There are many control strategy variants used across the wastewater industry. Each wastewater operating company has its own policy, and within this there are different models for different plants.
Since there are few generic control strategies, Shepherd relies on the client to provide a strategy or solution that works in their environment. Bactest is happy however, to work with the client to implement a suitable control strategy based on Shepherd outputs.
Although an intricate cascade model might suit some clients, where there exists a hierarchy of sensors used for control, one simple strategy uses a standard control system that is found in many plants with Shepherd intervention for aeration control, involving very few changes to the existing system. The existing control system is Dissolved Oxygen (DO) control, sometimes also integrated with Ammonia control in a more complex algorithm. Shepherd is essentially a BOD meter and there is a direct relationship between BOD and DO. This means that the control mechanisms can be readily interchanged with minor hardware and software changes. This simple approach is designed to allow rapid, cost effective integration of Shepherd with very little risk to the client.
The strategy involves setting up Shepherd to mimic a DO sensor input to the SCADA already in place. The SCADA system simply sees a signal from the sensor and reacts to adjust the aeration accordingly. This does not involve complex algorithms and leaves the responsibility for the control algorithm with the operator, reduces risk, and has the benefit that the operator is familiar with the algorithm.
Process information can be communicated from the local control cabinet to SCADA or other monitoring and control systems via 4-20mA analogue or digital Modbus (RTU or TCP) signal for process control purposes.
A SCADA interface typically translates the measurements of DO (active 4-20mA current loop) which is then used to control the aeration via an algorithm that is specific to the plant. Shepherd has an internal Input/Output (I/O) board that can interface with this system.
Process algorithms are designed to revert to high aeration in the event of a signal failure or alert from Shepherd. In this scenario, the process is protected by the high aeration while operators investigate the problem.
Set point adjustment
Shepherd software allows only small reduction steps for aeration, based on decreasing BOD demand. This means that operators cannot accidentally reduce the aeration in one rapid step and put the process at risk.
In the event of a problematic situation, the software allows the aeration to rise immediately when an issue is identified so that the process is not limited by oxygen, only the event itself.