MBR advantages are clear but may not be cost-efficient in all cases
In wastewater treatment, the membrane bioreactor (MBR) is a hybrid of biological and membrane treatment processes that has become increasingly popular due to its superior effluent quality and small footprint. While the advantages are unmistakable, MBR may not be the best choice in all cases. CAPEX and OPEX can be higher due to membrane costs, membrane maintenance, and higher energy consumption. Fortunately, costs have come down, and new infrastructure delivery modes have stepped in to make MBR financially viable for a greatly expanded market.
MBR’s costs, relative to those of traditional activated sludge processes, once limited its adoption to smaller projects, but that dynamic has changed due to dropping membrane costs and MBR’s lower energy requirements. These, together with a global trend toward tighter environmental regulation, are increasingly favoring MBR. Today, many municipal MBR plants with peak daily flows above 25 million GPD (100 million L/d) dot the map. When exactly do the advantages of MBR tip the scales?
MBR’s High Effluent Quality
Where superior quality is required, MBRs produce an effluent of exceptionally high clarity. Compared to conventional activated sludge plants, MBR’s effluent features significantly reduced pathogen concentrations, lower total suspended solids (TSS), lower biochemical oxygen demand (BOD), and lower phosphorus. It can be safely discharged into sensitive ecosystems. And, the process generates less sludge.
As an integral part of a water reclamation program, MBR effluent can be used for urban or agricultural irrigation, toilet flushing, or many other nonpotable applications. The effluent produced by MBR is of such high quality that it complies with the most exacting regulatory frameworks, and some direct potable reuse systems include it as a stage of treatment, replacing processes such as microfiltration and ultrafiltration.
MBRs have been successful in both industrial and municipal applications, especially for streams with organic loads that are readily biodegradable, which explains why the food and beverage industry has extensively adopted MBR. Due to MBR’s long solids retention time, which boosts the effectiveness of biological treatment, it has also been successfully applied to pharmaceutical wastewater, leachate from landfills, and other wastewater streams that are difficult to biodegrade.
Many regulatory bodies have a positive stance on MBR projects. Seven Seas’ Texas subsidiary AUC Group, for instance, has noted that the Texas Commission on Environmental Quality tends to fast-track MBR project permitting, completing it in a quarter of the time it takes for conventional projects to make it through the process.
Space-Saving Wastewater Treatment
MBR systems are more compact than traditional wastewater treatment processes because they replace the bulky clarifier with a membrane module. The advanced membranes reject suspended solids, doing the job of a clarifier but in much less space. A long solids retention time also means more concentrated biomass, so solids can be contained in less space.
MBR’s smaller footprint can be an advantage where an upgrade is needed but a building cannot be enlarged, or where space comes at a premium. Additionally, MBR systems not only take up less room, but they generate less noise and odor than traditional processes.
Where MBR May Not Be Advantageous
The primary disadvantage of MBR reactors is that they typically cost more than traditional systems, both in terms of initial capital outlay and long-term operation and maintenance costs. In jurisdictions where environmental quality standards can be met adequately by conventional processes or where there are no space constraints, MBR may be a needless expense.
The microscopic pores of MBR membranes also tend to clog due to membrane fouling. Wastewater streams containing free oil, whether vegetable or mineral, can be a challenge for MBR, and pretreatment with a separate plate separation or dissolved air flotation stage may be needed to protect the membranes from fouling. Adding extra stages may, however, increase cost and negate the advantage of MBR’s small footprint. Replacement membranes can be rather costly.
MBRs also have relatively high energy requirements due to several factors. Energy-intensive air compression is required for the fine-bubble diffusion that supplies dissolved oxygen to the heterotrophic bacteria. Energy is also required to pump sludge within the system, and the permeate (treated water) also requires pumping. Heating and pumping of backflush and cleaning agents add to energy costs, as do process control equipment and instrument operation.
MBR is a complex and technically demanding process, and the economic and operational viability of MBR for any given scenario should be weighed in consultation with experienced wastewater treatment experts.
Overcoming MBR Disadvantages
While CAPEX/OPEX may be higher for MBR, its smaller footprint and superior effluent often offset the expense by reducing the need for construction and extra treatment stages. Moreover, MBR membrane fouling can be managed with regular maintenance and cleaning.
For many entities, it may be daunting to consider the increased capital outlay and a significant commitment to long-term maintenance for a technically demanding process that requires specialized personnel. However, new modes for delivering MBR have evolved to address such vital concerns.
Seven Seas Water Group is the original and the best at Water-as-a-Service® (WaaS®) agreements, which remove the CAPEX barrier with public-private partnerships, and build-own-operate (BOO) and build-own-operate-transfer (BOOT) agreements. Lease plant agreements are also available. Seven Seas also purchases and upgrades existing plants to MBR and operates them. WaaS® clients only need to pay for wastewater services — Seven Seas takes care of the rest, including all operating costs and responsibility for safety, environmental, and regulatory compliance.
Contact Seven Seas to learn more about MBR and how our services ensure low, stable, and predictable life-cycle costs for wastewater treatment plants.
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