The effects of feedwater degassing on the permeate flux of a small scale SWRO pilot plant

The effects of pressure release on feed water cavitation have been studied using a small pilot scale SWRO system. The presence of dissolved atmospheric gases in seawater leads to a potential for cavitation within the porous membranes used in high pressure processes. The rapid application and release of applied pressures in the range of 10–60 atm was found to cause visible cavitation throughout the bulk solution phase. This phenomenon was not related to increased gas solubility under the applied pressure, since no additional gases were allowed into the system. It was found that almost complete removal of the initial dissolved atmospheric gases prevented this cavitation. Earlier laboratory scale studies had reported that removal of cavitation by feed water de-gassing enhanced permeate flow rates by 3–5% but this level of improvement was not observed in the pilot scale study operating at an applied pressure of 38 atm with seawater feed. It is possible that larger effects may be observed with the use of more hydrophobic membranes and at higher working pressures. Pre-heating seawater feed to enhance RO efficiency may also lead to greater cavitation within the RO membrane. This study has also demonstrated that pre-treatment using hollow-fibre membranes with efficient vacuum pumping systems can readily produce a high flow rate of 99.5% de-gassed seawater. Feed water de-gassing at these high levels also has the advantage of reducing both inorganic and biological fouling and reduces oxidative degradation of the polymeric membranes

The effect of de-gassing on the efficiency of reverse osmosis filtration

Water permeated porous membranes, under a large hydrostatic pressure differential, are likely to create opportunities for vapour and dissolved gas cavitation, which will restrict water flow through the membrane void spaces. The formation of cavities within the voids may be facilitated by the presence of dissolved gases in water. Removal of these gases might, therefore, reduce or prevent cavitation and thereby increase the flow rate through the porous membranes used to purify water. Reverse osmosis (RO) membranes operate under high pressure differentials and will be very susceptible to cavitation reduced flow. It is well established that RO membranes have a lower water flux than expected and the reasons for this have yet to be identified. This study presents direct experimental evidence, obtained with a commercial RO unit, in support of an average increase in product flow-rate of up to 3-5%, on de-gassing the feed water, consistent with the proposed cavitation model

Unusual properties of water: Effects on desalination processes

Recent research on the fundamental properties of liquid water has been applied to demonstrate potential new improvements to the three main technologies for desalination. They have demonstrated that the so-called 'inert' atmospheric gases, oxygen and nitrogen, have a significant influence, even at their relatively low level of solubility. For example, the almost complete removal of these dissolved gases enhances the dispersion of fine oil droplets in water, simply by shaking, without the need for added surfactants. Even more importantly, the almost complete degassing of liquid water significantly enhances its natural electrical conductivity. These discoveries have recently been applied to the development of ideas for the improvement of reverse osmosis and electrodialysis. In addition, investigations of the effect of salt on the dispersion of bubbles in water have allowed the construction of a high density air bubble column to transfer water vapour under controlled sub-boiling conditions

The hydrophobicity of non-aqueous liquids and their dispersion in water under degassed conditions

Several recent studies have shown that many oils, such as hydrocarbons, fluorocarbons, silicone and natural oils, are more readily dispersed as fine (micron-sized) droplets in water when the mixtures are almost completely degassed. These observations have not yet been fully explained and so this paper examines the nature of hydrophobicity of a wide range of oils and considers both the cavitation process and the surface charging expected during the separation of hydrophobic materials in water. Cavitation inside porous hydrophobic solids immersed in water is also considered. We also introduce a quick, easy and alternative method to freeze–thaw degassing, by which enhanced dispersions can be formed, which gives further support to the central role of degassing

The effects of feed water de-gassing on the permeate flux of a small scale SWRO pilot plant

The effects of pressure release on feed water cavitation have been studied using a small pilot scale SWRO system. The presence of dissolved atmospheric gases in sea water leads to a potential for cavitation within the porous membranes used in high pressure processes. The rapid application and release of applied pressures in the range of 10-60atm was found to cause visible cavitation throughout the bulk solution phase. This phenomenon was not related to increased gas solubility under the applied pressure, since no additional gases were allowed into the system. It was found that almost complete removal of the initial dissolved atmospheric gases prevented this cavitation. Earlier laboratory scale studies had reported that removal of cavitation by feed water de-gassing enhanced permeate flow rates by 3-5% but this level of improvement was not observed in the pilot scale study operating at an applied pressure of 38atm with sea water feed. It is possible that larger effects may be observed with the use of more hydrophobic membranes and at higher working pressures. Pre-heating sea water feed to enhance RO efficiency may also lead to greater cavitation within the RO membrane. This study has also demonstrated that pre-treatment using hollow-fibre membranes with efficient vacuum pumping systems can readily produce a high flow rate of 99.5% de-gassed sea water. Feed water de-gassing at these high levels also has the advantage of reducing both inorganic and biological fouling and reduces oxidative degradation of the polymeric membranes

The hydrophobicity of non-aqueous liquids and their dispersion in water under degassed conditions

Several recent studies have shown that many oils, such as hydrocarbons, fluorocarbons, silicone and natural oils, are more readily dispersed as fine (micron-sized) droplets in water when the mixtures are almost completely degassed. These observations have not yet been fully explained and so this paper examines the nature of hydrophobicity of a wide range of oils and considers both the cavitation process and the surface charging expected during the separation of hydrophobic materials in water. Cavitation inside porous hydrophobic solids immersed in water is also considered. We also introduce a quick, easy and alternative method to freeze–thaw degassing, by which enhanced dispersions can be formed, which gives further support to the central role of degassing

The hydrophobicity of non-aqueous liquids and their dispersion in water under de-gassed conditions

Several recent studies have shown that many oils, such as hydrocarbons, fluorocarbons, silicone and natural oils, are more readily dispersed as fine (micron-sized) droplets in water when the mixtures are almost completely degassed. These observations have not yet been fully explained and so this paper examines the nature of hydrophobicity of a wide range of oils and considers both the cavitation process and the surface charging expected during the separation of hydrophobic materials in water. Cavitation inside porous hydrophobic solids immersed in water is also considered. We also introduce a quick, easy and alternative method to freeze-thaw degassing, by which enhanced dispersions can be formed, which gives further support to the central role of degassing

Removal of Glass Particles from Compost Mixtures at Laboratory and Pilot Scales

The contamination of compost with glass presents a serious problem for the handling and re-use of this type of waste material. Most refuse can be turned into useful by-products, yet compost that is highly contaminated with glass is considered low grade and useful only for a limited range of applications, such as at turf farms and mine sites. A novel method for removing particulate glass from general waste is proposed and tested here. The two-stage process consists of a flotation and a filtration step. A rotating flotation vessel was used to separate glass particles from partially separated solid waste. A back-flushable, asymmetric, inorganic filter was tested for the treatment and reuse of the water in the separation process. The novel design proved to be robust and the separation process was found to be capable of producing high quality compost from glass contaminated samples, in both batch and continuous operation modes. The asymmetric filter produced water suitable for re-use in the same separation process. The filter performance could be restored, without damage, by short, low-pressure back-flushing. The production method for the asymmetric filter is presented in this article and the filtration and flow characteristics of a range of these novel filters are also reported

Quorum quenching bacteria can be used to inhibit the biofouling of reverse osmosis membranes

© 2017 Elsevier Ltd Over the last few decades, significant efforts have concentrated on mitigating biofouling in reverse osmosis (RO) systems, with a focus on non-toxic and sustainable strategies. Here, we explored the potential of applying quorum quenching (QQ) bacteria to control biofouling in a laboratory-scale RO system. For these experiments, Pantoea stewartii was used as a model biofilm forming organism because it was previously shown to be a relevant wastewater isolate that also forms biofilms in a quorum sensing (QS) dependent fashion. A recombinant Escherichia coli strain, which can produce a QQ enzyme, was first tested in batch biofilm assays and significantly reduced biofilm formation by P. stewartii. Subsequently, RO membranes were fouled with P. stewartii and the QQ bacterium was introduced into the RO system using two different strategies, direct injection and immobilization within a cartridge microfilter. When the QQ bacterial cells were directly injected into the system, N-acylhomoserine lactone signals were degraded, resulting in the reduction of biofouling. Similarly, the QQ bacteria controlled biofouling when immobilized within a microfilter placed downstream of the RO module to remove QS signals circulating in the system. These results demonstrate the proof-of-principle that QQ can be applied to control biofouling of RO membranes and may be applicable for use in full-scale plants