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 holing” (i.e., pulling supernatant through the sludge, which results in thin sludge). For example, the off-time may be 30 minutes and the on-time may be 10 minutes. If the sludge blanket level is known, the time interval can be trimmed slightly so that the blanket remains at a normal level. Typically, adjustments to time because of blanket level should be made infrequently, with a single change made each day or so because the blanket level typically does not change quickly.
7.4.2 Sludge Removal Control—Valve to Common Pump
If more than one primary clarifier shares a dedicated pump, the control gets more complicated. In this instance, each primary sludge tank valve is controlled with an “open time”. The sludge pump (along with any grinders associated with the pump) will typically run continuously or as long as any associated tank’s sludge valve is open. The valves for each clarifier will typically cycle in order, with the clarifier valve that has been closed the longest opening next. In general, the open-time frequency for each valve should be short enough so that the clarifiers waiting to remove sludge do not cause plugging at the pipe inlet and to prevent rat holing. For example, four clarifiers sharing the same pump might each have their valve open time at 10 minutes, resulting in a closed time of 30 minutes for each clarifier valve. If sludge blanket levels are known, the relative open time for each clarifier can be trimmed slightly so that the blankets remain close across all clarifiers. As stated previously, adjustments to time because of blanket level should typically be made infrequently, with a single change made each day or so because the blanket level typically does not change quickly.
7.4.3 Scum Trough Operation Control
Rotating scum troughs are turned to allow the scum to enter the trough. The main control is frequency and timing of the rotation. The trough needs to rotate at the time the scum rake or flight is approaching the trough to maximize the amount of scum collected. In addition, it is important to have enough water along with scum to help the scum travel along the trough and to the pit so that the scum does not accumulate in the trough. At the same time, too much water could overload the scum pit and downstream scum removal and/or treatment processes.
Typically, a discrete position indicator for the skim arm (i.e., circular clarifier mechanisms) or flight (i.e., rectangular clarifier mechanisms) is provided to indicate when the position of the flight is optimal to lower the trough. The controller will count these events and lower the trough when the count reaches the setpoint. The count setpoint can be lower (perhaps 1) when there is a large scum accumulation and set higher when there is less accumulation. There is also a “down-time” setpoint. When the trough is at the low position for a time equal to the down-time setpoint, it is raised again.
7.4.4 Scum Pit Level Control
Scum pit level control is similar to other well-level pumping. Typically, constant-speed pumps are used and there is a high “start-level” setpoint and a low “stop-level” setpoint. When the level reaches the start-level setpoint, the scum pump is started. When the level reaches the stop-level setpoint, the scum pump is stopped.
Scum tends to stratify in the scum pit, with the scum rising to the top and the water falling to the lower level. In some systems, the pit is mixed to allow a more homogeneous mixture, which will be easier to pump than the concentrated scum. In these instances, the mixer is started when the level reaches the “start pump” setpoint and is run for a time equal to a setpoint. When the mixer stops, the scum pump is started.
8.0 ACTIVATED SLUDGE SYSTEM CONTROL STRATEGIES
8.1 Dissolved Oxygen Control
8.1.1 Process Description
The aerobic section
