Is high-temperature pyrolysis key to the PFAS puzzle in biosolids?

This continues GHD’s ongoing thought leadership series on PFAS, including most recently: PFAS in Biosolids: 5 Considerations for Wastewater Treatment Plant (WWTP) Operators.

Turning a waste product into something useful is generally seen as a good thing. This can include taking the waste products of municipal wastewater treatment – biosolids and bioliquids – and diverting them away from landfills to deposit on farmland, with the added benefit of higher crop productivity. Approximately 4.75 million dry metric tons (dmt) of biosolids are generated annually, with about 2.4 million dmt biosolids being land applied. What is clear, not all wastes are created equally.

There’s growing concern that substances in the biosolids and bioliquids may pose future threats to the environment and people. Waste streams that produce biosolids, including municipal sewage, often contain per-and poly-fluoroalkyl substances, or PFAS. This is a class of manufactured substances, commonly referred to as “forever chemicals,” which are widely used in common goods, including non-stick cookware, breathable outerwear and textiles, cosmetics, food packaging, and other products. There’s increasing evidence about the health impacts of PFAS, such as increased cholesterol levels and decreased vaccine response in children, including those found in the waste stream.

There are several ways to reduce the risk of PFAS by breaking up the constituent molecules. The challenge lies in finding practical, effective and affordable ways to do this, as the structure of PFAS by design makes it a challenging compound to break down.

Turning up the heat on PFAS

As noted above, PFAS molecules tend to be extraordinarily durable, partly because they include a carbon-fluorine bond, which is one of the strongest in nature, hence the common nickname for these compounds is “forever chemicals.” But if PFAS can be separated into smaller constituent molecules and ultimately the elementary atoms making up those molecules, the potential risks to human health and the environment can be mitigated and the challenge of managing the risk can be significantly reduced.

One particularly promising technology is pyrolysis, which involves heating organic matter in the absence of oxygen to high temperatures -- in the range of 500 to 800 degrees Celsius -- which has been shown to break PFAS molecules.

GHD is at the forefront of applying pyrolysis to municipal wastewater treatment. One such example is the design of a pyrolysis system coupled with biodryers for a 3.8 million gallon per day (MGD) wastewater treatment facility in Ephrata, PA, USA.

The innovative biodrying and pyrolysis process will replace an existing anaerobic digestion system that currently produces less stabilized biosolids. We have completed the design of the new system, with construction expected to start in early 2022 and the system up and running by mid 2023.

Once completed, dewatered biosolids will be reduced ten-fold in the new biodryers and pyrolysis reactor, producing about 200 pounds of biochar for every ton of dewatered cake solids. While not the primary driver for this project, knowing that the pyrolysis technology will potentially address future PFAS regulations was a key reason the client selected this technology over several others considered.

While the project in Ephrata is still in its relative infancy, pyrolysis technology looks promising as part of a comprehensive solution to the PFAS puzzle. Other pyrolysis pilots and demonstration projects underway also show similar success in potentially eliminating PFAS from biosolids.

Biochar: a valuable by-product of pyrolysis

The pyrolysis of biosolids not only breaks down PFAS but also produces a valuable by-product know as biochar - a form of charcoal. Widely used as an amendment to boost soil productivity, biochar is important for increasing crop yields, decreasing emission of carbon dioxide, and adsorbing environmental contaminants.

Many municipalities must currently pay to dispose of the biosolids they generate. Turning that costly waste product into something they can sell reverses the economics completely while creating a valuable product that ultimately feeds into a more sustainable solution and pathway towards an increasingly circular economy around this waste. It has the potential to make the solids disposal operation cost neutral or even profitable, potentially recouping some of the operational costs of biosolids treatment.

Another benefit of biochar becomes more important as entities consider the carbon impact of their operations. Biochar production and incorporation into agricultural soils are carbon negative. This is because the pyrolysis process diverts the carbon present in the biosolids feedstock, and concentrates that carbon in the biochar, instead of sending it up the stack as carbon dioxide or landfill gas.

The same cannot be said of landfill decay and other higher temperature thermal processes such as gasification and incineration that occur in the presence of oxygen, consequently producing higher levels of carbon dioxide. Biochar is a stable means of sequestering carbon in the ground for hundreds if not thousands of years, resulting in a net negative greenhouse gas profile.

Another benefit of pyrolysis is that due to the high temperature and holding time, the process also destroys biological contamination, such as viruses and pathogens. Biological contamination has been a long-standing concern regarding biosolids handling and reuse.

Pyrolysis may be of particular interest to solid waste facilities, food waste anaerobic digesters, WWTPs, and other entities that need to handle and treat biosolids and who are concerned about the impact of PFAS, other contaminants, and current energy intensive biosolids processing operations.

We welcome partners interested in learning more to determine if pyrolysis can be a viable solution to their PFAS problem and other aspects of biosolids management.

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