Membrane hybrid systems in wastewater treatment for reuse : optimisation of backwash technologies

Publication Type:
Thesis
Issue Date:
2006
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NO FULL TEXT AVAILABLE. Access is restricted indefinitely. ----- As a consequence of increasingly stringent standards for wastewater disposal and reuse, various new membrane technologies have emerged. Membrane processes such as reverse osmosis and nano-filtration can remove the majority of the pollutants, including dissolved organics, but their operational costs are high because of high energy requirements and membrane fouling. Microfiltration and ultra-filtration require low energy and thus are cost effective options, however they still cannot remove dissolved organic matter due to their relatively larger pore sizes. A solution to this inadequacy is to couple powder activated carbon (PAC) adsorption with these membrane systems. PAC usage adsorbs organic matter leading to a significant increase in the removal efficiency of dissolved organic matter, reducing membrane fouling and the organic loading to the membrane. However, membrane fouling still occurs and can not be totally alleviated. Membrane fouling leads to permeate flux decline and the requirement of the frequent cleaning or replacement of membranes. This significantly increases the maintenance and operating costs of the membrane system and restricts the wider application of this technology. Therefore, the aims of this study are: (i) optimise the frequency and duration of cleaning techniques on a cross-flow ultrafiltration membrane system to minimize membrane fouling; (ii) investigate the effectiveness of cleaning techniques and PAC concentrations on membrane fouling and the removal efficiency of a membrane adsorption hybrid systems at lab and pilot scales using synthetic and real wastewater; and (iii) develop new adaptive membrane fouling controls to enhance the productivity and performance of membrane adsorption hybrid systems under unsteady operating conditions that are expected as they are used in larger and more widespread applications. An optimal frequency and duration of cleaning techniques in experiments with the lab-scale cross-flow ultrafiltration membrane system using synthetic waste water resulted in a net productivity increase of 14.8% and more than doubled the operational lifetime of the membrane. Through monitoring the transmembrane pressure increase over time, a periodic backwash was found to be more effective than periodic relaxation in the control of membrane fouling. However, it was recognized during these experiments that setting a pre-determined backwash frequency and duration faced significant shortcomings for larger and more widespread applications, especially under unsteady operating conditions. Two new control systems were then developed to optimize the duration and initiation of the backwash and to operate in an optimal manner under unsteady operating conditions. The new control systems used continuous monitoring of the transmembrane pressure to initiate and control the backwash cycle, rather than fixed times. The new backwash duration control system reduced the required backwash permeate by 25%. The new backwash initiation control system reduced the backwash permeate by up to 40% and helped to alleviate throughput problems commonly experienced with rain in sewage treatment plants, through only backwashing as required.
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