen without the use of any moving parts. Flow enters the separation chamber creating a circular motion that permits the water to move through the cylindrical mesh screen, at the same time driving debris toward the middle of the chamber. The water vortex will be regulated to ensure the tangential flow around the chamber is larger than the radial force trying to push particles outward. This will minimize solid accumulation on the mesh screen. Solids and floatables are collected in their respective compartments until the flow stops where they will be removed later.

When fine screens are used, some sort of mechanically cleaned bar screen or trash rack should be installed upstream to protect the fine screen. Because of their robust design, some internally fed rotary drum screens will not require protective devices to be installed upstream. Communitors or channel grinders should not be installed up stream of fine screens as they will decrease the removal efficiency of the fine screens. It would be a good idea to install a minimum of two fine screens (one duty, one standby), both should be able to handle the peak flow capacity. Fine screens are susceptible to grease build up, therefore flush water should be available nearby to dislodge collected grease and solids.

To obtain the head loss through a fine screen, the following equation is used (notice the head loss equation for the fine screen is different the bar screen):

                                            h=1/2g(Q/CA)²

where: h = head loss, m (ft)

          C = coefficient of discharge for the fine screen (typically this value for a clean screen is                  0.60)

          g = acceleration due to gravity,

          Q = discharge through the screen, m³/s (ft³/s)

          A = effective open area of submerged screen, m² (ft²)

The C and A value will be dictated by the screening design factors and shall be obtained by the manufacturer.