Using VFDs to Maximize Digester Performance

Reducing industrial equipment’s power consumption and increasing energy efficiency continues to be an important energy policy goal globally, with power consumption representing the highest single operating cost for most facilities. In the wastewater industry, targeting the energy efficiency of treatment processes has been an integral part of the effort to create net-zero energy wastewater treatment facilities. For hydraulic mixing systems, the use of a variable frequency drive (VFD) can help maximize energy efficiency, without sacrificing system performance. To demonstrate this, take a look at the benefit of using a VFD to optimize energy consumption of a typical hydraulic mixing system for an anaerobic digester.

Anaerobic Digesters & Cogeneration Anaerobic digesters are present in most larger wastewater facilities. While they are not the most energy intensive treatment process, the biogas they produce can be effectively used to offset power requirements in other parts of the facility. This is achieved by either using biogas to feed boilers or through cogeneration. In some more advanced facilities, the energy produced by cogeneration is substantial enough to offset the entire power usage of the facility. In order to maximize the energy output from an anaerobic digester, it is important to produce as much biogas as possible while reducing energy needed to sustain the digestion process. For anaerobic digestion, the most common design is a continuously stirred tank reactor (CSTR), which relies on continuous mixing to achieve a homogeneous process volume, providing consistent temperature and pH, dilution of inhibitory compounds, and improved biological contact. Mixing also prevents settlement and deposition of suspended solids, which can reduce digester volume and lead to expensive clean-out costs. Hydraulic mixing systems are commonly used to provide mixing for CSTR anaerobic digesters. They consist of multiple nozzle assemblies within a tank, with flow often driven by a chopper pump. This produces an efficient, even distribution of mixing energy throughout the entire volume, while keeping moving parts out of the digester where maintenance can be more easily performed. For this example, consider a hydraulic mixing system, which centers on a chopper pump, and is designed to mix a typical 1 million gallon (MG) digester. For most hydraulic mixing systems, as flow to the nozzles is varied, the amount of energy transfer to the fluid also varies. Therefore, through manipulation of the operating curve of the chopper pump with a VFD, an operator can control the amount of mixing energy in their process. Image 3 shows a typical relationship between mixing energy and digester performance. As mixing energy increases, digester performance also increases as temperature and pH become more consistent and inhibitory compounds are diluted. Eventually, the digester reaches a point where additional energy no longer improves performance and inefficiency occurs. At even higher mixing energies, overall performance may begin to decline due to mechanical foaming and sludge bulking. In most digesters, a mixing energy between 0.15 and 0.35 horsepower (hp)/103 cubic feet (CuFt) will provide the most efficient volatile solids reduction per unit mixing energy. Mixing Energy Required In general, the mixing energy required to prevent deposition is significantly higher than the energy necessary to provide a homogeneous process volume. However, it is rarely necessary to continuously maintain the energy required to prevent deposition. In the example of this hydraulic mixing system, some applications may require as little as two hours per day at full mixing energy to allow for resuspension of settled solids. The remaining time can be spent operating at a lower mixing energy as required to maintain stable digestion. By varying the mixing energy in this way, a significant savings can be achieved. Typical design parameters for a 1 MG digester mixing system are provided in Image 4. At full-speed operation of 60 hertz (Hz), the mixing system is operating at 46.8 hp. This will serve as the baseline energy consumption. If instead a VFD is used to operate at 45 Hz, the mixing system will now be operating at 19.8 hp. Assuming a daily operating schedule of two hours at 60 Hz and the remaining at 45 Hz, this brings the average operating power to 22.1 hp—a reduction of almost 53 percent. Assuming a power cost of $.08/kilowatt (kW), this produces an annual operational savings of $12,982 without reducing system performance. However, in some cases, more mixing energy may be required to maintain digester performance, which will reduce the possible savings. It is ultimately up to the operator to determine the ideal operating schedule for the facility and process. In addition to anaerobic digesters, VFDs can be used to increase efficiency in any process hydraulic mixing system that has variable mixing energy requirements. This could be batch tanks with variable fluids or equalization tanks with variable level. As VFDs become more common and cost effective, a growing number of applications may benefit from VFDs to improve the control and efficiency of process mixing. Benefits of Variable Speeds For wastewater treatment plants concerned with the bottom line, the use of a VFD means they do not have to sacrifice mixing power to save on operating costs. Instead, by incorporating a VFD, energy costs can be reduced by 50 percent or more, while providing a complete mix to the digester and other tanks. In this case, variable mixing can lead to consistent savings.