Reduction of turbidity and hardness in coal power plant water: Investigation of the implication of flocculation and crystallisation mechanisms
Van der Linde, J.C.
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Water is a scarce commodity in South Africa and one of the largest consumers of water is the industrial sector. Within the industrial sector, power generation uses a vast amount of water. This water is necessary for cooling equipment, as well as for steam generation for turbines. In this study chemical treatment processes adequate for the treatment of raw water as well as RO-reject water from the Grootvlei power station were developed through identification of suitable coagulants and flocculants as well as optimum operating conditions. Such initiative was carried out to assist Eskom in achieving its goal of zero liquid effluent discharge policy; thereby, to minimise its environmental footprint and reduce the amount of water abstracted from rivers. Two mechanisms of water treatment were considered, based on the nature of pollutants in the water, namely, the flocculation mechanism, which involves charge neutralisation and agglomeration of flocs, and the crystallisation mechanism, whereby super saturation occurs to promote crystal growth. The flocculation mechanism was applied to the treatment of raw water. Coagulants and flocculants were used to remove dissolved solids from the water; therefore, reducing turbidity of the water. Coagulants were used to neutralise the charge of the particles in the water, and flocculants were used to aid in the agglomeration of the particles. The optimal conditions were achieved using polyaluminium chloride (PAC) as a coagulant, at a dosage of 30 mg/L, and ARFLOC100 as a flocculant, at a dosage of 0.8 mg/L. This combination yielded turbidity as low as 2 NTU. Settling was relatively slow and a duration of 30 minutes was needed to achieve a 10 mL floc bed. With the treatment of RO-reject water, the crystallization mechanism was considered whereby lime and NaOH were used to reach super-saturation. With the super saturation, scaling agents, which are the main problem with the RO-reject water, would be insoluble, and crystals will form and precipitate. The optimal conditions when using lime were Rheofloc5023, 0.5 mg/L, ARFloc100, 0.2 mg/L and lime, 220 mg/L, at 60°C. This combination yielded a conductivity removal of 36%, a turbidity increase of 59%, a total hardness removal of 54% and an alkalinity removal of 71%. When NaOH was used, the treatment was more efficient at 40°C. Rheofloc5414 was found to be the best flocculant to use with a dosage of 220 mg/L NaOH. This yielded a conductivity removal of 1.26%, a turbidity removal of 58.75%, a total hardness removal of 20.3% and an alkalinity removal of 50.6%. The settling velocity and stability of the crystals were, however, superior at higher temperatures with lime and NaOH, the Rheofloc5414 with the NaOH being more stable and quicker to settle and precipitate. The difference between these two mechanisms could be seen clearly in this study, as flocculation occurred more rapidly, and less slow mixing time was necessary to treat the raw water. It was clear that super-saturation was necessary for the crystallisation process to take place during the treatment of RO-reject water. The latter mostly contains ion pollutants, while raw water contains mostly organic pollutants. It can also be concluded that temperature is important in the crystallisation process initiated by lime, as the treatment was more efficient at higher temperatures. Lastly, it was seen that crystals are more stable and settle faster than the flocs that formed in the treatment of the raw water.
- Engineering