By Dillon Devitt
With the growing demand for smaller, more efficient package wastewater treatment systems, membrane bioreactors (MBRs) are increasingly being applied in the larger water industry footprint. Applying MBRs in lieu of conventional activated sludge systems delivers higher effluent quality while eliminating secondary clarification processes. These systems incorporate flat-sheet or hollow-fiber membranes with pore sizes measured in micrometers or nanometers, which allow for complete retention of solids inside the treatment system. Effluent total suspended solids (TSS) concentrations below conventional detection limits are often achieved, depending on influent values and system design parameters.
This high filtration level allows MBR systems to push the limits of conventional activated sludge treatment and provide multiple design benefits for project applications, including higher organic loading rates, typically smaller footprints, decreased sludge production, and overall better biotreatment.
MBRs are not necessarily a solution for all applications, and they can have higher capital cost than conventional treatment processes. However, with expert engineering, an MBR system can be designed to perform at optimum efficiency in terms of both biological treatment and energy consumption. There are some design considerations to bear in mind that can often be overlooked during the design and implementation of an MBR system for a particular project. These areas include diffusion and managing the relationship between treatment facility footprint, loading rates and energy efficiency.
Fine Bubble vs. Coarse Bubble Diffusion
One of the primary design considerations for suspended growth activated sludge treatment is the aeration system. The aeration system must achieve four objectives: provide adequate mixing to keep all solids in suspension; supply enough oxygen to meet both the organic and inorganic oxygen demand of the influent wastewater; satisfy the oxygen demand for endogenous respiration of the biomass; and maintain an aerobic environment with a minimum dissolved oxygen concentration throughout the aeration tank. In many situations, the design comes down to one important question: fine bubble diffusers or coarse bubble diffusers?
The difference between the two can be explained by looking at one value: standard oxygen transfer efficiency (SOTE). This is the value that expresses a diffuser’s ability to transfer the oxygen from air into the water. Fine bubble diffusion generally maintains a higher SOTE than coarse bubble diffusion, which means it requires less total volume of air while in operation; however, the drawback is that fine bubble diffusers are generally more expensive. When selecting an aeration system during the project design, the SOTE, along with other parameters, should be evaluated to find the most efficient and economical application for the treatment system as a whole.
These methods to estimate aeration requirements have been widely accepted for the design of conventional activated sludge systems. When they are applied to MBR systems, however, another value called Alpha can become just as important. Alpha is defined as the ratio of the oxygen transfer rate in mixed liquor to that in clean water. This value is considered as a correction factor and varies with the type of aeration device, aeration intensity, wastewater characteristics, and mixed liquor concentrations, along with other parameters.
MBR systems with mixed liquor concentrations three to four times higher than other activated sludge systems can operate with Alpha values much lower than conventional systems, and the degree at which the Alpha value is decreased can sometimes be overlooked. The general understanding that fine bubble diffusers are more efficient at transferring oxygen in water may not hold true in a system with high suspended solids concentrations. To illustrate this relationship, Figure 1 demonstrates the effect of mixed liquor suspended solids (MLSS) on Alpha and Figure 2 shows the relationship of aeration requirements for both fine bubble and coarse bubble with increasing mixed liquor concentrations.
As one can see in Figure 2, the airflow requirement for coarse bubble is lower than fine bubble with increased MLSS concentrations. This gives coarse bubble diffusers the advantage in both oxygen transfer and energy efficiency for MBR systems. Although there are many other factors that need to be considered, with a little experience and proper engineering consideration, an MBR can be designed for optimum efficiency, making it an even more attractive alternative in the packaged wastewater treatment market.
Finding a Balance
Packaged MBR systems maintain multiple advantages over packaged conventional activated systems. Membrane filtration allows a system to build up MLSS concentrations more effectively than with secondary clarification and, in many applications, operate at MLSS concentrations three to four times higher or above 10,000 mg/L. Higher MLSS concentrations allow for smaller tanks, longer solids retention times, and in most cases more stable and treated water. Although a higher concentration seems to provide only benefits, at what point does it not?
As any engineer will tell you, designing is the art of considering the pros and cons of every aspect of a system and finding the best possible solution. When considering the application of an MBR system, there are a few key parameters that need to be considered to find that solution. These parameters include footprint, organic loading rate, and energy efficiency.
Footprint refers to the physical size of the system tank and ancillary equipment. Because MBR aeration tanks nominally operate at higher organic loading rates, namely MLSS concentrations three to four times higher than conventional activated sludge aeration tanks, the MBR tank is generally three to four times smaller.
Taking it a step further, could an MBR be designed to handle loading rates six to eight times more? The short is answer is no. Aside from the mechanical constraints, some physiochemical changes occur at high MLSS concentrations that can be undesirable. As explained earlier, higher MLSS concentrations will decrease the Alpha factor for the aeration system requiring more total airflow to obtain the same oxygen supply. Secondly, higher MLSS concentrations increase water viscosity. High-viscosity water is difficult to filter through membranes and will require more total membrane surface area to filter the same amount of water.
Forcing an MBR to treat the maximum possible organic load for the sake of maximum efficiency and lowering the system footprint ultimately defeats the purpose. The organic loading rates to a system are typically measured as pounds of biological oxygen demand (BOD) per day per volume. So respectively with increased MLSS concentrations and smaller tank volumes, a system will have higher organic loading rates. MBR systems that try to operate with higher MLSS concentrations using high organic loading rates (of 80 lbs/d-kft3) and above, will be forced to operate at shorter-than-desired solids retention times. When an MBR is operating with low solids retention it can easily be upset by fluctuating flows, temperatures, or organic loads and may have issues with nitrogen removal.
What makes a system energy efficient? There are many pieces of equipment within an MBR that require energy but the largest energy consumer is the blower. The higher the oxygen requirement, the higher the energy requirement, and as discussed previously, as MLSS increases so does the amount of air required to provide the same amount of oxygen. This concept can result in a double negative when also decreasing the footprint of a system. The increased aeration required and decreased tank volume in which to inject the air creates a large standard cubic feet per minute (SCFM) per unit volume or energy density. High energy densities will create highly turbulent environments inside an aeration zone, causing small biological particle sizes that can decrease water’s filterability through membranes. So its generally safe to say that systems that operate at lower MLSS concentrations are more energy efficient.
So how do we make a system as effective as possible? The answer to this will vary, and each system will have its own definition of effective. Some systems will try to fit within the smallest possible footprint by operating at MLSS concentrations 16,000 mg/L and above, sacrificing energy efficiency. Some would rather operate at high organic loading rates, above 80 lbs/d-kft3, with the same high MLSS concentrations, sacrificing reliability. Smith & Loveless design prefers to find the balance between the MBR footprint and organic loading rates to find the most energy-efficient system. We find that an MBR operating with an MLSS concentration ranging from 10,000 to 15,000 mg/L provides a good balance of energy efficiency and biological stability while generating reliable, high-quality effluent.
About the Author: Dillon Devitt is the supervisor of the process engineering group at Smith & Loveless Inc. He received both his bachelor’s and master’s degrees in civil engineering from South Dakota State University.
