Thermal Mass Flowmeter Provides Cost Effective Solution for Digester Flow Measurement

The San Leandro Wastewater Treatment Plant was constructed in 1939 to support about 14,000 residents. Today the plant, with its design capacity of 33 million gal per day (mgd), supports 85,000 residents and includes farms and commercial industries among its thousands of businesses. The plant is undergoing major reconstruction to modernize processes for its average influent flow of 5 mgd, while supporting spikes up to 23 mgd.

The plant includes primary and secondary treatment processes, and chlorine disinfection and has three 110-kw engines that run on methane to produce electricity and heat. The San Leandro plant, as with most wastewater facilities, actively seeks technologies that improve process efficiency or result in cost savings.

Using Flowmeters

Flowmeters are used in several locations at wastewater plants, but one area where they can have an impact on the bottom line is the digesters—considered a wet gas, condensing gas or biogas environment. A digester is a large tank that holds the organic sludge removed from other processes.

Microorganisms or “bugs” break down the sludge into stable compounds. During this process, methane is produced that contains approximately 6.5% water saturation. The methane is captured and cleaned so that any residual carbon dioxide, water vapor and trace gases are removed before it is used as a fuel source. Excess methane is sent to a flare. The U.S. Environmental Protection Agency is poised to require wastewater operators to monitor the amount of methane produced and compare it with the amount used as fuel and the amount flared.

Thermal mass flowmeters provide excellent measurement capabilities in dry gas flows. They have proven durability, accuracy and repeatability. However, thermal flow meters are sensitive to liquid droplets in the gas stream. For a thermal flowmeter to work accurately in the digester environment, the probe must be placed far enough downstream so the entrained water in the gas stream has condensed onto the pipe wall.

Optimal placement is typically not an option and, in the real world, probes are inserted very close to the digester. In this very wet environment, any condensing liquid (commonly referred to as mist or fog) contacting the sensor probes causes a high reading due to the liquid vaporizing on the heated portion of the sensor. As a result of this deficiency, digester measurements with conventional thermal meters are largely ignored when moisture levels rise because there is no confidence that the measurements are truer.

Accurate Application

Kurz engineers conducted a study that lead to a unique, new design that allows thermal flowmeters to perform accurately within a wet digester gas environment. Product comparison tests performed over several months at the San Leandro and other wastewater plants demonstrated that the Kurz WGF flowmeter is the only thermal meter capable of consistently and accurately tracking the dry gas in a wet gas environment.

By improving digester management utilizing this new technology, the San Leandro Wastewater Treatment Plant has had the opportunity to improve their efficiency and decrease operation costs.

Accurate digester measurements can show indications of digester imbalances, enabling early corrective action and leading to increased gas production. Optimizing the digester process allows a facility to recover maximum digester gas because correlations can be made between gas production and sludge feeding.

Monitoring the true gas flow can facilitate less gas being diverted to the flare and more directed toward recovery processes. Engines can be properly sized for facilities, and managing the engines can be more accurately controlled. Monitoring the gas coming out of the digester ensures that greenhouse gas emissions are reported with lower and more accurate numbers because gas production can be matched with gas usage.

Robert Morpeth is a technical writer for Kurz Instruments. Morpeth can be reached at [email protected] or 800.424.7356, ext. 333.