Properties of Magnesium Hydroxide for Wastewater Treatment - Collection System

DOUG KELLEY \ December 16, 2022

Once sewage enters the wastewater collection system, its chemical properties can change depending on numerous factors, the most dominant being the effect of wastewater microorganisms when the water is held for a long detention time. Like all living things, wastewater bacteria need oxygen to survive. The most readily available form of oxygen is molecular oxygen (O2), which may be present in wastewater as dissolved oxygen (DO) during times of moderate to high flows – likely during business hours and early evening hours of the day. Once the flow from businesses and homes slows down in the late evening, the bacteria can consume all of the DO within a lift station sump or within a force main. At that time, certain facultative bacteria can alter their activity to get the oxygen they need from other ions present in the water (such as sulfate, SO4 and nitrate, NO3).

Sulfate is ubiquitous in most wastewater streams, as well as being a common contaminant in industrial discharge due to the use of sulfuric acid. Therefore, in the absence of DO sulfate-reducing bacteria (SRBs) present within the sewer line can strip the four O-atoms from SO42- to generate hydrogen sulfide, H2S.  Being an acid, when the concentration of H2S in the sewer line increases the pH of the wastewater can decrease, according to the equation below:

H2S  ↔  HS  +  H+
Pros & Cons of nitrate-based products to control H2S odor & corrosion in the collection system

As a weak acid, hydrogen sulfide is in equilibrium in two forms:  H2S and HS. While H2S is the volatile molecule that stinks and causes gas-phase corrosion, HS is a nonvolatile, water-soluble sulfide ion. Being nonvolatile, it stays in solution; therefore, not contributing to odor or gas-phase corrosion. Other acid-producing bacteria are also active under anaerobic conditions, which can further decrease the wastewater pH. The more acidic the wastewater becomes, the more the equilibrium shifts in the direction of the H2S molecule, releasing more H2S into the gas phase, resulting in terrible odor, dangerous breathing conditions, and accelerated gas-phase corrosion in the collection system. This is one of the reasons that sewer districts are modifying their SIU permits to adjust their effluent pH to a slightly higher range – to help hold sulfide in solution. For instance, when the pH is 6.5, the amount of hydrogen sulfide in the H2S form is about 90%, whereas at pH 7.5 the equilibrium shifts so that H2S is present at about 50% with the remainder being the nonvolatile HS anion.

 

One popular method to minimize H2S odor and corrosion in the collection system is to feed an alternative oxygen source. By feeding a nitrate-based product (Bioxide, Ca(NO3)2, being the most widely known) into the collection system, the growth of denitrifying bacteria is fostered.  Using this treatment method, the wastewater bacteria are to leave SO42- alone and instead strip the O-atoms from NO3 to generate N2 gas – which is odor-free, noncorrosive, and 80% of the atmosphere we breathe.  From an environmental perspective, this is a very “green” treatment approach.

However, there can be negative side effects in the use of a nitrate-based product to achieve optimum odor control performance. The first side effect is that in order to achieve optimum performance, the nitrate dose needs to be significantly higher than is indicated by chemical stoichiometry.  This is because the wastewater bacteria need to experience an abundance of NO3 in order to “ignore” the SO22- that is also present. For this reason, a relatively high dose of nitrate is required, which can be cost-prohibitive and can result in a significant residual carrying into the treatment plant. If the wastewater influent flows into a primary clarifier, the excess NO3 can encourage the growth of denitrifiers in the clarifier, resulting in the formation of N2 gas that can release bubbles from the settled sludge, causing the solids to float instead of settle.  Secondly, many municipalities are being challenged with new NPDES permits that include a Total Nitrogen limit. Therefore, the high dose of nitrate in the collection system can result in added costs to remove the excess nitrogen flowing into the treatment plant.

The second side effect of feeding nitrate is related to the growth of denitrifying bacteria in the collection system. Accumulation of biomass and FOG (fats, oils, & grease) can reduce the pipe’s inner diameter and restrict flow. In Washington state, it is common for sewer districts to treat for odor only during the summer months.  We have been informed by various nitrate users that shortly after the initiation of chemical feed, they begin to see grease build up in downstream manholes and lift stations.

The mechanism for how magnesium hydroxide mitigates odor and corrosion in the collection system is unlike that for nitrate. Instead of feeding an alternative oxygen source to the microorganisms, magnesium hydroxide simply increases the pH of the wastewater, shifting the H2S equilibrium to favor the nonvolatile HS anion. By maintaining the pH at ~7.5 with IER’s ALKA-Mag+ (60% Mg(OH)2) it is common that there will be no detectable H2S odor present downstream of the feed point. At first glance, this is surprising, since at pH 7.5 about 50% of the sulfide in the wastewater should be in the volatile (stinky) H2S form. One theory as to why we see quantitative odor control at this slightly alkaline pH is that there remains a significant amount of undissolved Mg(OH)2 particles in the water. These particles have a surface pH of around 10.5-11.0. As the dissolved H2S molecules in the wastewater interact with the undissolved Mg(OH)2 particles they become deprotonated into the nonvolatile HS anion and remain in solution. Therefore, no odor and no gas-phase corrosion.

Figure 1

Figure 2.

Benefits of ALKA-Mag+ to replace nitrate-based products in the collection system

Unlike nitrate-based products, the use of ALKA-Mag+ has side benefits – one for the collection system and the other for the treatment plant. As biomass and FOG can build up over time in the collection system, the use of ALKA-Mag+ causes these to become dislodged from pipe walls and flow into the treatment plant, leaving the treated sewer line (lift station, force main, etc.) clean of debris. To confirm this, we measured the flow rate from a lift station pump operating at about 105 gpm prior to the feed of ALKA-Mag+. However, as is shown in Figure 1, just a few weeks after the initiation of the ALKA-Mag+ feed the pump flow rate had increased to 135 gpm (with the maximum pump flow rate being 140 gpm). In another instance, after the initiation of ALKA-Mag+ feed in September there was an increase in the influent TSS entering the treatment plant, which stopped once the magnesium hydroxide feed was stopped in October when the rains came and there was no longer a need for the odor control treatment. The cause for this biomass/FOG cleaning effect by ALKA-Mag+ is that the release of hydroxide ions can accelerate the hydrolysis (or breakdown) of large insoluble carbonaceous compounds, such as fats, oils, and greases, as well as the polysaccharide film that adheres biomass to pipe walls. These hydrolysis reactions also called saponification, result in reducing large insoluble molecules into small, water-soluble molecules, such as volatile fatty acids, that do not adhere to the pipe walls and are beneficial food for the wastewater microorganisms within the treatment plant.

A second benefit of the use of magnesium hydroxide in the collection system is that the boost in alkalinity can be beneficial to maintain sufficient alkalinity within the treatment plant, especially if there is a need to perform nitrification to meet a Total Nitrogen limit. Figure 2 shows the final effluent alkalinity values at a water resource recovery facility (WRRF) in Western Washington. As can be seen, the alkalinity values are quite low because the water used by the city is surface water from snowpack in the Cascade Mountains. However, after the initiation of magnesium hydroxide feed out at the lift station, the final effluent alkalinity from the wastewater treatment plant rose about 40% – sufficient to support nitrification. So, in this case, by replacing the use of a nitrate odor control product in the collection system, the wastewater plant did not need to feed an alkaline additive for nitrification.   

Conclusion: Multiple benefits of IER’s ALKA-Mag+  in the collection system 

In summary, the use of ALKA-Mag+ in the collection system resulted in addressing three challenges with a single additive: replacement of a nitrate-based treatment resulted in effective odor and corrosion control of the treated force main entering the wastewater plant headworks, and the collections staff was pleased that they no longer needed to “pig” the force main to recover diminished pumping ability, while the wastewater plant staff were thrilled that they did not need to feed an additional chemical (especially one that is hazardous!) at the plant for nitrogen removal. 

We like to call this “killing three birds with one stone” 

1) odor/corrosion control

2) biomass/FOG control

3) pH/alkalinity control.

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Interested in finding out more about the properties of magnesium hydroxide in wastewater treatment? Click the link below to read the previous article:

Part One: Industrial Wastewater Pretreatment

Already read that one? Check out the next article in this three-part series:

Part Three: Wastewater Treatment Plants

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