After you flush your toilet, drain, or do your laundry, wastewater travels through the sewer system and into a wastewater treatment facility. This creates a residual solid waste called sewage sludge, which can contain heavy metals, chemicals and pathogens. The large amount of sewage sludge produced globally creates environmental problems due to high levels of pollutants. However, researchers from the University of Cordoba in Spain have developed a sustainable and simplified method to convert sewage sludge into high-value activated carbon that can be used in a wide range of industrial and residential sectors.
In recent years, pyrolysis (high-temperature thermal decomposition of materials in an inert atmosphere) has attracted interest as a method of converting sewage sludge into valuable activated carbon. Because there are still large knowledge gaps about whether this process is feasible, the researchers started here.
Using raw sewage sludge treated through a biological process (which uses microorganisms to purify wastewater), the researchers first dried the sludge in a furnace to remove its high moisture content. The dried product is then ground into a powder in a grinder and mixed with an activator.
Activators activate or accelerate thermochemical reactions and are essential for obtaining activated carbon from sewage sludge. The researchers chose potassium hydroxide (KOH) because it is low-cost and non-polluting, and tried to use lower proportions to make the process more sustainable and reduce resource consumption, environmental pollution and final production costs. After activation, the sludge powder is pyrolyzed and carbonized under anaerobic conditions, and then treated with hydrochloric acid to purify and eliminate certain minerals.
The researchers studied different mixing ratios of sludge to KOH, pyrolysis time and temperature targets to determine the best way to produce high-surface activated carbon from sludge. The researchers found that reducing the amount of KOH by at least 50%, a 3:1 sludge to KOH ratio, and a maximum temperature of 800°C (1,472°F) were optimal, yielding 0.63 kg (1.3 lb) of activated carbon per kilogram (2.2 lb) of sewage sludge. This also makes activated carbon more porous and has a higher carbon content (62%). Since activated carbon is used for air and water purification, odor control, and precious metal recovery, its porous nature is important because it increases its ability to adsorb chemicals in gases and liquids.
María Ángeles Martín, corresponding author of the study, said: "From a practical point of view, it is important to come up with a solution that can be implemented on an industrial scale. This is one of the simplest procedures in the literature, using technology that already exists on the market at an industrial scale."
The researchers calculated energy, mass and economics and estimated that the cost of producing activated carbon from wet sludge would be €17.53 ($18.91) per kilogram. They said the reason for the high cost is that the water content of the sludge is as high as 92%. If centrifugal separation is used to reduce the humidity to 80% in the wastewater treatment process, they estimate that the cost per kilogram of activated carbon will be reduced by more than 50% to 8 euros ($8.63).
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 research was published in the Journal of Environmental Management.