Study Examines Opportunities for >MBR Energy Optimization
In order to address growing demand and significantly reduced effluent permit limits, the Village of Dundee, MI, upgraded its wastewater treatment plant (WWTP) from sequencing batch reactor (SBR) technology to membrane bioreactor (MBR) technology in June 2005.
By Mark Stone and Dennis Livingston
In order to address growing demand and significantly reduced effluent permit limits, the Village of Dundee, MI, upgraded its wastewater treatment plant (WWTP) from sequencing batch reactor (SBR) technology to membrane bioreactor (MBR) technology in June 2005. Following the upgrade, effluent quality significantly improved and capacity increased to 1.5 mgd from 0.75 mgd. As expected, plant energy demand also increased, roughly doubling SBR requirements.
Although the plant has not been optimized for energy efficiency, the average demand of 1.44 kWh/m³ (MBR Zone demand < 0.3 kWh/m³) in 2006 was less than some comparable greenfield facilities. For evaluation purposes, monthly energy demand was trended for roughly two years of continuous operation. The trended energy data shows a clear relationship between energy efficiency and treated flow. For example, the plant (total including HVAC, etc.) average energy usage in January (2006, 2007) was 0.95 kWh/m³ at an average flow of 1.29 mgd. However, during periods of low flow (< 0.6 mgd), energy requirements have been as high as 2.69 kWh/m³. The primary reason for the disparity in efficiency appears to be system turndown capabilities and control limitations.
Although the plant has performed well (no violations) for over two years at a constant air scour intensity of 0.015 SCFM/ft²the setpoint was recently increased to compensate for a low recycle to flow ratio at peak demand. Proportional aeration is now being considered for optimizing MBR Zone energy requirements between 0.3 kWh/m³ and 0.52 kWh/m³ while the same time addressing known hydraulic constraints.
As the system has been optimized for energy efficiency, power costs have nearly doubled since the upgrade. This paper evaluates long-term MBR System performance and proposes methods for improving plant energy efficiency by ~ 40%.
MBR System Overview
Influent goes through 1/8" screen and de-gritted influent flows into a single AX Zone (used for alkalinity recovery) before flowing by gravity into a PA Zone. The PA Zone is fitted with fine-bubble diffusers (FBD) that provide oxygen for biological treatment. From the PA Zone mixed liquor is conveyed to a common channel that feeds four MBR Zones.
For solids separation, each MBR Zone is equipped with 11 EK-400 submerged membrane units (SMU) and has space to add two more SMUs. Mixed liquor, thickened during filtration, flows into a common return channel where it is recycled (pumped) back to the head of the plant. Filtered effluent (permeate) is pumped into a baffled contact tank where alum is added for phosphorous removal.
The total volume of wastewater treated in 2006 was 19% more than the 2001 total. More importantly from a filtration standpoint, flows in January (highest monthly average flow since 2001) have more than doubled over the last six years.
During wet weather months such as January, the plant routinely treats flows equivalent to 3.0 mgd for several hours a day in order to accommodate inflow and infiltration (I&I) in addition to sewage. However, even during drier months such as May, rain events can cause membrane flux to bounce between low and high setpoints for hours as I&I surcharges the plant (see Figure 1).
Plant Energy Costs
Plant energy requirements can be roughly divided into two categories; the MBR System and the Digestion System. All equipment not associated with the Digestion System is included as part of the MBR System, including the lift (transfer pumps). A usage power matrix is provided in Table 1.
Using the power data in Table 1, a power distribution chart (Figure 2) can be created to identify areas where energy efficiencies might be gained. Based on plant data, the MBR blower motors were routinely run at roughly half speed (900 rpm) until April 2007. At 900 rpm the MBR blowers provide ∴570 SCFM to each reactor and draw on the order of 30 bhp. In addition, at ADF conditions, filtration (membrane) capacity fluctuates between high flow and idle or Intermittent Mode to accommodate typical diurnal flow variations. Treating more flow in shorter periods of time reduces the uptime or utilization of the MBR blowers.
In January 2006, at an average flow of 1.07 mgd, the actual total energy requirement was 1.23 kWh/m³ or 32% higher than projected value (0.84 kWh/m³). However, in January 2007 at a flowrate of 1.5 mgd, actual energy usage was 0.66 kWh/m³: roughly 21% lower than the estimate. Taking an average of the two months suggests the proposed energy model is representative of actual operating conditions with an actual demand of 0.95 kWh/m³.
Using Figure 2 as a guide, there are three main subsystems (components) that present energy optimization opportunities: the MBR blowers, the PA blower and the Digester blower. Before considering how to reduce energy demand, a review of current operating strategies is presented below.
MBR Blower Operation
Until April 2007, the membrane air scour intensity was roughly 0.015 SCFM/ft²equating to an air flow rate of 570 SCFM/MBR (or a total flow 2,273 SCFM). This is the amount of air introduced into the process via coarse-bubble diffusers at the bottom of the SMU. At low flow (low demand) the processor automatically shuts down one or both trains and puts them in what is called an Intermittent Mode. In Intermittent Mode, the MBR blowers pulse air into the reactors at the selected air scour rate.
Several years ago when the system was designed it was believed that the upper and lower cassettes should perform differently due to more effective air scouring closer to the water surface. However, research and field data now suggest that any discrepancy is small and that the approach may increase overall energy cost by reducing system rangeability or turndown. For example, at Dundee one train (two Permeate Pumps) acts as a single unit. In order to better match low summer flows and avoid over aerating (lost energy), additional piping and controls could be added to allow each MBR to act independently.
At a constant air scour intensity (0.015 SCFM/ft²) fluxes up to PHF values (23.1 gfd) have been sustainable for over two years. However, as IR at PDF is relatively low, the risk of localized dewatering can increase in the presence of sustained high fluxes and given the possibility of unequal distribution.
A strategy to adjust air scour intensity as a function of calculated flux is currently being implemented by the System Supplier. Referred to as Proportional Aeration, the plant computer will automatically adjust air scour intensity using a mathematical correlation and based on three operator inputs defining Low, Medium and High setpoints. For example, in Figure 3 air scour intensity (color coded) is varied according to influent flowrate shown as Q (MMF). Assuming this type of diurnal profile, energy requirements can be reduced on the order of 20% or more using proportional aeration.
Automate PA Blower Control
Implementing DO monitoring and control could significantly reduce PA blower demand. Presently the ORP control system is not functioning and blower speed is set manually. Assuming design loading conditions, the total oxygen demand can be as low as 1,500 lb/day. Roughly 70% of the demand is from endogenous decay given the SRT. Even at a reduced air scour intensity of 0.015 SCFM/ft²the MBRs can contribute upwards of 2,000 lb O2/day. Theoretically, the PA Zone is not even necessary to meet design limits for most of the year. In other words, if a wastewater could bypass the PA Zone altogether, or if the PA Zone volume were significantly reduced, sufficient nitrification would occur to meet the stated ammonia limit.
Digester Blower Optimization
The Digester blower is currently operated based on a manually set timer. Typical settings are 6hr ON/ 4 hr OFF or 8 hr ON and 2 hr OFF. However, in some situations the blower is left on 100% of the time to ensure proper stabilization and to avoid septic conditions in preparation for hauling. Similar to the PA blower, implementing real-time feedback control based on one or more online measurements (e.g. DO) should improve efficiency of the Digestion blower.
MBR System (Plant) Layout Modifications
The system is not equipped with offline equalization and has virtually no surge capacity. In fact, under typical operating conditions, the average side water depth in the PA Zone is less than 12 in. from the top of the tank wall and fluctuates over a 3 in. band.
The lack of surge capacity and limited turndown capabilities of the system limit the ability of the system to efficiently handle seasonal and diurnal flow variations. The effect of limited turndown is clearly shown in Figure 4 where energy usage drops below 1.0 kWh/m³ at flows near ADF but increased significantly as flows decrease. For example, at 0.53 mgd the monthly energy consumption was 2.69 kWh/m³.
By compartmentalizing the existing AX/PA Zone into two process trains and a dedicated Equalization Zone, the system could be programmed to exactly match incoming load under most conditions.
Using Proportional Aeration in conjunction with better flow pacing, energy usage could approach the calculated demand of < 0.84 kWh/m³ without impacting performance. Based on historical costs, such an energy savings could translate into >$50,000/yr.
Overall the Dundee WWTP upgrade has met and far exceeded design objectives. Water quality has significantly improved and capacity doubled without increasing the plant footprint and for a total installed cost of $6.55M. However, now that the plant has been successfully operating for more than two years, there is room for optimizing performance and reducing operating costs. In particular, energy consumption could be significantly reduced with minor modifications to the plant and control strategy. A summary of overall findings and recommendations is provided below.
- Plant performance in terms of effluent quality has exceeded design objectives. Plant effluent has not violated any permit limits since upgrading to MBR technology.
- Energy usage data suggests that optimization efforts could reduce the combined MBR and Digestion System demand by ∴30–40% to an estimated value of 0.84 kWh/m³.
- The three most significant ways of increasing plant flexibility would be to implement Proportional Aeration, automate Digester Blower/Preaeration operation and compartmentalize the process tankage into inline equalization. –
- Alford, Andeer and Jackson (2006), Dundee Wastewater Treatment Plant: A Brief Examination, CEE 592, W06
- Schuler, et al. (2007), Small Treatment Plants, MBR Applications, WEF Webcast
About the Authors:
- Mark Stone received his BS in Chemical Engineering from Lamar University in 1987. In 2005, Stone took over the Enviroquip pilot program and has done several municipal/industrial studies. He regularly participates in WEF sponsored workshops and seminars covering MBR technology. He is now the R&D Manager for the MBR Systems Division of EWT in North America.
- Dennis Livingston is Operations Director for the MBR Systems Division of EWT in North America. He was a contributing author of the WEF Manual on Membrane Systems for Wastewater Treatment and The MBR Book. He is also the principal author of numerous technical and trade articles on MBR technology and regularly participates in WEF sponsored workshops and seminars covering MBR technology.