Transparent exopolymer particles: detection and role in membrane based systems

Transparent exopolymer particles (TEPs) have be en a rather unknown component of the extracellular ¨polymeric substances. Lately, they received more¨ attention because of some remarkable characteristi cs, such as their transparency and their stickines s (defined as high probability of adhesion upon co llision). Since the development of their detection ¨method, they have been found to be ubiquitous in¨ natural waters, where they are primarily formed fr om polysaccharides secreted by microalgae and bact eria. Several recent studies have pointed out an i mportant role of TEPs in the biological and colloi dal fouling of membranes during water purification . An important factor to be considered is that TEP ¨particles are not formed in biofilms on surfaces, ¨but in the bulk of the water itself. There they f orm microclimats for micro-organisms and can even¨ form a kind of protobiofilm which can accelerate t he fouling of surfaces significantly. The research ¨performed on TEPs is hindered by the fact that th ere are several possible detection methods. Since¨ the development of the first method to quantify TE Ps in 1993, a number of alternative methods have b een developed in an attempt to improve the origina l method. Some of these alternative methods deviat e strongly from the original method and determine¨ a different fraction of TEPs present in the same i nvestigated sample. This makes comparison between¨ results of studies very difficult or even impossib le. In Chapter 1}, an overview and a possible cate gorization of TEPs is presented to elucidate which ¨fraction is measured by which method and how this ¨fraction relates to the ones measured in other st udies. In this PhD, several systems where TE Ps could be relevant were experimentally investiga ted. These sytems were (1) an ultrafiltration wate r purification plant, where TEPs were suspected to ¨contribute to the irreversible fouling, (2) dead- end microalgae filtration for harvesting, where TE Ps were hypothesized to be a determining factor in ¨filtration, (3) a continuous microalgae growth an d harvesting installation with submerged membrane¨ filtration, where TEPs, along with other factors,¨could have an influence on productivity and growth , and (4) a submerged membrane filtration system w here parameters, such as membrane characteristics¨ and coagulant addition, were varied for better mic roalgae harvesting. A full-scale ultrafiltra tion installation, operated by The Watergroep at H arelbeke, Belgium, for the purification of surface ¨water was monitored for 8 months with measurement s of several parameters, including TEP quantity (C hapter 2). A correlation study and membrane autops y showed a complex fouling mechanism with interact ions between algae, iron and TEPs. Overall, algae¨ related parameters, like chlorophyll concentration s, correlated stronger than TEPs with irreve rsible fouling rates. The presence of TEPs could h ardly be detected on the membrane surfaces and the ir overall role in membrane fouling therefore seem ed limited in the studied ultrafiltration plant. Microalgae growth and harvesting is ano ther area where TEPs could be of importance, since ¨high correlations were found in natural environme nts between microalgae blooms and TEP-concentratio n peaks. Therefore, the role of TEP particles on m embrane fouling during dead-end filtration of diff erent Chlorella vulgaris broth solutions¨ was investigated (Chapter 3). Different fractions¨ of the broth (e.g. the soluble and bound fractions ¨and the cells separated from these fractions) wer e also used as filtration feed. The relation betwe en the feed properties and their filterability ove r three membranes was determined. The statistical¨ analysis disclosed that no universal sample variab le and fouling parameter could solely explain the¨ filtration performance. However, soluble compounds , TEPs and carbohydrates, seemed of high importanc e for flux-decline when using low-pressure microfi ltration membranes. This was inconclusive for ultr afiltration membranes, where the higher pressures¨ presumably pushed all cells in a dense cake layer¨ which determined the permeance. In the area¨ of microalgae growth and harvesting, membrane foul ing is not the only important factor to be conside red. TEP concentrations and other parameters were¨followed together with biomass density and product ivity of microalgae in a membrane photobioreactor¨ with continuous growth and harvesting of microalga e (Chapter 4). Results showed that TEPs were secre ted during the algae cell growth, and that their p resence is thus inevitable in a continuous system. ¨In addition, substances such as counter ions and¨ unassimilated nutrients accumulated in the system. ¨Also bioflocculation was observed. Although the d irect cause was not determined, it is likely that¨ a combination of these factors limited the algae g rowth, which indicates that there is a limit on th e number of time the growth medium can be recycled . When microalgae harvesting is ¨done with the aid of coagulants or flocculants, p ossibly in combination with membrane filtration, t here is the possibility that negatively charged al gogenic materials could interact with the positive ly charged coagulants. Supposing that membrane fou ling is mainly due to algogenic material rather th an algae cells themselves, filtration systems coul d be optimized using coagulants. In a last study,¨ filtration tests were performed in an effort to de termine optimal parameters for the preparation of¨ membranes via phase inversion and for the addition ¨of coagulants to obtain a maximal flux and minima l fouling during filtration of Chlorella vulga ris (Chapter 5). In all, TEPs are a gro up of ubiquitous particles that, for multiple reas ons, are not easy to define or quantify. Their rol e in the studied systems did not seem as large as¨ was expected initially, based on earlier reports.status: publishe