Advanced sludge treatment affects extracellular polymeric substances to improve activated sludge dewatering

The management of wastewater sludge (biosolids) accounts for a substantial portion (25-50%) of the total cost of wastewater treatment. Dewatering is a major hurdle in sludge handling. Advanced sludge treatment (AST) processes, including thermal/thermochemical treatments and chemical oxidation (e.g., using hydrogen peroxide), have been developed to improve dewatering and facilitate disposal. Extracellular polymeric substances (EPS), a hydrated biofilm matrix embedding microorganisms, are crucial for pollutant removal, bioflocculation, settling, and dewatering. This paper investigates how AST affects EPS characteristics and improves dewaterability. The authors have previously conducted extensive lab-scale, semi-pilot, and pilot studies on various AST methods.

The production of EPS is a common characteristic of microorganisms in various environments. EPS comprises polysaccharides, proteins, nucleic acids, lipids, and other polymers found within or outside microbial aggregates. EPS contribute significantly to the structural and functional integrity of aggregates, influencing their physicochemical and biological properties. In activated sludge, EPS can constitute up to 80% of the mass. While polysaccharides were initially believed to be the dominant component, proteins are often found in higher concentrations, possibly due to exoenzymes and cell lysis. The high water-binding capacity of EPS affects sludge dewaterability; optimizing EPS levels can improve dewatering by enhancing flocculation, but excessive EPS hinders dewatering due to increased water retention. Water binding mechanisms involve electrostatic interactions and hydrogen bonds between water and functional groups in EPS.

The study used sludge samples from a municipal sewage treatment plant (approximately 6% dry solids). Various AST methods were applied: thermal hydrolysis (neutral, acid, alkaline), and peroxidation using hydrogen peroxide and Fe(II) salts. The optimum treatment conditions for each method were determined based on previous studies by the authors. The protein and polysaccharide content of the untreated and treated sludges were analyzed using modified Lowry and anthrone methods, respectively. The size distribution of sludge flocs was measured using a Malvern Mastersizer laser diffractometer to assess flocculation changes. An economic assessment was conducted for a 300,000 equivalent-inhabitant wastewater treatment plant to compare traditional and AST methods.

The results showed that AST methods improved sludge dewaterability by two primary mechanisms: 1. **EPS Degradation:** All AST methods degraded both proteins and polysaccharides in the sludge. Acid thermal hydrolysis showed a 23% reduction in protein and polysaccharide mass, while peroxidation caused a 27% reduction. Peroxidation, even at ambient temperature, was most effective at degrading EPS. This degradation reduced water retention, thus increasing dewaterability. 2. **Flocculation Enhancement:** Most AST methods, except thermal hydrolysis alone, increased the size of sludge flocs, indicating improved flocculation. Acid and alkaline treatments at ambient conditions, and peroxidation all increased the average floc size significantly. Acid thermal hydrolysis and alkaline thermal hydrolysis compensated for the detrimental effect of heat on flocculation by optimizing pH for improved particle interactions. The increased floc size reduced the amount of fine flocs, thereby improving dewatering. The economic assessment revealed that peroxidation, while having higher initial costs due to the use of corrosion-resistant materials and chemicals, resulted in significant savings due to a considerable reduction in sludge volume and improved dewaterability. For a 300,000 IE WWTP, the estimated annual savings exceeded 340,000 EUR, or 52 EUR/ton DS.

The findings demonstrate that AST methods effectively improve sludge dewaterability by targeting EPS. The dual action of degrading EPS to reduce water retention and enhancing flocculation to minimize fine flocs is crucial for efficient dewatering. Peroxidation emerges as a particularly promising technique, offering significant economic benefits despite higher operational costs. The results support the understanding of EPS's role in sludge dewatering and provide insights for optimizing AST processes.

Advanced sludge treatment methods, particularly peroxidation and thermochemical treatments, enhance dewaterability by degrading EPS and improving flocculation. These improvements translate into significant cost savings in sludge management. Further research could explore the optimization of AST processes for different sludge characteristics and the potential for integrating AST with other sludge treatment technologies.

The study focused on a specific municipal sewage treatment plant. The generalizability of findings to other wastewater treatment plants with different sludge characteristics may require further investigation. The economic assessment involved estimations and may vary depending on specific plant conditions and disposal costs.