Magnesium Hydroxide for Improved Membrane Bioreactor Performance

Conventional and MBR Wastewater Systems

The conventional process for treating wastewater involves five main stages:

Screening and grit removal:

Separation of large objects, sand, and dirt from the incoming wastewater.

Primary clarification: 

A large volume basin with low flow velocity and long detention time, allows suspended particles to settle to the bottom to be removed as sludge.

Aeration:

Soluble and suspended contaminants remaining in the wastewater are consumed by microorganisms, which grow into microorganism colonies called “activated sludge.

Secondary clarification:

A large volume basin with low flow velocity and long detention time, allowing the activated sludge to settle to the bottom, to be pumped back to the front of the aeration process to re-initiate the consumption of wastewater contaminants; a fraction of this secondary sludge is regularly removed in order to maintain an optimized microbiological process.

Disinfection:

Overflow from the secondary clarifier is treated with disinfectant or ultraviolet light to reduce microorganisms in the final effluent below a regulated level.

This conventional wastewater treatment process has been shown for decades to consistently produce high-quality water for discharge back to the environment. However, there are situations where this treatment approach is not convenient or even feasible. For instance, primary and secondary clarification processes require a large footprint for tankage that might not be available within certain communities. There are numerous instances where communities have grown completely around a WRRF, making it impossible to add more clarifiers to accommodate the increased volume entering the facility.

Another factor is the growing need for improved water quality, in order to meet a more stringent discharge regulation or to allow the final effluent to be reused in the environment, rather than simply discharged back to a river or ocean. In regions where potable water is becoming scarcer, an additional tertiary process may be required on the back end of a conventional system in order to provide a final polish of contaminants to meet new regulations for water reuse applications. The cost and available space for this tertiary process may not be feasible.

The membrane bioreactor was invented a few decades ago to address these issues and has steadily improved through advances in microfiltration and ultrafiltration technologies. The MBR concept involves the use of membranes instead of clarifiers for solids removal.

The primary benefits of MBR systems are:

• Small footprint: Large clarifiers replaced by compact aeration and membrane systems
• High-quality water: allowing for reuse applications without tertiary treatment

The following diagram from Philos, a South Korean membrane manufacturer, provides a conceptual view of the smaller footprint afforded by an MBR versus a conventional activated sludge (CAS) system.

(See diagram and study here)

Membrane Fouling

One of the biggest technical challenges with MBR systems is membrane fouling. By the time wastewater flows through an aeration zone, the great majority of wastewater contaminants (carbon, nitrogen, and phosphorous) have been consumed by bacteria, resulting in clean water mixed with an extremely large microorganism population – called activated sludge. The goal of the MBR is to separate the activated sludge particles from the clean water using either microfiltration (~0.1 micron) or ultrafiltration (~0.01 micron). As most bacteria are between 0.2 and 10 microns long, either of these filtration systems will remove the great majority of the bacteria from the filtered water.

MBR Case Study

A study was performed over the past three years at the King County Brightwater Membrane Bioreactor Treatment Plant in Woodinville, WA. The Brightwater MBR facility began operations in 2011 serving the northeast suburbs of Seattle. Effluent from the plant is either sent to Puget Sound or reused for industrial purposes or irrigation.

From the beginning, the plant consistently struggled to maintain stable pH and alkalinity control for full nitrification using caustic soda. The need for pH control was especially observed when dewatering anaerobic sludge, where the ammonia-rich centrate stream was returned to the aeration process, causing nitrifying bacteria (Nitrosomonas and Nitrobactor) to rapidly convert NH3 into nitrate (NO3-). This process rapidly consumes alkalinity, resulting in a dramatic decrease in pH within the aeration zone that required a rapid increase in the caustic feed rate. The resulting pH swings were not optimal for overall microorganism activity.

Rather than using 50% NaOH, which can freeze at temperatures as high as 55oF, the facility was using 25% NaOH. During periods of high chemical demand, the plant needed 1-2 truckloads per day of 25% NaOH, causing logistical problems, concerns about handling the highly hazardous chemical, and contributing to a large carbon footprint.

While the Brightwater MBR process typically performed well during the summer months, with the onset of winter weather the filterability of the MBR system significantly declined. This phenomenon occurred during the winter months every year, resulting in a reduction of hydraulic capacity that often required rerouting sewage to other King County wastewater treatment facilities at an increased cost of operations.

Transition from Caustic Soda to Magnesium Hydroxide

Not long after start-up the Brightwater staff observed that the daily sCOD (“superfine” chemical oxygen demand) values were dramatically reduced, indicating that there were fewer superfine particles in the stream, along with improved floc structure in microscopic analyses of the activated sludge. These findings helped to explain the ability of the membranes to maintain unchanged filterability and flow even during the coldest months of the year, as well as a reduction in membrane cleaning frequency, unlike with caustic soda where every winter season there was a significant loss in membrane filterability and throughput.

This ability to maintain treatment capacity through the winter months is a tremendous benefit, as those are the months of the year with the highest rate of influent flow (rain and snow melt), when the need for optimum capacity is most important. The reduction in membrane cleaning cycles resulted in significant energy cost savings and helped to further reduce the Brightwater carbon footprint.

Could Magnesium have an effect on biofilm?

In an attempt to better understand the observation of year-round optimized filterability and flow capacity with Mg(OH)2, we researched the known literature on the effect of water temperature on membrane fouling. In a recent study by Weihua et al, a membrane ultrafiltration system was operated with river water at 4 °C and 25 °C, showing the growth of a denser biofilm and reduced flux at the lower temperature.1 The chemical composition of the biofilm at low temperatures had a higher proportion of molecules with polysaccharide C═O and O═C–O functional groups, which intensified the binding strength of the biofilm to the membrane surface. This formation of a “dense biofilm” during the cold months could be one factor that partially explains why the membrane flux decreases during the winter months and then recovers again in the spring. Apparently, the feed of caustic soda for pH control did not disrupt this “biofilm densification” process during the winter months.

Interestingly, numerous studies have shown that Mg(OH)2, MgO, and Mg2+ each possess the ability to inhibit biofilm formation. A recent article by Demishtein et al provides a summary of “recent advances in understanding the antimicrobial properties of magnesium ions with an emphasis on their effect on biofilm formation”.2  Within this summary article are references to numerous studies showing the antimicrobial effects of different magnesium species.

For instance, a 2009 collaboration between researchers at the Universities of Massachusetts and Illinois found that the magnesium ion triggered the activity of a synthetic antimicrobial additive to bind to a lipid membrane, indicating the activity to enhance the antimicrobial effect of the additive.3

In another study, researchers were looking for nontoxic, antibacterial coatings to apply to prosthetics in order to minimize infections. In their studies, they found colony-forming units of Staphylococcus epidermis that were 10,000 to 100,000 times lower on magnesium-coated surfaces as compared to titanium-only surfaces. (4)

While the mechanism of activity remains unknown, this disruption of biofilm formation by the magnesium ion is very well documented. Our finding of improved MBR flow during the winter months when feeding magnesium hydroxide upstream of the membranes is simply another data point that is consistent with the overall scientific understanding of how magnesium ions inhibit biofilm growth.

Maintenance of optimum performance during the cold weather months was observed when caustic soda was replaced with magnesium hydroxide for pH control of a membrane bioreactor wastewater system. Possible reasons for this benefit were the observed reduction in sCOD and concomitant improvement in microorganism floc formation, suggesting that the magnesium ion was providing a coagulation function to bridge anionic suspended particles together. Another possible contributing factor for the maintenance of optimum MBR flow during the feed of magnesium hydroxide is that the Mg2+ ion exhibits antimicrobial properties that inhibit the formation of biofilm, allowing the membrane surfaces to remain clear of exopolymeric substances (EPS) that can otherwise become dense and restrictive of flow.

References

1. Li W., Siddique M., Graham N., Yu W., Environ. Sci. Technol. 2022, 56, 12, 8908-8919.

2. Demishtein K,, Reifen R., Shemesh M., Nutrients 2019, 11, 10, 2363.

3. Som A., Yang L., Wong G.C., Tew G.N., J. Am. Chem. Soc. 2009, 131, 15102-15103.

4. Zaatreh S., Haffner D., Strauss M., Wegner K., Warkentin M., Lurtz C., Zamponi C., Mittelmeier W., Kreikemeyer B., et al. Biofouling 2017,33, 294-305.

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