Please use this identifier to cite or link to this item: http://hdl.handle.net/2445/66617
Title: Effect of ultrasound, low-temperature thermal and alkali treatments on physicochemical and biological properties of waste activated sludge
Author: Ruiz Hernando, María
Director: Labanda, Jordi
Llorens Llacuna, Joan
Keywords: Llots de depuradora
Reologia
Polímers
Sewage sludge
Rheology
Polymers
Issue Date: 17-Jun-2015
Publisher: Universitat de Barcelona
Abstract: [spa] El desarrollo de procesos para la reducción o reutilización de los lodos resulta determinante para una apropiada gestión medioambiental. Tratando por separado el lodo activado del primario (y no la mezcla como suele hacerse) se favorece el uso del lodo activado en agricultura tras su digestión, ya que el lodo activado contiene el doble de nutrientes y menor carga contaminante que el primario. No obstante, el lodo activado (también denominado lodo secundario o biológico) es más difícil de deshidratar que el primario, ya que presenta una estructura interna organizada de flóculos unidos por las sustancias poliméricas extracelulares (EPSs, por sus siglas en inglés), las cuales atrapan el agua. En este contexto, la reología, que es la ciencia que estudia las relaciones entre la fuerza y la deformación de los materiales, es una herramienta muy útil para la caracterización de la viscosidad o estructura interna de los lodos. El lodo activado es también más difícil de digerir anaeróbicamente debido a las cadenas de glicanos presentes en las paredes celulares de las bacterias. En resumen, para poder deshidratar y digerir un lodo activado será necesario acondicionarlo previamente mediante tratamientos, con el fin de solubilizar las EPSs y aumentar su biodegradabilidad. Si además el lodo quiere destinarse a agricultura deberá estar higienizado. En esta tesis se ha estudiado el efecto de tres tratamientos (ultrasonido, térmico a baja temperatura y alcalino con NaOH) sobre la reología, deshidratación, higienización y digestión anaerobia de lodos activados. Los tres tratamientos redujeron considerablemente la viscosidad del lodo debido a la solubilización de la las EPSs. Así mismo, la solubilización de la EPSs facilitó la expulsión del agua confinada dentro de los flóculos del lodo, mejorando la deshidratación mediante centrifugación. El tratamiento alcalino con dosis de NaOH elevadas fue el que más EPSs solubilizó, de manera que fue el que permitió extraer más agua y el que más aumentó la producción de metano, además de higienizar el lodo. No obstante, este tratamiento tiene el inconveniente de incrementar la concentración de sodio en el digestor, lo cual podría llegar a inhibir el proceso de digestión. Los ultrasonidos redujeron notablemente la viscosidad, además de mejorar la deshidratación y la digestión del lodo. Sin embargo, este tratamiento no fue capaz de higienizar el lodo. El tratamiento térmico a baja temperatura aumentó muy ligeramente la producción de metano (posiblemente debido al corto tiempo de exposición) y permitió deshidratar e higienizar el lodo.
[eng] 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.
URI: http://hdl.handle.net/2445/66617
Appears in Collections:Tesis Doctorals - Departament - Enginyeria Química

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