The basic functions of a headworks system is to remove trash and grit from wastewater prior to pumping and treatment.
That task is generally accomplished with two separate components: bar screens (as described below) and a grit removal systems. Removing grit and trash prior to pumping or treating wastewater prolongs the life of pumps and motors while reducing costly chemicals and extra electricity otherwise required in the wastewater treatment process.
The District also believes that the headworks will cut down on odors from the Peterson Wastewater Facility and increase treatment plant efficiency. A low interest loan for $2 million was secured through the Colorado Water Resource and Power Development authority for the project.
Bar 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 not 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 bar’s 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’s 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.
Types of Bar Screens
There are various types of bar screens available for installation, they include but are not limited to chain bar screens, reciprocating rake bar screens, catenary bar screens, and continuous belt bar screens.
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. While some WWT plants still use manually cleaned bar screens, because they are so labor intensive the trend is to move toward mechanical bar screens. Mechanical bar screens are the more routinely used type because of their ability to operate automatically.
Coarse Versus Fine Bar Screens
Moreover, coarse bar screens (or bar screens) are distinguished from fine screens by the space opening. Coarse screens usually have a spacing of 6 millimeters (mm) (or 0.25 inches [metric system conversion]). and larger, whereas fine screens spacing is usually between 1.5 mm (or 0.059 inches) through 6 mm (or 0.25 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 nozzles. 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.