Energy Auditing, Part II
In the Spring 2010 issue of Membrane Technology, Enviroquip discussed its increased focus on improving energy consumption and energy efficiency in membrane bioreactors (MBRs). A number of strategies were identified for improving energy efficiency for new or existing systems. These strategies included utilizing equalization, increasing blower and pump turndown, using underutilized trains as thickeners, improving design-flow efficiency and installing new technologies.
In this article, Enviroquip reviews a preliminary energy audit of an operational MBR system. The intent of the preliminary audit is to:
- Compare actual versus design hydraulics;
- Compare expected versus actual energy consumption and energy efficiency;
- Analyze operating data for inconsistencies or trends; and
- Serve as a basis for proposing a path forward for improving the energy efficiency of the system.
For the real world example, Enviroquip called the plant “Etown WWTP.” The preliminary audit began with a collection of historical data. Ideally, two years’ worth of flow data, energy consumption and operational data, including MBR mixed liquor suspended solids, is supplied for the preliminary audits in order to provide enough data to establish trends and form a reliable basis for analysis. For Etown, a little less than two years’ worth of data was available, including energy bills, influent flow rates and suspended solids.
The first and most significant piece of data was the discrepancy between the design system capacity and the actual plant influent flow. The Etown system is only running at approximately 20% of its design capacity. So, the portion of the system that was sized to support the biological process at a given throughput (specifically mixers and mixed liquor recycle pumps) was oversized relative to the actual conditions. Thus, without examining any other data, the efficiency of that portion of the system, in kilowatt hours (kWh) per cu meter, was five times higher.
When the system’s energy bills were analyzed against their actual throughput, Etown’s plant efficiency equaled 8.34 kWh/cu meter. This compares with an expected efficiency at design influent flow volume of about 2.5 kWh/cu meter. A summary of Etown’s significant electrical load contributors is shown in Table 1, below.
Table 1: MBR Subsystems and Electrical Loads
Significant Electrical Load(s)
Influent lift station
Fine screens and CWC
Transfer pumps, mixer
Anoxic (AX) zone
Membrane bioreactor (MBR) Zone
MBR aeration system
PA aeration system
Internal recycle (IR) system
Feed forward pumps (no VFDs)
Membrane CIP system
Sludge transfer duties shared with FF pumps
Effluent transfer pumps
PC, exhaust fan, lights
Analysis, Conclusions and Recommendations
Enviroquip input Etown’s system configuration data into its EQProSim simulation tool. The EQProSim simulation tool was developed to support the design of new systems to determine optimum equalization requirements and predict energy utilization for any number of custom diurnal flow conditions. For an existing system like Etown, EQProSim utilizes information on installed equipment and, along with actual historical influent flow information, provides a comparison between the system performance and 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.
The Etown plant has been running for more than two years, and some specific adjustments were made to the EQProSim to account for the running pumps and blowers, including runtimes, meter-recorded air and liquid flow rates, motor efficiencies, VFD usage and wasting rates. Additionally, non-process-related equipment, located in the operations building, were included as well (it was determined that energy consumption for the operations building consisted of a PC; one “always-on” exhaust fan, 0.33hp; a motor control panel; and several fluorescent lights for the plant—relatively small energy users).
Using the actual influent diurnal information, influent totals and actual equipment set points and runtimes from Etown’s historical process trend data, the predicted efficiency for the Etown plant totaled 5.75 kWh/cu meters. This is more than 30% lower than its reported efficiency of 8.34 kWh/cu meters. A breakdown of Etown's expected power consumption, based on installed equipment, duty-point electrical draws and reported run times, is shown in Chart 1.
Chart 1: Etown Power Consumption Breakdown
The Etown plant design, like most similar sized systems, was not optimized for energy consumption and the flexibility of the system to run efficiently at low flows is limited. That being said, there are fairly low-cost, easily implemented options to reduce energy demand off design points once an energy profile is known and compared against known benchmarks.
While strategies like the use of equalization and proportional aeration can provide incremental improvements in a system's energy efficiency, the primary focus, and perhaps the biggest opportunity, for possible improvement of Etown's energy consumption is to understand the difference between expected and actual energy consumption at present conditions. Based on analysis of the energy and flow data trends and interviews with site personnel, the most likely causes of the discrepancy are failed energy usage meters or flowmeters/totalizers, and unaccounted energy drains in the system, including inefficient motors and unlisted equipment.
Although specific MBR strategies are not yet being recommended for Etown, the preliminary energy audit has yielded a significant discrepancy between what is expected versus what is reported. Just knowing that the Etown plant may be operating 30% worse than expected (before any advanced tune-ups) reveals a potential gap in savings that could, if recognized and resolved, potentially yield thousands of dollars per year. And the analysis provided by the preliminary audit will serve as a basis for further improvement activities.