You Don't Have to Break the Bank to Tackle Odor and Corrosion Issues

When asked, “what is the root cause of odor in a sewer line?”, many wastewater operators reply, “Hydrogen Sulfide” or H₂S.  

But in actuality H₂S is the symptom of the problem – not the root cause.  H₂S is simply a by-product of microbiological processes that can occur within the collection system.  Just like nearly everything else in the world of wastewater treatment, it’s all about the bugs and how they are maintained.  The root cause relates to how they are maintained.

The root cause of a typical sewer line odor problem is the presence of a septic or anaerobic condition that can exist under certain conditions.  The most typical reason for the existence of an anaerobic condition is low flow or long detention time.  A common example of this condition is the extremely low flow rate of sewage during the late evening and early morning hours that can occur each day in certain collection system pump stations.  Under these long detention time conditions, the residual oxygen in the water can be completely consumed by both chemical and microbiological activity, resulting in a water that contains no free O₂.  When no free oxygen molecules are present bacteria can display a facultative behavior, allowing them to convert their activity from aerobic to anoxic and anaerobic – being able to metabolize ions such as nitrate (NO3–) and sulfate (SO4-2) as sources of the O-atom needed for their continued growth and health.  These microbiologically-driven chemical reactions result in stripping the O-atoms from the ions, leaving behind molecular nitrogen (N₂) and hydrogen sulfide (H₂S).

2 NO₃^–   🡪    N₂

  SO₄^-2   ↔   H₂S  

The microbiological reaction with nitrate (called denitrification when it occurs at the treatment plant) results in the release of nitrogen gas, which is odorless and noncorrosive – N₂ is actually 80% of the atmosphere we breath every day.  Certainly not an odor problem.

However, it’s the microbiological reaction with sulfate (SO₄^-2) that forms the rotten-egg H₂S odor.  Under these anaerobic conditions there are additional microbiological processes that generate acids (called volatile fatty acids).  As these acids are being formed, the pH of the wastewater can decline – called going “septic”.  For instance, it’s not uncommon to find restaurant grease traps covered with a thick layer of grease where the pH of the water beneath the cap is as low as 4 or 5.  Also, consider fire sprinkler lines.  While they are filled with fresh city drinking water, when flushed they exude a tremendous H₂S odor from the microbiological conversion of sulfate into sulfide.  They also run black with corrosion product – what used to be the pipe wall!  This mechanism of odor and corrosion is not unlike what is happening in certain sewer lines when long detention times are the norm.

Hydrogen sulfide is also a weak acid in water that exists in two possible forms: H₂S and HS^–.  

  H₂S    ↔    HS^–

        volatile     nonvolatile

When the water is acidic (having pH < 7), the most prevalent species is H₂S – the volatile, foul-smelling gas that we’re trying to avoid.  When the water is basic (pH > 7), the most dominant species is HS–, which is nonvolatile (something to keep in mind as we dive further into this discussion).

Interestingly, when sewage is confined within a force main the SO₄^-2 🡪 H₂S process can be catalytic – meaning that sulfate can be microbiologically converted into H₂S, which in subsequent microbiological reactions within the pipe can be converted back into sulfate, and then back into H₂S, and so on.  Unfortunately, in most cases the reactions that convert H₂S back into SO4-2 can cause accelerated corrosion within the system.  

Within a force main or any enclosed section of the sewer line where the gas cannot escape, H₂S can be converted into SO4-2 by the activity of thiobacillus bacteria or by simple chemical oxidation reactions in the pipe.  In this reaction, a most common initial species formed is sulfuric acid (H₂SO₄), which can very rapidly react with the pipe wall (concrete or steel) to cause corrosion, and releasing SO₄^-2 back into the water to be converted again into H₂S by sulfate reducing bacteria (SRBs).  Again, recall the stinky black water when a fire sprinkler system is flushed – corrosion accelerated by the microbiologically-driven catalytic cycle of SO₄^-2 to H₂S to SO₄^-2, and so on.

Shock Treatment

This approach typically involves slug dosing a highly caustic chemical (such as Caustic Soda) to drive the pH up to an extreme level, or slug dosing of a strong oxidizer (such as Bleach) to drive the biocide concentration up to an extreme level.  In either case, this approach provides a short-term benefit by causing large amounts of biomass to slough off the pipe.  However, with no continued treatment the biomass and corresponding odor rapidly return causing the need for another dose.  This observation of rapid microorganism recovery teaches us how incredibly resilient these organisms are.  Consider the drinking water distribution system, where biofilms have been shown to survive and build despite the maintenance of a continuous residual of chlorine or chloramine.  Ask yourself, why do we get “brown water” downstream from a location where a city worker has accidentally bumped a drinking water pipe?  If we’ve been putting clean drinking water into what was originally a clean pipe, shouldn’t the pipe remain clean?  Should be able to bump it all day without consequence.  Not, so.  Microorganisms are incredibly resilient!

Precipitation

Another well used odor control method is to cause the precipitation of H₂S into a mineral that becomes a part of the suspended solids being carried along in the system.  The most common precipitating agents used are iron salts, such as ferric chloride or ferrous sulfate, resulting in the formation of iron sulfide precipitates.  The biggest benefits of this treatment are that iron salts are relatively inexpensive and they react rapidly with H₂S to give a quick knock-down of odor.  On the negative side, these highly acidic products can be very hazardous for operators to handle, can depress the wastewater pH flowing to the treatment plant, and can cause a significant increase in suspended solids entering the plant.

Oxygenation

As was stated at the beginning of this discussion, the root cause of the odor and corrosion problem is a lack of oxygen in the system that results in anaerobic conditions.  Therefore, numerous technologies have been invented to treat the collection system with a source of oxygen with a goal of maintaining a residual of dissolved oxygen in the system.  The biggest benefit of this approach is that it does not involve the purchase of chemical (unless the oxygen source is liquid O₂).  The struggles with this technology have been with the capital and maintenance costs of the oxygenation systems, along with the need to control the dose of oxygen so that not too much nor too little is being added.  Too much gaseous oxygen can result in tripping and clogging of air release valves or water hammer and pump cavitation.  In addition, numerous treatment plants rely on the presence of acids (volatile fatty acids, VFAs) generated by the anaerobic microorganisms as readily available food for the treatment process.  Maintenance of a DO residual in the collection system negates the formation of VFAs.

pH Adjustment

Another way to control gas-phase odor and corrosion from H₂S is to not allow it to escape from the water-phase in the first place.  This can be done by simply adjusting the pH of the water in the alkaline direction.  As was mentioned previously, hydrogen sulfide is a weak acid always in equilibrium with its counter-anion, HS^–.  By boosting the pH in the collection system, the equilibrium shifts in favor of HS^–, which is a non-volatile ion.  Being nonvolatile, it does not leave the water-phase, completely mitigating odor and gas-phase corrosion.  In addition, the added pH and alkalinity is typically beneficial for the downstream treatment plant.

There are numerous chemical methods to increase the pH of water.  However, many of those chemical options have severe negative implications:

• Caustic Soda (NaOH):  can provide extremely rapid increase in pH, but can be difficult to control pH spikes or loss in pH when underfed.  Not a strong alkalinity buffer.  Also, highly hazardous for operators to store, handle, and feed.
• Soda Ash (Na₂CO₃):  can provide rapid increase in pH along with buffering benefit, but typically fed as a powder involving cumbersome and dirty feed systems.  It also adds 2 moles of sodium per dissolved molecule to the treated water, with sodium and TDS levels becoming an increasing effluent concern.
• Lime (Ca(OH)₂):  can provide rapid increase in pH along with buffering benefit, but can result in calcium carbonate scale formation that can restrict flow and minimize pumping capacity.  Is also hazardous and cumbersome to handle and feed.  For these reasons, calcium-based products are less desirable for feed into a collection system.

Unlike the other alkaline additives, magnesium hydroxide (Mg(OH)₂) does not provide a rapid pH increase.  Instead, it dissolves relatively slowly in water and provides a powerful, long-lasting buffer.  In addition, it is completely nonhazardous – commonly called “milk of magnesia” – allowing operators the confidence to handle and feed without safety concerns.  

Replacing a sodium-based additive (NaOH or Na₂CO₃) with a magnesium additive is a significant improvement in TDS, and the Mg²⁺ ion is beneficial for aquatic and terrestrial life (Mg²⁺ is the core element of chlorophyll which drives photosynthesis).  Finally, no other liquid alkaline additive can release a larger amount of hydroxide (OH^–) into the water from a single dose than a 60% Mg(OH)₂ product.  For instance, as shown by the following stoichiometric equations, a feed of 60 lbs of 60% Mg(OH)₂ will release the same number of moles of hydroxide (OH-) as 100 lbs of 50% NaOH:

The following graphs provide real-world data demonstrating effective H₂S mitigation from the feed of magnesium hydroxide into collection systems.  Figure 1 provides data from an Australian odor control application of a 60% Mg(OH)₂ product named ACTI-Mag (from Calix ltd.), showing a dramatic drop in H₂S.  Dosing was into a lift station about 10 miles from the treatment plant, resulting in excellent odor and corrosion control in the force main, along with a reduction in the need to clean the lift station for build-up of fats, oils, and greases (FOGs).  Because of the increased pH and alkalinity entering the treatment plant, they were able to eliminate lime usage and reduce the amount of alum being fed for phosphorous reduction.