Electrochemical treatment of sanitary landfill leachates

Sanitary landfill leachate is one of the major environmental problems concerning water pollution, since it is a very complex wastewater containing different heavy metals, organic and inorganic compounds, some of them refractory and toxic, which possesses colour and odour. Optimal leachate treatment, in order to fully reduce the negative impact on the environment, is today a challenge, since the conventional treatment methods used are not enough to reach the level of purification needed. For this reason, several advanced technologies have been studied for the treatment of sanitary landfill leachates and among them electrochemical methods have received great attention. In fact, electrochemical technologies have shown high efficiency in the elimination of persistent pollutants and several studies have reported its application in wastewater treatment. The objective of the work described in this thesis was to evaluate the application of two electrochemical methods, electrocoagulation and electrochemical oxidation, in the treatment of sanitary landfill leachates. Studies were performed with different leachate samples, collected at different sanitary landfill facilities, at different points of the treatment plants existed and in different seasons of the year. Different cell configurations and electrode materials were experimented. Operational variables such as applied current/potential, stirring, flow rate and electrolysis time were studied. Pollutants removal, mineralization and biodegradability indexes and energetic costs were also assessed. The results obtained demonstrated that electrocoagulation and electrochemical oxidation are effective technologies to treat leachates from sanitary landfills. Depending on the leachate characteristics, electrochemical treatments can be applied as pre-treatment or post treatment of biological processes. For leachates with high content in organic matter, electrochemical oxidation was more effective when applied after the biological treatment, eliminating the refractory organic matter remaining. Energy consumptions of 15 and 21 W h (g COD)-1 were achieved at laboratory and semi-pilot scales experiments, respectively. For leachates designated as “old” or with low biodegradability index, electrochemical oxidation process was more efficient when applied to the raw leachate, without any kind of pre-treatment. Chemical oxygen demand removals above 90% were achieved with energy consumptions of 78 W h (g COD)−1. For leachates with a high amount of solids, the application of an electrocoagulation process before the electrochemical oxidation enhanced the treatment efficiency. Electrocoagulation assays performed led to reductions in organic load of 50% with energy consumptions of 2 W h (g COD)−1. Furthermore, the combined electrocoagulation/ electrochemical oxidation treatment, when applied to raw leachates, enhances the biodegradability of the organic pollutants, improving the performance of the subsequent biological process. An increase in the biodegradability index from 0.3 to 0.9 was attained for the combined assays performed, with chemical oxygen demand removals above 95%. Moreover, this combined treatment has the advantage of being able to use the simultaneous cathodic reduction to remove heavy metals from the leachate, since these processes reduce the metal ions by depositing them onto the cathode. However, when solids content is low, it is preferable to apply only the electrochemical oxidation process, since it does not have the disadvantage of sludge production. The experiments using Ti/Pt/PbO2 anodes showed that this material can be successfully used for the treatment of sanitary landfill leachates, leading to lower energy consumptions than those obtained with boron doped diamond anodes. Both anode materials presented similar chemical oxygen demand removal kinetics and, despite boron doped diamond anodes yields higher mineralization indexes, Ti/Pt/PbO2 promotes higher levels of total and ammonia nitrogen removals