removal are used to remove large materials and abrasive grit to protect downstream equipment.
6.1.1 Bar Screens
Bar screens stop material that is too large to fit through bar openings. Facilities typically have a number of bar screens in parallel. More than one bar screen is typically needed so that availability is maintained during maintenance. In addition, optimal velocity through the bar screen should be maintained between 0.4 and 0.9 m/s (1.25 to 3.0 ft/sec). This prevents grit settling in the chamber at low velocity and material being pushed through the screen at high velocity.
Many bar screens have rakes that can operate intermittently. The rake must operate frequently enough to ensure that material does not build up on the rack and blind it; however, if the rake operates too frequently, it will increase wear on the equipment, which will increase the frequency of maintenance and likely reduce its availability.
6.1.2 Grit Removal
There are several variations of grit removal equipment. Grit removal maintains settling velocity in a range that will allow materials with a high specific gravity to settle out while not allowing organic matter to settle. All grit removal equipment contains mechanical equipment that must operate to move grit from the units. Many modern systems consist only of grit pumps and cyclones to produce grit that can be disposed.
6.1.3 Conveyance Systems
In most instances, bar screens and grit removal systems discharge to a conveyance system of some sort to transport screenings and grit to a container for removal from the system. These conveyors are typically some sort of belt system with a driver and rollers.
6.2 Process Variables
The following are typical process variables needed for control:
• Bar screen channel velocity,
• Bar screen channel level,
• Bar screen rake operating time,
• Bar screen channel differential level,
• Grit system transport equipment operating time, and
• Conveyance system equipment operating time.
6.3 Controlled Variables
The following are typical controlled variables used for automatic control:
• Bar screen channel gates (open–close),
• Bar screen rake (on–off),
• Grit system channel gates (open–close),
• Grit system transport equipment (on–off),
• Aerated grit air supply (valve position),
• Air or water jet valve (open–close), and
• Conveyance system transport equipment (on–off).
6.4 Control Strategies
6.4.1 Bar Screen Velocity Control
Where possible, the number of bar screens in operation is controlled to maintain the velocity through each screen. This is typically accomplished by calculating velocity based on total flow and the channel level. The gates of the appropriate bar screens are opened or closed as needed to accomplish this task. This is often done in a lead–lag series so that it is known which screen to start or stop next.
Before opening the channel of a bar screen, the rake of that bar screen is typically operated for one cycle to ensure that the screen is clear. Similarly, the rake is operated after closing the channel to ensure that it will be ready for the next cycle.
6.4.2 Bar Screen Cleaning Control
Rake frequency for each rake is typically controlled by both time and differential level across the screen. Time between rakes is typically the cause of raking the bar screen. For example, a rake strategy may provide for a raking cycle every 10 minutes under normal operation. A rake action by differential level provides protection from screen blinding. For example, a rake cycle may be initiated if a differential level greater than 0.15 m (6 in.) for more than 25 seconds occurs. If the differential level exceeds normal, it is a signal that the rake is likely beginning to blind and more frequent rake action is needed.
During high facility flows, it is common to greatly increase the frequency of rake operation. This is attributable to flushing of the facility’s collection system, which tends to present a much higher concentration of screenings material. Often, when facility flow is much greater than normal, all the rake action is set for continuous operation. Each rake cycle typically is stopped when the rake reaches a safe position.
6.4.3 Grit Settling Velocity Control
Grit settling velocity control varies by type. Aerated grit systems control settling velocity primarily by controlling the airflow to each unit to maintain organic materials in suspension. For these units, it is typical to control the airflow to each unit to a constant value. For both aerated and other types, the flow through the units is typically kept within a range according to the specification of the units to maintain control and avoid flooding of the system. In some instances, channel level can be used to indicate that more grit units are needed. The grit units in service are controlled by opening the gates to each unit.
6.4.4 Grit Removal Control
Most grit systems have removal systems that do not operate continuously. Rather, the equipment is typically controlled to run on time. For example, a typical strategy may provide for grit removal for 10 minutes of operation every 1 hour under normal operation. In some systems, there may be several components of grit removal that work in series. For example, an auger may move the grit slurry to an area where the grit pump can pump the grit to a grit washer. In this event, there is typically a time lag between each component of the grit removal system. For example, the auger may start 3 minutes before the pump start and the washer may start 30 seconds after the pump start and continue for 2 minutes after the pump stops.
In some systems, an air or water jet may also be used to loosen compacted grit before pumping. In this instance, the air or water valve is typically opened for several seconds before starting the grit pump.
During high facility flows, it is common to greatly increase the frequency of grit removal operation. This is attributable to flushing the facility’s collection system, which tends to yield a much higher concentration of grit. Often, when facility flow is much greater than normal, the grit removal equipment operation is set for continuous operation.
6.4.5 Conveyance System Control
Conveyance is typically controlled to run in conjunction with the equipment (screenings, grit, or both) that is providing material for conveyance. Typically, there is some delay between the start of the screen rake or grit removal equipment and the conveyor to account for the time it takes for the material to reach the conveyor. Likewise, the conveyor typically continues operating after rake or grit removal equipment has stopped operation to allow time for the material to reach its destination.
