Year

2017

Degree Name

Doctor of Philosophy

Department

School of Civil, Mining and Environmental Engineering

Abstract

Membrane distillation (MD) has several important attributes that are ideal for strategic desalination applications. These attributes include the ability to concentrate feed solutions to their saturation points with negligible flux decline, the exclusion of a high hydraulic pressure and hence a reduction in capital and operational costs, and the compatibility with low-grade waste heat and solar thermal energy. However, current MD applications are still restricted to lab-scale and pilot demonstrations. The realisation of MD for practical desalination applications has been challenged mostly due to high energy demand and membrane fouling and scaling that lead to membrane pore wetting. This thesis focuses on optimising small-scale MD systems with respects to thermal efficiency and membrane fouling and scaling for the desalination of seawater and saline produced water from coal seam gas exploration. The thesis also aims at manifesting the viability of MD for the regeneration of hyper saline liquid desiccant solution used in air conditioning system.

Membrane scaling and mitigation techniques during air gap membrane distillation (AGMD) of seawater were investigated using a lab-scale system. The results showed a strong influence of AGMD operating temperature on not only the process water flux but also membrane scaling and subsequent membrane cleaning efficiency. Elevating feed/coolant temperature from 35/25 to 60/50 ºC increased water flux, but also escalated membrane scaling of the AGMD process. Membrane scaling was more severe, and occurred at a lower water recovery (68%) when operating at 60/50 ºC compared to 35/25 ºC (78%) partly because of increased concentration polarisation effect. Operating temperature also affected the efficiency of the subsequent membrane cleaning. Membrane scaling that occurred at low temperature (i.e. 35/25 ºC) was more efficiently cleaned than at high temperature (i.e. 60/50 ºC). In addition, membrane cleaning using vinegar was much more efficient than fresh water. Nevertheless, vinegar cleaning could not completely restore the membrane surface to the original condition. Scaling material remaining on the membrane surface facilitated scaling in the next operation cycle. On the other hand, anti-scalant addition could effectively control scaling. Membrane scaling during AGMD of seawater at 70% water recovery and 60/50 ºC was effectively controlled by anti-scalant addition.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.