Effect of coagulant types on textile wastewater reclamation in a combined coagulation/ultrafiltration system

A combination of coagulation and ultrafiltration (UF) processes for textile wastewater reclamation was investigated using various types of coagulating chemicals such as polyamine, alum, polyaluminum chloride (PACl), and ferric salts. The potential of the combined system was evaluated to replace the existing treatment processes which are composed of a flotation tank (primary step) and a series of filtration beds including sands, granular carbons, and diatomaceous powders. Regardless of the type and dosage of the coagulants, the UF system achieved substantial colloidal particle removal (>97% of turbidity was removed), but membrane fouling was mitigated in a different manner. The degree of fouling reduction was highly dependent upon the type of coagulants used, even though the turbidity and organics removal efficiencies were nearly the same. The polymeric coagulant aggravated membrane fouling, whereas the inorganic coagulants always helped reduce fouling. A residue of the coagulating polymer, which was added in the primary step and its concentration in the effluent was at an immeasurable level, was found to cause serious membrane fouling. The use of polymeric coagulants should be prevented or minimized if UF is considered for textile wastewater reclamation. Polymerized aluminum was found to be the most effective among the coagulants tested, although ferric salt was better than alum in controlling fouling. In particular, it seemed that the characteristics of coagulation chemistry and the coagulated particles had a great impact upon membrane fouling.

A combined coagulation/UF system for textile wastewater reclamation was tested to investigate the possibility of replacing the current filtrationbased processes. In order to select an appropriate membrane pore size and coagulants, dead-end and crossflow membrane filtration experiments were performed while analyzing the quality of membrane permeate in terms of turbidity and TOC removal. In particular, the effect of organic (polymeric) and inorganic coagulants on fouling control was assessed during crossflow UF. The following conclusions were drawn. (1) Direct MF/UF processes accomplished substantial turbidity removal (>97%), although TOC removal was minimal. The permeate from the UF alone was enough for reuse in the water jet process, but the addition of various coagulants in the UF system was not able to enhance significantly the quality of water. (2) The sizing agent (PVA) and polymeric coagulant (polyamine), which are currently being used in textile wastewater treatment plants, had a different effect on membrane fouling during stirred-flow UF. Unlike the sizing agent, the polymeric coagulant caused fouling at a larger dosage of 4 mg/L. Accordingly, membrane fouling became worse with a small dose of polyamine during continuous crossflow UF. The use of polymeric coagulants should be reconsidered in the combined system, since the residual polymer may affect clogging of the membranes. (3) Among the inorganic coagulants tested, it was found that PACl controlled membrane fouling the most even at a small dosage (1.0 mg/L as Al). This is because PACl can provide a relatively high degree of destabilization while mitigating floc break-up under shearing conditions. (4) The efficiency of turbidity and TOC removal did not account for the control of membrane fouling during crossflow UF. This is because colloidal fouling is closely associated with the nature of aggregate flocs such as size, structure, and stability. Further studies on the chemistry and characteristics of coagulated flocs are needed.