Nanofiltration : fouling and chemical cleaning

Thesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2004.The challenge of providing developing rural areas in South Africa with sufficient potable water is substantial. North West Province, among others, is water-stressed, semi-arid, and largely rural with a high dependence on groundwater as a strategic resource. Some parts of the province are having poor water quality which ends up affecting households, farming and livestock. The aim of this study is to evaluate the performance of nanofiltration (NF) membranes in detrimental ion (fluoride, nitrate and sulphate) rejection and to monitor fouling on membranes with their subsequent chemical cleaning. Five commercial membranes (D12, D11, CTC1, NF90 and NF70) were characterized using scanning electron microscopy (SEM), single salt retentions and clean water permeation studies. The three-layered structure of the membranes was observed using SEM, viz.: smooth dense layer, loosely networked sublayer and the support. 012, D11 and CTC1 showed higher water flux than NF90 and NF70. Membranes showed more retention of divalent ions than of monovalent ions. All tested membranes showed a negative surface charge density. During treatment of sampled rural water, all the membranes tested (D12, D11, CTC1, NF90 and NF70) gave different ion retention results and were mostly influenced by water composition. All tested membranes satisfactorily rejected sulphate. NF70 effectively rejected all the ions of interest (fluoride, nitrate and sulphate) from rural water, indicating that NF70 behaves more like a reverse osmosis (RO) than an NF-membrane. During fouling experiments, it was found that salts crystallize on the membrane surface, thus decreasing the membrane performance. Cake formation was observed on the membranes fouled with rural water. During chemical cleaning, acid was not an effective cleaning agent. Alkali and surfactant solutions separately proved to be moderate cleaning agents (flux recovery ranged from 50% to 75%) while their combination (alkali and surfactant) gave the best results (100% flux recovery)Master