Removal of organic pollutants from landfill leachate by electrochemical oxidation - Assessment of performance and applicability in Northern Norway

Abstract

Landfill leachate treatment by advanced oxidation processes has gained much attention in the past decades. The removal of organic pollutants from landfill leachate in particular has been in focus due to their detrimental effects on the environment. This fact also leads to increasingly stringent regulations put in place by the authorities regarding the pollutant removal and adherence to treatment goals. More stringent regulations for landfill leachate treatment are also expected to be effective in Norway in the near future. The new regulations contain a list of priority substances that need to be removed below their detection limit in order to release the effluent after treatment into nearby waterbodies. An appropriate treatment process that is able to accomplish the treatment goals and takes the given local climate conditions into consideration is required. Low average temperatures in subarctic climates provide a significant challenge.

The motivation of the present thesis was to find a suitable landfill leachate treatment process that is able to remove priority substances while withstanding cold operating temperatures. An extended literature study was the basis for choosing electrochemical oxidation, an advanced oxidation process, as a suitable treatment process. The majority of electrochemical oxidation studies have been carried out at room or elevated temperatures, while there is a lack of focus on wastewater treatment in cold climates. Furthermore, none assessed the application of advanced oxidation processes. This thesis bridges this knowledge gap and contributes to the applied field of electrochemistry by assessing its applicability under cold operating temperatures. The work is therefore of great interest in light of the increased focus on the technology in arctic regions.

This thesis consists of three separate studies:

1. A mechanistic study that helps to understand the ongoing different oxidation processes during the electrochemical oxidation of organic pollutants.

2. A laboratory scale study with a model organic pollutant from the priority list where the influence of different parameters on its oxidation was assessed, with a special focus on temperature.

3. A laboratory scale study with pre-treated landfill leachate that contains the same model organic pollutant, where the matrix effect and more applied parameters were assessed under cold climate conditions.

The mechanistic study of the model pollutant salicylic acid confirmed that three different oxidation pathways take place during the electrochemical oxidation: Direct electron transfer from the pollutant to the electrode surface, electrochemical oxygen transfer reaction from hydroxyl radical to the organic pollutant, and mediated oxidation via an intermediately formed oxidant, such as active chlorine. Density functional and natural bond theory were able to successfully predict the salicylic acid intermediates that were formed during electrochemical oxidation.

In the second study, Bisphenol A (5 µM) was used as the model compound as it is listed on the Norwegian priority substance list. Complete removal of Bisphenol A was achieved at low temperatures (6 °C) with the major drawback of extended treatment times. Besides temperature, pH also had a significant effect on the removal of Bisphenol A, and an alkaline pH (10) was found to be favourable. The anode material was found to have a major impact on the formation of disinfection by-products, favouring perchlorate formation on BDD anodes and trihalomethanes on Pt.

The final study confirmed increased treatment times at low operating temperatures (6 °C). The study further showed that a relatively high current density (43 mA/cm2 ) has to be applied in order to achieve complete Bisphenol A removal (5 µM) from the landfill leachate. The matrix effect of the landfill leachate disclosed lower Bisphenol A degradation rates compared to the ones obtained in the second study in clean electrolyte (3.3 mM NaCl & 0.3 mM Na2SO4). Formation of disinfection by-products increased with the application of higher current densities (10 – 86 mA/cm2 ) and temperatures (6 – 20 °C) while the anode material influenced their nature as previously.

The results of the three studies show that electrochemical oxidation is able to remove Bisphenol A from landfill leachate under cold operating temperatures. Treatment goals regarding Bisphenol A, given by the Norwegian authorities, were satisfied and indicate that results are transferrable to other organic compounds on the priority list. The major drawback are the increased treatment times, which subsequently result in higher energy demands, and ultimately in higher costs. Norway is a country driven by hydropower and lower electricity prices than the rest of Europe, so electrochemical oxidation is still a sustainable and economically feasible choice. Attention has to be paid to the choice of electrode material as they are a major matter of expense as well as influencing the disinfection by-products that are formed. This work sets iii precedent with regards to applicability of electrochemical advanced oxidation processes under cold operating temperatures.