Floc sludge could become electrode material

Chinese researchers found that floc sludge could be used as an electrode material for high-end capacitors.

With the acceleration of urbanisation and industrial development, the treatment rate of urban sewage and industrial wastewater has increased yearly. When examining China, there were over 3000 sewage treatment plants in 2015 with an output of 40 million tons of sludge with a moisture content of 80 per cent. 53 per cent of sludge was not effectively and safely disposed of. Floc sludge is a solid waste produced in treating urban sewage and industrial wastewater, which is a serious environmental and human health hazard.

In industrial wastewater treatment, sludge often contains elevated concentrations of harmful contaminants, such as suspended solids, metal ions and organic pollutants. However, traditional sludge disposal methods are insufficient to prevent secondary pollution caused by leaching, diffusion, and resuspension in the environment. Therefore, effective stabilisation and hazard-free treatment are highly needed.

Floc sludge more useful than previously thought

Using acidification and KOH activation at high temperature, the researchers from the Institute of Process Engineering and the University of Chinese Academy of Sciences have transformed floc sludge into porous carbon matrix composites and used it as an electrode material for application in capacitors, compared the effects of different treatment processes on the electrochemical properties of sludge materials, and analysed the environmental impacts by life cycle assessment. This study entitled “Recovery and reuse of floc sludge for high-performance capacitors” is published online in Frontiers of Environmental Science & Engineering in 2022.

As a complex heterogeneous material composed of organic fragments, bacteria, colloids, inorganic particles, etc., sludge contains a large amount of carbon and metal salts. Consequently, much attention has been drawn to their excellent electrochemical properties. Furthermore, it has been noted that various types of pretreatments can greatly improve the electrochemical properties of carbon materials. It indicates that sludge has good application potential in the electrochemical field for reuse.

However, there remain many questions that require further investigation. The feasibility of facile fabrication of capacitors from floc sludge is an ongoing question. There need to be investigations into the effects of different treatment processes on the electrochemical properties of sludge materials. Others have looked at the environmental impacts such as resource consumption and toxicity risk.

Research Outcomes:

The research team found that the pore structure has formed via acidification and KOH activation at high temperatures. They compared the crack effect of three different acids. They chose to investigate the activation influence on the pore structure and electrochemical performance at four mass ratios of KOH/C further.

Their study revealed that the conductivity increased by Fe. The mass transfer efficiency was enhanced by more carboxyl functional groups. New pores created by the KOH activation method produced better properties for electrochemical applications. Their investigation showed that after KOH was activated, the sludge material had good double-layer capacitive behaviours. It was more reversible and beneficial for electrolyte ion diffusion and charge transfer. Finally, it pointed out an optimum treatment process after comparing the energy storage properties. They also assessed the long-term stability and the normalised environmental impacts with the others synthetically.

This study developed a facile, clean and healthy resource utilisation method by reusing floc sludge for carbon matrix electrodes with porous channels and high conductivity. This work provides a cost-effective strategy to synthesise hierarchical porous carbon as an electrode material for high-performance supercapacitors and offers a promising method for the large-scale fabrication of energy storage materials from hazardous industrial waste.