Membrane screening to identify optimal properties when separating organic matter from Bolmen water

Abstract: In many places across the northern hemisphere, drinking water treatment plants (WTPs) need to avert the effects of deteriorating quality in surface water sources caused by increasing lev-els of natural organic matter (NOM). The two major factors responsible for this phenomenon, also called brownification, are the greenhouse effect and changes in soil acidification. Additional treatment steps needs to be investigated and implemented in order to ensure safe drinking water in the future. Membrane technology is a candidate that has proven to be effective in terms of organic matter removal as well as functioning as a microbiological barrier. The company responsible for producing drinking water in southern Sweden, Sydvatten, has experienced a change in raw water quality and as a consequence, an increasing demand for dosing coagulant chemicals in order to treat the incoming water. They want to investigate the possibility of introducing membrane filtration units as part of the treatment process. The efficiency of membranes are very site specific. This thesis is therefore executed with water from Lake Bolmen, which is the raw water source for the largest WTP owned by Sydvatten; Ringsjöverket. A comparative study of 12 different membranes was performed with the purpose of identifying a suitable membrane, or membranes, for removing organic matter. The membranes, ranging between tight nanofiltration to ultrafiltration, were all flat sheet membranes with the exception of one hollow fibre membrane. Screening tests with cross flow filtration were executed using a batch unit with the capacity of testing three membranes at once. An additional bench scale batch unit was used for the hollow fibre membrane. The test procedures were designed to accommodate the studying of flux, fouling and the effect of changing different operational parameters. Samples of the surface water and permeate were collected and analysed in terms of UVA254, total organic carbon (TOC) and colour to estimate the retention capabilities of the membranes. Initial screenings identified two promising membranes, Alfa Laval NF99HF and DOW FILMTEC(TM) NF270 (hereinafter "NF270"), both displayed a prominent flux trend combined with efficient removal of NOM with a TOC retention of approximately 94% and 95% respectively. However, the two membranes also showed signs of irreversible fouling which would affect the efficiency and lifetime of the membrane. Changing the transmembrane pressure (TMP) affected the membranes with larger pore sizes where the permeate quality decreased at higher pressures. For the tighter membranes, NF99HF and NF270 included, this effect was negligible and the TOC retention remained at the same level. After the initial screenings, spiral wound module tests were performed with the NF99HF membrane which achieved the same level of NOM separation. However, the fouling behaviour and the flux capacity somewhat contradicted previous results where the flux was halved and the cleaning managed to restore the permeate flux completely. The combined results from the different experiments showed that Alfa Laval NF99HF could potentially be suitable in terms of achieving a high flux without compromising the permeate quality. However, there are certain contingencies concerning flux and fouling behaviour as well as drawbacks such as the inability to treat the membrane with disinfectant chemicals containing chlorine which is an advantage from a hygienic perspective.