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The impact of assimilable organic carbon on biological fouling of reverse osmosis membranes in seawater desalination
Dissertation   Open access

The impact of assimilable organic carbon on biological fouling of reverse osmosis membranes in seawater desalination

Lauren A. Weinrich
Doctor of Philosophy (Ph.D.), Drexel University
01 Feb 2015
DOI:
https://doi.org/10.17918/etd-6397
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Abstract

Seawater--Organic compound content Fouling Environmental Engineering
Biological fouling is neither well understood nor consistently prevented and continues to be a challenging problem in seawater reverse osmosis (SWRO) desalination. Biofouling occurs from bacterial proliferation and leads to operations and water quality challenges. Despite use of biocides (i.e., disinfectants) to control microbial growth, biofouling has not been well-managed in the seawater desalination industry. This project used a biological assay, the assimilable organic carbon (AOC) test to evaluate pretreatment effects on the nutrient supply. The AOC test provided a useful surrogate measurement for the biodegradability or biofouling potential of RO feed water. Biofouling observed in full scale and in controlled conditions at the bench- and pilot-scale resulted in correlations between AOC and operational effects, such as increased differential pressure and decreased permeate flux through the membrane. Increased differential pressure was associated with RO membrane biological fouling when the median AOC was 50 [mu]g/L during pilot testing. In a comparison test using 30 and 1000 [mu]g/L AOC, fouling was detected on more portions of the membrane when AOC was higher. Biofilm and bacterial deposits were apparent from scanning electron microscope imaging and biomass measurements using ATP. Chemicals used in seawater RO plants such as antiscaling, cleaning, and dechlorinating agents increased AOC, and therefore the biodegradability of the seawater. AOC was also a byproduct of reactions with commonly used disinfectants, such as chlorine, chlorine dioxide and ozone. AOC was increased by 70% in seawater with 1 mg/L humic acid and a chlorine dose of 0.5 mg/L Cl2. Increases in biodegradability and AOC were often not mirrored by a the total organic carbon (TOC) measurement; TOC varied less than 3%. TOC is not an informative tool for the plant operators to predict biofouling potential, which is problematic because it is often the only organic carbon parameter used in SWRO monitoring. Polyphosphonates and polymer-based antiscalants increased AOC less than 30 [mu]g/L; however, phosphate-based antiscalants increased AOC levels nearly 100 [mu]g/L. Depending on the active chemical or inherent impurities, antiscalants may increase biofouling potential of the RO feed despite the targeted application for controlling inorganic fouling. Better operational practices such as removing the chlorine residual prior to dosing the antiscalant would alleviate the adverse effect of AOC byproduct production. TOC removal efficiency is typically very poor and the pretreatment impacts on AOC levels should be controlled in SWRO plants that experience biological fouling problems on the RO membranes. Besides creating more effective organic carbon removal, minor pretreatment configurations and monitoring programs in the plants are recommended to help control AOC levels in the RO feed.

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