Recently, Jiang Yizhang, a Ph.D. candidate in Environmental Engineering Program and a BSc graduate of Biotechnology and Food Engineering (Class of 2022) at Guangdong Technion - Israel Institute of Technology (GTIIT), published a research paper titled “Unraveling the Bound-Water Removal Mystery: Preoxidation Enhances Sludge Dewatering via Cell Permeabilization and Transformation of Intracellular Polymeric Substances to Extracellular Polymeric Substances” in Environmental Science & Technology (EST, IF=12.2), a top international journal in the field of environmental science and technology. Jiang Yizhang is the first author, and Associate Professor Xiao Keke, in Environmental Engineering Program at GTIIT, is the corresponding author.

The study systematically traced the coupling relationship among “oxidation intensity-cell viability-migration of organic components-dewatering performance” during sludge preoxidation. It proposed a mechanism for enhancing sludge dewatering centered on cell membrane permeabilization rather than cell lysis, and identified the optimal oxidation dose, providing monitorable criteria for deep sludge reduction and engineering control.

About the Study

The production of waste activated sludge continues to rise with the expanding scale of wastewater treatment. Its high moisture content and large volume make sludge dewatering difficult, which is a major challenge in sludge reduction. On September 22, 2022, the National Development and Reform Commission of the People’s Republic of China issued the notice “Implementation Plan for the Harmless Treatment and Resource Utilization of Sludge” (Fagai Huanzi [2022] No. 1453), emphasizing the orderly promotion of sludge incineration. As a pretreatment step for sludge incineration, sludge dewatering can increase the calorific value of sludge and significantly reduce operational energy consumption.

Water in sludge exists in two forms: free water and bound water. Conventional flocculation and mechanical dewatering can remove most of the free water, but bound water “trapped” by extracellular polymeric substances (EPS) and cellular structures is difficult to remove. Preoxidation, which uses reactive groups such as hydroxyl radicals and sulfate radicals to disrupt EPS-water interactions, is regarded as the most promising enhancement approach. However, for a long time its mechanism has mostly been attributed to assumptions such as “cell lysis and release of intracellular water” and “oxidation that is too mild is ineffective while excessive oxidation is harmful,” with a lack of direct evidence.

The study used waste activated sludge as the substrate and set a dosage gradient of 0–0.4 mmol/g volatile solids of an oxidizing agent (i.e., oxone). A series of analytical methods, such as flow cytometry (to distinguish free viable cells, free dead cells (permeabilized cells), and bacterial cells attached to aggregates and cells disintegrated in debris (lysed cells)) and confocal laser scanning microscopy (to distinguish live and dead cells) were used to measure cell viability. This study analyzed the migration of organic fractions among four fractions: soluble, loosely bound, and tightly bound extracellular polymeric substances (EPS), and intracellular polymeric substances, and established their correlations with sludge dewatering performance.

The results showed that 0.2 mmol/g volatile solids of oxone was the optimal dosage: the moisture content of the dewatered sludge cake decreased from 73.3% in the raw sludge to 63.6%. At this point, the proportion of free viable cells was the highest and cells disintegrated in debris (lysed cells) was the lowest. When the dosage increased to 0.3–0.4 mmol/g volatile solids, the sludge flocs were significantly disintegrated, the median particle size decreased, and hydrophilic substances continued to accumulate, with dewatering performance deteriorating accordingly. It can thus be seen that what determines success is not “how strong the oxidation intensity is” but “to what extent the cells are damaged.”

At the mechanistic level, the improvement in dewatering performance mainly originated from the transfer of low-molecular-weight (<20 kDa) proteins and polysaccharides from the intracellular to the extracellular compartment, as well as the selective degradation of glucose and peptidoglycan, the most hydrophilic components of extracellular polymeric substances. The organic matter migrating from intracellular to extracellular locations mainly came from cellular components of Gram-negative bacteria. The study also revealed that intracellular substances significantly affect dewatering and act through mechanisms different from those of extracellular polymeric substances: as oxidation intensity increased, the protein/polysaccharide ratio in loosely bound extracellular polymeric substances continued to rise, whereas it declined intracellularly; carboxyl and lactone groups decreased in soluble and loosely bound extracellular polymeric substances, but carboxyl groups in intracellular polymeric substances increased; and the hydrophobic fraction of intracellular polymeric substances was higher than that of extracellular polymeric substances at all dosages. Excessive oxidation caused cell rupture and the accumulation of hydrophilic debris, which was detrimental to sludge dewatering.

This work turns “moderate oxidation” from an empirical operation into monitorable mechanistic criteria: in engineering practice, the adequacy of oxidation can be judged by the ratio of permeabilized cells to cell debris and by changes in the composition of intracellular and extracellular organic matter, thereby avoiding “excessive oxidation” that damages dewatering performance. At the same time, the results also show that intracellular organic matter in sludge—a component long overlooked—should not be underestimated in its role in sludge dewatering.

Paper Link

https://doi.org/10.1021/acs.est.6c14464

IntroductionEnvironmental Science & Technology is published by the American Chemical Society (ACS). Founded in 1967, it has a history of nearly 60 years and is widely recognized as one of the flagship journals in environmental science and engineering. It enjoys major international influence in areas including environmental chemistry, pollution control, environmental engineering and sustainable technologies, serving as a key venue for high-quality original research in environmental disciplines.

Funding

This research was supported by the Seed Grant Between Guangdong Technion-Israel Institute of Technology (GTIIT) and the Technion-Israel Institute of Technology, the National Natural Science Foundation of China (52170133), the Characteristic Innovation Projects of Ordinary Colleges and Universities in Guangdong Province (2024KTSCX160), the GTIIT Changzhou Innovation Institute Seed and Startup Funds, the Guangdong Provincial Key Laboratory of Materials and Technologies for Energy Conversion (MATEC2022KF011), and the Scientific Research Innovation Capability Support Project for Young Faculty (Project No. SRICSPYF–BS2025045), among others.

Text: Xiao Keke, GTIIT News & Public Affairs

Photos: Xiao Keke