RO Membrane Mineral Scaling In the Presence of a Biofilm

RO Membrane Mineral Scaling in the Presence of a Biofilm

John Thompson¹, Nancy Lin¹, Eric Lyster¹, Ronit Arbel², Tom Knoell³,
Jack Gilron² and Yoram Cohen¹

¹University of California, Los Angeles, Water Technology Research Center, Los Angeles, CA, USA, 90095, yoram@ucla.edu

²Ben-Gurion University, Zuckerberg Institute for Water Research, Sede Boqer, Israel, 84990, jgilron@bgu.ac.il

 ³Orange County Water District, Water Production Group, 18700 Ward Street, Fountain Valley, California 92708, tknoell@ocwd.com

            Two of the most severe types of RO membrane fouling are biofouling and mineral scaling. Biofouling and mineral scaling are of particular concern in the desalination of secondary wastewater, as well as agricultural wastewater. The high concentration of microorganisms in wastewater can also cause significant membrane biofouling during the RO desalting step. Studies have shown that even by removing 99% of microorganisms through MF/UF pretreatment processes, microorganisms can still invade and colonize on RO membranes to cause biofouling [1-2]. Inland water desalination requires operation at high recovery in order to increase the needed potable water portfolio while reducing the challenge of RO concentrate management volume of generated RO concentrate. At high product water recoveries, however, solution supersaturation conditions can be reached in the membrane channel (with respect to sparingly water soluble mineral salts). As a consequence, mineral salt crystallization in the bulk and deposition onto the membrane surface, as well as mineral crystallization on the membrane surface can lead to severe flux decline and shortening of the useful membrane life. It has also been reported that biofouling could have a significant impact on membrane mineral scaling [3]. A number of recent studies have suggested that biofouling can result in enhancement of concentration polarization [4-5] which in turn could exacerbate the problem of mineral scaling. Accordingly, the present study explores the impact of biofouling on mineral scaling.

      The present study focused on direct observation gypsum scaling in the presence of a pre-existing biofilm, arising from municipal wastewater effluent (1100 mg/L TDS), on the nucleation and crystal growth patterns of calcium sulfate dihydrate (gypsum) on reverse osmosis membranes. The study was carried out on-site at the Orange County Municipal Water District utilizing the tertiary wastewater effluent for generating the biofilm. The thickness of the density of the biofilm was ascertained via a fluorescent staining technique and confocal laser scanning microscopy (CLSM). Subsequent flux decline measurements, over the period of biofilm growth (~3 days), demonstrated significant flux decline, with about 60% of the total flux decline occurring within the first 12 h of biofilm growth. Real-time monitoring of biofouling and mineral scaling was accomplished in-situ via a novel UCLA patented optical mineral scale detection system [6]. Studies of mineral scaling in the presence of biofilm were carried out using feed solution undersaturated with respect to gypsum.  RO desalting under operating conditions of initial supersaturation of gypsum (Saturation index~2) at the membrane surface. For the same level of initial gypsum supersaturation at the membrane surface, permeate flux decline for the biofouled membrane was significantly greater than in the absence of the biofilm. Analysis of the fouled and scaled membrane surface, via focused ion beam scanning electron microscopy (FIB-SEM), elemental analysis and optical imaging, revealed that the majority of gypsum crystals nucleated and grew within the biofilm matrix. The crystal number density (on the membrane surface) was ~2-3 times higher on the biofouled membrane suggesting a higher level of crystal nucleation for the biofouled membrane. The scaled membrane surfaces were also analyzed in the context of classical nucleation theory [7] indicating that the solution within the biofilm matrix was of a higher level of concentration polarization relative to the biofilm free membrane. The results of the study suggest that where biofouling is significant in RO elements (e.g., in lead elements where biofouling is typically expected to occur first) there may be an increased propensity for mineral scaling in the same regions due to enhancement of local concentration polarization associated with the biofilm.

 

References

 

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