SAMCO: Can a membrane bioreactor be right for your municipal wastewater treatment facility?

Can a membrane bioreactor be right for your wastewater treatment facility? Membrane bioreactors (MBRs) are a type of secondary wastewater treatment technology that combines membrane filtration with biological treatment. Newer and generally more expensive than conventional biological treatment options, MBRs have traditionally been reserved for smaller-scale wastewater treatment applications. Since the late 1990s, however, MBR use has been on the rise at facilities of all sizes.

So how do MBRs compare to conventional wastewater treatment technologies? In this article, we’ll explain the major benefits and drawbacks of MBRs for municipal wastewater treatment applications and explore how MBR technology might fit your municipal wastewater treatment needs.

Benefits of MBRs

Much like conventional activated sludge, MBR can be deployed as a secondary treatment step, the goal of which is to degrade the organic content of the stream and separate out suspended solids. Conventional approaches call for multiple steps to settle the solid wastes out of solution, while MBR achieves solids separation more quickly and completely by using a hybrid approach that marries biological treatment with membrane filtration. As a result, the key advantages of MBRs are consistent, high-quality effluent streams and a compact size. Read on for more about these benefits.

High-quality effluent

MBRs are highly effective for the removal of common wastewater stream constituents like biochemical oxygen demand (BOD), bacteria, total suspended solids (TSS), and even nutrients such as nitrogen and phosphorous. That MBR produces high-quality effluent streams is one of its biggest assets, as it allows wastewater treatment plants to safely discharge to surface waterways, meet stringent discharge regulations, or even reuse treated wastewater for other applications.

The suitability of MBR technology for water reclamation applications is likely one of the biggest factors that have driven the growth of MBR use at wastewater treatment plants across the world—in short, as water recycling efforts have become more popular, so too have MBRs.

Small footprint

Another benefit is that MBR systems are very compact. MBRs require only about 25-50% of the space that a conventional wastewater treatment train would, as it combines activated sludge, clarification, and media filtration into one step. Part of the reason that MBRs are able to achieve this level of efficiency is that the bioreactor and filter media components are designed to maximize surface area. As a result, an MBR system is able to maintain a larger biomass population in a smaller area compared to conventional activated sludge systems, a design feature that both saves space and enhances biodegradation efficiency. As such, MBRs can be a good fit for municipalities where limited space is available.

Consistent effluent quality

MBRs and conventional activated sludge systems share in that they are both biological treatment methods that rely on living biomass to perform the work of degrading dangerous organic constituents in a stream. Conventional activated sludge systems rely on the formation of flocs for separation purposes. In short, as the biomass breaks down organic material, it forms biological solids that flocculate into larger clumps, or flocs, that can then be settled out and disposed of as solid waste. When biomass activity is compromised due to changes to the makeup of the wastewater stream, floc formation may not happen as intended, and the resulting effluent will be of lower quality.

By contrast, MBRs do not depend on the formation of flocs. Instead, MBRs use filtration membranes for separation purposes. For this reason, MBRs offer advantages such as greater stability in effluent quality and greater resistance to a system upset in comparison to conventional activated sludge systems. As such, MBRs can be a good fit for facilities whose wastewater streams are variable or poorly degradable.

Drawbacks of MBRs

While MBRs offer some great benefits, they’ve got a few downsides too. In short, the main disadvantages of MBRs are higher cost and greater operational complexity compared to conventional activated sludge systems, as detailed below.

Cost

Perhaps the biggest drawback of MBRs is their high capital costs and high operational costs relative to conventional wastewater treatment technologies. These costs can mostly be attributed to the cost of membranes, energy for pumping and aeration, and skilled labor for operations and system maintenance.

But the seemingly high cost of MBR technology is only part of the story. For some facilities, cost analyses have shown that investment in MBRs can actually be more cost-effective in the long term, as compared to conventional wastewater treatment. The reasons for this are many and can include reduced energy and water consumption, a smaller footprint, as well as cost advantages related to water recycling, such as the ability to sell or reuse treated effluents for irrigation, industrial process water, or other purposes.

US municipalities who are interested in MBRs—but wary of the cost—may also be able to find financial assistance through various grant and loan programs administered by their state and/or the US Environmental Protection Agency (USEPA). Many such programs have existed for decades, but the significant additional investment is likely to come with the expected passage of the Drinking Water and Wastewater Infrastructure Act of 2021. Several funding programs included in the proposed legislation aim to increase water reuse or otherwise improve efficiency, both of which align well with the benefits of MBR technologies. MBRs are also well-suited for small-scale installations, and the earmarking of a significant share of federal funding for small and underserved communities will potentially put MBRs in reach for communities that may have overlooked the technology in the past.

Operations and maintenance demands

One of the most significant expenses associated with an MBR is the membrane element. While all membranes will degrade over time and eventually require replacement, proper care and maintenance of the MBR system are essential as a means of preventing fouling—and maximizing cost-effectiveness for the MBR system as a whole by extending the life of the membrane element.

As such, one of the downsides of MBR units is that they require a moderately high level of effort for operations and maintenance. MBR units require a regular cleaning regimen, which may include both physical cleaning (e.g., air scouring, backwashing) and/or chemical cleaning (e.g., application of oxidants, bases, or acids to remove foulants from the membrane). The precise cleaning and maintenance schedule is specific to each installation, but will generally include routine weekly cleanings at minimum, as well as deeper cleaning cycles a few times per year. Facilities considering MBR technology should be sure to consider how costs associated with cleaning chemical use and disposal, system downtime, and skilled labor will affect the overall viability of MBR as a wastewater treatment solution.

Summary

It is important to note as regulations become tighter such as removal of micro nutrients, pharmaceuticals, mercury and PFOA / PFOS we will see a higher demand for membranes and other advanced treatment technologies.

As membranes advance, we will see both capx and opx lower over time.  A good example of this is the advancements of ceramic membranes, which are now at or better in life cycle cost to polymeric membranes.

To know more, please check SAMCO.