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Method for Direct Observation of Biofilm Formation During Operation on Forward Osmosis Membranes

Jacob I. Deneff Department of Chemical & Biomolecular Engineering University of Connecticut

Storrs, CT, USA Jacob.Deneff@uconn.edu

Jeffrey R. McCutcheon Department of Chemical & Biomolecular Engineering University of Connecticut

Storrs, CT, USA Jeffrey.McCutcheon@uconn.edu

Leslie M. Shor Department of Chemical & Biomolecular Engineering

Center for Environmental Sciences and Engineering

University of Connecticut Storrs, CT, USA

Leslie.Shor@uconn.edu

Abstract— The purpose of this study is to describe a method

for direct visual observation of biofilm formation on membranes used in a forward osmosis flow cell. Until recently biofouling research has focused on biofouling during reverse osmosis, and generally examined biofilms after, not during, operation. Direct observation allows for more sensitive measurements of fouling than flux data and membrane autopsies alone. The flow cell proposed here can be used to examine the mechanism of membrane fouling and to systematically examine the effects of surface coatings, membrane materials, and cleaning methods on biofouling using a broad range of process conditions.

Keywords—biofilm; biofouling; forward osmosis; membrane separations; water treatment

I. INTRODUCTION Forward osmosis is a separation process which uses an

osmotic pressure differential to drive water across a semi-permeable membrane. Unlike reverse osmosis, where external pressure is applied to move pure water against a concentration gradient, forward osmosis utilizes a high-osmolarity draw solution to draw pure water from a less-concentrated to a more-concentrated solution. Forward osmosis can be performed at low pressures, requires less energy than reverse osmosis, and can be implemented in a range of water

treatment applications including seawater desalination, point-of-use water treatment, wastewater treatment, and production of pharmaceuticals.

One major hindrance of forward osmosis processes, and many membrane-based water separation processes, is biofouling. Biofouling is the formation of communities of bacteria embedded in a hydrated gel [1]. Bacteria naturally produce biofilms to provide themselves with greater tolerance against desiccation and antimicrobial exposure, to aid in nutrient sequestration, and to promote cell-cell coordination via quorum sensing [2]. Biofilms can grow to a thickness of several hundred micrometers, and can be a source of hydrodynamic resistance. More importantly, biofilms can reduce water flux severely, in some cases to 20% of its original value [3].

The traditional method for observing the effects of biofilms has been to perform a “membrane autopsy” following operation. Depending on the techniques employed to visualize bacteria on a membrane surface, this analysis may require desiccation or chemical treatment which can alter the physical morphology of the biofilm. Another limitation of the autopsy approach is it can only show the final condition of the membrane [3], and does not elucidate the changing biofilm coverage with declining performance over time.

Here we report a custom-machined direct observation forward osmosis flow cell designed to permit direct visualization of bacterial biofilm formation in real time as the membrane is operated in a forward osmosis process. The approach employs inverted wide-field light microscopy to image proliferation of green-fluorescence protein (GFP)-expressing bacteria during operation without disrupting the membrane or interrupting the process. The flow cell can accept membranes of different formulations. Operation parameters permit realistic process conditions of crossflow velocity and transmembrane flux. By observing the membrane continuously, we can link the degree of fouling with membrane performance at any given time. We can use this flow cell to examine how variables like membrane structure, draw solution composition, and crossflow velocity affect the growth of the biofilms.

Fig. 1. Example of biofouling of fluorescent bacteria on a membrane surface. Bacteria are visible as white spots with the membrane support structure visible behind them. Image taken using a 5x objective (Zeiss ECPlan-NEOFLUAR, 5x/0,16 ∞/0,17, Carl Zeiss Inc, Germany) .

II. FLOW CELL DESIGN AND OPERATION The flow cell consisted of three separate pieces, two of

which held the membrane between them and provided the structure of the cell, and a cover piece that secured a glass slide and sealed the device. The flow cell created for these experiments was fabricated from stainless steel in order to make it possible to use an autoclave for sterilization. It was designed to use a glass slide as a window through the bottom of the cell, allowing visual observation of the membrane surface on an inverted microscope. The top of the device was left solid, making it necessary to use fluorescence imaging to observe the membrane from the bottom. All inlets and outlets were located on the top of the device in order to prevent tubing from interfering with the objectives of the microscope. The effective membrane area was 9.24 cm2.

Cross flow on both sides of the membrane was provided by a single, two channel peristaltic pump. This ensured that no pressure gradient existed across the membrane to alter the performance during testing. The range of flow rates tested using this apparatus was 10 ml min-1 to 50 ml min-1.

III. PURPOSE AND USES The main purpose of this design is to visually observe the

process of biofouling as the membrane is being used in forward osmosis. A number of studies have focused on direct visual observation of membrane fouling, both with inorganic foulants [4] [5], and with organic foulants and biofouling [6] [7]. Direct visual observation of fouling has been proposed as a method to study critical flux, below which fouling is negligible, in forward osmosis systems. Microscopic observation has been found to be more sensitive in determining the onset of fouling in forward osmosis than measurements of flux [6], and a correlation has been found between visible membrane coverage and observations of critical flux [4].

Direct observation of biofouling may also have utility in determining anti-adhesive and antibacterial properties of

membranes and membrane coatings. A recent study used visual observation of bacterial deposition on nanocomposite membranes to determine the effectiveness of embedded silver nanoparticles as antibacterial agents [7]. Similar methods using polydopamine coatings have been proposed as well [9].

This technique is well-suited to systematic investigation of membrane cleaning and reclamation. Backwashing has been proposed as an effective means to clear fouled membranes and restore initial flux levels [9]. Surface modifications may also have an effect on membrane recovery, either by making mechanical cleaning more efficient or by allowing the removal of the coating to which foulants have become attached [8]. As previously noted, flux measurements alone may not be adequate for capturing initial deposition of foulants in forward osmosis, and visual observation may make it possible to more accurately determine effectiveness of treatments designed to prevent initial colonization of the membrane surface

IV. CONCLUSIONS Biofouling is one of the major stumbling blocks in

improving implementation of forward osmosis technology in various water treatment applications. Until recently, little research has been done on forward osmosis biofouling, with most biofouling studies focusing on reverse osmosis. Important differences in process conditions and ion gradients make it important to initiate careful studies on the initiation and progression of biofouling in forward osmosis systems. The flow cell proposed here may enable this research and contribute to better implementation of forward osmosis for sustainable water treatment.

REFERENCES [1] A. Matin, Z. Khan, S. M. J. Zaidi, M. C. Boyce, “Biofouling in reverse

osmosis membranes for seawater desalination: phenomena and prevention,” Desalination, vol. 281, pp. 1-16, October 2011

[2] H. C. Flemming “Biofouling in water systems – cases, causes and countermeasures,” Appl Microbiol Biot, vol 59, pp. 629-40, September 2002

[3] M. Herzberg, M. Elimelech, “Biofouling of reverse osmosis membranes: role of biofilm-enhanced osmotic pressure,” J Membrane Sci, vol. 295, pp. 11-20, May 2007

[4] S. Zhao, L. Zou, C. Y. Tang, D. Mulcahy, “Recent developments in forward osmosis: opportunities and challenges,” J Membrane Sci, vol. 396, pp. 1-21, April 2012

[5] Y. Wang, F. Wicaksana, C. Y. Tang, A. G. Fane, “Direct microscopic observation of forward osmosis membrane fouling,” Envir Sci Tech, vol. 44, pp. 7102-7109, September 2010

[6] S. Zou et al, “Direct microscopic observation of forward osmosis membrane fouling by microalgae: critical flux and the role of operational conditions,” J Membrane Sci, vol. 436, pp. 174-185, June 2013

[7] Y. Liu, E. Rosenfield, M. Hu, B. Mi, “Direct observation of bacterial deposition on and detachment from nanocomposite membranes embedded with silver nanoparticles,” Water Res, vol. 47, pp. 2949-2958, June 2013

[8] D. J. Miller et al, “Short-term adhesion and long-term biofouling testing of polydopamine and poly(ethylene glycol) surface modifications of membranes and feed spacers for biofouling control,” Water Res, vol. 46, pp. 3737-3753, October 2013

[9] B. Mi, M, Elimelech, “Organic fouling of forward osmosis membranes: fouling reversibility and cleaning without chemical reagents,” J Membrane Sci, vol. 348, pp. 337-345, February 2010

Fig. 2. Schematic representation of the three-part structure of the direct observation flow cell developed in this study.