Impact of carboxylic acid ultrafiltration recycle streams on coagulation

This research identified unintended consequences of integrating ultrafiltration and its required ancillary cleaning systems within conventional surface water treatment facilities. Carboxylic acids, used in ultrafiltration membrane chemically enhanced backwashes, were demonstrated to interfere with ferric chloride and alum coagulation if recycled into coagulation basins at sufficient acid to coagulant (A/C) molar ratios. Tricarboxylic citric acid and monocarboxylic acetic acid were shown to interfere with conventional coagulation process performance. Significant changes in settled water turbidity, true colour and metal concentrations were observed in jar tests designed to simulate full-scale water treatment plant operations. A threshold A/C molar ratio for coagulation interference was identified for three surface waters in the United States based on sedimentation basin performance goals established by the US Environmental Protection Agency. Citric acid interfered with coagulation at A/C molar ratios as low as 0.028; whereas acetic acid negatively influenced coagulation at A/C molar ratios in excess of 18.0.