Abstract
We present operando small-angle neutron scattering (SANS) experiments on silica fouling at two reverse osmose (RO) membranes under almost realistic conditions of practiced RO desalination technique. To its realization, two cells were designed for pressure fields and tangential feed cross-flows up to 50 bar and 36 L/h, one cell equipped with the membrane and the other one as an empty cell to measure the feed solution in parallel far from the membrane. We studied several aqueous silica dispersions combining the parameters of colloidal radius, volume fraction, and ionic strength. A relevant result is the observation of Bragg diffraction as part of the SANS scattering pattern, representing a crystalline cake layer of simple cubic lattice structure. Other relevant parameters are silica colloidal size and volume fraction far from and above the membrane, as well as the lattice parameter of the silica cake layer, its volume fraction, thickness, and porosity in comparison with the corresponding permeate flux. The experiments show that the formation of cake layer depends to a large extent on colloidal size, ionic strength and cross-flow. Cake layer formation proved to be a reversible process, which could be dissolved at larger cross-flow. Only in one case we observed an irreversible cake layer formation showing the characteristics of an unstable phase transition. We likewise observed enhanced silica concentration and/or cake formation above the membrane, giving indication of a first order liquid–solid phase transformation.
Highlights
Desalinated inland brackish groundwater has become an important source of drinking water in arid and semi-arid regions and is increasingly used to resolve the worldwide water shortage
We explore the fouling behavior of two polyamide Reverse osmosis (RO) thin film composite (TFC) membranes exposed to colloidal silica dispersions of different size, concentration, and ionic strength
We presented several examples of operando small-angle neutron scattering (SANS) experiments on silica fouling at two RO membranes under close to realistic conditions of RO desalination technique
Summary
Desalinated inland brackish groundwater has become an important source of drinking water in arid and semi-arid regions and is increasingly used to resolve the worldwide water shortage. Silica precipitation and scaling can be alleviated by using commercial antiscalants [8,9], or by reducing silica concentration in the feed water by alumina adsorbents [10], electrocoagulation [2], or softening through a coagulation [11]. Such pretreatments increase operational costs and can induce organic fouling and biofouling and various alternative approaches have been tested to deal with silica scaling. The presence of sodium alginate in the feed water reduces silica scaling during the desalination process [12]
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