Thesis submitted in fulfillment of the requirements for the degree Master of Technology Chemical Engineering In the Faculty of Engineering at the Cape Peninsula University of Technology / Wet textile processing (WTP), is faced with many challenges that are related to operating costs and market competiveness. WTP uses large amounts of water and electricity, which constitute a big portion of operating costs of dyehouses and other costs are related to releasing large quantities of water, high concentrations of dyes and chemicals into the textile effluents with possible effluents discharge limits (EDL) penalty charges if EDL are exceeded. EDL penalty costs had become a normative part of the operating costs for some WTP textile factories, making the EDL penalties, a monthly cost item, because water utilities and the effluent discharge are not controlled and optimized. Cotton dyeing is a complicated chemi-physical-sorption process that is not easy to perform efficiently. Inefficient dyeing (off-shades and un-level dyeing) sometimes results in several reprocessing steps, leading to mega litres of water and chemical usage. Inefficient dyeing can also lead to higher concentrations of dyes and chemicals in the dyeing effluents. The main objectives of this study were to investigate the applicability of electrocoagulation (EC) in treatment of reactive dyes textile effluents for safe discharge into sewers or forreuse and also to evaluate EC reaction kinetics in removal of various pollutants from reactive dyes textile effluent with a batch electrocoagulation reactor (ECR). To achieve these objectives; textile effluents to be used had to be created instead of using factory effluents because textile effluents vary between dyeing batches and reaction kinetics study require constant and consistent composition of effluents. This was done by following the standard commercial sample cotton-dyeing procedures. The dyeing and pre-bleaching procedures were formulated from literature sources. The dyeing and pre-bleaching were done to create the reactive dyes textile effluents with commercial sample dyeing machines; Washtec-P and Pyrotec-MB2 at liquor ratios of 10:1 and 20:1.

In this work, a novel treatment module capable of simultaneously enacting electrocoagulation and electrofiltration was designed, fabricated, and tested aiming for the reclamation of CMP (chemical mechanical polishing) wastewater. In general, CMP wastewater contains sub-micron particles and has high alkalinity, turbidity, total solids content, and silica content. Discharge of CMP wastewater without proper treatment would pose a great threat to the environment and ecology. In this investigation, oxide CMP wastewater and mixed CMP wastewater were obtained from a wafer fab in Taiwan. They were characterized by various standard methods. In this study, the efficiency of this dual-function treatment module (using aluminum as the sacrificing anode and stainless steel as the cathode) was evaluated in terms of applied electric field strength (0 ~ 112.5 V/cm), influent velocity (112 ~136 cm/s), and transmembrane pressure (1.0 ~ 3.0 kgf/cm2) on permeate qualities. Experimental results have shown that the contents of total solids of permeates could be reduced to about 180 mg/L and 426 mg/L, respectively for oxide CMP wastewater and mixed CMP wastewater. The respective values of turbidity and total organic carbon could also be reduced to below 1 NTU and 1.5 mg/L. Therefore, the treated water could be reused as the feed water for the ultrapure water production system. In this study, an empirical equation was established to relate the quantity of filtrate and applied electric field strength when CMP wastewater was subjected to electrofiltration alone. It was found that the theoretical aluminum concentration released to the reaction chamber was much greater than the actual one. This would explain why the efficiency of electrocoagulation needs to be improved in this treatment module. Experimental results also have indicated that proper backflushing would be beneficial to the flux of permeate and saving of membrane cost.

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Thesis (Ph. D.)--University of Sydney, 2003. / Title from title screen (viewed Apr. 28, 2008). Submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the Dept. of Chemical Engineering, Faculty of Engineering. Degree awarded 2003; thesis submitted 2002. Includes bibliography. Also available in print form.

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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.

Electrocoagulation was compared to centrifugation at pilot scale for harvesting Nannochloris oculata and Nannochloropsis salina for biodiesel production. The pilot scale testing is a proof of concept and no optimization was conducted. Testing used the KASELCO commercial electrocoagulation system. The KASELCO electrocoagulation system successfully coagulated microalgae in laboratory testing. Aluminum and stainless steel electrodes successfully recovered algae in laboratory testing. Electricity consumed was lowest using aluminum electrodes in laboratory testing, but inconsistently coagulated microalgae at the pilot scale. Stainless steel electrodes consistently recovered algae and were selected as the primary electrode to treat microalgae at the pilot scale. Scaling power settings to pilot testing using laboratory data was successful following KASELCO’s proprietary guidelines. The KASELCO electrocoagulation system showed an electrical reduction in pilot scale operational cost for harvesting. Economic analysis using the Algae Income Simulation Model concluded that the KASELCO electrocoagulation system increase net present value of a commercial algae farm by $56,139,609 using a discount factor of 0.04. The KASELCO electrocoagulation system was calculated to use 26 kWh/ton at a commercial algae farm. However, cultivation and extraction processes are energy intensive, resulting in minimal electrical savings for the algae farm. The increase in net present value reduced production costs at the algae farm by 1%. The probability of success for the microalgae farm was zero for all scenarios analyzed. While a reduction in capital and operational costs were observed, several improvements, including harvesting using electrocoagulation, in cultivation, extraction, and conversion are necessary for economic success for biodiesel production using algae farms.

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.