Many wastewater operators consider themselves “zoologists”.  However, unlike at zoos, where the species being looked after are lions, tigers, and bears (oh, my!), wastewater operators are responsible for maintaining the stability and growth of “critters” they can’t even see – microorganisms. But, oh, what amazing life and activity that can be seen when examining a wastewater sample under a microscope! It’s a whole different world!

The processes involved in recovering clean water and valuable carbon, nitrogen, and phosphorous by-products from wastewater are actually very elegant. In the traditional, conventional approach, as wastewater enters a facility, the biggest contaminants are removed using grit filters and screens, followed by primary clarifiers where the heaviest of the suspended solids are allowed to settle to the bottom as “primary sludge”, allowing the remaining soluble and suspended contaminants to travel onward with the water. In the next stage, hungry microorganisms consume the remaining suspended organic contaminants in the wastewater through aerobic and anoxic processes, and in so doing multiply to tremendous numbers – called “activated sludge”. In a properly designed secondary treatment process, the activated sludge so effectively removes the wastewater contaminants that the bacteria can actually become cannibalistic toward each other. Upon recycling this activated sludge to the front end of the process, the extremely hungry bacteria are ready to consume the fresh wastewater contaminants entering the system, and the whole cycle repeats itself – with extremely clean water being the end result.

Over time, the volume of activated sludge grows so large that some need to be removed from the system. Therefore, a percentage of the activated sludge is separated from the secondary system and joined with the primary sludge from the beginning of the process. These two sludges are typically fed into another microbiological system called an anaerobic digester, to be reduced in number and converted into methane–biogas, “green” energy.

How wastewater microorganisms can lower the pH

All microorganisms display certain types of metabolic activity, meaning that they have the ability to chemically react with certain contaminants in the wastewater to reduce the contaminant into a smaller substrate. These chemical reactions driven by wastewater microorganisms can have an impact on the pH of the overall water system. For instance, the microbiological activities involved in removing nitrogen from wastewater are called nitrification and denitrification. In nitrification, bacteria convert ammonia and similar nitrogen compounds into nitrite (NO₂^–) and nitrate (NO₃^–).  These reactions consume alkalinity, which can result in a drop in pH. If not countered by feeding an alkaline additive into the wastewater, the pH can fall to a level that is below that for the bacteria to continue to perform their functions. If the pH falls too far, it can actually lead to microorganism death. 

As was mentioned previously, the industry standard for treating low pH is to feed caustic soda (NaOH), which is hazardous for the employees needing to store and feed it. It is also hazardous for the microorganisms, as NaOH kills any bacteria it encounters upon entering the wastewater stream until it becomes diluted to the desired pH. At that point, its job has been completed and it has no lingering buffering ability. Lastly, the increased sodium ion (Na⁺) concentration can become a detriment to microorganism activity and can interfere with the formation of beneficial flocs for solids settling and dewatering. 

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 SO₄^2- 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.

Significant industrial users (SIUs) are regulated industries within a municipality or sewer district that discharge either: 

1) a volume of wastewater that is above a minimum threshold as being representative of a certain percentage of flow entering the treatment facility, or 

2) a wastewater stream containing a contaminant(s) that could adversely impact the operations of the treatment facility.  

SIUs are commonly regulated on wastewater contaminants such as TSS (total suspended solids), BOD (biological oxygen demand), pH, and flow.  For instance, a common pH range for acceptable discharge is from 6.0 to 9.0.  Therefore, if an industry has a wastewater stream that has a pH less than 6.0, it will be required to feed an alkaline additive to bring the final effluent pH to above 6.0 prior to discharge to the municipality.

More recently, sewer districts are requiring SIUs to increase the pH of their effluent wastewater, perhaps from a minimum of 6.0 up to 6.5 or 7.0.  This is being done to minimize the impacts of further pH reduction and corrosion that can occur while the sewage passes through the collection system (which will be discussed in the next section).

The industry standard alkaline additive used to increase the pH of acidic wastewater is sodium hydroxide (NaOH), also called caustic soda, and it is typically sold as a 50% concentrate.  In colder climates, the typical product concentration is 25% NaOH, which freezes at the same temperature as water, while 50% NaOH freezes at 60oF.  

All about caustic soda: the benefits and the drawbacks 

One of the benefits of caustic soda is that it is relatively inexpensive, being a byproduct of the chlor-alkali process that produces chlorine gas.  Another benefit is that it is a pure liquid and a strong base that reacts instantly within the wastewater stream to give the desired pH within seconds.  As a liquid, it is relatively easy to feed using a standard chemical metering pump. 

The negatives of caustic soda are also related to its high rate of reactivity.  First, if the dose in the wastewater is too high, the pH can readily spike up to a value above 12, resulting in a need to feed an acid-based product to bring the wastewater pH back down into regulatory compliance.  That means the wastewater operator has to handle two hazardous products – a strong base and a strong acid.  This brings us to the next concern with sodium hydroxide:  it is a highly hazardous chemical to handle.  The primary industrial use for NaOH is as a cleaner, because of its rapid hydrolysis reaction to break down proteins and carbohydrates that can line the walls of storage tanks and transfer lines.  That is why “caustic cleaners” are extremely common products in the sanitation market.  Unfortunately, our skin and eyes consist of proteins and carbohydrates that are equally susceptible to the extreme reactivity of caustic soda, and serious caustic burns are far too common in the industry.  For this reason, most wastewater operators do not enjoy working with caustic soda.  

In addition, an industrial wastewater operator typically does not have the same amount of resources, support, or training available to them as compared to a municipal operator.  At a municipal wastewater treatment facility, the entire purpose is to achieve a high-quality effluent stream that meets or exceeds their state permit.  The effluent being discharged to the environment is their “product” – something to be extremely proud of.  However, for industries like food processing or metal plating, a clean wastewater stream is not a sellable product.  Their products are pork chops and chrome fenders, and the need to pretreat the wastewater for discharge is purely an overhead expense.  Therefore, the Environmental Manager responsible for the industry’s pretreatment process often runs a leaner operation with less personnel and training.  For these reasons, it’s in their best interest to help their wastewater operator to work with less hazardous chemicals.  From the very beginning of IER, this has been our primary mission: to help industrial wastewater operations convert from the use of caustic soda to magnesium hydroxide, to make the life of the wastewater operator a little safer each day.

Various forms of antacids have been used to calm upset stomachs for decades, and the benefits of magnesium hydroxide for acid indigestion have been known since the early 1800s. The brand Phillips’ Milk of Magnesia was first established in 1872 as an 8% magnesium hydroxide medicinal product. Despite being known for two centuries as a well-respected medicinal treatment, the use of magnesium hydroxide to treat low pH and low alkalinity wastewater has only recently become accepted in the wastewater treatment marketplace.

The reason for mentioning the medicinal benefits of Milk of Magnesia is that the biological processes that occur in our stomachs are quite similar, though on a much smaller scale, to what happens aerobically and anaerobically in a wastewater treatment facility. Whether in our upset stomachs or within a multi-million-gallon wastewater treatment plant, the pH and alkalinity need to be sufficient and stable for bacteria to optimally convert carbonaceous wastewater contaminants into methane and carbon dioxide, and ammonia into nitrogen gas.

In the following series of articles, we will focus on the properties of magnesium hydroxide that make it the most powerful, and yet most gentle, additive in the market for providing sufficient and stable pH and alkalinity for wastewater treatment.

The articles will focus on three primary wastewater applications: industrial wastewater pretreatment, the wastewater collection system, and the wastewater treatment plant.

To read the articles, click the links below:

Part One: Industrial Wastewater Pretreatment 

Part Two: Wastewater Collection System

Part Three: Wastewater Treatment Plants

Sources:

1.) Glasdam S.M., Glasdam S., Peters G.H.  The Importance of Magnesium in the Human Body: A Systematic Literature Review. Adv. Clin. Chem. 2016, 73, 169-193. 

2.) De Baaij J.H., Hoenderop J.G., Bindels R.J.  Magnesium in man: Implications for health and disease. Physiol. Rev. 2015, 95, 1-46.

It is very common to hear someone in the wastewater treatment industry refer to something called “MagOx” as an alkaline additive they use to control pH or low alkalinity conditions.  Often they are simply using this term as a nickname when talking about either Magnesium Hydroxide or Magnesium Oxide, which is the chemical precursor to Magnesium Hydroxide, but which has very different properties.  So, when looking to obtain an additive that will provide the most cost-effective pH control performance for your wastewater treatment application, it is important to understand the relationship between Magnesium Oxide and Magnesium Hydroxide, and how not all products are equal.

First of all, the chemistry of Magnesium Oxide (MgO) and Magnesium Hydroxide (Mg(OH)₂) is based on the Magnesium di-cation (Mg²⁺).  Magnesium is an essential macronutrient and is the core element in chlorophyll – the stuff that makes plants green and drives photosynthesis.  While MgO and Mg(OH)₂ are very similar, knowing the differences in the manufacturing processes for each will help to understand the physical and chemical differences – which is very important in the selection process of the chemical best suited for your application.

Magnesium Oxide (MgO) is typically obtained from the calcination (heating) of Magnesite ore (Magnesium Carbonate or MgCO₃) in much the same way as Quicklime (CaO) is formed from Limestone (Calcium Carbonate or CaCO₃).  By changing the temperature and speed of how fast the MgCO₃ passes through the heat zone, one can control the structure, porosity, and reactivity of the resulting MgO particles.  You may ask why this matters?  The more porous the particulate structure, having more available surface area, the more reactive the MgO.  The ability to control structure and porosity can result in the development of a wide range of MgO products with diverse properties for numerous industrial applications.