Energy Efficiency of MBR
Submerged membrane bioreactors (MBRs) are often considered to be the best available technology for the treatment of wastewater. However, MBR systems are also often seen as more expensive to own and operate than competing conventional activated sludge (CAS) technologies. In particular, MBR energy consumption has been reported in the literature as being 10% to 100% greater than for comparable CAS plants,2 and in some cases the numbers are even higher.
The simple and oft-cited explanation for increased energy usage at MBR plants is membrane air scouring. However, upon closer inspection, plant energy efficiency in MBR systems is really a result of overall plant design, operation and, specifically, turndown.
Submerged membranes typically require coarse bubble aeration (air scouring) to remove foulants and sustain filtration capacity.1 If designed or run inefficiently, coarse-bubble aeration systems can significantly impact the overall turndown capabilities of a system and drive up energy bills; but again, this is only one piece of the puzzle. For example, it is common practice to set up MBR systems like CAS plants, pumping internal recycle streams back from MBRs to the head of the plant (e.g., the anoxic zone). While on paper this strategy appears to reduce pumping energy, the effect is to increase air scouring requirements (due to blower turndown), thereby reducing plant efficiency.
An alternate strategy of pumping forward allows for semi-batch treatment at low flows (generally off design duty point) and allows operators to better match treatment capacity to demand. In other words, if a trickle of flow is coming into a plant, why not store it until there is enough to treat instead of blasting blowers on for a few seconds? This is just one example of the how the overall approach to MBR system design and operation will ultimately determine a utility bill.
Drawing on the experience of more than 100 North American MBR facilities and thousands of international partners, the Energy ProTM system addresses turndown issues and improves the overall energy performance of MBR plants.
In simple terms, an Energy Pro system is an MBR plant designed to operate efficiently over a range of conditions instead of at a single duty point. The basic components of an Energy Pro system must be carefully selected to meet the needs of each new application and include the following: simultaneous nitrification and denitrification (SNdN) in one or more process zones; automation to incrementally match process capacity, air scouring and filtration rates to plant (pollutant) loading; air delivery systems, permeate control and recycle components with sufficient flexibility to meet system turndown requirements (>12:1); and turbo fan technology (optional).
Although the basic building blocks of each system are similar, it is the automation of the various plant subsystems that determines real efficiency. The operation of an Energy Pro system can be best described as “treatment on demand.” The control strategy used to calculate and match plant demand to treatment generally employs two levels of optimization: incremental adjustments in filtration capacity to match demand and proportional air scouring. In proportional air scouring, automated adjustments to air flow rates are made based on filtration rate. As filtration rate (flux) increases, air scouring intensity is also increased, and vice versa.
The design of an MBR system generally determines how efficient a plant will be at one or more specific duty points. However, the reality of running a wastewater treatment plant of any kind is that flows and loadings are constantly changing and may never stay at predefined or specified capacities. The ability of systems to efficiently operate over a wide range of conditions is the key to real energy efficiency. Take, for example, the Dundee and Delphos plants.
At the Dundee Enviroquip MBR plant in Michigan (not an Energy Pro sytem), a clear trend between capacity and energy efficiency highlights the need for turndown in MBR systems. As Figure 1 shows, at or near design capacity (ADF = 1.5 million gal per day), the MBR system is more efficient than the conventional plant it replaced by about 20%. However, at low flows, energy requirements go up significantly due to lack of turndown at this retrofit installation.
The total plant energy usage at the Delphos, Ohio, MBR system (equipped by Enviroquip) varies substantially depending on operator setpoints and site conditions. Figure 2 shows how normalized energy usage at the Energy Pro system was reduced from 5.38 kWh/m3 to 1.59 kWh/m3 in large part by optimizing control parameters and making use of the plants effective 30:1 turndown capabilities.
The actual energy efficiency of MBR systems is more a function of overall plant design and component selection than the need for membrane air scouring. Moreover, design concepts and components that work well in conventional activated sludge plants may not result in the most energy-efficient operation of an MBR plant. An integrated system approach is necessary to realize lower energy bills.