About Mechanical Bar Screens

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Mechanical bar screens ( or mechanical screens ) have been used at the headworks of most medium to large wastewater treatment plants for the past 50+ years. More recently, mechanical bar screens are appearing in pump stations and combined sewer systems. In wwtp applications such as these, it is very common to provide two bar screens. At smaller treatment facilities where is mechanical bar screens are used, they are usually the duty bar screen while manual bar screens are used as standby units.

Trash Racks

Manual Bar Screens

Mechanical Bar Screens

Chain Bar Screens

The standby bar screen will be brought online when the mechanical bar screen is inoperable or down for routine maintenance. At larger treatment facilities there may be more than two mechanical bar screens, having two or more duty as well as standby units. Stop logs should be installed to allow each channel to be isolated, so that the channel can be dewatered and the bar screens maintained.

The clear bar spacing for mechanically cleaned bar screens is usually between 6 mm to 38 mm (or 1/2 to 1 1/2 inches [metric system conversion]). The amount of debris removed from the bar screen can be directly linked to the bar space opening. The bar screen should be positioned at a 45 to 90 degree angle from the horizontal, with 60 degree being the most common configuration. This will increase the screening surface area, which in turn will expedite cleaning and eliminate clogging. This configuration will augment screening surface area up to 100%, it will also facilitate cleaning and eliminate escalating head from matting on the bar screen.

Reciprocating Rake Bar Screens

Catenary Bar Screens

Continuous Belt Bar Screens

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When designing any wastewater treatment process at a wastewater treatment facility, head loss calculations should be performed. To obtain the head loss through the bar screen the follow equations is used:
h = 1 ((V²- v²)/2g)
0.7

where:   h = head loss, m (ft)
    v = approach velocity, m/s (ft/sec)
               V = velocity through bar opening, m/s (ft/sec)
            g = acceleration from gravity
0.7 = head loss coefficient

This equation assumes that bar are completely free of rags or other stringy material, as experience teaches us, this is usually not the case. As the mechanical bar screen traps solids and debris, the bar screens starts to become matted. Mechanical bar screens are supplied with some type of cleaning mechanism. Once solids accumulate on the bar screen, the cleaning mechanism is initiated.

The mechanical device that cleans the bar screen is activated by a manual start-stop switch, a timer, a overload switch, a actuator which senses pressure differential between points upstream and downstream of the bar screen, and a float that turns on when head across the bar screen is greater than some predetermined elevation. In wastewater treatment process combustible gases such as methane are emitted. To avoid catastrophic failure, all motors and controls are to be explosion proof.

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In addition to head loss considerations, velocity must also be optimized to ensure successful operation of the mechanical bar screen. The flow should be uniformly distributed across the bar screen, this is done by provided an straight approach channel this is several feet ( about 6 in) length. This approach should be used for both manual and mechanical cleaned bar screens. To reduce grit deposition, the minimum approach velocity in the channel should be no less than 0.40 m/s (or 1.25 ft/sec). The velocity through the bar screen should not exceed 0.62 m/s (or 2 ft/sec) at average flow. To reduce break through at peak flow, the velocity through the bar screen should not exceed 0.9 m/s (or 3 ft/sec).

Mechanical bar screens offers several advantages over the trash rack or manual bar screen. With mechanical bar screens labor cost from maintenance are usually alot lower, screenings capture is more efficient, and larger debris are removed with less hastle.These are a few ways mechanical bar screens show improvement over manual bar screens. More recent bar screens are using more robust materials such as stainless steel to stand up against resist corrosion.

There are various types of mechanical bar screens available for installation, they include but not limited to chain bar screens, cable bar screens, reciprocating rake bar screens, and catenary bar screens.

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Always consult with the manufacturer or your local equipment representative if you have specific questions about bar screens. This site is done by a random individual to help get you informed, but is not to substitute for professional opinion. Please read our disclaimer.