Operators are required to get daily flow readings from their plants. This is done by either reading the totalizer reading or calculating the flow rate from an elapsed time meter. Wastewater flow is measured as a quantity (gallons) moving past a point (primary device) during a specific time interval. The Florida Department of Environmental Protection (FDEP) requires recording this flow, and plants larger than 1 MGD are required to measure the flow continuously. The most common primary devices used in wastewater treatment are V-notch weirs (Figure 1), rectangular weirs, and Parshall flumes.
Additionally, many small wastewater facilities use lift station pump times as their flow meter. In this case, the primary device would either be the lift station wet well or the pump discharge pipe, depending on how the flow rate was calibrated.
All secondary devices communicate to a monitoring device where the operator finds the totalizer to record the flow reading (Figure 2). The secondary devices most used at wastewater plants are mechanical floats, ultrasonic transducers, and magnetic devices. The elapsed time meter becomes the secondary device when a small wastewater facility uses a lift station to record its flow. According to FAC Chapter 62-600.200(25)(a)(b), these totalizers must be calibrated at least every 12 months. Accurate flow measurements are not only crucial for reporting reasons but also for precise process control and troubleshooting. Many process control calculations use flow measurements, and all treatment facilities are designed for specific hydraulic retention times. Inaccurate flows can lead to improper decision-making when managing solids or troubleshooting. Accurate flow calibrations are essential, and so is knowing what can go wrong with calibrations and the causes of inaccurate flow measurements.
According to FDEP rules, plants smaller than 0.100 MGD can use elapsed time measurements on lift station pumps or dosing pumps. These calibrations are performed by either a draw-down measurement on the wet well or by using a portable clamp-on meter to calculate the flow of each pump. When using either of these methods, it is essential to measure the flow using the normal operating depth of the wet well. If the wet well is near empty, the measured flow will be less than normal, and if the well is higher than normal, the extra head pressure will cause a higher reading. A common problem with a draw-down calibration is performing the measurement when the wet well is higher than a pipe invert. For a draw-down measurement to be accurate, no pipe invert inside the wet well can be partially or entirely submerged in the liquid. This will cause an inaccurate reading due to the extra time it takes to pump the liquid out of the attached pipe.
If it is not possible to perform a draw-down measurement below all the pipe inverts, then the other method of using a portable clamp-on meter should be used instead. Another standard error when using elapsed time meters for flow measurement is when a pump becomes clogged with a rag and runs longer, and the reduced flow output from the pump is not factored in the calculation. Additionally, when a pump is replaced before the 12-month expires for calibration needs to be performed. This is true even if the replacement pump is any exact model.
A newer pump will usually pump more flow than an older one. Regarding open channel primary devices, it is imperative for whomever is calibrating the flow meter that they use the correct measurement of the primary device. It is often hard to differentiate with the naked eye the difference between a 45-degree v-notch from a 60- degree v-notch. Also, if any corrosion on the device has taken place, the v-notch may no longer be at the correct angle. Another common error in open channel flow meters is an improper installation of the ultrasonic transducer. The transducer should be at least 12” upstream from the weir and located over a calm area or in a stilling well, so it is not affected by waves or turbulence.
The ultrasonic device also needs to be between 12-18” above the high-water mark. (Figure 3) shows a current transducer installation.
It is also important to make sure the transducer doesn’t get knocked around to become unlevel or lose its initial height.
Transducers should be perpendicular to the liquid surface. Make sure to read the totalizer using the correct units. The last number on the totalizer could change every ten gallons, 100, or even 1,000 gallons. Knowing how many zeroes, if any, to add to the number is imperative for a correct flow total.
One last type of flow meter I haven’t discussed yet are closed channel devices. The primary device with these types of meters is the pipe itself. Closed channel meters measure the velocity of flow going through a known pipe diameter. Most wastewater plants use turbine, ultrasonic, magnetic, or venturi meters. One common reason for inaccurate readings from this type of meter is the installation.
Disturbances located upstream or downstream of the flowmeter, such as pipe elbows and control valves, can adversely affect measurement accuracy. It is common practice to install the meter at a minimum of 10 times the pipe diameter upstream from any obstructions and five times downstream. There is no universal flow meter. Each type of flow meter has its own specific application and installation constraints. The way to select your flow meter is to use the application as your guide, not the technology. It is also imperative to enter the correct pipe data into the meter. If an ultrasonic device is used, it is important to make sure pipe preparation is done before installing the transducers.
The pipe must be dry and free of rust or paint. The recommended compound for the transducers should also be used. The purpose of the compound is to make a smooth connection between the face of the transducers and the pipe. This connection also inhibits any air or moisture from getting between the transducer and pipe. Over time, the compound will have to be reapplied.
Finally, it is important for most flow meters that the section of pipe that holds the meter be full of liquid. If a turbine meter has a partially filled pipe, less resistance will cause the meter to spin faster, recording a higher flow than what is going through the pipe. A partially filled or empty pipe will cause an error fault to register on an ultrasonic device, and all the flow will not be properly recorded. Air can also interfere with some ultrasonic devices.
If you are concerned at all about the accuracy of your wastewater flows, or need help in choosing the right meter, don’t hesitate to contact Tom Stirtzinger!