Effect of ultrasound, low-temperature thermal and alkali treatments on physicochemical and biological properties of waste activated sludge

• Autores: María Ruiz Hernando
• Directores de la Tesis: Jorge Labanda Angulo (dir. tes.), Joan Llorens Llacuna (dir. tes.)
• Lectura: En la Universitat de Barcelona ( España ) en 2015
• Idioma: inglés
• Tribunal Calificador de la Tesis: J. Giménez Farreras (presid.), Miquel Rovira Boixaderas (secret.), Josep Sabaté Reboll (voc.)
• Materias:
  • Ciencias tecnológicas
    • Ingeniería y tecnología del medio ambiente
      • Eliminación de residuos
      • Tecnología de aguas residuales
• Enlaces
  • Tesis en acceso abierto en:  TDX  Dipòsit Digital de la UB 
• Resumen

  • The development of treatment processes to reduce or reuse sewage sludge is crucial for a proper environmental management. Segregating primary from secondary sludge allows for better reuse of secondary sludge, since digested secondary sludge is better suited for agricultural application, containing about double concentration in nutrients but significantly less contaminants. However, secondary sludge (also called waste activated sludge (WAS)) is difficult to dewater compared with primary sludge due to the existence of colloidal materials and extracellular polymeric substances (EPSs), which strongly retain water. WAS is also more difficult to digest under anaerobic conditions due to the presence of glycan strands in microbial cell walls. The low dewaterability and digestibility of WAS can be improved by applying treatments, which can partially disintegrate the WAS by disrupting flocs and cells and solubilising the EPSs. The treatments studied in this thesis were ultrasound, low-temperature thermal and alkali with NaOH. Specifically, it has been analysed the effect of the aforementioned treatment on WAS rheology, dewatering, hygienisation and anaerobic digestion. Rheology is the study of stress-strain relationships of viscoelastic materials. All the sludges analysed (untreated and treated) behaved as non-Newtonian pseudoplastic fluids, which indicates that the viscosity decreases with the applied shear rate. The sludges were also thixotropic (showing a hysteresis area in the plot of stress versus shear rate), which means that the viscosity is time dependent. The three aforementioned treatments resulted in the reduction of the steady state viscosity and the hysteresis area. Complementarily, a rheological structural model was proposed to examine the variations in the thixotropic behaviour of WAS after the application of the treatments. Sludge dewatering is essential to obtain a product dry enough to allow a reduction in storage volume. The three conditions of the thermal treatment (60, 80 and 90 ºC; 1 h, including the heating time to reach the temperature) and the higher intensities of ultrasound (27,000 kJ/kg TS) and alkali (157 g NaOH/kg TS) treatments improved sludge dewatering by centrifugation. To further evaluate the effect of treatments on the sludge dewatering, the EPSs contained in sludge were extracted and characterised. The EPSs were fractionated to generate two different EPS fractions: loosely bound EPSs (LB-EPSs) and tightly bound EPSs (TB-EPSs), where the latter is the innermost fraction and harder to release. An analysis of the untreated sludge revealed that the proteins, followed by the humic acids and polysaccharides, were the major constituents of the EPSs. Each of these components was primarily observed in the TB-EPS fraction. The treatments, particularly the alkali treatment, thoroughly solubilised the EPS, as indicated by the increase in the TOC, protein, humic acid and polysaccharide contents in the LB-EPS fraction. As a result, the dewaterability of WAS was improved. Hygienisation of the sludge is of uttermost importance if the sludge is intended for agriculture. The level of hygienisation was evaluated using three indicator organisms: Escherichia coli, somatic coliphages (SOMCPH) and spores of sulfite-reducing clostridia (SSRC). The thermal treatment at 80 ºC and the alkali treatment at 157 g NaOH kg TS allowed the hygienisation of the sludge, thus satisfying normal levels accepted by the EPA and the 3rd official draft from the EU. Conversely, the conditions tested for the ultrasound treatment barely reduced the levels of microbial indicators. Mesophilic anaerobic digestion is a widely used method to stabilise the sludge. The alkali pre-treatment (157 g NaOH/kg TS) exhibited the greatest methane production increase (34%) followed by the ultrasonication (27,000 kJ/kg TS; 13%), whereas the thermal pre-treatment (80 ºC, 15 min) presented a methane potential similar to the untreated sludge. However, the use of NaOH as a pre-treatment is rather limited because of the rising sodium concentration in the digester.