UF membranes operated on paper machine wastewater : fouling tendencies and characterisation

[摘要] ENGLISH ABSTRACT:At the Mondi Kraft paper mill in Piet Retief, paper machine effluent is pre-treated by meansof dissolved air flotation (DAF) and a mierostrainer prior to ultrafiltration (UF). Despite therigorous pre-treatment of the effluent, severe fouling of the UF membranes still persisted,resulting in a sharp decrease in operational flux. In an attempt to improve the fluxperformance of the UF membranes an investigation was launched into the possible causes ofmembrane fouling. The study yielded the following results:Ultraviolet-visible (UV-Vis) spectrophotometric analyses of varIOUS effluent samplescollected from different locations at the mill indicated the presence of aromatic compounds.Lignosulphonate appeared to be the main constituent in all the samples analysed.UV-Vis spectrophotometry was also performed on fouling extracted from membranes Inorder to evaluate the different approaches attempted to reduce membrane fouling. Most ofthe UV-Vis spectra obtained did not show the absorbance maxima below 210 nm that wereseen for the paper machine effluent, DAF product, lignosulphonate and mierostrainer product.This indicated that the compounds with absorbance at lower wavelengths did not foul themembranes to the same extent as the aromatic substances with absorbance maxima between230 and 400 nm.The influence of pH on the absorption of the various effluent samples was also investigated.An increase in pH resulted in (1) a shift in the wavelength scans from a lower to a higherwavelength, suggesting ionisation (deprotonation) with a subsequent delocalization ofelectrons and (2) an increase in the turbidity.The increase in turbidity which accompanied the Increase in pH could be explained bycomplex formation between the carboxylate ions, phenolic groups and divalent metal ionspresent in the effluent. Inductively coupled plasma analyses of several effluent samples withpH values 7 and 13 indicated the presence of significant amounts of Ca2+ ions in the effluent.There was a significant decrease in the Ca2+ levels with an increase in pH, which supportedthe hypothesis that Ca2+ might contribute to complex formation. This resulted in a decreasein solubility and an increase in turbidity. The addition of a chelating agent(ethylenediaminetetra-acetic acid disodium salt) to an effluent solution at pH 13 redissolvedthe precipitate and considerably reduced the turbidity. The subsequent addition of CaChagain induced precipitation and increased turbidity, confirming the role of Ca2+ in complexformation.Gel permeation chromatographic analyses of mierostrainer product at pH 13 showed theformation of high molecular mass organo-calcium complexes. The exact molecular mass ofthe complexes present in the mierostrainer product could not be determined by electro spraymass spectrometry because of their poor ionisation ability.Atomic force microscopy and scanning electron microscopy (SEM) showed distinctdifferences in the membrane surface texture before and after fouling. Furthermore, SEMimages of the UF membranes exposed the limited ability of the 30 urn microstrainer, installeddownstream from the DAF unit, to remove residual fibres from the DAF product.Static fouling experiments performed on all the flocculants and coagulants used In thepaper-making process at the mill showed that none of these substances fouled the UFmembranes.Cleaning of the UF membranes with Triton XIOO®, a nOn-IOnIC surfactant, caused atemporary increase in the operating flux to values higher than that of the initial flux.Mechanical cleaning of the UF membrane surface with spongebalIs proved to be one of themost effective and successful methods to prevent flux loss caused by fouling.Pre-coating of the UF membranes with Plutonic FI08, another non-ionic surfactant, did notpromote membrane productivity. Evaluation of various types of membranes indicated thathydrophilic or negatively charged membranes withstood membrane fouling more effectivelythan hydrophobic UF membranes under the same operating conditions.

[发布机构] Stellenbosch University