Coagulation of humic acid by aluminium sulphate in saline water conditions

Chemical coagulation has been applied in practice for the pre-treatment of saline waters in the process of water treatment by membrane desalination. The relative magnitude of the ionic strength and individual ion concentrations (cations and anions) in saline waters leads to differences in the nature of coagulation by hydrolysing metal salts compared to the treatment of freshwaters with respect to colloid destabilisation, coagulation mechanisms, and colloidal removal. In this study, the coagulation of saline solutions containing humic acid by aluminium sulphate has been examined systematically. Laboratory experiments have been undertaken over a wide range of pH (4 to 10) and alum dose (31000 muM Al) conditions. Coagulation has been assessed principally in terms of colloidal particle removal (change in UV-absorbance) and colloid surface charge (zeta potential). The experimental results have shown that the coagulation of humic substances in seawater has some distinctly different characteristics compared to the well-established coagulation behaviour in surface freshwaters. These differences can be attributed to the solution chemistry and its modification of the aluminium hydrolysis reactions and the nature of the precipitates formed. Based on the results, coagulation performance domains (Al dose vs. pH) are defined, and the mechanisms of coagulation are discussed.