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Improved energy efficiency in membrane processes - water desalination and purification

Résultats exploitables

The primary objective of this project was to increase the applicability of membrane processes for a variety of feed waters and therefore to develop methods to improve design and operation of membrane water-treatment facilities. This project has included the development of advanced feed pre-treatment facilities based on the use of an ultra-filtration membrane pre-treatment process, the development of improved fouling control techniques based on dosage of anti-scalants and, the development of criteria for selecting optimal pre-treatment facilities depending on the types of raw water and membrane. A special test section has been constructed which allows on line optical microscopic observations to be performed during membrane filtration or back-washing. The section is of a narrow channel configuration and employs flat sheet membrane pieces. Optical observations are conducted through a glass window that forms part of the channel. The pressure and hydrodynamic conditions cover practical micro-filtration or ultra-filtration applications. Any type of polymeric MF or UF membrane can be employed. Such a test section is considered a useful tool for simulating membrane filtration applications and performing fundamental studies of membrane fouling since, in addition to membrane flux measurements, information about the development of fouling deposits is obtained.
The primary objective of this project was to increase the applicability of membrane processes for a variety of feed waters and therefore to develop methods to improve design and operation of membrane water-treatment facilities. This project has included the development of advanced feed pre-treatment facilities based on the use of an ultra-filtration membrane pre-treatment process, the development of improved fouling control techniques based on dosage of anti-scalants and, the development of criteria for selecting optimal pre-treatment facilities depending on the types of raw water and membrane. Experimental results covering a wide range of important parameters in membrane colloidal fouling have been obtained. Moreover, a review and evaluation of the literature on colloidal fouling and its modelling has been performed. Based on these, the improved understanding gained on the key factors, such as fluid shear, permeation flux and physicochemical interactions, has motivated an effort to develop better tools for assessing the colloidal fouling propensity of RO feed waters. Such improved testing equipment and procedures that will better simulate the hydrodynamic and physicochemical conditions prevailing in actual RO membrane operations are needed to overcome the drawbacks of the existing empirical fouling indices.
The primary objective of this project was to increase the applicability of membrane processes for a variety of feed waters and therefore to develop methods to improve design and operation of membrane water-treatment facilities. This project has included the development of advanced feed pre-treatment facilities based on the use of an ultra-filtration membrane pre-treatment process, the development of improved fouling control techniques based on dosage of anti-scalants and, the development of criteria for selecting optimal pre-treatment facilities depending on the types of raw water and membrane. Detection of a scaling threshold limit by flux decline measurements and evaluation of anti-scalant effectiveness by a recycle technique require precipitation of sufficient material on the membrane. An overriding consideration is, therefore, the inventory of scaling material in the recycling solution. Three different laboratory techniques have being developed for characterisation of scaling propensity and anti-scalant effectiveness: a batch recycle technique, a once through technique and an intermittent recycle technique. The simplest technique is the batch recycle technique in which the onset of scaling is simply determined by detecting the water recovery level at which there is a sharp permeability decline. This technique is suitable for a relatively more soluble scaling species such as CaSO4. With very sparingly soluble salts such as CaCO3, the very low inventory of CaCO3 forming species in the recycling solution cannot provide sufficient material to clog the membrane and enable convenient detection of scaling threshold limits in a practical laboratory system. Two procedures were developed for overcoming the inventory limitation of sparingly soluble salts. In the once-through technique the membrane is continuously fed with water having a specified scaling potential. There are no inventory limitations at all and scale propensity at different levels of the scaling potential, with and without anti-scalants, can be readily evaluated by both the rate of permeability decline and the rate of scale precipitation. In the less demanding intermittent recycle technique, scaling propensity is characterised by evaluating scale deposition rates from changes in the composition of a recycling solution. The solution is periodically replenished to increasing super-saturation levels by adding fresh feed and bleeding permeate. The above techniques determine an upper limit of the water recovery at which scale precipitation will occur immediately. A technique was also developed for determining the lower water recovery limit at which scaling is prevented or at least delayed for a long period of time. The technique is based on a fundamentally based method for correlating induction time measurements with the water recovery level.
The primary objective of this project was to increase the applicability of membrane processes for a variety of feed waters and therefore to develop methods to improve design and operation of membrane water-treatment facilities. This project has included the development of advanced feed pre-treatment facilities based on the use of an ultra filtration membrane pre-treatment process, the development of improved fouling control techniques based on dosage of anti-scalants and, the development of criteria for selecting optimal pre-treatment facilities depending on the types of raw water and membrane. Foreground information coming from the pilot testing are the criteria for the design and operation of membrane processes for high fouling waters. Result provides information about the capabilities of several types of membranes and membrane processes, particularly as pertains to combined systems, so that they may be able to make more informed decision about selecting the technology without requiring extensive pilot testing efforts. These criteria concerns all the information requested from the set-up of membrane processes and from the hydraulic purpose to water quality objectives considering the economical aspects. Studies by partners suggest that optimised design and operation of seawater desalination can lead to reduced specific energy consumption (to ~4 kWh/m3) with a total water cost ~0.5 EURO/m(3). Regarding problematic water sources, such as polluted river and brackish waters, and the effluent from municipal treatment plants, the results indicate a cost figure of approx. 0.4 EURO/m(3) but with a specific energy consumption of only ~1.5 kWh/m3. Treatment of other types of water (not requiring desalination) may be even more energy and cost efficient (0.1 kWh/m3, 0.25 EURO/m(3). Novel processing trains promoted by this research were established in the treatment of such polluted feedwater to achieve: -Maximum Product Safety; application of reliable technologies capable of removing biological hazards while minimising or avoiding the formation of dangerous disinfection by-products (e.g. THM) and of other hazardous chemical compounds. -Cost Effectiveness; development of novel designs insuring high water recovery under modest energy expenditure; selection of appropriate pre-treatment methods in connection with near-optimum overall process conditions is necessary to mitigate fouling and scaling problems thus improving process efficiency. -Environmental Friendliness; pre-treatment methods to remove hazardous compounds and to mitigate fouling/scaling should be implemented with the minimum of additives; RO desalination should be applied at reduced specific energy usage.