Biofouling of Spiral Wound Membrane Systems

Biofouling of spiral wound membrane systems High quality drinking water can be produced with membrane filtration processes like reverse osmosis (RO) and nanofiltration (NF). Because the global demand for fresh clean water is increasing, these membrane technologies will increase in importance in the coming decades. One of the most serious problems in RO/NF applications is biofouling - excessive growth of biomass - affecting the performance of the RO/NF systems due to e.g. (i) increase in pressure drop across membrane elements (feed-concentrate channel), (ii) decrease in membrane permeability, (iii) increase in salt passage. These phenomena result in the need to increase the feed pressure to maintain constant production and to clean the membrane elements chemically. In practice, the first phenomenon is most dominant. The objective of this study was to relate biomass accumulation in spiral wound RO and NF membrane elements with membrane performance and hydrodynamics and to determine parameters influencing biofouling. The focus of this research was on the development of biomass in the feed-concentrate (feed-spacer) channel and its effect on pressure drop and flow distribution. These detailed studies can be used to develop an integral strategy to control biofouling in spiral wound membrane systems. Problem analysis Studies to diagnose biofouling in 15 full-scale RO and NF membrane installations with varying feed water types showed that (i) highest biomass concentrations were found at the installation feed side, (ii) the biomass related parameter adenosine-tri-phosphate was suitable for biofouling diagnosis in membrane element autopsies, (iii) measurements of biological parameters in the water were not appropriate in quantifying biofouling, and (iv) there is a need for a representative monitor and sensitive accurate pressure data to enable a reliable evaluation of the development of biofouling (Chapter 2). Based on the practical observations it was decided to develop a set of tools to study biofouling at controlled conditions. Method development A monitor was developed (Chapter 3) in combination with testing of a sensitive differential pressure drop transmitter (Chapter 4). This small monitor named Membrane Fouling Simulator (MFS) uses the same membranes and spacers as present in commercial membrane elements, has similar hydrodynamics and is equipped with a sight window. The MFS is an effective scaled-down version of a full-scale system and allows to study the biofouling process occurring in the first 0.20 m of RO/NF elements. Magnetic Resonance Imaging (MRI) provided in-situ, non-invasive, and spatially-resolved measurements of biofouling and its impact on hydrodynamics and mass transport in spiral wound membrane elements as well as in the MFS (Chapter 5). A three-dimensional computational model was developed to simulate biofouling in membrane elements, with feed spacer geometry as used in practice (Chapter 6). The model combines fluid dynamics, solute transport and biofouling. The methods described in the first part of the thesis have been used to increase the understanding of fundamental aspects of biofouling. Basic studies The development of biomass and related increase in pressure drop was not influenced by the permeate production in the elements (Chapter 7). Irrespective whether a flux was applied or not, the feed-concentrate channel pressure drop and biofilm amount increased in RO and NF membranes in monitor, test-rig, pilot and full-scale installation. Mass transport calculations supported that permeate production plays a minor role in the development of biofouling. Since fouling occurred irrespective of permeate production, the critical flux concept stating that “below a critical flux no fouling occurs” is not applicable to control RO/NF biofouling in extensively pretreated water. In essence, biofouling is a feed spacer channel problem (Chapter 8). This observation is based on (i) practical data and supported by (ii) in-situ visual observations of fouling accumulation using the MFS sight window, (iii) in-situ non-destructive observations of fouling accumulation and velocity distribution profiles using MRI, and (iv) differences in pressure drop and biomass development in monitors with and without feed spacer. MRI studies showed that already a restricted biofilm accumulation on the feed channel spacer influenced the velocity distribution profile strongly, leading to a strong decrease of the effective surface area in the membrane module and probably increasing the salt concentration in the dead-zones of the element leading to increased salt passage. Three-dimensional numerical simulations of biofilm formation and fluid flow were executed and compared with MRI and MFS studies (Chapter 9). The simulations showed similar (i) pressure drop development and (ii) patterns in flow distribution and channelling as observed in MRI and MFS studies. Feed spacers showed to have an essential role in biofouling, and are considered a prime target for improving the membrane elements. Based on the gained insights several potential methodologies to minimize the impact of biofouling have been studied and described in the last chapters of the thesis. Control studies The effect of substrate concentration, linear flow velocity, substrate load and flow direction on pressure drop development and biofilm accumulation was investigated in MFSs (Chapter 10). The pressure drop increase was related to the amount of accumulated biomass and linear flow velocity. Biomass accumulation was related to the substrate load. A flow direction change in the pressure vessels instantaneously reduced the pressure drop, accentuating that hydrodynamics, spacers and pressure vessel configuration offer possibilities to restrict the pressure drop increase caused by accumulated biomass. The impact of flow regime on pressure drop, biomass accumulation and morphology was studied (Chapter 11). In RO and NF membrane elements, at linear flow velocities as applied in practice voluminous and filamentous biofilm structures developed in the feed spacer channel, causing a significant increase in feed channel pressure drop. The amount of accumulated biomass was independent of the applied shear, depending on the substrate load. A high shear force resulted in more compact and less filamentous biofilm structure compared to a low shear force, causing a lower pressure drop increase. A biofilm grown at low shear was easier to remove during water flushing compared to a biofilm grown at high shear. Flow regimes manipulated biofilm morphology affecting membrane performance, enabling new approaches to control biofouling. Phosphate limitation as a method to control biofouling was investigated at a full-scale RO installation, characterized by low phosphate and substrate concentrations in the feed water and low biomass amounts in lead membrane modules. MFS studies showed that phosphate limitation restricted the pressure drop increase and biomass accumulation, even in the presence of high substrate concentrations (Chapter 12). Outlook Most past and present methods to control biofouling have not been very successful. Based on insights obtained by the studies described in this thesis, an overview is given of several potential complementary approaches to solve biofouling (Chapter 13). An integrated approach for biofouling control is proposed, based on three corner stones: (i) equipment design and operation, (ii) biomass growth conditions, and (iii) cleaning agents. Although in this stage chemical cleaning and biofouling inhibitor dosing seem inevitable to control biofouling, it is expected that in future – also because of sustainability and costs reasons - membrane systems will be operated without or with minimal chemical cleaning and dosing.