When to Consider An Alternative to Thermal Dispersion Meters

Maintaining a firm grip on blower output within digesters at wastewater treatment plants is the key to stable dissolved oxygen (DO) levels that support an optimal biological cycle. By monitoring air flow at critical points, with a flow meter placed at each aeration section, an operator can detect a blockage or equipment issue often before the DO level falls noticeably. However, the most common type of device used to measure aeration in the activated sludge process — thermal dispersion flow meters — is not always the best fit.

Thermal dispersion flow meters are based on heat transfer and contain one of two types of temperature sensors, known as resistance temperature detectors or RTDs. Air passing by the RTDs gives a cooling effect, with the resulting temperature change then correlated to the pipe diameter to help determine the flow.

The downside is that moisture can build on the RTDs and impact performance, but the good news is that wastewater plant operators have multiple alternatives to RTD-enabled thermal dispersion meters.

Evaluating the Possibilities

Several types of technology are available for retrofitting an existing plant or designing a new facility. Variable-area flow meters, which measure the pressure in a line via a float that rises within a tube, are the most cost effective. However, they don’t provide precise readings.

Mechanical devices rely on what’s basically a pinwheel. With turbine flow meters, for example, the mechanical energy of the air flowing past the turbine causes rotation. The downside to turbine flow meters and similar mechanical devices is that they contain moving parts, so any buildup of foreign material can cause the meter to stop working.

Vortex flow meters contain a shedder bar mounted across the diameter of a pipe. The shedder bar creates vortices, which are measured by twin piezoelectric crystals. The faster the flow past the shedder, the greater the frequency of the vortices. The shedder bar is comparable to a flag pole and the piezoelectric crystals are like a flag. The faster the wind blows around the flag pole, the faster the flag will flap in the breeze.

The cost of vortex and thermal dispersion is comparable. Even when insertion vortex meters are not a direct fit, an adapter can allow replacement mounting to be accomplished.

Contaminants in the air don’t create problems for a vortex meter, as there is no moisture or foreign matter sensitive parts, so they offer significantly lower maintenance issues and perform well in humid regions.

By contrast, some plants in highly humid areas face a monthly maintenance nightmare of removing and cleaning dozens of thermal dispersion meters. Those that have switched to vortex meters have eliminated a massive hassle.

Another upside to vortex flow meters is that they are easy to swap out. When replacing a thermal dispersion meter with a vortex meter, the fittings are usually identical (allowing plant operators to simply unscrew the old meter and screw in the new one). Even when there isn’t an exact-fitting match, making modifications for a vortex replacement is a relatively easy task.

Vortex flow meters can also be used to measure liquids and steam, such as biogas created in a digester, which tends to be dirty and contain a significant amount of moisture. This task can be challenging for thermal dispersion flow meters.

Vortex meter manufacturers offer a wide variety of options, including an in-line version available for smaller pipes, as well as insertion, flange, and remote head versions. Within the last two years, the vortex meter industry has also made its products more attractive by introducing pressure and temperature measurements as an add-on option. Vortex flow meters can also be used effectively for measuring gas created at landfills, HVAC in apartment buildings and high rises, and natural gas lines. 

When it comes to accuracy, vortex and thermal dispersion meters are about the same, and each is susceptible to turbulence. Vortex meters are less equipped for low-flow environments, however, as thermal dispersion meters can reach slightly lower flow rates.

Overcoming Barriers

Precision control is key to maintaining proper DO concentrations in activated sludge, thereby promoting an optimal and stable biological process, but the best technologies to accomplish this don’t always come to the forefront.

Although vortex or other flow meters could provide a more effective solution than thermal dispersion flow meters at many wastewater plants, they’re not commonly considered as a replacement at existing facilities. Additionally, most specifications for new construction continue to be written for thermal dispersion flow meters.

Plant operators and consulting engineers should carefully review all the flow meter options before choosing to go with the status quo replacement or when designing a new site. Selecting the most appropriate technology can significantly improve metering performance and reduce the need for extensive maintenance.