Air Distribution System
The name VACOMASS® encompasses numerous components of a measurement and control system that are put together as building blocks – they can be used individually or combined to a complete system for application in wastewater treatment plants. At its most basic, it could simply be an airflow meter for aeration air, or an air control valve. As a system, it starts as a local control loop for dissolved oxygen (DO) and can expand to complex control systems combining multiple local control loops, and including variable pressure control or air distribution control, with flow simulation in the measurement and control section of the piping. The VACOMASS® system integration and the precise calibration of the air distribution systems in our CAMASS® flow calibration laboratory always ensure optimal interaction of the system components and ultimate precision and security for the control of aeration air at lowest possible operating costs.
Applications in biological treatment
When the biological stage of a wastewater treatment plant is under-aerated it will lead to process disruptions and effluent limits can be exceeded. On the other hand, an over-supply of air wastes blower energy and results in uneconomical operation of the plant. Additionally, the denitrification process can be compromised by surplus dissolved oxygen in the recycle flow. Only a well-designed aeration control system based on actual oxygen demand can ensure both a controlled and economical plant operation.
Aeration air has to overcome static and dynamic resistance, e.g. changes in water level, condition of the aerators and pressure drop of the piping, on the way to the treatment tank. These factors change over time and are hard to control. Therefore, even very small changes will have a significant influence on air distribution.
This is precisely where the VACOMASS® concept can be applied: Each VACOMASS® system continuously monitors the local air supply and can immediately detect the smallest deviation from the set point. The local control immediately intervenes and compensates for the external disturbances of the air distribution system. VACOMASS® provides – as a function of loading and oxygen demand – an load-based and steady supply of aeration air to the various trains, basins or control zones, or the load-based control of aerated and anoxic phases during intermittent denitrification.
Conventional control systems are usually based on measurement and control of DO, and larger systems often overlay additional process parameters, such as ammonium and/or nitrate concentration. Due to basin size, system inertia, or incorrect sizing of blowers and control elements, and the use of butterfly valves or gate valves with inadequate or limited control capability, DO control systems can experience deviations of up to 1.5 mg/l from the set point.
Negative deviations of this magnitude can lead to an under supply of oxygen to the activated sludge, with negative consequences for sludge properties and effluent values, particularly ammonium.
Positive deviations lead to over-aeration of the biology and increased energy costs. Over-aeration can also have negative consequences for the process, such as oxygen contamination of anoxic zones (inhibited denitrification, increase of nitrates and total nitrogen in the effluent) or mineralization of the activated sludge. These negative consequences can occur quickly in under loaded plants.
The best control performance and maximum flexibility are achieved using cascade control systems. Blowers provide sufficient aeration air at the necessary pressure in the header pipes (possibly using variable pressure control). The process controller ensures that the DO is maintained at the set point while accounting for additional process parameters. When there is an error signal (deviation from the set point) the VACOMASS® system uses the airflow as the control variable – based on actual DO, the DO set point and the actual airflow rate it calculates the desired airflow rate and adjusts the control element accordingly. This allows the system to react to loading changes immediately; it does not have to wait for changes to the control element to have an effect on the measured DO. Deviations from the set point are reduced and over or under supply of oxygen are largely prevented.
Layout of the measurement and control section
The layout of the system can vary depending on the available piping and the choice of control valve (diaphragm valve or jet control valve).
When using diaphragm control valves, the pipe diameter is typically reduced at the start of the measurement and control section, and expanded at the end. The VACOMASS® jet control valve can normally be installed in the pipe without any reduction/expansion.
Standard Layout: When sufficient straight piping is available, the airflow meter can be positioned far enough away from the control valve that it is not influenced by flow profile changes caused by control valve actions.
Compact Layout: The airflow meter is positioned 350 mm in front of the diaphragm control valve, or 0.5*D in front of the jet control valve. When using a VACOMASS® square diaphragm control valve or an elliptic diaphragm control valve, the airflow measurement has to be corrected for the actual valve opening position. Depending on the pipe layout ahead of the airflow meter and the required accuracy, an automatic correction for the valve stroke of the VACOMASS® jet control valve may or may not be necessary.
When wastewater treatment plants are operating under partial load or low load conditions, the oxygen demand is reduced, airflow rates are lower and the pressure drop in the piping goes down. When using constant pressure control, this needs to be compensated by the control valves. Instead of throttling the airflow with the control valves, it makes more sense to adjust the system pressure. The VACOMASS® econtrol functionality in the flexcontrol cabinet monitors the condition of the aeration system and determines the minimum pressure that is needed to satisfy the oxygen demand of the plant. Reducing the blower pressure reduces the electricity consumption, ensuring economical operation under partial load conditions.
In the air distribution system, one valve is always wide open (most-open-valve control). Instead of controlling the system pressure, the controller determines the load-based airflow demand, which has to be provided by the master control of the blowers.