Whitepaper: Understanding the Effects of Wastewater on Concrete Surfaces

Understanding the Effects of Wastewater on Concrete Surfaces
By Dustin Young, Technical Services Specialist, SSPC

Wastewater treatment facilities present severe environments for high-performance protective coatings. The facilities offer an abundance of different deterioration issues, inducing hazards that affect not only the performance of the protective coating, but also the substrate. Due to the corrosion hazards being so severe, it is important to identify problems and remediate as soon as possible because this corrosive environment can rapidly deteriorate a structure. 

Understanding Concrete in Wastewater Facilities
To understand corrosion in a wastewater facility you must first understand the structure that holds the wastewater. Concrete is the most popular material used to construct a wastewater treatment facility. Concrete itself is a strong alkaline product and while it is externally dense, it contains many pores through a channel of capillaries formed from the hydration phase of concrete hardening. These capillaries are the main culprit of concrete permeability and act as channels in wastewater facilities for chemical deterioration of building materials.  

A common reaction that results from unprotected concrete being exposed to wastewater is the breakdown of the cement paste that holds the aggregate in place. This happens when the chemicals from wastewater enter the capillaries in the concrete structure. Another common form of concrete deterioration in wastewater facilities occurs when the chemicals found in wastewater react with the concrete to reduce its pH, which can result in a breakdown or weakened adherent sand matrix at the surface of concrete. 

This explains why the concrete industry is more focused on the mix design (formulation) of concrete to reduce the chances of corrosion. A proper mix design can reduce the number of capillaries found in the concrete structure. Reducing the number of capillaries can then reduce the amount of moisture that enters the concrete, which is the main villain involved in concrete deterioration. While a solid concrete mix design can reduce the amount of permeability in a concrete structure, the use of high-performance protective coatings is a more effective approach to ensuring a longer service life for a wastewater facility. The decision to use a protective coating on a wastewater facility should be an easy one given the corrosive environment, but a harder, more time-consuming decision is which protective coating system should you use? 

Root Causes of Deterioration in Wastewater Facilities
When determining which protective coating system to use in a wastewater facility you must first consider the corrosive environment of wastewater itself. The most common corrosive exposures in a wastewater facility include, but are not limited to, chemical attack, abrasion erosion, chloride-ion induced corrosion, and freeze-thaw conditions. 

Chemical Attack
Chemical attacks can occur both naturally and synthetically in wastewater facilities. A synthetically induced chemical attack happens when manufactured acids are mixed with the wastewater to lower the pH, initiating an acid attack on concrete. A naturally occurring chemical attack commonly found in wastewater facilities includes sewage that forms a layer of sludge. The sludge contains sulfate-reducing bacteria known as SRB. The SRB will react with oxygen in the sulfate ions (contained in sewage) to form sulfide ions. The sulfide ions will then react with hydrogen to form hydrogen sulfide, which will create a hydrogen sulfide gas that reduces the pH of concrete. Once the pH of concrete is reduced from roughly 12 (normal) to 9.5, sulfuric acid can be formed. Sulfuric acid will attack Portland cement concrete. In addition to the atmosphere in wastewater facilities harvesting or creating an abundance of moisture and oxygen, those elements combine with the lower pH creating SOB, or sulfur oxidizing bacteria. The SOB will begin to colonize on the concrete substrate. It uses the oxygen- and hydrogen-rich atmosphere to again form sulfuric acid causing an acid attack. 

Abrasion Erosion
Wastewater can contain foreign materials, such as sand, rocks, ice, or silt. These materials will contact the substrate during unstable water flow conditions. This repetitive contact will cause the concrete to breakdown and produce a smooth wear pattern on the substrate.

Chloride-Induced Corrosion 
Chloride-induced corrosion can happen in any environment, not just wastewater facilities. Moisture will often enter the pores of a concrete surface and penetrate the protective film of the steel reinforcement (rebar). The chloride ions found in the moisture will break down the protective film and cause the steel to rust. The rust will gradually increase to cause the concrete to expand. This expansion will cause the concrete to crack or spall.

Carbonation
Carbonation occurs naturally in all concrete exposed to the atmosphere. It involves the reaction of atmospheric carbon dioxide with the hydrated components of Portland cement paste, especially carbon dioxide. The reaction of the carbon dioxide with concrete gradually reduces the cement paste’s pH. As the pH of cement paste is reduced, the reinforcement steel (rebar) no longer has alkaline protection and begins to corrode. 

Freeze-Thaw Deterioration
Regulating thermal conditions in a wastewater facility is nearly impossible.  Concrete will deteriorate over time when exposed to varying thermal conditions (hot and cold) and humidity cycling (dry and wet) on opposite sides of the structure. Irregular water immersion along with freezing temperatures can result in concrete deterioration. 

Protective Coatings Commonly Used in Wastewater Facilities 
Now that we have addressed common root causes of concrete deterioration in wastewater facilities, we can discuss which protective coatings are often successful in these types of environments. A few things to consider when selecting the right protective coating include exposure time of wetness, physical conditions (temperature, flow, etc.)  of the service environment, chemical conditions of the environment, and the condition of the prepared substrate. The protective coating system must also meet the wastewater facility owner’s criteria. Penetrating sealers, thin-film coating systems, thick-film coating systems, or linings may be required. 

• Penetrating sealers are commonly used to penetrate the concrete substrate and seal the pores from moisture. 

• Thin-film coating systems commonly used with a primer have a DFT ranging from 10-30 mils. 

• Thick-film coatings or lining systems typically have a DFT range of 40-120 mils 

While penetrating sealers in wastewater treatment facilities are often used as primers, thin-film coating systems provide moderate chemical resistance and perform well against sulfide attack. 

Examples include:
• Epoxies (typically high solids, amine-cured)
• Polyurethanes (typically high solids, two-component)
• Vinyl Esters

Thin-film systems hold up in moderately exposed wastewater treatment facilities, a more severe environment will require a thick-film/heavy-duty lining. Severe exposures to biogenic corrosion, sulfate attack, chloride-induced corrosion, and carbonation require a coating system that will not only resist these hazardous exposures, but also resist being constantly immersed in wastewater. 

Examples include:
• Epoxies (Typically high build, high performance, amine-cured)
• Polyurethanes (Typically high build, aromatic)
• Polyureas

While none of the aforementioned coating systems offer a permanent solution to deterioration in wastewater facilities, these coatings provide the best performance given the corrosive environment. 

It is also important to acknowledge that properly trained applicators should apply these coating systems. A common mistake encountered in the field is using unqualified contractors to apply high performance protective coatings. SSPC’s Painting Contractor Certification Program (PCCP) evaluates contractor capabilities to successfully complete protective coating application work. SSPC-QP 8 specifically evaluates the qualifications of contractors that coat concrete. In addition to training contractors, SSPC offers a Concrete Coating Inspection (CCI) course that qualifies inspectors specifically for inspection of protective coatings applied on concrete surfaces. 

Summary
Wastewater treatment facilities provide an abundance of threats hazardous to the stability of concrete as a material of construction used in wastewater facilities. This article has provided an overview of concrete deterioration and the root causes of such deterioration, along with suggestions for protective solutions to minimize corrosion threats when concrete is used in wastewater facilities. Wastewater treatment facilities pose not only chemical but physical threats to concrete as well. It is critical to understand the properties of concrete and how these threats affect concrete deterioration. Once these topics are understood, the job of selecting a protective coating system becomes much easier. 

References
1.) Bayne, Heather, “The Basics of Deteriorating Concrete at Wastewater Plants: Tips on Causes, Repair, and Resources.” Journal of Protective Coatings and Linings (JPCL), September 2009, pp 47-54.
2.) Nixon, Randy and Drisko, Richard, “The Fundamentals of Cleaning and Coating Concrete.” SSPC: The Society for Protective Coatings, 2001.
3.) SSPC-Paint 44, “Liquid-Applied Organic Polymeric Coatings and Linings for Concrete Structures in Municipal Wastewater Facilities, Performance-Based,” SSPC: The Society for Protective Coatings, 2013.
4.) Morris, Kevin, “Advice from a Wastewater Expert,” PaintSquare News, September 2017.
5.) Bennett, David C. and Nixon, Randy, “Corrosion and Materials Fundamentals for Engineers in Wastewater Treatment Plants & Collection Systems,” NACE International, 2016. 
6.) Ault, J. Peter, Greenfield, Kyle, and Hosein, Susane. “Applying Linings to Concrete Surfaces in Water and Wastewater Environments,” Protective Coatings for Water and Wastewater Facilities, SSPC: The Society for Protective Coatings, 2006.