Same same but different

A 2023 study tested biosolids from sewage treatment plants operated by six northern Queensland councils. It looked beyond nutrients and the usual heavy metals, testing for PFAS, antibiotics, antidepressants, other medicines and chemicals used in soaps, detergents and personal-care products.

https://www.sciencedirect.com/science/article/pii/S0269749123007881?via%3Dihub

The study was limited. It reported seven biosolids groups, generally using at least three samples from each treatment plant. It did not test Unitywater biosolids and cannot tell us what would arrive at Glan Devon. What it does show is the possible scale of variation.

Different sources produced different biosolids

One agricultural and industrial catchment recorded zinc at about 2,430 milligrams per kilogram and copper at about 1,050 milligrams per kilogram. Other biosolids contained higher levels of antibiotics, antidepressants and synthetic fragrances.

The clearest comparison involved two sewage treatment plants operated by the same council. One produced biosolids containing an average of about 29.3 micrograms per kilogram of PFOS. The other contained about 253 micrograms per kilogram—nearly nine times as much.

The researchers linked these differences to the industries, hospitals, homes and known contaminated areas connected to each sewer network. In simple terms, biosolids carry part of the chemical history of the communities that produced them. And that chemical history changes by both location and by the season.

This means “biosolids” is a material category, not a reliable chemical specification.

What do the PFAS levels mean?

A PFOS concentration of 253 micrograms per kilogram is equal to 253 milligrams in each dry tonne. Processing 10,000 dry tonnes at that concentration would involve about 2.53 kilograms of PFOS.

That is a tiny share of the total material, but PFOS matters because it is persistent and can accumulate through repeated environmental loading.

Australia’s current PFAS National Environmental Management Plan is national guidance rather than a stand-alone Queensland law. It provides a default restricted-use screening level of 31 micrograms per kilogram for PFOS plus PFHxS, with lower values where larger safety margins are applied. The sample containing 253 micrograms per kilogram of PFOS alone was therefore more than eight times the default restricted-use value. The 29.3 result was close to the default value but remained above the more precautionary levels.

That comparison does not decide whether a particular biosolids batch can be applied to a particular paddock. The amount already present in the soil, the application rate and the total contaminant mass per hectare also matter.

For metals, the paper’s authors concluded that most samples were Grade B, mainly because of zinc and copper. That generally means restricted use rather than automatic prohibition. Queensland’s Biosolids End of Waste Code remains the main state instrument controlling when biosolids can be supplied and used as a resource.

Many of the pharmaceuticals and synthetic fragrances measured in the study do not have simple numerical limits in that code. Their presence does not automatically prove a legal breach, but it shows that modern testing can identify a wider range of chemicals than the regulated list covers.

Why changing the material may change contaminant behaviour

Many chemicals attach themselves to organic matter, minerals and fine particles in biosolids in a process called “sorption”. Certain components in biosolids act as a trap, capturing some chemicals in preference to others.

How strongly a chemical is held can change when the pH, moisture or mineral composition changes. The paper discusses earlier experiments in which adding lime made some medicines more mobile while causing others to remain more strongly attached to the solids.

This matters because Glan Devon proposes to mix dried biosolids with mineral fertiliser and then make a pellet or granule. The application does not yet clearly define the binders, conditioners, liquids or chemicals that may be added.

Those additions may affect more than pellet strength. They may also change how PFAS, medicines and metals move between solids, water, fines and other process streams. The paper does not prove that a future Glan Devon additive will cause such a change, but it shows why the additives and process chemistry must be identified and assessed.

Drying is not the same as carbonisation

At Glan Devon, the Xetrov unit would process non-recyclable waste to provide heat. That heat would dry the biosolids before they were mixed with mineral fertiliser.

Drying mainly removes water. It does not demonstrate that metals, PFAS, pharmaceuticals or microplastics have been destroyed. Metals remain, while organic contaminants may remain in the dried product, move into condensate or air, attach to dust or partly transform. The outcome depends on the actual process conditions.

At the same time as the BYV Organics Glan Devon proposal is being assessed, Unitywater has its own alternative biosolids project under consideration at Yandina. Unitywater proposes to dry and then carbonise the biosolids themselves using a high-temperature process, converting them into biochar rather than retaining them as dried biosolids blended with mineral fertiliser.

The difference is important:

BYV Organics: use heat mainly to remove water, then retain the biosolids in a fertiliser product.

Unitywater: thermally transform the biosolids themselves into a different carbon-rich material.

Neither approach should be accepted merely because it involves heat. BYV Organics needs evidence showing where contaminants go during drying and pellet manufacture. Unitywater needs evidence showing destruction efficiency, breakdown products, emissions and the quality of the resulting biochar.

Why this matters to the whole Glan Devon project

The BYV Organics proposal is often described as a simple resource-recovery process:

> Receive biosolids, dry them, add mineral fertiliser, make pellets and sell the product.

The northern Queensland study suggests a more complicated reality. The facility may receive chemically different biosolids from different sewage treatment plants and different sampling periods.

That variability affects the whole project:

* which biosolids can be accepted;

* what happens during drying;

* whether contaminants enter the product, condensate, dust or emissions;

* how pellet additives change contaminant behaviour;

* whether fines and recycled material have the same composition as the finished product;

* how failed batches are managed;

* whether the final material meets an End of Waste pathway;

* and how contaminant mass is tracked onto individual paddocks.

The study does not prove that Glan Devon’s process will fail or cause harm. It does show that Queensland biosolids can contain complex mixtures of contaminants and that their concentrations can vary sharply between treatment plants.

The central question is therefore not simply whether Glan Devon can dry biosolids and make pellets. It is whether the proposal has defined and tested a complete system capable of controlling variable contaminants from the sewage treatment plant through to the final receiving land.

https://www.dcceew.gov.au/environment/protection/publications/pfas-nemp-3 "PFAS National Environmental Management Plan 3.1"

https://www.business.qld.gov.au/running-business/environment/waste-management/regulated-waste/eow-codes"End of waste codes"

https://www.unitywater.com/en/about-us/projects-in-your-area/major-projects/bioresource-recovery-facility "Bioresource Recovery Facility"

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