Automation of Water Resource Recovery Facilities. Water Environment Federation

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. For example, a screenings conveyor may only start after a rake has completed one cycle; it may continue for 3 minutes after that time.

7.0 PRIMARY SLUDGE AND SCUM CONTROL STRATEGIES

      7.1 Process Description

      Sludge and scum must be removed from primary clarifiers to avoid raking mechanism overload, septic conditions, and plugging.

7.1.1 Sludge Removal

      Sludge that reaches the bottom of a primary clarifier is typically raked to a sump area from which it can be piped for removal. Often, there are valves outside the tank that control removal. Most systems have pumps following the valves to pump sludge to the solids processing portion of the facility. Many systems have the ability to valve more than one tank to the same pump. Typically, sludge needs to have at least some thickening within the primary clarifier to avoid overloading downstream solids processing systems. However, if the sludge is allowed to become too thick, it may cause the sludge removal mechanism to overload. In addition, the sludge cannot become too thick for the sludge pump to handle. Therefore, the amount of time that the sludge remains in the primary clarifier should be limited to avoid septic conditions in the primary clarifier.

7.1.2 Scum Removal

      Scum is typically pushed to one section of the primary tank surface by flights or a surface rake for removal. From that section, the scum drops by gravity into a scum pit. The drop may be caused by a slotted pipe (scum trough) that rotates down for the scum to enter the pit or a beach where the rake pushed the scum up and into the drop pipe during each rotation of the rake. Scum is typically pumped from the pit to the solids processing section of the facility. The scum pit might have a mixer that allows mixing of the scum before pumping. Like primary sludge, scum must be thin enough that it can be pumped, but not so thin that downstream processes are overloaded. Scum must also have a limited retention time to avoid septicity.

7.2 Process Variables

      The following are typical process variables needed for control:

      • Sludge blanket level within the clarifier,

      • Sludge removal time,

      • Scum accumulation on surface,

      • Scum rake or flight position,

      • Scum pit level, and

      • Scum pit water/scum interface.

7.3 Controlled Variables

      The following are typical controlled variables used for automatic control:

      • Sludge removal valve (open–close),

      • Sludge pump (on–off),

      • Scum removal trough (up–down),

      • Scum pit mixer (on–off), and

      • Scum pit pump (on–off).

7.4 Control Strategies

      7.4.1 Sludge Removal Control—Dedicated Pump

      If each primary clarifier has a dedicated pump, control is easier. In this instance, primary sludge pumping is done with “on-time” and “off-time”. If there are automatic valves and/or grinders associated with the pump, they will need to be included in the on–off cycle with the pump. In general, the off-time frequency should be short enough to prevent plugging at the pipe inlet. In addition, the on-time frequency should be short enough to prevent “rat