A new control system design uses monitoring components to optimize the wasting from activated sludge treatment. The control system is versatile enough to follow whatever wasting strategies the wastewater treatment plant uses.
The Model 7700 Systems control units are supplied with algorithms that can be customized for each installation based on dynamic properties of the plants activated sludge treatment train. The systems maker, Royce Instruments, has said that in field installations it has provided a 10 to 15 percent reduction in MLSS concentrations without a negative impact on ammonia removal. It also can improve sludge settling properties and reduce surface foaming. Lab tests have on average been reduced 80 percent, according to Harry Wilkinson, Director of Marketing for Royce.
Customization follows parameters developed from a questionnaire, which each customer fills out at the start of the project.
The system is a complete monitoring and control package. It includes two in-situ solids density analyzers, a portable solids calibration meter, and a process controller that provides a user-friendly interface.
It continuously monitors the concentration of solids in the aeration train, the concentration of the activated sludge (in the return path), and waste sludge flow. This information is processed to control the flow of waste sludge in a variety of selectable strategies to provide continuous optimized control of sludge retention time (SRT).
The control system is designed to be located within each train of a wastewater treatment plants aeration process area. Two Model 7011A Total Suspended Solids Analyzers are the primary input devices for the controller unit, along with the customers waste flow meter.
A Royce Model 73A Color-Compensating sensor is used to monitor the mixed liquor suspended solids (MLSS). The activated sludge suspended solids concentration for the train can be measured in the RAS line to provide continuous data. Either a Royce Model 73A or the Model 74A High-Density Pipe Insertable Sensor can be used to provide this data.
Both sensor models have a self-cleaning option to help minimize maintenance. All electronic elements of the SRT control system, including the control unit, are housed in NEMA 4X enclosures suitable for outdoor mounting.
The system also has one 4-20mA output that can be used directly to control a waste pump or valve. A programmable PI algorithm in the controller can regulate waste flow to the target value using the waste flow meter as feedback.
Alternately, a 4-20mA output indicating target flow can be sent to a separate controller if the customer prefers to use external valve or pump speed control hardware. One relay output can be used to start and stop a waste pump, while another relay can be wired to an alarm panel to alert the operator of system problems.
Jim Dartez, president of Royce, calls the control system an "expert system," meaning it uses a safeguard module which sorts the data. If a reading is way off, it gets thrown out.
