1. PFAS and Their Behaviour in Wastewater Treatment

Per- and polyfluoroalkyl substances (PFAS) are a large group of synthetic chemicals widely used in industrial and consumer applications. Due to the strength of the carbon–fluorine bond, they are highly persistent in the environment and are often referred to as “forever chemicals”.

In wastewater treatment processes, PFAS are difficult to remove. Conventional treatment technologies show limited effectiveness in degrading or eliminating these compounds. Instead, PFAS tend to partition between the liquid and solid phases, with a significant proportion accumulating in sludge. The exact distribution depends on factors such as molecular structure, chain length and process conditions.

As a result, while treated water quality may meet discharge standards, certain contaminants are effectively transferred to the sludge phase, increasing the complexity of downstream sludge management.

2. Regulatory Trends and Shifting Sludge Disposal Pathways

Growing awareness of the environmental and health risks associated with PFAS has led to tightening regulations worldwide, with direct implications for sludge disposal.

In the United States, regulatory agencies have introduced strict limits for key PFAS compounds such as PFOA and PFOS, and several states have restricted or banned land application of biosolids. In the European Union, proposals for broader PFAS restrictions are under discussion, alongside existing limits in drinking water directives. In China, PFAS-related substances have been included in the list of priority controlled emerging pollutants, with regulatory frameworks continuing to evolve.

Under these conditions, traditional sludge reuse pathways – particularly agricultural land application – are facing increasing uncertainty. When PFAS concentrations exceed regulatory or application thresholds, alternative disposal routes such as landfill or incineration become necessary.

These alternatives typically involve higher costs and stricter operational requirements. In some projects, sludge disposal costs have already increased significantly, reflecting the impact of these regulatory changes.

3. Implications for Sludge Dewatering Systems

Although PFAS are primarily a chemical and regulatory concern, their effects are increasingly influencing sludge treatment and dewatering processes.

3.1 Changing Performance Requirements

Traditionally, sludge dewatering performance has been evaluated mainly by cake moisture content, which directly affects transportation and disposal costs.

As disposal pathways shift towards thermal processes such as incineration, lower moisture content becomes even more critical. Reduced water content improves calorific value and reduces the need for auxiliary fuel.

This trend places higher demands on sludge dewatering systems, which must achieve not only stable operation but also improved dewatering efficiency to meet evolving downstream requirements.

3.2 Filtrate Recirculation and System Load

While a substantial portion of PFAS accumulates in sludge, certain fractions remain in the liquid phase and are returned to the treatment system via filtrate. This can lead to recirculation and potential accumulation within the plant.

As regulatory limits become stricter, filtrate streams may require additional treatment, such as adsorption, ion exchange or membrane processes. This introduces new considerations for system design, where sludge dewatering is no longer an isolated unit but part of an integrated treatment approach.

3.3 System Adaptability and Process Integration

PFAS-related constraints are accelerating changes in sludge management strategies. Systems that rely on a single disposal route are becoming more vulnerable, while those designed for flexibility are better positioned to adapt.

In this context, sludge dewatering must be considered in conjunction with downstream processes such as drying, incineration or thermal treatment. The characteristics of the dewatered sludge – including moisture content and stability – directly influence the performance of these subsequent processes.

4. From a Single KPI to System-Level Optimisation

PFAS do not fundamentally change the principles of sludge dewatering, but they are redefining its role within the overall treatment system.

Moisture content alone is no longer sufficient as a performance indicator. Greater emphasis is now placed on system stability, compatibility with downstream processes and overall lifecycle cost.

The key question is shifting from “how to dewater sludge” to “how to ensure stable and efficient operation under changing disposal conditions”.

5. Outlook

As PFAS-related regulations continue to evolve, further adjustments in sludge management practices are expected. Anticipating these changes and optimising system design accordingly can help mitigate operational risks.

From an engineering perspective, developing adaptable and integrated sludge dewatering solutions is becoming increasingly important for long-term system reliability.