Water Treatment Waste Creates Stronger Sewer Pipes

Australian engineers have discovered that mixing discarded water treatment sludge with industrial slag creates concrete alternatives that are more than 50% stronger than traditional materials used in sewage infrastructure.

The new alkali-activated materials resist acid-eating bacteria that cost Australian taxpayers nearly $70 billion annually in repairs and maintenance. This development could transform how cities build sewage systems while solving a growing waste disposal problem.

The research, published in the Journal of Building Engineering, demonstrates how alum-based water treatment sludge (AWTS)—typically destined for landfills—can be combined with ground granulated blast furnace slag to create superior construction materials for corrosive environments.

The Hidden Enemy in Sewage Systems

Concrete sewage pipes face a relentless biological assault. Sulfate-reducing bacteria in sewage produce hydrogen sulfide gas, which rises into the pipe’s headspace and lowers the concrete’s surface pH from 12 to below 7. Once this happens, sulfur-oxidizing bacteria colonize the surface and produce sulfuric acid, creating a destructive cycle that eats through pipe walls.

This microbiologically influenced corrosion (MIC) process creates gypsum—a mushy, non-adhesive material that lacks structural integrity. The formation of gypsum and ettringite causes significant volume expansion within the cement matrix, leading to cracking and structural failure that requires costly repairs across Australia’s 9,300 kilometers of sewage infrastructure.

“Sludge is usually disposed of in landfill sites, which not only reduces available land for other uses, but also harms the environment, creating CO₂ emissions from transporting the waste,” explains Weiwei Duan, the University of South Australia PhD candidate who led the research.

Engineering a Biological Defense

The research team tested their new materials using live sulfur-oxidizing bacteria sourced from actual sewage treatment plants. They created controlled laboratory conditions that mimicked real sewer environments, including the pH changes and bacterial growth patterns found in functioning sewage systems.

The key innovation lies in the material composition. By replacing 20-40% of blast furnace slag with processed water treatment sludge, the researchers created alkali-activated materials with fundamentally different chemical properties:

• Reduced calcium availability – limiting the raw materials bacteria need to form destructive gypsum
• Lower porosity – creating physical barriers that prevent bacterial penetration
• Hybrid gel formation – developing both C-A-S-H and N-A-S-H gels that enhance acid resistance

The Science Behind Superior Strength

The research revealed a sophisticated chemical defense mechanism. Traditional concrete relies heavily on calcium-rich phases that bacteria can easily attack. The new material creates a dual-gel system where calcium-based C-A-S-H gel provides early strength while sodium-aluminosilicate-hydrate (N-A-S-H) gel contributes long-term durability through lower solubility in acidic environments.

When exposed to bacterial attack for 56 days, samples containing 20-40% water treatment sludge maintained over 50% higher compressive strength compared to conventional blast furnace slag concrete. Equally important, they showed dramatically reduced formation of destructive gypsum deposits.

The researchers used sophisticated analytical techniques including X-ray diffraction, infrared spectroscopy, and electron microscopy to understand exactly how the bacterial attack progresses. They discovered that the improved materials create a “sandwich-like” structure during corrosion, with protective silica-rich layers that slow acid penetration.

Breaking the Bacterial Assault Pattern

Perhaps most significantly, the study revealed why some materials resist bacterial damage better than others. Using energy-dispersive spectroscopy, researchers found high concentrations of phosphorus—a key indicator of bacterial activity—within corroded samples. This showed that sulfur-oxidizing bacteria can penetrate protective layers and continue producing acid directly against the concrete matrix.

However, materials with 20-40% water treatment sludge content showed lower porosity that effectively limited bacterial migration. The reduced calcium content also meant less raw material for gypsum formation, breaking the destructive cycle that typically accelerates pipe deterioration.

The optimal composition appears to balance multiple factors: sufficient calcium for early strength development, adequate aluminosilicate content for long-term durability, and controlled porosity that blocks bacterial penetration without compromising structural integrity.

Environmental and Economic Benefits

Professor Yan Zhuge, who supervised the research, emphasizes the broader implications: “This has the potential to extend the service life of sewage pipes, reduce maintenance costs, and promote the reuse of water treatment byproducts, thus contributing to the circular economy. The construction industry is one of the world’s biggest greenhouse gas emitters, so if we can cut down on the need for cement, we will be helping to lower carbon emissions.”

The environmental benefits extend beyond carbon reduction. Water treatment facilities worldwide generate millions of tons of aluminum-rich sludge annually. Current disposal methods typically involve landfilling, which consumes valuable land resources and generates transportation-related emissions.

The research showed that calcining water treatment sludge at 800°C for two hours optimizes its reactivity for concrete production. This thermal treatment decomposes hydrated phases and creates reactive amorphous aluminosilicate phases that enhance the final material’s performance.

Real-World Applications Ahead

While laboratory results are promising, the researchers acknowledge challenges in scaling up production. Consistent quality control of water treatment sludge, uniform pre-treatment processes, and validation in real sewer environments remain important next steps.

The work has already gained recognition—Duan recently received the 2025 Australian Water Association’s Student Water Prize, marking the first time a University of South Australia student has received this national honor in 60 years.

As cities worldwide grapple with aging infrastructure and mounting environmental pressures, solutions that simultaneously address waste disposal and infrastructure durability could prove invaluable. The research suggests that sometimes the answer to complex engineering challenges lies not in developing entirely new materials, but in finding innovative uses for the waste streams we already produce.