Hydrothermal treatment of sewage sludge under oxidative and nonoxidative conditions

Abstract

In this doctoral thesis, the application of hydrothermal treatments both oxidative (wet oxidation) and non-oxidative (thermal hydrolysis) to stabilize activated sludge has been studied. Specifically, the purpose of this work was to analyse the effect of these treatments on the chemical, biological and rheological properties of sludge, in order to investigate in more depth the reactions involved in this process. As a preliminary part of this study, an extensive literature review of the research works related to the topic was carried out. Consequently, it was observed that there are numerous published data on valuable industrial products formed during the hydrothermal treatments of sludge, but those were too scattered in the wide literature about the subject. Therefore, a review article dealing with this topic was prepared, which in turn will serve as a complementary part to the introduction section. Afterwards, the effect of the operating variables for both treatments on the physical, chemical and rheological properties from sludge was analysed. Empirical equations to model the effects of solubilisation of solids by thermal hydrolysis on the rheological behaviour of the sludge were stablished. Likewise, a reaction mechanism was proposed for sludge wet oxidation, which included two stages in series: firstly, the solubilisation of the solid organic matter; secondly, the oxidation of the solubilised material. The next step was to compare and analyse the differences between both hydrothermal techniques applied to the sludge treatment. The results showed that wet oxidation represents a more effective technique to treat sewage sludge, providing better results in terms of solubilisation, settleability and mineralization than thermal hydrolysis. According to the above mentioned, the following part of the research was focused on studying in more detail the treatment of the sludge by wet oxidation. For that purpose, the sludge was separated into its structural components, which correspond to: soluble microbial products, loosely bound extracellular polymeric substances, tightly bound extracellular polymeric substances and naked cells. Each fraction was subjected separately to treatment by wet oxidation and the results were compared with those obtained from the treatment of the raw sludge. In this way, it was identified that the oxidability of the biopolymers that make up the sludge depends on the location of these in the floc structure. Therefore, greater refractoriness to the oxidation was observed on those fractions located more externally to the cells. Moreover, the analysis of the molecular size distribution of the biopolymers during wet oxidation showed that the larger molecules were degraded by thermal hydrolysis and oxidation reactions, obtaining an effluent composed mainly of low molecular weight polymers (<35 kDa) at the end of the treatment. In the final stage of the research, the effect of wet oxidation on the main biopolymers of the sludge was analysed, carrying out that study for each structural component of the sludge. From these results, a kinetic model that includes the effects of oxygen concentration and temperature of the process, as well as the rates of formation and reaction of the main solubilised biopolymers was proposed and validated. In addition, a study addressed to analyse the effect of wet oxidation on physicochemical characteristics and composition of different landfill leachates (young, old and biologically stabilised) was carried out. Liquid Chromatography Mass Spectrometry was used to identify the organic compounds present in the leachates and their transformations during the wet oxidation treatment.