Blowers are the largest single user of energy at treatment plants representing up to 70% of the
Aeration is important for providing sufficient DO for aerobic organisms performing BOD removal and nitrification in activated sludge plants, as well as keeping the biomass in suspension. The nitrification capacity can be varied in relation to DO control in two ways: by adjusting the aeration intensity or by adjusting the aerated volume. Apart from DO concentration, several other factors have been reported to affect nitrification rates including inorganic substrates, solids retention time (SRT), temperature, pH and toxic inhibition. Other control handles which also will affect the nitrogen removal, and hence might have an impact on the DO control loops, are return and waste activated sludge flows and nitrate recycle or external carbon dosage for plants with denitrification.
Important Elements in Dissolved Oxygen Control:
Air supply system
The blowers are the first stage in a diffused air system. Adequate blower system design is required for efficient control of the DO concentration. During the design process it is important to consider that the air flow demand varies over the day, week and year as well as along the aeration tank. The flexibility of a blower system is crucial for the performance of the aeration system since plants need to handle a large variation in load.
Historically, inlet vanes or outlet dampers have been used to meet a varying demand but not in an energy efficient manner. Today, High speed Turbo blowers supplied with variable frequency drives (VFD) allow turndown of the aeration capacity. Apart from blower capacity control, blower control also includes start–stop procedures for multiple blowers as well as safety procedures during start-stop and surge control which prevents instability at low flows by maintaining a minimum flow.
The air passes through a valve before it is diffused into the aeration basin. Butterfly valves, damper valves, globe valves, plug valves, etc. have different mechanical design. The flow dynamics of the valve describes the flow rate as a function of the valve position. The flow characteristics for a fixed pressure drop over the valve can either be linear (flow is proportional to valve lift), equal percentage (flow is proportional to the first derivative of the flow with respect to the valve lift) or quick opening (a small change in valve lift produces a large change in flow).
Control structure design is about how to set up the control system, namely which variables to control, which variables to manipulate and how to combine these two sets of variables to create control loops. Depending on the process at hand different types of controller structure can be considered in a process control scheme; There is no unique way to categorise the control structures for aeration control. The most popular structures can be considered as the following four categories:
- DO cascade control;
- Ammonium-based supervisory control (Feedback control-Feed forward–feedback control);
- Advanced single input single output (SISO) and multiple input multiple output (MIMO) controllers;
- Control of the aerobic volume;
Advantages of an Automated Aeration System
Automating an aeration system is considered an indispensable upgrade when improvements to any Wastewater Treatment Plant’s aeration system are being considered. One of the primary benefits is the optimization of efficiency and extended life of the High Speed Turbo Blower which the immediate and long term costs of a waste water treatment plant.
An automated aeration system consists of a Master Control Panel (MCP) “See Diagram 1- typical Network Diagram” that constantly monitors and controls High Speed Turbo Blowers operation and DO level and air flow in each aeration zone. This is done through the use of flow meters, DO probes and modulating flow control valves in each aeration zone. The MCP will be controlling the High Speed Turbo Blowers’ operation to deliver the required flow according to precisely calculated air flow requirements. This optimizes the High Speed Turbo Blowers’ performance and control thereby delivering DO requirements and significantly reducing power consumption and costs since there will be no needless air produced.