| Summary: | Feed flow reversal (FFR) is a novel approach that can potentially mitigate the primary challenges of fouling/scaling for membrane treatment processes. The concept is particularly effective for highly concentrated industrial wastewaters whereby fouling/scaling is a severe concern. However, the innovation has not been widely adopted for industrial applications so far due to the additional system cost and uncertain effectiveness for the operation. Previous studies on the FFR typically utilized wastewater/synthetic solutions with relatively low levels of total dissolved solids (TDS) only, and the reversal period is consequently long due to the slow time scale of scaling formation and solute crystallization. The present study focuses on highly concentrated feedwaters instead and using fast FFR (FFFR)-incorporated NF membrane systems for treatment. The FFFR operations have shorter flow reversal intervals as the name indicates, to tackle the issue of much shorter induction times due to the higher solute concentrations. A literature review of highly concentrated industrial wastewaters from shale gas extraction and zero liquid discharge (ZLD) processes as well as the approach of FFR itself is first carried out. An experimental study is then conducted with FFFR using NF membranes, treating feedwaters having mixtures of solutes, including NaCl, MgSO4, and CaCl2, and with high concentrations up to TDS ~70 g/L exceeding those in previous FFR studies. The experimental results of permeate recovery, salt rejection, Energy recovery efficiency (ERE) and specific energy consumption (SEC) under the operating conditions used in this study are found to be ~5.7-22.6%, ~47.5-73.4%, 90-95% and 0.86-2.03 kWh/m3, respectively. Subsequently, a mathematical model is developed to predict how the changes in the solute concentration both spatially and temporally inside the membrane system, which enables the prediction of permeate flux under different FFFR operational conditions. Modelling predictions show good agreement with most cross-flow filtration (CFF) cases, however, was found incapable of predicting the desalination performance for FFFR under a specific condition of R > 1, where R is the ratio between the retention time and reversal period. Finally, R was experimentally verified to be important in incorporating the flow reversal concept into membrane desalination processes. R in a range of 1.16-1.43 was achieved, and the permeate flux decline percentage was as high as 18.5%. In summary, the various operating conditions of applying the FFFR concept and the associated performance results obtained in this study can be used as reference in future applications. The mathematical model developed can be used to predict both spatial and temporal changes in the solute concentration during the designing stage for FFFR applications with highly concentrated industrial wastewaters so that the operation can be optimized. In addition, ratio R is suggested to be carefully monitored in avoidance of negative effect on the permeate production due to concentration accumulation.
|
|---|