screens are typically at the headworks
(entrance) of a wastewater treatment plant (WWTP), bar screens are used to remove large
objects such as rags, plastics bottles, bricks, solids, and toy action figures
from the waste stream entering the treatment plant. Bar screens are vital to the
successful operation of a
plant, they reduce the damage of valves, pumps, and other appurtenances.
Floatables are also removed at the entrance to a treatment plant, these are
objects that "float" on the surface of the water and if aren't removed end up in
the primaries or aeration tanks. It is not uncommon to see floatables hanging
over the weirs of some clarifiers. Though they don't diminish the function of
those processes, floatables are rather unsightly.
Another place where floatables
can become are problem is at combined sewer overflow (CSO) outfalls, which is
also where trash racks and bars screens are used. During wet weather events when a treatment plants
capacity is overloaded flow may be diverted to an outfall where raw sewage and
rain water are discharged to a body of water. Both trash racks and bar screens may be installed at an
upstream location of the outfall, in some instances they are installed in
tandem. The trash rack is installed upstream of the bar screen to prevent damage
to the unit. Even though the bar screens are very durable, they are not designed
to withstand the impact of some of the debris that enter a combined sewer. In
many cases screening is the only treatment the combined sewer flow will see
prior to being discharged.
Although this site focuses primarily on coarse
bar screens, some WWTPs utilize both coarse bar screens and fine
Coarse bar screens (or bar screens) are
distinguished from fine screens by the space opening. Coarse screens usually
have a spacing of 6 mm (or 0.25 in [metric system conversion]). and larger, whereas fine screens spacing is
usually between 1.5 mm (or 0.059 in) through 6 mm (or 0.25 in [metric to inches]). Fine screens are
installed at some wastewater treatment plants that do not have primary treatment
to minimized clogging of downstream liquid and solid processes. Fine screens
have been used for "effluent polishing" which increases secondary effluent to
tertiary effluent quality. They also are installed upstream of the trickling
filters to minimize clogging and fouling of distributor
Typically bar screens fall under two classification, mechanical
bar screens and manual bar
screens (trash racks can either be manually
cleaned or mechanically cleaned). Both manual and mechanical screens contain
equally spaced vertical or inclined bars that span the width of a channel.
Design considerations for both mechanical and manual screens include: bar
spacing, bar size, geometry of bar, channel width, angle of screen and approach velocity.
Some WWTPs plants still use manually cleaned bar screens, but
because they are so labor intensive, the trend is to move toward mechanical bar
screens. Mechanically bar screens are the more routinely used type because of
their ability to operate automatically.
In addition to the vertical or inclined
bars, mechanical bar screens are equipped with rakes or some type of cleaning
mechanism for removing collected debris from the face of the unit. Once
screenings are collected from the unit, they are usually dewatered and hauled
away to a landfill.
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
professional opinion. Please read our disclaimer.