The treatment of water and wastewater is a critical element of municipal responsibility. An increase of public and private awareness of water quality, availability, and cost is a driving force behind the demands for better efficiency and economy in these processes. Whether local needs call for new facilities or improvements to existing facilities, thermal flow meters can help meet these demands. At the same time, wastewater or aeration basin meters will eliminate undesirable system pressure drops and the high maintenance costs that relates to older technology: differential flow meters and rotary meters.
A common use of thermal flow meters at Wastewater Treatment Plant (WWTP) facilities is to measure the air (or oxygen) flow necessary for the secondary treatment of the activated sludge process. For this step, air and “seed” sludge mix into wastewater to facilitate decomposition. Next, air pumps into large aeration tanks where the wastewater and sludge mix. This stimulates the growth of aerobic bacteria and other organisms that are present in the sewage.
Operators must carefully monitor and adjust the rate of air flow, as necessary, throughout the tanks and throughout the overall process for optimal efficiency. Adding too much or too little air can have a noticeable negative impact on this important step of the treatment process. So, proper balance and distribution of air / oxygen supply in an aeration system is critical in any effective WWTP. Providing accurate measurement of the air flow is often the primary application for thermal flow meters at treatment plants.
After additional steps, such as settling and recirculating, the sludge is subjected to anaerobic treatment where the sludge is placed in digesters (oxygen-free tanks). It then heats for a number of days to stimulate the growth of anaerobic bacteria. This digestion process is necessary to convert as much of the sludge as possible into water and a mixture of carbon dioxide and methane gas called digester gas or biogas. This presents another excellent and increasingly critical opportunity to incorporate thermal flow meters into the plant operations.
Potential points of measurement in a WWTP air system are:
Water and wastewater treatment processes, such as aeration and pumping, are energy-intensive. Energy costs are comhttps://psctexas.com/digester-flow-meter-for-landfill-municipal-gas-flow-measurement/https://psctexas.com/digester-flow-meter-for-landfill-municipal-gas-flow-measurement/https://psctexas.com/digester-flow-meter-for-landfill-municipal-gas-flow-measurement/monly the second leading expense of WWTP facilities behind only facility staffing. However, the digester gas from the anaerobic process typically contains 60–80% of methane gas. Methane gas presents a set of environmental problems and restrictions in the atmosphere. So, rather than allowing the gas to escape into the atmosphere, operators capture methane for use as an energy source. Methane can drive turbines that produce electricity or can directly drive other plant equipment. The gas can also power boilers, thus providing heat for the facility’s buildings. In turn, all of this helps reduce the need for purchasing natural gas from another source.
A typical digester system captures gas in a storage tank and acts as a buffer to balance fluctuations in the production of gas in the digesters. Production is usually greater in summer than in winter which is often the opposite of the facility’s pattern of usage. As supplies dwindle, natural gas must supplement digester gas. And it is here that the accurate measurement of both digester gas and natural gas have a critical impact in the cost-effective operation of the treatment facility.
Closely monitoring the use of both gases as it is distributed through the treatment facility provides the information needed for efficient operation and for the reporting of the cost savings derived from the capture and use of the digester gas itself.
Constant temperature thermal mass flow aeration basin meters, such as those from Eldridge Products Inc. (EPI), operate on the principle of thermal dispersion or heat loss from a Resistance Temperature Detector (RTD) to the flowing gas. Two active RTD sensors operate in a balanced state. The first acts as a temperature sensor reference. Meanwhile, the other RTD is the active heat sensor. Heat loss to the flowing fluid tends to unbalance the heat flow sensor and makes it go back into balance using the electronics.
With this method of operating the constant temperature sensor, the fluid flow heat loss only affects the skin temperature. This allows maintenance of the sensor core temperature. Furthermore, it produces a fast response to fluid velocity and temperature changes. Additionally, because it can apply power only as necessary, this technology has a wide operating range of flow and temperature. The heated sensor maintains an index of overheat above the environmental temperature sensed by the unheated element. The molecular heat transfer and sensor temperature corrections virtually eliminate the effects of variations in density. Overall, use thermal mass flow aeration basin meters improve efficiency and reduce maintenance costs.
Operators must consider a number of factors when selecting and specifying any instrumentation. This is also true for thermal mass flow meters for WWTP systems. To specify the best configuration, operators must determine:
All flow meters have minimum and maximum flow limits for every pipe size as well as temperature and pressure ranges for the physical construction. Assuring that flow meters meet these basic requirements is the first step in specifying proper wastewater and aeration basin meters. These parameters will determine the calibration scale and accuracy, in addition to helping identify potential issues with overall installation.
The flow readings will be most accurate where the gas flow profile in the pipe is uniform and consistent. In this way, the sensor output at the point of measurement is truly representative of the overall flow through the pipe. All instrumentation manufacturers recommend straight run requirements for the installation of flow meters. In turn, these recommendations help end-users determine suitable locations for the flow meters. However, it is important to recognize that these are only guidelines and not guarantees of optimal positioning.
By its nature, most digester gas is considered to be “wet”. Simply put, thermal mass flow meters will not read accurately* if water droplets come into contact with the sensor RTDs. Although there are successful strategies for minimizing the potential for problems, manufacturers strongly advise installing flow meters at a location where the gas is dry.
*The heat loss to a liquid such as water droplets is so much greater than the heat loss to a dry gas that the meter’s flow signal will typically rise to a higher value, thus producing an error that will remain until the heated RTD is dry again. Although these droplets will affect our flow meters, they will not cause damage to the flow sensor. Non-condensing water vapor in the gas is acceptable.
Master-Touch thermal flow meters comply with the requirements of 40 CRF 98.34 (c)(1). They undergo calibration in our own NIST-traceable facility using CH4/CO2 gas mixtures for the best accuracy. Our flow meters provide real-time mass flow measurement and totalization for continuous air and digester gas flow monitoring. In addition, EPI Master-Touch flow meters can store as many as four independent flow ranges for multiple gas mixtures. These thermal mass flow wastewater treatment and aeration basin meters also have two fully-programmable relays for a variety of notification functions and/or pulse totalization of flow. A variety of sizes and configuration are available to meet virtually any installation requirement.
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