Sustainable process turns sewage sludge into high-value activated carbon

Sustainable process turns sewage sludge into high-value activated carbon
Researchers have developed a simple, sustainable way of converting sewage sludge to activated carbon
Researchers have developed a simple, sustainable way of converting sewage sludge to activated carbon
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Researchers have developed a simple, sustainable way of converting sewage sludge to activated carbon
Researchers have developed a simple, sustainable way of converting sewage sludge to activated carbon

Researchers have developed a simplified, sustainable method of converting sewage sludge from wastewater treatment into high-value activated carbon that can be used in a wide range of industrial and residential applications.

After you flush the toilet, drain the sink, or do the laundry, the wastewater travels through the sewer system to a wastewater treatment facility. This results in leftover solid waste – sewage sludge – that can contain heavy metals, chemicals, and pathogens. Given its high pollutant content, the large volume of globally produced sewage sludge causes environmental problems.

However, researchers from the University of Córdoba, Spain, have developed a sustainable, simplified way of converting sewage sludge into high-value activated carbon.

In recent years, pyrolysis, the thermal decomposition of materials at high temperatures in an inert atmosphere, has generated interest as a way of converting sewage sludge into valuable activated carbon. Because there are major knowledge gaps about whether the process is feasible for this purpose, the researchers started there.

Using raw sewage sludge that had been treated using a bioprocess where wastewater was purified using microorganisms, the researchers first dried it in a stove to remove the high moisture content. The dry product was then ground to a powder in a mill and mixed with an activating agent.

The activating agent, which activates or accelerates the thermochemical reaction, is crucial to obtaining activated carbon from sewage sludge. The researchers chose potassium hydroxide (KOH) due to its low cost and non-polluting properties, and experimented with using lower proportions to make the process more sustainable and reduce resource consumption, environmental pollution, and final production cost. After activation, the sludge powder was carbonized by pyrolysis in the absence of oxygen, and a hydrochloric acid treatment was added that purified and eliminated certain minerals.

Different mix ratios of sludge with KOH, pyrolysis time, and temperature targets were studied to establish the optimal method for producing high-surface-area activated carbons from the sludge. The researchers found that reducing the amount of KOH used by at least 50%, with a sludge-to-KOH ratio of 3:1 at a maximum temperature of 800 °C (1,472 °F) was optimal, producing a yield of 0.63 kg (1.3 lb) of activated carbon per kilogram (2.2 lb) of sewage sludge. This also resulted in a more porous activated carbon with a higher carbon content (62%). Because activated carbon is used for air and water purification, odor control, and precious metal recovery, its porosity is important as it increases its capacity for adsorbing chemicals from gases and liquids.

“From a practical point of view, it is important to propose solutions that can then be carried out on an industrial scale,” said the study’s corresponding author, María Ángeles Martín. “[I]t is one of the simplest procedures in the literature and uses technologies that already exist on the market on an industrial scale.”

When the researchers performed energy, mass and economic calculations, they estimated that the cost of producing activated carbon derived from wet sewage sludge was €17.53 (US$18.91) per kg. They say that the high cost is due to the 92% moisture content of the sludge. If centrifugation were used during wastewater treatment to reduce humidity to 80%, they estimate the cost would decrease by more than 50% to €8 ($8.63) per kg of activated carbon.

Now that the simplified process has been tested to verify the quality of activated carbon obtained from sewage sludge, the researchers plan to develop applications for this material.

The study was published in the Journal of Environmental Management.

Source: Universidad de Córdoba

The city of Milwaukee, Wisconsin has been turning it into a high quality fertilizer called Milorganite™ for many decades.
It seems like an unreliable source of carbon; it's mixed with all sorts of crap (pun intended). My guess is their test samples are carefully selected, maybe not even from actual sewage. Real sewage will have waste plastics, heavy metals, various minerals (HCL probably won't remove fluorides from toothpaste), cell phones (maybe partially ground up?). I'm sure there are cleaner sources of carbon, not significantly more expensive than sewage.

My guess is that the company is pushing a pyrolysis sewage treatment plant and "activated carbon" is a faddish marketing term to make investors think the process is better.
Another problem with this product: a big market for activated carbon is water and air filters. How will consumers react to filters made from sewage?
@TechGazer, you really should read the sources linked at the bottom of each article before going off on wild speculations. The study addresses the source of the sludge and any potential contamination. Fluoride is not a health concern, except for conspiracy theorists. Cell phones? How many people flush their cell phones? You bring up heavy metals without noticing that they've already addressed it. As for consumer acceptance, I would be far more confident about using activated carbon made from sludge that has been sterilized by pyrolysis than eating foods where unsterilized sludge was used as fertilizer – the source of quite a few outbreaks of listeria, shigella, campylobacter salmonella, E coli and other food contamination.
@CraigAllenCorson. There are problems with Milorganite spreading PFAs and PFOs and PFAAs. Unfortunately, a really great closed loop process is at risk because of the chemicals we use in many food preparation products, like compostable cutlery and especially cling wrap with PFA plasticisers.