Go with the flow
A recent installation of air flow meters in a wastewater treatment facility in the US has helped deliver energy efficiencies, says Stephen Cox, a senior member of technical staff at Fluid Components International
A fast-growing city in the arid desert regions of the western USA 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 increased from less than 3,800m3/d to more than 5,600m3/d. In planning for the wastewater treatment plant’s expanded capabilities, the city’s water engineers identified minimising aeration basin compressed air energy costs as a goal.
In wastewater treatment plants, a variety of processes are employed to eliminate organic pollutants from water to ensure it meets sanitary requirements for future use. One of the most common processes is activated sludge.
This process involves pumping compressed air into the aeration basins where a diffuser system ensures the air is distributed evenly for optimum treatment. The energy needed to provide compressed air is a significant cost in the operation of a wastewater treatment plant.
Tiny micro-organisms in the aeration basins decompose biologically degradable organic solids in the wastewater. These micro-organisms depend on the aeration system to provide the right amount of air necessary for them to thrive and consume the suspended solids in the wastewater.
These solids are eventually removed downstream of the aeration basin and can be digested to create energy at the plant. Large amounts of air are required to ensure the aeration process operates effectively to treat the wastewater adequately before it can be moved along to clarifying basins, filtering, disinfection, and other treatment processes.
Controlling the proper amount of air that is released into the aeration basins is essential since the air-flow controls the growth of micro-organisms that treat the wastewater. Flow meters are typically installed in the aeration system piping to measure the amount of air-flow, with their analog or digital output connected to the facility’s control system.
In most plants, each of several aeration basins is configured with numerous diffuser systems, and individual air-flow monitoring with independent control is generally required for each diffuser system. The compressor system must run at all times to maintain the optimum amount of air to the diffuser systems and the aeration basins, with flow rates changing throughout the day based on demand.
Positioning flow meters
The engineers at the city plant needed to place air flow meters in a rugged area of the facility with an underground vault, which required remote access. The installation site was further challenged with straight pipe run limitations, hazardous gases present, a wet/dirty environment and with fluctuating wastewater levels. The flow meters needed to be installed in a 610mm line for 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 (DO) in the aeration basins and proper treatment of the wastewater.
The meters also needed to provide accuracy over a wide flow range, because the facility requires flow rates to increase from 1,800m3/d to 11,360m3/d with future growth.
After reviewing various flow meter technologies, the city’s plant engineers selected the ST98 Flow Meter from Fluid Components International (FCI) because of its accurate performance over a wide flow range. The ST98 flow meter includes a two-way HART protocol interface for direct communication with the plants distributed control system. This allows plant engineers to receive multiple process variables and configure the flow meter remotely from the control room.
The highly stable constant power flow sensor design allows the meter to be used in applications where upset flow conditions such as sudden changes in flow, temperature, or moisture are present. Its highly reliable thermal mass sensing element delivers precision mass flow rate, totalised flow and temperature measurement to the city’s engineers. It is ideal for air/gas flow measurement in wastewater treatment applications and offers high accuracy to ±1% of reading, 0.5% of full scale. Repeatability is ±0.5% of reading.
In wastewater treatment facilities, the cost of air flow can easily be one of the facility’s largest energy expenses. The cost of energy to produce compressed air continues to rise along with fuel costs. Optimising the aeration process by measuring and controlling the aeration system’s air flow with an accurate, reliable flow meter reduces energy costs and overall plant operational costs. In addition, the flow conditioner can reduce piping costs and the low pressure drop characteristics minimize compressor/pump power requirements.
Fitting out wastewater treatment aeration systems, such as this city’s plant, with a mass flow meter will result in improved process effectiveness and reduced energy consumption. Looking carefully at measuring accuracy and range needs, installation conditions and maintenance requirements will assist in selecting the most cost effective flow metering solution.
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