Sludge in wastewater treatment is the thick, semi-solid byproduct that settles out from the treated wastewater. This sludge can be broken down by bacteria in a process called anaerobic digestion, producing biogas that can be used as fuel to power the wastewater treatment plant.

However, anaerobic digestion of sludge is challenging because the sludge is dense and contains complex materials that bacteria can’t easily break down. Therefore, pretreatment processes like thermal hydrolysis are used to break these materials into simpler forms, making it easier for bacteria to digest the sludge and produce more biogas.

Other than increasing biogas production, the thermal hydrolysis process kills off major pathogens in sand, reducing the time for digestion, and decreasing the tank size of the digester required. However, the high temperatures employed in current processes lead to high energy consumption and the production of refractory organics, which cause problems with UV disinfection and nitrogen removal downstream.

Advanced thermal hydrolysis introduces oxygen into the process with the goal of reducing required operating temperatures and times, lowering the energy consumption and the amount of refractory organics. The current reagents employed are peroxides, which although effective, reduce biogas production and make the exiting material less biodegradable. If that’s the case, why don’t you just pump in pure oxygen gas instead of using peroxides? They did try that*, and that’s what I’m talking about today.

A summary of what the team did: placed some sludge in a high-pressure mixing tank (or a fancy pressure cooker), pumped in a mixture of nitrogen and oxygen gas, sealed the system and heated it up to temperature while stirring. After the timer went off, they took out the processed sludge for analysis and digestion trials. What they considered were how different hydrolysis temperatures and oxygen concentration affected sludge composition, the effects of longer treatment time, and whether any can be linked to changes in biogas production.

It was found that the degree of solubilisation of sludge increased with harsher operating conditions (higher temperature, more oxygen, longer time). But employing these also led to excessive levels of ammonia nitrogen (NH3-N) and inhibitory volatile fatty acids (propionic and butyric acid), which negatively affect the prokaryotes involved in biogas production. The team concluded that the best yield could be achieved at 145 °C with 20 bar oxygen gas after 15 minutes. Importantly, this advanced thermal hydrolysis process could be run at temperatures lower than current thermal hydrolysis processes and required less time (the cited commercial process required 165 °C for 30 minutes). Additionally, they concluded that operating under these conditions could prevent the formation of the problematic refractory organics.

These advancements are incredibly exciting and important for both the industry and the scientific community. The ability to produce more biogas with less energy consumption and fewer harmful byproducts means more sustainable and cost-effective operations. For scientists, the breakthroughs in understanding how to optimise conditions for biogas production open new avenues for research and innovation. This progress not only improves environmental outcomes but also enhances the feasibility of renewable energy solutions, making a significant impact on the future of waste and wastewater management and sustainable energy.

– Basil

Article link:

A novel strategy for integration of oxidation within advanced thermal hydrolysis of sludge
doi.org/10.1016/j.chemosphere.2023.140676