Membrane Bioreactor Sets Stage for Future Adaptability

Construction of the Membrane Bioreactor (MBR) treatment technology is underway at the Water Resource Recovery Facility (WRRF) this month. This forward-thinking capital investment follows years of planning, holistic systems analysis, and a detailed alternatives analysis. Ultimately, MBR beat out the WRRF’s Conventional Activated Sludge (CAS) process based on several feasibility criteria, including setting the stage for potable reuse in the future. The construction is part of the comprehensive SLO Water Plus upgrade that will continue through 2023.

CAS is a biological process used to reduce the organic matter and nutrients present in wastewater. The WRRF’s existing CAS equipment has been operational since 1995. During the SLO Water Plus design phase, the project team performed a comparative evaluation to determine which was better—upgrade the CAS system to enhance its treatment capacity, or replace it with MBR. The preferred system would need to meet the City’s effluent criteria and be able to treat higher volumes of wastewater flows during extreme weather events.

The City evaluated both systems using the following criteria:

Cost and Energy Demand

First, the City evaluated infrastructure capital costs, operations and maintenance costs, as well as the cost of energy needed to run the equipment.

Upgrading the CAS equipment would have required additional infrastructure: the existing two bioreactors and secondary clarifiers, plus two additional clarifiers and five additional bioreactors. This additional equipment was needed to biologically treat, then physically settle, the solids from the process. By comparison, MBR only needs two additional bioreactors and no secondary clarifiers—a significantly reduced footprint for the same volume and a higher level of wastewater treatment. MBR uses a membrane to remove solids.

Though MBR requires less structures, it has more mechanical equipment making the cost comparable to a CAS upgrade. Both systems have roughly the same energy demand, as well as similar operations and maintenance costs.

System Footprint

The SLO Water Plus upgrade design was approached holistically, with consideration given to how the team could optimize the WRRF’s existing footprint. Smaller footprints offer several opportunities, from reduced operational costs to space for future expansion as the City’s needs change. MBR offers a much smaller footprint compared to CAS because it uses less infrastructure. In choosing MBR, the City gained the flexibility to support indirect or direct potable reuse expansion in the future.

System Reliability

Both the CAS and MBR systems utilize a biological process to reduce the organic matter and convert nutrients. System optimization depends on maintaining the best biological conditions day after day, which can be challenging as the characteristics and flow of the plant influent varies. Consumer trends, population changes, and major events like COVID-19 or drought can affect the characteristics of the plant influent variations significantly. MBR’s advantage is that it uses a physical barrier after biological treatment to prevent particles from passing through to downstream treatment processes. This results in a more consistent and higher quality effluent.

Water Quality

Water quality and environmental regulations are constantly evolving. MBR allows the City to produce a much higher quality effluent that just wasn’t feasible with CAS and other technology. The pore size of the MBR membrane fibers is so small that it can even filter out bacteria. In fact, MBR is so effective it will eliminate downstream filtration and reduce the size of the downstream disinfection process while fulfilling more water quality requirements than CAS is capable.

“By putting MBR up front, we hope to reduce the cost of the disinfection stage of treatment later because MBR produces a much higher quality of water more consistently,” said Chris Lehman, WRRF Supervisor.

The higher quality effluent is good news for local ecosystems too. Read how MBR is resulting in better Creek health

Adaptability—Setting the Stage for an Expansion of Water Reuse

Climate change may result in unpredictable weather patterns including longer and more frequent droughts. The City has secured three local surface water sources to provide added reliability if one water supply is unavailable, such as during a drought. Accordingly, the City is exploring opportunities to better use the water it already has, such as with the expansion of water reuse. Currently, recycled water is used only for irrigation. In its technical memorandum, the City described the SLO Water Plus investment in MBR as “the most complimentary to future potable reuse goals.” Because of its water treatment capabilities, MBR will reduce the scope of future recycled water infrastructure, as fewer chemicals will be necessary further down the treatment train.

© City of San Luis Obispo 2019