As membrane bioreactors (MBRs) have continued to mature as an established best-available technology for treating municipal and industrial wastewater, focus on improving energy consumption has yielded dramatic improvements in the energy efficiency of MBR systems.
Using field experience regarding the interdependencies between the biological process and plant hydraulics (biohydraulics), the water industry is continuing to better understand how specific configurations, automation strategies, subsystems and components impact overall energy consumption. In turn, Enviroquip has been able to develop system configurations that optimize design-point efficiency and provide significant system turndown by incrementally responding to demand with permeate and air scour capacity. As a result, MBR systems designed today have a best efficiency point between 0.40 and 0.50 kWh/cu meters, with process turndown capabilities exceeding 8:1. In addition to improving overall process efficiency, specific plant configurations can turn “idle” capacity into opportunities to reduce sludge handling costs, further improving operating costs.
The reason for focusing on energy efficiency becomes obvious when the impact of efficiency on operating costs is examined. For every kilowatt-hour per cubic meter per million gallons per day of flow that a system’s efficiency can be improved, approximately $100,000 of operating costs can be saved.
Potential Savings
While configurations for improving system efficiencies are being incorporated into new system designs as strategies continue to be developed, the question has become how to use this evolving knowledge to improve existing systems. With more than 100 operating MBR systems designed over the last 10 years, the ability for existing systems to run efficiently varies widely. This is due to one or more of the following three reasons:
1. Timing. Because the strategies employed today have been developed over time, many existing systems did not incorporate these strategies, as the features did not exist when the systems were designed.
2. Design vs. reality. Systems are typically designed with a specific hydraulic and/or biological loading basis. If actual loadings are substantially lower than planned and insufficient flexibility was designed into the system, the ability for those systems to run efficiently at lower duty points will be limited.
3. Priorities. Some systems were simply designed with a focus on cost (or value-engineered) as opposed to designing for flexibility or efficiency.
To address the gaps in efficiencies between older and current treatment systems, Enviroquip recently kicked off a program called EQIDEAS (Intelligent Decisions for Environmentally Advanced Systems). Through EQIDEAS, the company is applying its accumulated knowledge of energy-efficient designs to existing systems by conducting an audit of a facility’s maintenance costs, labor requirements, chemical consumption and normalized energy usage. By auditing these systems and comparing them to current design practices, opportunities for improving the energy efficiency of older systems are being identified and expected energy savings quantified.
The heart of these energy audits is the EQProSim simulation tool that was developed by Enviroquip to support the design of new systems. For new system designs, EQProSim is able to determine optimum equalization requirements and predict energy utilization for any number of custom diurnal flow conditions. For an existing system, EQProSim utilizes information on installed equipment and, along with actual diurnal/seasonal influent flow information, compares the system performance with the performance of a new system that would be designed today for the same conditions. At the same time, the impact of actual versus expected diurnal flows can be quantified, providing a basis for improvement strategies.
Audit Process
These energy audits begin with the collection of historical data—ideally, two years worth of flow data, energy consumption and operational data, including MBR mixed liquor suspended solids provided by the operating plant. The flow data, along with the plant configuration information (number of trains, number of MBR basins, equalization volumes and installed permeate capacity) is entered into the EQProSim simulation tool.
The simulation run then provides minute-by-minute information on all the equipment in the treatment system, including run status and operational duty points. Using this information, a list of baseline data can be established: expected energy efficiency at design flows, expected energy efficiency at actual flows and discrepancy between expected and actual energy consumption. From this baseline analysis, it can be determined quickly whether the system energy consumption is consistent with predicted consumption (even though it may be high). At that point, the audit can turn toward analyzing the possible benefits of going back in and applying current energy strategies and design configurations. These strategies can include:
- Increasing equalization. MBRs run most efficiently at higher flow rates. The amount of energy that they use for air scour and permeate production per gallon of water produced is highest at low fluxes and lowest at high fluxes. If a plant is significantly hydraulically under-loaded, increasing equalization can allow collection of influent volumes for treatment at higher fluxes instead of running at low fluxes constantly.
- Increasing turndown. New Enviroquip systems are designed with Energy Pro and proportional aeration features. These configurations provide for incremental response of both permeate capacity and air scour flow based on influent flows. The benefit to a system with widely varying influent flow rates is the ability to utilize only as much energy as is necessary for permeate production.
- Using underutilized trains as thickeners. New systems in which the influent flows are expected to have large seasonal variations or have large peaking factors due to storm-related infiltration are designed to use underutilized/unutilized capacity as a thickener. Therefore, an N+1 train’s mixers and fine-bubble aeration power can be utilized for reducing solids-handling costs instead of maintaining unneeded biological volume and permeate capacity.
- Improving design-flow efficiency. Forty percent of Enviroquip’s existing plants operate with gravity filtration through the MBR membranes. Though gravity systems offer operational simplicity relative to systems that use pumps, normal gravity systems use a higher MBR basin side water depth to assure adequate driving head. This additional side water depth increases the power associated with air scour. New Enviroquip systems often are designed with a pump-assisted gravity (PAG) configuration. With this configuration, the MBR basin can operate as gravity systems at lower side water depths because pumps are available to resolve issues that can impact flow in gravity systems, including high TMP and air entrainment. PAG systems can reduce the energy associated with air scour and permeate production up to 25% when compared with a gravity system. So the opportunity exists for some gravity plants to change to PAG configurations to reduce the MBR-related energy consumption.
- Installing new technology. New MBR systems benefit from being able to incorporate the latest in available equipment technology. An example of this is utilization of high-efficiency turboblowers in place of positive displacement blowers. Enviroquip has begun integrating KTurbo blowers into its designs, which typically save up to 15% in blower energy in new systems. When applied to an existing treatment system, the opportunity for reducing blower-related energy is even greater because the new turboblowers can be sized for their best efficiency point, where the existing blowers are sometimes legacy equipment in place prior to the MBR systems.
By utilizing the EQProSim tool, Enviroquip is able to run multiple flow scenarios, with and without each of these configurations, and project the resultant energy savings. This data provides the return-on-investment analyses for installation of new tanks, controls, valves, instrumentation, piping changes or blower upgrades.
