Benefits Beyond pH and Alkalinity

To achieve sufficient phosphorus removal in a wastewater treatment process, two different treatment approaches are used.  The first approach is to foster the growth of a healthy, diverse biomass that can absorb as much of the phosphorus as possible.  The other approach is to chemically capture the remaining phosphorus.  Typically, this is done with aluminum or iron based salts which tend to be acidic and can result in the need for pH supplementation in order to meet the effluent pH permit limit.

You may be asking where magnesium hydroxide fits into the Phosphorus Removal equation.  While magnesium hydroxide is technically a chemical addition, its primary role is to enhance the microbiological treatment process.  With the alkaline buffering strength of Mg(OH)₂, an environment can be created where the pH is boosted for healthy biomass growth, while the strong alkalinity prevents pH swings.

As seen in the graph below, running a pH of around 7 to 7.5 provides the best environment for wastewater microorganisms to gobble up all the available food, rather than wasting energy to protect from low or high pH swings.

Recently, a WWTP began using ALKA-Mag⁺ (60% magnesium hydroxide) to help achieve a new pH limit of 6.9.  They previously had a lower pH limit (6.0) that did not require chemical treatment.

Prior to feeding ALKA-Mag⁺ the plant had seen an average of ~83% removal of total phosphorus in the treatment process, with wide swings in performance from month-to-month ranging from 66% to 93%.

The addition of ALKA-Mag⁺ increased the % total phosphorus removal to be consistently >90%, demonstrating that by providing an optimum pH along with strong alkaline buffering, the bacteria can perform to their fullest ability.  To learn more about this study click here.

The reduction in biofilm or biomass accumulation on surfaces by magnesium hydroxide and the magnesium ion (Mg²⁺) has been well documented.  Demishtein et al published a summary of how the antimicrobial properties of magnesium may provide opportunities to develop healthier food (Nutrients. 2019 Oct; 11(10): 2363.), with the focus of their review on magnesium’s “ability to impair bacterial adherence”.

The ability of magnesium to reduce biofilm adhesion is a benefit that can be seen in a few different places in the wastewater stream.  The first place would be in the collection system.  It has often been observed that upon feeding ALKA-Mag⁺ (60% Mg(OH)₂) for odor control in a collection system, in the first month there is an increase in fines (total suspended solids, TSS) coming through the system.  This observation is likely due to the effect of Mg(OH)₂ and Mg²⁺ to cause sloughing of biomass and FOG present in line.

In a study done at the Birch Bay Sewer and Water District, this line cleaning effect from the feed of ALKA-Mag⁺ was documented through the observation of increased pumping efficiency.  Birch Bay had been treating their force mains for years with Bioxide (nitrate), which fosters the growth of biomass (denitrifiers) in the sewer line.  When ALKA-Mag⁺ was fed to a line that had not been cleaned in over a year, and was operating at 105 gpm, within one month the flow rate in the line improved to its maximum flow rate of 135 gpm, demonstrating the role played by magnesium in the gradual reduction of biofilm and debris buildup in the line. To learn more about this study click here.

Another interesting example of biofilm reduction was observed in a membrane bio-reactor (MBR) facility when replacing Caustic Soda with ALKA-Mag⁺.  It was noted that every winter they saw an increase in biofilm growth on the membranes that negatively affected flow through the membranes.  Upon conversion from NaOH to ALKA-Mag⁺ they noted that the seasonal drop in flow capacity of the membranes did not occur, pointing to the anti-biofilm activity of magnesium.  Additionally, before the trial they were having significant foaming issues and soon after feeding magnesium hydroxide this foaming problem went away and has not returned.  For these reasons, they have not stopped feeding ALKA-Mag⁺ and are in the process of installing a permanent bulk storage and feed system.  To learn more on this trial click here.

You may be asking how magnesium hydroxide affects biofilm growth and accumulation.   As mentioned above, there are numerous studies that demonstrate the anti-biofilm effects of different magnesium species, though the mechanism of this activity is not entirely clear.  A possible chemical explanation for biofilm removal is that the gradual release of the hydroxide ion from Mg(OH)₂ can drive continuous hydrolysis reactions that cleave large FOG and biomass molecules into smaller volatile fatty acids (VFAs), resulting in the gradual sloughing of biomass from the internal surfaces of the force main and downstream sewer line.

One of the most common comments made to IER staff after an industrial or municipal wastewater facility begins the feed of ALKA-Mag⁺ is that the operators observe a significant improvement in sludge dewatering.  It is common to find that the percent solids in the dewatered sludge is 2-4% higher.  In a recent trial, ALKA-Mag⁺ was used to replace Caustic Soda feed into the return activated sludge line.  The secondary sludge was then processed along with the primary sludge in their anaerobic digestion system.  The result was an increase in cake solids that allowed them to cut their monthly sludge hauling bill in half – a large savings due to the size of this particular water resource recovery facility!  To learn more about this case study click here.

Why does magnesium hydroxide improve solids handling?  We believe there are two factors involved.  The first factor brings us back to the magnesium ion (Mg²⁺), which can provide coagulation benefits when compared to the sodium ion (Na⁺), which cannot.  The graphic below shows that as the charge field around the ion gets smaller (called “charge density”) the coagulation effect of the ion increases.

This is why many coagulants are based on aluminum (Al³⁺), as it has one of the strongest coagulation effects of any ion.

The ferric ion (Fe³⁺) has a similar charge density, and is also an excellent coagulant.  The Mg²⁺ ion is not as strong as Al³⁺ or Fe³⁺, but it still has a significant coagulation effect.

As a result, Mg²⁺ can interact with the negatively charged suspended particles in the wastewater to aid in floc formation and improve sludge dewatering.

What separates magnesium hydroxide from the Al³⁺ and Fe³⁺ coagulants is its dual functionality – providing coagulation benefits while also providing a pH and Alkalinity boost to the treatment system.

The second coagulation effect that magnesium hydroxide provides gets us back to the basic attributes of boosting the pH and alkalinity.

As we observed with phosphorous reduction, by optimizing the pH and then locking it in with strong alkalinity, a healthier mixture of wastewater microbiology results.  Healthier bacteria tend to floc better, settle better, and are easier to dewater.

When combining the coagulation benefits afforded by the Mg²⁺ ion and the enhanced coagulation from optimized pH and Alkalinity for stronger floc formation, IER’s ALKA-Mag⁺ provides wastewater operators much more than a simple and safe boost in pH.

In most cases, the entire wastewater treatment process is enhanced!