Year

2013

Degree Name

Doctor of Philosophy

Department

School of Civil, Mining and Environmental Engineering

Abstract

The occurrence of trace organic contaminants (TrOCs), both from anthropogenic and naturally occurring origins, in the aquatic environment is of concern from environmental and human health protection perspective. Many of these TrOCs are ubiquitous in domestic wastewater and advanced treatment processes are required to ensure their removal to a safe level if the reclaimed water is intended for indirect potable water recycling applications. This thesis work investigated the removal of TrOCs by three integrated membrane processes for indirect potable water recycling applications. The results reported in this thesis indicate that a combination of membrane bioreactor (MBR) with nanofiltration (NF) or reverse osmosis (RO) membrane filtration can complement each other very well to efficiently remove a wide range of TrOCs. Forward osmosis (FO) is an emerging treatment technology and results reported here also showed some promising aspects of this process for the removal of TrOCs. The innovative combination of FO in combination with MBR in the form of osmotic membrane bioreactor (OMBR) for the removal of TrOCs was also investigated in this thesis work. The results are preliminary but demonstrate the potential of this approach as a low energy process for the production of high quality treated effluent, particularly when discharging into the ocean (i.e. seawater is readily available as the draw solution).

The removal of TrOCs by a hybrid treatment process incorporating an MBR with NF/RO filtration was investigated. Using a laboratory scale MBR system and a cross-flow NF/RO system, experiments were conducted with 40 organic compounds representing the major groups of TrOCs found in wastewater. The results suggest that the MBR system could effectively remove hydrophobic and biodegradable trace organic compounds, while the remaining trace organic compounds (mostly hydrophilic) were effectively removed by the NF/RO membranes. The combination of MBR and a low pressure RO membrane resulted in more than 95% removal (or removal to below the limits of analytical detection), for all the compounds investigated in this study. Results reported in this research component also suggest that fouling mitigation of the NF/RO membranes can be adequately controlled.

The rejection of TrOCs by an osmotically driven membrane filtration process was also investigated using a set of 40 compounds. Their rejection by an FO membrane specifically designed for the osmotically driven process and a tight NF membrane was systematically investigated and compared under three different operating modes, namely forward osmosis (FO), pressure retarded osmosis (PRO), and reverse osmosis (RO). The results revealed that the FO membrane had a considerably higher water flux than the NF membrane when operated in either the FO or PRO modes. However, the NF membrane consistently rejected the contaminants better than the FO membrane. In the RO mode, electrostatic interactions played a dominant role in governing the rejection of charged compounds, whereas in the FO and PRO modes, their rejection was governed by both electrostatic interaction and size exclusion. On the other hand, the rejection of neutral compounds was dominated by size exclusion, with rejection increasing with the molecular weight of the component. The PRO mode resulted in a higher water flux but a notably lower rejection of TrOCs than with the FO mode. It is also noteworthy that the rejection of neutral compounds in the FO mode was higher than in the RO mode. This behavior could be attributed to the retarded forward diffusion occurring in the FO mode.

The removal of TrOCs using an innovative OMBR system was also investigated. Following an initial gradual decline, a stable permeate flux value was obtained after approximately four days of continuous operation, although the biological activity of the OMBR system continued to deteriorate, possibly due to the build-up of salinity in the reactor. The OMBR mostly removed the large molecular weight trace organic compounds by above 80% and was possibly governed by the interplay between the physical separation of the FO membrane and biodegradation. Whereas, the removal efficiency of smaller trace organic compounds by OMBR was scattered and appeared to depend mostly on biological degradation.

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