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Anaerobic Digestion Controls
Control Strategies PIDs
Control Loop Descriptions Digesters and Sludge Transfer P&ID
Digester Feed
Thickened primary sludge is sent to the primary digester, and thickened secondary sludge is sent to the secondary digester. The primary and secondary sludges are pumped through individual flow meters to monitor the quantity of sludge from each area. 
Local and Remote Controls
Local and remote control of each feed source is covered in their individual section of this eOM manuals. For additional information on the individual process review the following areas:
Digester Mixing
The gas mixing system was supplied as a package, and includes gas filters, two sliding vane compressors, oil separators, pressure switches, pressure relief valves, motorized unloading valves, flow meters, control panels, gas distribution balancing devices and the diffusers within the digesters. Literature provided by the package system supplier should be consulted for operation of the system.
Digester Heating  
Digester heating is controlled by the Hot Water System, which is explained in detail by following the link.
Hot Water Boiler
The boiler is started manually from the boiler control panel and is intended to be left on continuously. Once it has been manually started, the burner will automatically fire and shut off in response temperature. It will normally operate on digester gas. Dur­ing the coldest times of the year, when a digester is out of service, or when sludge is being digested at a reduced rate, the demand for heat will exceed the amount avail­able from the digester gas. The floating cover on Digester # 2 will come to rest on its corbels and the pressure in the system will begin to drop.

The plant's PLC receives two gas pressure transmitter signals, one from the gas with­drawal piping from Digester # 2 and another on the gas withdrawal piping from Digester # 3. The operator selects at the PCMS one of these signals to be used for both boiler control and waste gas burner control. If the pressure read by the selected pressure transmitter (presently # 2) falls below the adjustable set point (presently 7.5 inches), the PLC automatically switches the boiler to operation on natural gas. When the pressure returns to a value greater than the set point and remains there for an adjustable time period (presently 3 hours), the boiler automatically returns to operation on digester gas. If the pressure rises above a second adjustable set point (presently 8.7 inches), a signal is sent to the waste gas burner control panel that initi­ates an automatic startup sequence. The waste gas burner will remain in operation until the pressure drops below another adjustable set point (presently 8.1 inches).

When the demand for heat is below the amount available from the digester gas, gas will accumulate. The floating cover on Digester # 2 will rise to its maximum height and the gas pressure in the system will begin to rise above the normal operating value. The rise in pressure above an adjustable set point, which is presently set an 8.7 inches water column, will automatically start the waste gas burner. The waste gas burner uses natural gas as a pilot fuel.

Additional control systems and panels were provided as a package with the boiler and with the waste gas burner. Literature provided by the package suppliers should be consulted for operation of these systems.
Digester Draining
To drain the #1 or #2 digesters for cleaning or maintenance, the digester circulation pump or the sludge transfer pump is used. The H&H pump may also be used to pump the digester with the setup of a stand pipe and a truck load area. Raw sludge must be pumped directly to the number two digester.

When draining the #2 digester, the primary digester must be operated in a carefully controlled fill and draw cycle utilizing the emergency overflow to the pump station wet well. Digester number 3 may also be valved to take flow from digester #1. The rings in digester #1 outlet box should be set to prevent overflow to the secondary and also should be plugged.

When draining of the digesters, the sewage gas piping system must be valved off from the digester to be cleaned. Be sure that the gas pressure-vacuum relief valve on top of the digester to be drained is working properly to admit air to the digester as the liquid contents are withdrawn.

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Last Updated: 9/16/2013 11:24:12 AM
Version 4.0.1