Electrocoagulation is an empirical (and largely heuristic) water treatment technology that has had many different applications over the last century. It has proven its viability by removing a wide range of pollutants. The approach to reactor design has been haphazard, however, with little or no reference to previous designs or underlying principles. This thesis reviewed these reactor designs, identifying key commonalities and synthesising a new design hierarchy, summarised by three main decisions: 1. Batch or continuous operation; 2. Coagulation only or coagulation plus flotation reactors, and; 3. Associated separation process if required. This design decision hierarchy thereby provides a consistent basis for future electrocoagulation reactor designs. Electrochemistry, coagulation, and flotation are identified as the key foundation sciences for electrocoagulation, and the relevant mechanisms (and their interactions) are extracted and applied in an electrocoagulation context. This innovative approach was applied to a 7 L batch electrocoagulation reactor treating clay-polluted water. Structured macroscopic experiments identified current (density), time, and mixing as the key operating parameters for electrocoagulation. A dynamic mass balance was conducted over the batch reactor, for the first time, thereby enabling the extraction of a concentration profile. For this batch system, three operating stages were then identifiable: lag, reactive, and stable stages. Each stage was systematically investigated (in contrast to the previous ad hoc approach) with reference to each of the foundation sciences and the key parameters of current and time. Electrochemical behaviour characterised both coagulant and bubble generation. Polarisation experiments were used to determine the rate-limiting step at each electrode�s surface. Consequently the appropriate Tafel parameters were extracted and hence the cell potential. At low currents both electrodes (anode and cathode) operated in the charge-transfer region. As the current increased, the mechanism shifted towards the diffusion-limited region, which increased the required potential. Polarisation experiments also define the operating potential at each electrode thereby enabling aluminium�s dissolution behaviour to be thermodynamically characterised on potential-pH (Pourbaix) diagrams. Active and passive regions were defined and hence the aluminium�s behaviour in an aqueous environment can now be predicted for electrocoagulation. Novel and detailed solution chemistry modelling of the metastable and stable aluminium species revealed the importance of oligomer formation and their rates in electrocoagulation. In particular, formation of the positively trimeric aluminium species increased solution pH (to pH 10.6), beyond the experimentally observed operable pH of 9. Thereby signifying the importance of the formation kinetics to the trimer as the active coagulant specie in electrocoagulation. Further leading insights to the changing coagulation mechanism in electrocoagulation were possible by comparison and contrast with the conventional coagulation method of alum dosing. Initially in the lag stage, little aggregation is observed until the coagulant concentration reaches a critical level. Simultaneously, the measured zeta potential increases with coagulant addition and the isoelectric point is attained in the reactive stage. Here a sorption coagulation mechanism is postulated; probably charge neutralisation, that quickly aggregates pollutant particles forming open structured aggregates as indicated by the low fractal dimension. As time progresses, pollutant concentration decreases and aluminium addition continues hence aluminium hydroxide/oxide precipitates. The bubbles gently sweep the precipitate through the solution, resulting in coagulation by an enmeshment mechanism (sweep coagulation). Consequently compact aggregates are formed, indicating by the high fractal dimension. Flotation is an inherent aspect of the batch electrocoagulation reactor via the production of electrolytic gases. In the reactor, pollutant separation occurs in situ, either by flotation or settling. From the concentration profiles extracted, original kinetic expressions were formulated to quantify these competing removal processes. As current increases, both settling and flotation rate constants increased due to the additional coagulant generation. This faster removal was offset by a decrease in the coagulant efficiency. Consequently a trade-off exists between removal time and coagulant efficiency that can be evaluated economically. A conceptual framework of electrocoagulation is developed from the synthesis of the systematic study to enable a priori prediction. This framework creates predictability for electrocoagulation, which is innovative and original for the technology. Predictability provides insights to knowledge transfer (between batch and continuous), efficient coagulant and separation path, to name just a few examples. This predictability demystifies electrocoagulation by providing a powerful design tool for the future development of scaleable, industrial electrocoagulation water treatment design and operation process.
| Identifer | oai:union.ndltd.org:ADTP/283050 |
| Date | January 2003 |
| Creators | Holt, Peter Kevin |
| Publisher | University of Sydney. Chemical Engineering |
| Source Sets | Australiasian Digital Theses Program |
| Language | English, en_AU |
| Detected Language | English |
| Rights | Copyright Holt, Peter Kevin;http://www.library.usyd.edu.au/copyright.html |
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