Concrete is the backbone of wastewater infrastructure, from sewer systems to treatment facilities. Despite its strength, concrete is an alkaline, permeable material that contains many pores formed from the hydration phase of concrete hardening. This permeable quality leaves concrete exposed to chemical and environmental deterioration.

To combat these issues, the concrete industry has focused on durability and waterproofing mix designs that reduce the risk of concrete failure in wastewater structures. A proper mix design can reduce the permeability of the concrete structure — and even self-heal any cracks that occur.

In this short article, we’ll explore the five most common problems that arise in concrete used in wastewater structures and explore preventative solutions to ensure the longevity of the structures and reduce repair and maintenance requirements.

5 Challenges with Concrete Used in Wastewater Treatment

Protecting concrete in corrosive wastewater environments requires understanding the specific challenges involved:

1. Chemical attack

Wastewater can contain various chemicals like chlorides, sulfates, and other chemicals that can degrade concrete over time. Chemicals from wastewater affect concrete in two ways:

1. Chemicals react with the concrete to reduce its pH, which can weaken the surface of the concrete.
2. Chemicals enter the pores of an unprotected concrete structure, and the cement paste that holds the aggregate in place can start to break down. 

A common chemical attack occurs when sewage forms a layer of sludge on the concrete. The sludge contains sulfate-reducing bacteria that consume oxygen in the sulfate ions in sewage and metabolize them into a hydrogen sulfide gas that reduces the pH of concrete. Once the pH of concrete is reduced from 12 to 9.5, additional bacterial action converts hydrogen sulfide into sulfuric acid that further attacks the concrete.

Chemical attack also happens when industrial or process-manufactured acids are mixed with the wastewater, lowering the pH and initiating a similar acid attack on the concrete. 

2. Abrasion and erosion

Wastewater can contain foreign materials such as sand, rocks, silt, or even ice in colder climates. These solid materials have an abrasive effect on the surface of a concrete clarifying tank during turbulent water flow conditions, eventually breaking the concrete down and leaving a smooth wear pattern on the substrate.

3. Carbonation

Carbonation involves the reaction of atmospheric carbon dioxide with the hydrated components of cement paste,  gradually reducing its pH. In other words, exposure to atmospheric CO2 naturally reduces the alkalinity of the concrete over time. 

As the pH is reduced, the rebar becomes more susceptible to chemical attack and begins to corrode. 

4. Freeze-thaw deterioration

Regulating thermal conditions in a wastewater facility is almost impossible, given the nature of the structures that support it. 

As such, it is inevitable that concrete will degrade over time when exposed to differential thermal (cold and hot) and humidity(wet and dry) conditions on opposing sides of the structure or to intermittent water immersion along with freezing temperatures.

5. Chloride-induced corrosion

When concrete is placed around rebar, the steel surface corrodes before a tightly adherent oxide film naturally forms over the surface to protect it from further corrosion — provided the film remains intact. However, when concrete comes under attack from chemicals, abrasion and erosion, carbonation, or freeze-thaw activity, moisture and oxygen can penetrate the concrete to reach the rebar. 

The chloride ions found in the moisture break down the protective film, causing the rebar to rust. The buildup of rust causes the concrete to expand, potentially leading to cracking or spalling and, once the concrete begins to crack, water, oxygen and aggressive chemicals can freely enter the concrete and further attack the embedded rebar, escalating the deterioration.

3 Ways to Prevent Concrete Deterioration in Wastewater Treatment Structures

Here are three effective strategies that leverage advanced materials and technologies to mitigate the common challenges of concrete deterioration in wastewater treatment:

1. Integral Crystalline Waterproofing

The use of an integral crystalline waterproofing admixture can be used to enhance the impermeability of concrete. When added to concrete, crystalline solutions react with water and unhydrated cement particles to form insoluble needle-shaped crystals that fill capillaries, micro-cracks, and pores within the concrete. 

This reaction not only blocks the pathways for water and harmful chemicals but also has the ability to self-seal any new cracks that may form, significantly reducing the risk of chemical attacks and ensuring the longevity of concrete structures in harsh wastewater environments.

Case Study: Las Maravillas Wastewater Treatment Plant Expansion, Tijuana, Baja California, Mexico

Las Maravillas Wastewater Treatment Plant #1 had a capacity of 40 liters per second and served around 10,000 people. However, its capacity was no longer enough, so Grupo Ruba began developing Las Maravillas #2.

The team added KIM to 2,000 m3 (70,629.33 ft3) of concrete and the Krystol Waterstop System to nearly 800 m (2,624.67 ft) of concrete with ease.

In the end, the results exceeded their expectations, with the lead engineer noting that his team experienced one of the best startups of a wastewater treatment plant they’ve ever had in recent memory. 

Using KIM and the Krystol Waterstop System also allowed them to save time and labor on manually applying an external waterproofing membrane. Instead, they could just add KIM directly into their concrete mix for immediate full-depth waterproofing and apply the Krystol Waterstop System to the nearby joints.

2. Integral Abrasion & Erosion Resistance or Hardener

Most hardeners are surface-applied and lose efficacy over time, especially if they are not applied correctly. 

Integral hardening admixtures are designed to increase concrete’s abrasion and erosion resistance. By integrating them into the concrete mix, the concrete’s surface life is extended in environments with high levels of physical wear, such as from sand, rocks, and other debris in wastewater treatment plants. 

This type of admixture is also particularly beneficial in protecting floors, walls, and other surfaces from the accelerated wear caused by the abrasive action of materials carried in wastewater, maintaining structural integrity and reducing maintenance costs.

Case Study: Aquatera’s Wastewater Treatment Plant, Grande Prairie, AB, Canada

In 2013, faced with a rapidly growing population in Alberta’s Grande Prairie, Aquatera urgently needed to upgrade its wastewater treatment plant — a $58 million project marked by tight timelines and challenging winter conditions. 

To ensure the project’s success, the construction team used Kryton’s KIM for integral waterproofing and Hard-Cem for integral durability. These admixtures eliminated the need for external waterproofing membranes and surface applications, simplifying construction while providing reliable waterproofing and durability. KIM protected various areas, including slabs, retaining walls, and structural concrete, guarding against potential chemical attacks and wastewater ingress. Hard-Cem fortified the centrifuge building’s slab-on-grade for resilience against chlorides and sulfate exposure. Combining both admixtures safeguarded the concrete composite of the plant’s steel deck against moisture ingress, chemical threats, and abrasive wear. 

3. Surface Protection

Surface-applied crystalline waterproofing treatments can be applied to freshly placed concrete slabs, such as those found in wastewater treatment facilities. 

This post-applied waterproofing solution works by penetrating the concrete and forming crystals within the pores and capillary tracts, effectively preventing water ingress and protecting against chemical degradation. It can also be used as a remedial solution for existing concrete, where it helps to repair and waterproof old or damaged concrete surfaces exposed to the aggressive wastewater environment.

Case Study: Mid-Halton Wastewater Treatment Plant, Oakville, ON, Canada

In preparation for a huge population boom, a regional government in Ontario began modernizing its aging wastewater network at the Mid-Halton Wastewater Treatment Plant. 

Hydrostatic pressure from groundwater was extremely high around the wastewater tank area, so the construction team recommended a permanent, high-performing concrete waterproofing system to prevent cross-contamination between groundwater and wastewater. 

The team applied Krystol T1 to the tanks’ concrete walls, protecting them from water intrusion. Krystol T1 is a surface-applied crystalline slurry treatment that transforms new or existing concrete into a permanent waterproofing barrier.  

For more details, visit Kryton’s extensive line of waterproofing and hardening solutions or contact one of our experts.