(778b) Membrane Bioreactor Process and Fouling Control for Water Reclamation and Groundwater Recharge Applications

The use of membrane bioreactor systems (MBR) for water treatment, wastewater treatment and water reclamation and reuse has drawn great interest from researchers and engineers. Despite its many advantages such as smaller footprint and better product quality, their wider application require better control of membrane fouling and permeate flux decline as foulant removal depends on chemical or physical cleaning. In spite of their excellent retention characteristics, there are still problems that slow down use of membranes in these appliactions. Over time, membrane fouling and subsequent cleaning cause deterioration of membrane materials, resulting in compromised permeate quality and reduced membrane life spans.  Chemical and biological fouling are major problems in membrane filtration due to reduced permeate flux, increased energy costs, and system downtime for maintenance.  Biofouling in MBR systems is caused by the  buildup of organic chemicals, microorganisms, and microbial communities at the membrane surface. Biofilms attached to a surface begin with cell adhesion, and progress to thick layers of extracellular polymeric substances (EPS), other organic chemicals, and a complex community of microbial cells that are often difficult to remove. Organic fouling due to the presence of natural organic matter (NOM) often leads to surface  and internal pore fouling.  More importantly, organic fouling progressively leads to biological fouling in so far as providing the organic nutrients for biofilm growth and sustenance.

The emerging use of  advanced oxidation processes such as ozone, hydrogen peroxide, and UV radiation as well as catalysts (such as titanium dioxide)  in water treatment offers new opportunities, because the AOPs are  able to decompose a number of  membrane foulants very efficiently, particularly the NOM.  The effect of  these processes on membrane fouling is difficult to predict due to the complex nature of NOM, and the major influence of the conformation of NOM and the decomposition products on organic and biological fouling.

In the context of water reclamation and reuse, the MBR process with ultrafiltration can be made more energy efficient, economical and versatile than reverse osmosis or nanofiltration by using exhibiting better transport properties, higher permeate flux, and greater fouling resistance. The MBR process  is specially designed for the control of membrane fouling and permeate flux decline by a combination of powder activated carbon (PAC) sorption and fluid management. The present research is intended to  overcome organic and biological fouling problems, and to  improve aqueous transport properties.  This aspect of the study required the investigation of fouling  mechanisms in the operation of MBR systems.

The flat-sheet membrane evaluation tests in batch-mode represent an economic means for evaluating membrane materials for fouling potential and rejection characteristics.  Preliminary flat-sheet membrane tests were performed in plate-and-frame cells using polyether sulfone (PES) membranes to obtain  a priori  information on the  nature of organic and biological fouling. These tests also included subjecting the feed solution to AOPs including ozonation, and combinations of ozone and hydrogen peroxide.  The breakdown of NOM constituents into organics of lower molecular weights and different structural characteristics shall influence the extent of surface fouling and internal pore fouling.  Another aspect was the evaluation of microbial fouling using Escherichia coli (E. coli) as model bacteria to obtain prior estimates of microbial fouling and permeate flux decline at different bacterial concentrations in the feed. In these tests,  the permeate flux and the effluent total organic carbon (TOC) and UV absorbance at 254 nm were determined to evaluate membrane rejection and flux decline patterns.    The reclaimed water used in these tests was obtained from a water reclamation facility in Southern California. 

The MBR continuous flow tests that followed the flat-sheet membrane tests employed polyether sulfone hollow-fiber membranes fabricated from the same material as the flat-sheet membranes.  These experiments simulated the conditions in an MBR system using PAC and acclimated microorganisms.  In these studies, the permeate flux and the effluent TOC and UV absorbance (at 254 nm) were determined to evaluate flux decline patterns in the hollow-fiber membranes. The information has been useful from  several standpoints  including the nature and mechanisms of  membrane fouling, and its impact on permeate flux decline, which will eventually lead to the design of superior hollow-fiber membranes for different MBR applications.  Furthermore, the MBR tests using reclaimed water subjected to AOPs would provide a good evaluation on the impact of NOM oxidation on membrane fouling and rejection.  It is anticipated that the insights gained from these studies will be useful in the development of novel membranes with superior fouling resistant and aqueous transport characteristics through material modification techniques.