High silica concentration in RO concentrate

A.H Haidari,G.J Witkamp,S.G.J Heijman

doi:10.1016/j.wri.2022.100171

A.H Haidari, G.J Witkamp + Show 1 more

Open Access

https://doi.org/10.1016/j.wri.2022.100171

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Abstract

Silica scaling is one of the major scaling challenges in Reverse Osmosis (RO). The safe operation practice is to keep the silica concentration below 150 mg/L in RO concentrate. This study addresses the effects of divalent cations such as calcium and magnesium on silica scaling in a seawater RO installation used as a pretreatment to Eutectic Freeze Crystallisation (EFC). Results showed that in the absence of antiscalant and divalent cations a sustained silica concentration of approximately 280 mg/L in concentrate is possible without declining membrane permeability. At a higher concentration of divalent cations, the membrane permeability decreased. Membrane autopsy and analysing destructed membrane showed a relatively low magnesium and a high calcium concentration on the membrane after adding divalent ions into the solutions. It is concluded that in absence of divalent cations and without antiscalant the limits of 150 mg/L silica can be extended to 280 mg/L for 6–8 h.

Highlights

Safe and clean water is in short supply across the globe
The current study aims to investigate the scaling potentials of the Reverse Osmosis (RO) installation at a high silica concentration without using antiscalant, and in the absence of divalent hardness cations
The effluent of Ion Exchange column (IEX) was collected in a tank, after which it was fed to seawater RO (SWRO) (SWC2540 from Hydranautics) as a means to decrease its volume and prepare it to be used by the Eutectic Freeze Crystallisation (EFC) unit

Summary

Introduction

Safe and clean water is in short supply across the globe. Reverse osmosis (RO) offers a solution to the water shortage because it offers an excellent barrier against source water constituents. It is predicated that in the coming years the usage of BWRO will increase due to significant and rapid climate change, and population increase This increases the necessity of finding techno-economic solutions to (i) reduce the membrane scaling, which is the main fouling mechanism in BWRO, and (ii) to prevent environmental damages that can occur as a consequence of discharging concentrate into the ground by inland installations [3]. Monosilicic acid, which is a weak acid, remains in the monomeric state for long periods in water at 25 ◦C as long as the con centration is below 2 mM (around 300 mg/L), but usually polymerises rapidly at higher concentrations due to the condensation reaction [10,11,12].

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