Thermal mass flowmeters’ performance in wastewater treatment aeration systems
In wastewater treatment facilities, the cost of supplying compressed air flow to aeration basins easily can be one of the facility’s largest energy expenses. The figures most often cited are that 40% to 50% of a wastewater plant’s total energy usage can be attributed to the aeration process. The cost to produce this compressed air continues to increase proportionally with energy cost increases. By measuring the system’s air flows with accurate, repeatable air flowmeters, the aeration process can be better controlled to optimize the process and minimize plant energy costs.
Beyond energy cost savings, choosing the right flowmeter for aeration systems also can result in saving thousands of dollars per instrument in installation, operation, maintenance and life-cycle costs. Carefully considering the requirements for performance, plant installation environment, installation ease, maintenance needs and instrument life can provide a stream of cost savings and a faster payback on the flowmeter investment.
Wastewater treatment plants employ a variety of processes to eliminate organic pollutants from water to ensure it meets sanitary requirements before being discharged. One of the most common processes is the activated sludge method, in which large aeration basins are deployed early in the process to biologically treat the wastewater. This process deploys large blowers and a network of piping to transport and distribute the air into the aeration basins and ultimately into a diffuser system to oxygenate the wastewater. The energy needed to operate the blowers and provide air is a major, continuous operational cost for a wastewater treatment plant.
In the aeration basins, bacterial micro-organisms, which decompose biologically degradable organic solids in the wastewater, depend on the aeration system to provide the right amount of oxygen necessary for them to thrive. Too little air results in added processing time in the basin, while too much air results in excess operation of the blowers, both resulting in wasted energy.
The aeration process therefore requires large amounts of air to ensure the aeration system operates effectively and efficiently to treat the wastewater before it can be moved along to clarifying basins, filtration, disinfection and other downstream treatment processes. Accurate and repeatable air flow measurement is the key to controlling the proper amount of air released into the aeration basins, which is essential to sustaining the micro-organisms while minimizing the energy required for its production.
Flow Sensor Technology
Three flow sensor technologies typically have been used in aeration air flow monitoring applications in wastewater treatment plants:
- • Differential pressure (orifice plates);
- • Vortex shedding technologies; and
- • Thermal dispersion (mass flow).
Within wastewater treatment plant aeration systems, it is now generally accepted that thermal dispersion mass flowmeters have the largest installed base for this application, and this trend is expected to continue. While there are legacy orifice plates and vortex shedding meters installed in aeration systems, they are gradually being replaced during retrofit and plant upgrade projects. There are several reasons for their replacement that relate to process optimization, energy cost reduction, ease of installation, performance and low maintenance.
Thermal Mass Flowmeter Advantages
Thermal mass flowmeters offer a number of advantages in wastewater treatment plant aeration applications. They are accurate over a wide flow range, provide direct mass flow measurement, are easy to install, cause virtually no pressure drop, are the lowest cost for line sizes 4 to 16 in. [100 to 400 mm], and require little maintenance, which ensures continuous, trouble-free operation over a long life.
Accuracy. Thermal mass flowmeters are generally accurate to ±1% of reading, ±0.5% of scale (or better, depending on the specific meter) and with repeatability to ±0.5%. This level of accuracy and repeatability adequately meets the needs of wastewater treatment aeration processes.
Wide Flow Range. With the capability to measure from extremely low flow rates to very high flow rates (<1 to 1,000 SFPS), thermal mass flowmeters support the requirements of wastewater aeration treatment plants. Their wide turndowns (100:1) also support variable operating conditions due to community demand fluctuations or seasonal changes in weather.
Direct Mass Flow Measurement. Thermal flowmeters directly measure mass flow with a simple design, which reduces their maintenance requirements. They do not require the addition of temperature or pressure sensors.
Low-Cost, Single-Tap Installation. Thermal flowmeters can be installed with a single tap point, as opposed to full bore (e.g., spool-piece section) meter technologies. There is no need to interrupt the process, shut down the line, or cut the pipe and re-weld. With its small, center-point mounted insertion probe, there is virtually no pressure drop. That means aeration blowers outfitted with thermal flowmeters can run at lower power levels that reduce energy needed and achieve further cost savings.
Overcome Limited Straight Run. In a crowded wastewater treatment plant retrofit, expansion projects or otherwise limited straight-run installations, insertion thermal mass flowmeters can be supplied with, and specifically calibrated for use with, flow conditioners. Flow conditioners can reduce straight-run requirements from 30 diameters down to as few as 7 diameters. High-quality flow conditioners eliminate both swirl and profile distortions and overcome flow profile changes that occur in transitional flow regions and minimize pressure drop. They also eliminate the need for long upstream/downstream straight pipe runs requiring expensive pipe and installation labor.
Low Maintenance and Long Service Life. With no moving parts, there is nothing to break, wear out, clean or repair with a thermal mass flowmeter. They can continue to operate untouched and trouble-free for many years. With advances in techniques, even calibration can be checked in-situ. This can be done either partially, with dry-check techniques, or completely, with a wet-check technique, in which the installed thermal flowmeter includes a ball valve with an in-situ automated calibration verification kit.
A fast-growing city in the arid desert regions of the Western U.S. recently addressed the need to expand its municipal service for new residents and subdivisions. In recent years, the city’s residential demand for wastewater treatment tripled from less than 1 million gal to more than 1.5 million gal per day (mgd).
New FCI thermal mass flowmeters were installed in a 24-in. line for measurement of blower air flow into the aeration basins. Accurate measurement of the air flow was necessary for the control system to maintain the correct level of dissolved oxygen in the aeration basins and to help reduce energy costs. The meters also needed to provide accuracy over a wide flow range because the facility requires flow rates to increase from 0.5 to 3 mgd with future growth.
Choosing the proper flowmeter technology for aeration systems in wastewater treatment plants will result in optimum process effectiveness at the lowest installed cost and reduced energy consumption. Looking carefully at measuring needs for accuracy, repeatability and range; signal output needs of today and tomorrow; installation conditions and complexity; and maintenance requirements will result in selecting the most cost-effective flowmetering solution.