Intro to Wastewater Treatment Plants
Wastewater treatment plants are the unsung heroes of our communities, ensuring that our water supply remains clean and safe.
However, within the confines of these facilities, a silent battle rages—one that threatens the very structures that keep our environment healthy.
Wastewater treatment plants operate in a hostile environment. The combination of hydrogen sulfide gas, aggressive chemicals, and perpetual moisture creates a perfect storm for corrosion.
These corrosive agents can silently eat away at steel, concrete, and HVAC/R systems.
In this comprehensive guide, we’ll explore effective strategies for corrosion-proofing the interior (and some exterior methods) of wastewater treatment plants.
From understanding the corrosive environment to implementing cutting-edge methods, we’ll equip you with the knowledge needed to protect these critical assets.
Let’s dive in!
What is Corrosion?
The Association for Materials Protection & Performance (AMPP) defines corrosion as the “deterioration of a substance because of a reaction with its environment”.
Although the term ‘substance’ can encompass any material susceptible to degradation, it is commonly used to denote materials that are manufactured or specifically engineered.
This might sound complex, but let’s break it down.
In the context of water and wastewater systems, this typically involves materials like non-ferrous metals, ductile iron, and concrete.
When it comes to metals, corrosion often involves the formation of a “corrosion cell”. This cell consists of five basic elements:
Anode: This is the part of the metal surface where the corrosion occurs. It loses electrons and undergoes oxidation.
Cathode: This is another part of the metal surface, but it does not corrode. Instead, it gains the electrons lost by the anode and undergoes reduction.
Metallic Path/Electrical Connection: This is the metal itself, which provides the path for the flow of electrons from the anode to the cathode.
Electrolyte: This is the surrounding environment (such as water) that facilitates the flow of ions.
Potential Difference: There must be a voltage difference between the anode and the cathode.
If any of these elements are missing, galvanic corrosion will not take place. This understanding is crucial for effective corrosion control!
There are several factors that can affect the rate of corrosion within a water and wastewater system:
Hydrogen Sulfide and Sulfuric Acid Attack: These substances can accelerate the corrosion process, especially in concrete structures.
Temperature Fluctuations: Changes in temperature can speed up chemical reactions, including those involved in corrosion.
Humidity Levels: High humidity can provide the moisture needed for corrosion to occur.
Continued Exposure to Various Process Chemicals: Certain chemicals used in water and wastewater treatment can contribute to corrosion.
Environmental Chloride: Chloride ions can penetrate concrete and reach the reinforcing steel, leading to corrosion.
Sulfate and Nitrate Ions: These ions can also contribute to the corrosion process.
These factors are crucial for the maintenance and operation of water and wastewater systems, as it allows for the implementation of effective corrosion control measures.
Unraveling the Mechanisms of Corrosion
The Role of Microbes
Microbes, such as bacteria, can instigate what is known as microbiologically influenced corrosion (MIC). This is particularly prevalent in wastewater treatment plants.
The process involves a series of biogenic sulfate reduction reactions, which lead to the production of sulfuric acid.
This acid reacts with the concrete structures, causing rapid deterioration.
Rising H2S: A Silent Menace
Hydrogen sulfide (H2S) has emerged as a silent menace in the subterranean world of wastewater treatment.
This toxic gas is a byproduct of the decomposition of organic matter in the absence of oxygen, a common scenario in the bowels of a treatment plant.
Elevated levels of H2S not only pose health risks but also accelerate corrosion, particularly in the form of sulfuric acid attack on concrete and metal structures.
The Impact of UV Radiation
UV degradation is a process where ultraviolet radiation causes the breakdown of certain materials.
In wastewater treatment, UV-based advanced oxidation processes (AOPs) have become a significant topic of discussion.
The strong oxidizing radicals produced during these processes can accelerate both microbial inactivation and pollutant degradation.
Chloride-Induced Deterioration
Chloride-induced corrosion is a process where chloride ions infiltrate the porous concrete substrate and reach the reinforcing steel, or rebar.
This can lead to the formation of corrosive iron oxide (rust), which induces mechanical tensile stress within the concrete, leading to cracks and spalling.
Concrete carbonation is a process where environmental carbon dioxide or carbonate ions diffuse into the concrete’s porous structure and react with the concrete, reducing the overall pH level.
Other Physical Factors
Other physical forces such as impact, abrasion, and erosion can also contribute to the corrosion of concrete. These forces can cause the concrete to crack and degrade, leading to structural issues.
The Galvanic Process
Galvanic corrosion is a phenomenon that occurs when two different metals come into electrical contact within a corrosive environment.
In wastewater treatment facilities, which are filled with various metal equipment, galvanic corrosion is a common issue.
The area where air and water meet near the top of the clarifier is particularly susceptible.
Galvanic Corrosion: A Closer Look
The Galvanic Series, developed in the late 1700s, is a list that ranks metals by their nobility in an electrolytic solution.
When two metals from this series are connected, the less noble one (anode) will succumb to corrosion, while the more noble one (cathode) is spared.
In the watery corridors of wastewater treatment plants, this phenomenon is not just a theoretical concern but a practical challenge.
For instance, if a steel bolt is used in a copper pipe, the steel will deteriorate as it gives up its electrons to the copper.
This sacrificial act, while noble, can lead to structural failures and unexpected maintenance nightmares.
The rate of galvanic corrosion is influenced by several factors:
Electrolyte Conductivity: The more conductive the solution, the faster the corrosion.
Difference in Nobility: The greater the difference in the Galvanic Series, the more aggressive the corrosion.
Area Ratio: The size of the anodic area relative to the cathodic area can accelerate the corrosion process.
Understanding the intricacies of galvanic corrosion is essential for engineers and maintenance teams.
It informs decisions on material selection, design, and protective measures to ensure the longevity and reliability of wastewater infrastructure.
Fighting Corrosion in a Wastewater Treatment Plant
The fight against corrosion in wastewater treatment plants is as old as the facilities themselves.
Historically, the use of iron and steel in these plants has been a double-edged sword: while providing the necessary strength and resilience for heavy-duty processing, they have also been prone to the relentless attack of corrosion.
But over the years, the industry has witnessed a significant evolution in the understanding and management of this persistent foe.
There are several strategies that can be utilized to curb or even eradicate the impacts of corrosion:
Corrosion-Resistant Metals and Alloys: One approach is to use metals and alloys that are inherently resistant to corrosion. These materials have been engineered to withstand the harsh conditions that typically lead to corrosion.
Corrosion Inhibitors: These are chemicals that can be added to a system to slow down or prevent corrosion. They work by forming a protective layer on the material’s surface, blocking the corrosive substances.
Design/Environment Modification: Sometimes, simply changing the design of the system or modifying the environment can help reduce corrosion. This could involve altering the system’s layout to minimize exposure to corrosive elements or changing the environmental conditions to make them less conducive to corrosion.
Galvanic or Impressed Current Cathodic Protection System: This method involves using a sacrificial anode (in the case of galvanic protection) or an external power source (in the case of impressed current protection) to protect the material from corrosion.
However, the most common and cost-effective method to decelerate corrosion is:
Protective Coating or Lining System: This involves applying a special coating or lining to the material’s surface. This layer acts as a barrier, protecting the material from the corrosive environment.
By comprehending and implementing these strategies, we can significantly extend the lifespan of our systems and structures, saving both time and resources in the long run.
Legislative Changes: Shifting the Tide
Legislative changes have played a pivotal role in the battle against corrosion. As early as the 1970s, regulations began to tighten in response to the environmental movement, leading to stricter standards for wastewater treatment.
The Clean Water Act in the United States, for instance, not only aimed to reduce pollution but also indirectly influenced the materials and practices used in wastewater infrastructure to prevent corrosion-related failures.
The battle against corrosion is ongoing, with new strategies and technologies continually being developed.
If interested in further exploring this topic, check out this extensive list of wastewater treatment technologies!
Interior Systems for Corrosion-Proofing a Wastewater Treatment Plant's HVAC/R
RTPF Coils
RTPF stands for Round Tube Plate Fin. These coils are a type of heat exchanger widely used in HVAC/R systems.
They consist of copper tubes mechanically bonded to aluminum fins. The fin pack is assembled with galvanized steel tube sheets and coil case.
This assembly has classic galvanic corrosion components with multi-metal bonds between the fin-and-tube and tube-and-tube sheet.
Methods to Prevent Galvanic Corrosion
1. Elimination of Bi-Metallic Couple
One way of preventing galvanic corrosion of RTPF coils is through the effective elimination of the bi-metallic couple. An example of this approach is the all-copper RTPF coil.
The use of an all-copper construction, i.e., copper tube/copper fin, virtually eliminates the presence of dissimilar metals, one of the necessary requirements for galvanic corrosion.
2. Use of Protective Coating
Another method commonly used to prevent galvanic corrosion is to isolate the two dissimilar metals from the electrolyte through the use of a protective coating.
The protective coating in effect creates a barrier between the dissimilar metallic couple and the electrolyte, thereby eliminating the electrolyte from this interface.
3. Insulation of Electrical Connection
A third way to prevent galvanic corrosion is to insulate the electrical connection of the copper and the aluminum using a pre-coated aluminum fin.
The pre-coating insulation removes the electrical contact of the dissimilar metals.
4. Novation® Heat Exchangers with Microchannel Coil Technology
Novation® heat exchangers with microchannel coil technology utilize several aluminum alloys in combination with a metallic coating.
The alloys are carefully chosen to extend the life of the coil. Furthermore, the coil has been designed so that any galvanic couple within the coil has been carefully chosen to provide the maximum life possible.
5. E-Coated Coils
E-coated cooling/heating coils have the same e-coating as the condenser coils.
All e-coated coils have a durable and flexible epoxy coating uniformly applied over all coil surfaces, including tube sheets and coil cases.
The coating provides a barrier between the coil surfaces and the corrosive effects of the atmosphere.
Benefits of Using These Methods to Prevent Galvanic Corrosion
Extended Coil Life: By preventing galvanic corrosion, these methods can significantly extend the life of the coils, reducing the need for frequent replacements and saving costs in the long run.
Improved Performance: Corrosion can degrade the performance of the coils. By preventing corrosion, these methods ensure that the coils continue to operate at their peak efficiency.
Reduced Maintenance: Corrosion can lead to increased maintenance needs. By preventing corrosion, these methods can reduce the time and effort required for maintenance.
Enhanced System Reliability: By ensuring the longevity and performance of the coils, these methods can enhance the overall reliability of the HVAC/R system.
Choosing the Best Method to Corrosion-Proof the Interior of your Wastewater Treatment Plant
Choosing the best method to prevent galvanic corrosion for your specific facility involves considering several factors. Let’s explore how to make an informed decision:
Facility Environment:
Assess the environment in which your RTPF coils will operate. Consider factors such as humidity, temperature variations, and exposure to corrosive substances.
If your facility is in a harsh environment (e.g., near the coast, in an industrial area, or where chemicals are present), more robust corrosion prevention methods may be necessary.
Budget and Cost:
Different methods have varying costs. Evaluate your budget constraints.
For example, all-copper coils are effective but can be more expensive than other options. E-coating may be a cost-effective choice.
Maintenance Requirements:
Some methods require less maintenance than others.
E-coated coils, once applied, provide long-lasting protection with minimal upkeep. On the other hand, protective coatings may need periodic inspection and touch-ups.
Longevity and Durability:
Consider the expected lifespan of your coils.
Novation® heat exchangers with microchannel coil technology are designed for extended life. If longevity is crucial, explore such advanced options.
Compatibility with Existing Systems:
Ensure that the chosen method aligns with your existing HVAC/R system.
For retrofitting, consider methods that seamlessly integrate with your current setup.
Performance Efficiency:
Evaluate how each method affects the performance of your coils.
All-copper coils may have excellent corrosion resistance but might impact heat transfer efficiency. Balance corrosion prevention with performance requirements.
Regulatory Compliance:
Check if there are any industry-specific regulations or standards related to corrosion prevention.
Some facilities (e.g., healthcare, food processing) have strict guidelines. Ensure your chosen method complies.
Consult Experts:
Seek advice from HVAC/R professionals or engineers.
They can assess your facility’s unique needs and recommend the most suitable method.
For more on protecting against environmental corrosion on your commercial condenser coils and heating/cooling coils, refer to Carriers Selection Guide.
Exterior Coating Systems for Enhanced Durability
Selecting the right coating systems is a critical choice that greatly influences the longevity and resilience of both concrete and steel structures.
How Coatings Protect
Coatings protect metal surfaces through three main mechanisms:
Barrier Protection: By creating a physical barrier, coatings prevent corrosive substances from reaching the metal surface.
Sacrificial Protection: Some coatings contain elements like zinc that corrode in place of the metal substrate, sacrificing themselves to protect the underlying surface.
Inhibitive Protection: Coatings can also include inhibitors that slow down the chemical reactions that lead to corrosion.
Optimal Coatings for Concrete and Steel Structures
Concrete structures benefit from coatings that act as a protective shield, safeguarding the porous substance from corrosive agents.
Epoxy-based systems are favored for their strength and resistance to chemicals. These coatings are generally applied in slender layers, approximately 10-15 mils dry film thickness (DFT), providing robust protection while maintaining structural integrity.
Steel structures gain from an array of coatings, each tailored to meet distinct environmental threats. For example, zinc-rich primers deliver sacrificial protection, oxidizing before the steel to avert rust.
Topcoats such as fluoropolymer offer superior color and gloss preservation, enhancing aesthetics while protecting against UV radiation and severe weather.
Coatings: The First Line of Defense
Paints and coatings serve as the first line of defense against corrosion.
They are complex mixtures composed of several key elements:
Resins: The backbone of the coating, providing adhesion, durability, and forming the protective film.
Pigments: These give the coating its color and opacity, and some types, like zinc, offer additional protection.
Solvents: These are used to dissolve the resins and pigments, aiding in application. Once the coating is applied, the solvents evaporate, leaving behind the protective layer.
Significance of NSF/ANSI 61 Certified Interior Liners for Drinking Water Tanks
For drinking water tanks, safety is of utmost importance. NSF/ANSI 61 certified interior liners are essential as they adhere to rigorous standards for drinking water contact.
These liners undergo testing to confirm they do not release harmful substances into the water, ensuring its cleanliness and safety for human use.
Employing NSF/ANSI 61 certified liners is not only a recommended practice but also a regulatory mandate, underscoring a dedication to public health and well-being.
Contemporary Coating Systems: Tnemec Series 435 Perma-Glaze
In settings with elevated hydrogen sulfide (H2S) levels, a prevalent and highly corrosive gas in wastewater treatment plants, cutting-edge coating systems like Tnemec Series 435 Perma-Glaze prove to be indispensable.
This state-of-the-art coating is formulated for exceptional H2S resistance, offering a sturdy, tile-like surface capable of enduring the harsh conditions present in these environments.
Its 100% solid composition guarantees a tough, impenetrable layer, blocking H2S infiltration and the ensuing corrosion.
Through judicious selection of suitable coating systems, the operational lifespan of wastewater treatment facilities can be substantially increased, ensuring their continued efficiency and safety for the foreseeable future.
Conclusion: The Last Line of Defense Against Corrosion
Selecting the appropriate corrosion-proofing methods is akin to choosing the best armor for a warrior. It’s about safeguarding the integrity of our infrastructure, ensuring that it can withstand the harsh conditions it faces daily.
Regular inspections, timely repairs, and adherence to safety regulations are the cornerstones of sustainable wastewater management. They are the practices that breathe longevity into the steel bones of our treatment plants.
By being proactive, we not only protect our present but also secure our future—a future where clean water flows freely, and our commitment to the environment stands uncorroded.
Let’s embrace the challenge and ensure that our wastewater treatment plants remain robust, efficient, and, most importantly, corrosion-free.
If you want to know more about keeping the interior of your wastewater treatment plant free of corrosion, click the banner below or visit the Engineered Temperature Solutions website and we can answer all of your questions.
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