The Witches Brew

Let's discuss exactly what's in this biosolids and sewage sludge stuff

Scott Dunham

10/28/202611 min read

What really goes into our wastewater treatment plants, what comes out, and how much do we actually know about what happens in between?

There is a simple principle in science worth remembering whenever somebody tells you they have found a clever way to make waste disappear. What goes in must come out.

Matter doesn't simply vanish. It might change form, react with something else or break down into simpler compounds. But the atoms are still there, and they have to end up somewhere. Keep that in mind as we take a look at something most of us would rather not think about: what happens to everything we flush down the toilet or pour down the drain. And I do mean everything.

Where does it all come from?

Most people probably imagine wastewater treatment plants receiving sewage from homes and businesses. Human waste, toilet paper and plenty of water. Not particularly pleasant, but fairly straightforward. The reality is considerably more interesting.

Our sewerage systems collect wastewater from households, hospitals, aged-care facilities, restaurants, laundries, workshops, laboratories, factories and numerous other businesses. Commercial and industrial wastewater is generally called trade waste. Depending on its source, it can contain fats, oils, detergents, solvents, metals, cleaning chemicals and industrial compounds.

There are rules governing what businesses can discharge. Some substances are prohibited, others have concentration limits, and some discharges require pretreatment. But permitted discharge doesn't mean chemically pure water. It means wastewater that meets the receiving utility's conditions.

Then there are wastes delivered by tanker. Septic tank sludge, chemical toilet waste, industrial wastewater and, surprisingly, landfill leachate. Yes, landfill leachate. We'll come back to that shortly.

In South East Queensland, both Unitywater and Urban Utilities have arrangements for receiving approved liquid wastes. Unitywater's published 2026–27 charges specifically include tankered landfill leachate. So the sewage arriving at a wastewater treatment plant can be rather more complicated than the contents of your toilet. Although, come to think of it, what's in your toilet is complicated enough.

The household contribution

Let's start with the obvious. Human faeces, urine and toilet paper. Except even toilet paper isn't quite as simple as it looks. Depending on the product, it can contain different cellulose fibres, recycled paper, dyes, bleaching residues and manufacturing additives - even PFAS according to a recent study.

And toilet paper is only the beginning. People flush wet wipes, tissues, paper towels, tampons, sanitary pads, condoms, nappies, cotton buds, dental floss, cigarette butts, cat litter, animal faeces, food scraps, small toys and occasionally even dead pets or unfortunate goldfish.

I remember having the septic tank pumped out at a house where we were the second owners. The hose went in, the pump started, and we were treated to a succession of strange noises. Occasional clanks and clatters, rather like a vacuum cleaner picking up missing buttons. Then the hose blocked completely.

We stopped the pump to investigate. The offending item turned out to be a Barbie doll. Without her head. Goodness knows what Barbie had done to deserve that particular fate, or how long she'd been down there. But it does make you wonder what else finds its way into these systems.

Then there's the blood, saliva, mucus, vomit, hair, skin cells and everything else our bodies produce. And not everything enters through the toilet.

From the bathroom come soaps, shampoos, cosmetics, toothpaste, fragrances, sunscreens, hair dyes, disinfectants and toilet cleaners. Some contain bleach, acids or strong alkalis. From the kitchen come food particles, fats, cooking oils, detergents and coffee grounds. Washing machines add fabric softeners, dyes, dirt and fibres shed from clothing, including tiny plastic fibres.

Then there are pharmaceuticals. Prescription medicines, painkillers, antibiotics, hormones, vitamins and recreational drugs all contribute residues. Much of this is perfectly normal: we consume substances, metabolise them, and excrete some of the original compounds or their breakdown products. Others arrive because someone has disposed of unwanted medicines, paints, pesticides or household chemicals down a drain.

Millions of ordinary activities create an extraordinary chemical mixture. And that's before we add anything from industry.

The changing contents of the household cupboard

There's another complication that's easy to overlook. The chemical inventory entering our wastewater systems isn't constant. It changes with time, sometimes quite dramatically.

Think back fifty years, to 1976. Australian households had soaps, detergents, cleaning products, medicines and cosmetics, but the collection of chemicals found in an ordinary supermarket today is very different. Consider something as simple as soap.

Traditional soap is made from fats or oils reacted with an alkali. During the twentieth century, manufacturers increasingly adopted synthetic surfactants: chemicals that clean, emulsify grease and produce foam without being soap at all. Sodium lauryl sulfate (SLS), sodium laureth sulfate (SLES) and linear alkylbenzene sulfonates (LAS) are familiar examples.

There's an interesting piece of history here. Early synthetic detergents used branched alkylbenzene sulfonates that resisted biological degradation. They worked well enough in washing machines but caused persistent foaming in sewage treatment plants, rivers and waterways. By the 1960s, manufacturers were moving towards more biodegradable linear compounds.

Think about that. We developed synthetic chemicals to make washing easier. They worked, but created an environmental problem, so we changed the chemistry to reduce it. And the replacement chemicals went down the drain instead. It was a genuine improvement, but it illustrates how solving one problem changes the chemical inventory that wastewater systems must manage.

Consider antibacterial soaps and hand washes. Compounds such as triclosan became widespread in consumer products during the late twentieth century. Concerns about safety, environmental persistence and antimicrobial resistance eventually led to restrictions and reformulation.

Or those tiny plastic beads once added to facial scrubs. They exfoliated the skin, then washed down the drain. Australia began a voluntary phase-out in 2016, and by 2020 a government-commissioned assessment found 99.3% of surveyed rinse-off products were free of microbeads.

Some substances disappear, others replace them, and entirely new products appear. We humans are incredibly inventive. Sometimes the replacements raise questions of their own. A revealing Australian study examined archived wastewater samples from 2010 to 2020 and found that concentrations of several older PFAS compounds declined while some shorter-chain compounds became more prominent. Researchers could actually observe changes in the community's chemical footprint by examining its wastewater.

Why does that matter? For one thing, our recollection of past risks is based on a different chemical world. Decades of apparently uneventful sewage disposal tell us little about substances that were rarely used, poorly measured or not yet manufactured.

Regulation also takes time to catch up. A chemical can become widely used long before its environmental effects are recognised and controls introduced. During that interval, it doesn't politely wait for science and legislation.

And what about the future? What new medicines, synthetic materials, cleaning products and household chemicals will be commonplace in 2036 or 2056? Which familiar chemicals will disappear, and which will still be turning up in wastewater decades after their use has ended? We're not dealing with a fixed recipe. We're dealing with one that keeps changing.

A gold mine without the gold

Now let's return to landfill leachate, because this is a particularly fascinating part of the story.

Imagine a landfill. Thousands upon thousands of tonnes of rubbish accumulated over decades. Food waste, plastics, paper, clothing, building materials, broken equipment, household chemicals, discarded containers, metals and whatever else the landfill has been permitted to accept.

Rain falls, water penetrates the waste, dissolves substances, reacts with minerals and chemicals, and carries contaminants into the liquid collected at the bottom. That liquid is called leachate. For anyone familiar with gold mining, the process should sound rather familiar.

In a gold heap-leach operation, crushed rock is stacked into a pile and irrigated with a chemical solution, commonly containing cyanide. The solution percolates through the rock, dissolves some of the gold and is recovered for processing. The chemistry and water movement are carefully managed because the purpose is to extract particular metals.

A landfill operates on a broadly similar physical principle, but with very different and far less controlled chemistry. Rather than a reasonably well-characterised pile of crushed rock, we have an extraordinarily heterogeneous mixture of waste. Rather than a deliberately formulated leaching solution, we have water reacting with whatever chemicals the landfill produces or contains. And rather than recovering gold, we're trying to manage whatever the water has picked up.

A landfill is effectively an enormous, slowly changing chemical reactor.

As organic matter decomposes, it produces compounds that alter acidity and metal solubility. Microbial activity changes chemical conditions, containers corrode, materials deteriorate, the waste settles, and water finds new pathways through the pile.

Then a storm arrives. Rainfall can dilute some contaminants while mobilising others, changing both their concentrations and the total mass leaving the landfill. Previously isolated materials may be exposed to new conditions.

The leachate collected today can reflect waste deposited decades earlier. It may contain ammonia, dissolved salts, metals, hydrocarbons, industrial chemicals, PFAS and numerous other substances. Modern engineered landfills collect and manage leachate, which may be treated on-site or transported elsewhere, including to wastewater treatment systems.

And here's another connection with our changing household chemistry. Something discarded in 1986 may still influence leachate collected in 2026. Even when a chemical disappears from supermarket shelves, it can continue entering wastewater through an entirely different route. The past and present can quite literally become mixed together.

Where does everything go?

Now we have our mixture of domestic sewage, industrial wastewater and other liquid wastes arriving at the treatment plant. What comes out?

First, treated water. Most of the original water leaves through this stream, carrying residual dissolved substances and any suspended material not removed during treatment. Depending on the plant, it may be discharged or reused.

Then there are screenings and grit: rags, plastics, wet wipes, sand and other debris. Fats and floating materials may also be separated. There are also gases and emissions. Biological processes can produce carbon dioxide, nitrogen gas, methane and other gases, while some volatile substances and aerosols can enter the air.

Finally, there is sewage sludge, comprising solids removed from the wastewater and biological solids produced during treatment. It contains organic matter, nutrients, microorganisms, mineral particles, chemical precipitates and substances associated with those solids.

After suitable stabilisation, this material is commonly called biosolids. Sounds rather more appealing than sewage sludge, doesn't it? Stabilisation can reduce pathogens, odours and biological activity. Dewatering makes the material easier to handle, while the remaining nutrients and organic matter may be useful in agriculture.

But there's an important distinction. Making sewage sludge biologically stable does not necessarily remove its chemical contaminants. Some are destroyed or transformed. Others remain in water, concentrate in solids or leave through gases and other residual streams. We know these general pathways reasonably well, but not necessarily the complete chemical inventory moving through each one.

The things we're only beginning to understand

For decades, wastewater treatment and biosolids management have concentrated on familiar problems such as pathogens, nutrients, metals and conventional organic pollutants. Now attention is turning towards contaminants of emerging concern.

PFAS is probably the best-known example. These fluorinated compounds include substances that persist for very long periods, behave differently during treatment and can transform into other persistent compounds.

Microplastics are another. Tiny fragments and fibres enter sewage from clothing, consumer products and numerous other sources. Some pass into treated water, while others accumulate in sludge or are removed with screenings.

Then there are pharmaceuticals, hormones and personal-care chemicals. Some degrade readily; others persist or produce different compounds as they break down. But the list is much longer: plasticisers, flame retardants, industrial surfactants, disinfectants, pesticides, synthetic fragrances, rubber additives, engineered nanoparticles and their transformation products.

There is also a biological dimension involving antibiotic-resistant bacteria, resistance genes and their potential movement through environmental microbial communities. And what happens when these substances interact? Regulatory limits generally concern individual chemicals or defined groups, while living organisms are exposed to mixtures whose combined effects can be difficult to predict.

Modern analytical chemistry has added another complication. A 2024 investigation of North American biosolids detected 451 recurring chemical features in most of its samples. Researchers could confirm or assign probable identities to only 92 compounds. The remaining signals weren't necessarily hazardous, but their identities and environmental significance were unresolved.

We can increasingly detect more substances than we can confidently identify, quantify or assess. Which brings us back to the cauldron.

What's happening inside the witch's brew?

A wastewater treatment plant is an impressive piece of engineering. Operators manage physical, chemical and biological processes to remove organic matter, control nutrients, reduce pathogens and produce suitably treated water. Much of the chemistry is well understood, but consider what happens when our extraordinarily variable mixture enters the plant.

Acids and alkalis neutralise one another. Buffering resists changes in pH. Metals dissolve or precipitate depending on the conditions. Some substances bind to particles while others remain dissolved.

Microorganisms consume compounds and produce others, altering acidity and redox conditions that in turn affect metal solubility and chemical behaviour. None of these processes operates independently. Organic compounds can bind metals and change their mobility. A change in redox conditions can dissolve one mineral and precipitate another, while microbial decomposition changes the chemical environment and influences other reactions.

Mixing and storage can dampen short-term variations, but also combine contaminants from different sources and spread the effects of intermittent discharges over longer periods. Meanwhile, new material is continually arriving.

Engineers understand and control many important processes. But nobody routinely measures every chemical entering the system, every reaction it undergoes or every compound produced. We understand a great deal about wastewater treatment. We understand considerably less about the complete chemical history of everything entering and leaving an individual plant. And that chemical history is continually changing.

What don't we know?

We can analyse sewage sludge and biosolids for regulated metals, selected organic chemicals, pathogens and emerging contaminants. We can determine whether the material meets prescribed quality requirements. But a laboratory report tells us about the substances actually tested. It doesn't establish that everything else is absent, nor does a sample necessarily reveal how composition varied last week, last month or across a large stockpile.

Some substances have poorly understood environmental behaviour, and many possible interactions remain uninvestigated. The mixture we're testing today also won't necessarily be the mixture entering the system twenty years from now.

Queensland recognises sewage sludge as regulated waste. Its End of Waste framework allows appropriately treated and characterised biosolids to be used as a resource under specified conditions. Those conditions establish regulatory compliance, but don't establish that every constituent has been identified or every environmental effect understood.

And when biosolids are applied to agricultural land, the consequences can persist. Crops remove some nutrients. Some contaminants degrade, while others remain in soil or move into water and organisms. Repeated applications can create cumulative loading.

The soil may become the final resting place for substances that began their journey in someone's bathroom, an industrial workshop or a landfill many kilometres away.

And what about Glan Devon?

This brings us to the proposed biosolids-to-fertiliser operation at Glan Devon, near Nanango.

The now-lapsed development application proposed receiving biosolids, drying them and blending the dried material with mineral fertiliser to produce a saleable product. A waste-fuelled thermal system would provide process heat. Recovering nutrients and organic matter from waste has obvious appeal, but what happens to everything else in the biosolids?

Drying removes water. It doesn't necessarily destroy chemical contaminants. Some remain in the dried material, some become more concentrated on a mass basis, and others may transfer into vapour, condensate or other process streams.

A March 2026 scientific paper investigated precisely this issue for PFAS. Laboratory drying experiments showed reductions of 43–74% in detectable PFAS in the dried solids. Researchers also recovered PFAS in condensate, while a substantial portion of the measured input remained unaccounted for. They could not determine what happened!

A reduction in one product doesn't necessarily mean a contaminant has been destroyed. It may simply have moved somewhere else. For Glan Devon, that makes the characterisation of incoming biosolids, dried product, condensate, non-condensable gases and other process streams particularly important.

So too are the representativeness of feedstock sampling, differences between supplying wastewater treatment plants and the ability of the process to handle changing chemical inputs. And the uncertainty extends into the future. An industrial facility may operate for decades. Its designers can characterise today's feedstock, but cannot know precisely which substances will become commonplace in wastewater twenty years from now, or how their regulation might change.

If the resulting fertiliser is applied to agricultural land, those constituents enter yet another complex chemical and biological system: the soil.

Which brings us back to where we started. What goes in must come out.

We collect an extraordinary mixture of waste from homes, businesses, industries and other waste facilities. We mix it, treat it and send the resulting water, solids, gases and residues back into the environment. We know a great deal about these processes, but substantial gaps remain concerning emerging contaminants, their transformations, interactions and long-term fate.

And we're dealing with a moving target. The chemical mixture of 2026 is different from that of 1976. The mixture of 2076 will almost certainly be different again.

Recovering something useful from waste doesn't make the other constituents disappear. Before we turn the witch's brew into fertiliser, shouldn't we have a pretty good understanding of what's actually in the cauldron — and where it's all going to end up?