Application of the Bioluminescent Saltwater Assimilable Organic Carbon Test as a Tool for Identifying and Reducing Reverse Osmosis Membrane Fouling in Desalination
Type: Scientific Investigation
Year Released: 2015
Total Investment: $275,963 (Cash: $98,452, In-Kind cash and service: $177,511)
Principal Investigator: Charles Haas, Ph.D., Drexel University
Biological fouling of reverse osmosis (RO) membranes continues to burden the seawater desalination industry and cause costly operation and maintenance issues that can lead to process train downtime. An AOC test was developed specifically for seawater using Vibrio harveyi. This seawater AOC test can be used as a surrogate measurement for biofouling potential and was evaluated for its relationship with effects from membrane fouling such as increases in transmembrane pressure and declines in permeate flux.
Goals and Objectives
A novel seawater assimilable organic carbon (AOC) assay was used to identify the relationship between RO membrane biological fouling and the biodegradability of seawater by the following objectives:
- Relating biofouling potential at full scale seawater RO plants with operational conditions and chemical dosing;
- Testing the impact of oxidants on AOC formation;
- Measuring the biofouling potential of pretreatment chemicals in sequential dosing order; and
- Monitoring biofouling in RO feed conditions having variable AOC.
This report evaluates the impact of biological fouling and operations of seawater reverse osmosis membranes (SWRO) using the assimilable organic carbon (AOC) test to:
- Identify relationships between biofouling potential, chemical dosing, operational data and AOC in full scale SWRO treatment plants.
- Evaluate organic carbon changes and AOC formation from three commonly used oxidants: chlorine, chlorine dioxide, and ozone.
- Evaluate pretreatment chemicals, including antiscaling, membrane cleaning and dechlorinating agents for organic carbon content and AOC formation after reaction with chlorine.
- Determine the influence of AOC on biological fouling in bench and pilot scale RO membrane testing.
Findings and Conclusions
For biological fouling to occur on RO membranes, bacteria and nutrients must be present in conditions conducive to growth and proliferation of the bacteria. Control of biofouling is typically conducted in seawater RO plants using biocides (i.e. disinfectants). However, biological growth and subsequent fouling has not been well-managed nor has pretreatment been focused on nutrient limitation. This project used the assimilable organic carbon (AOC) test for seawater to evaluate the impact of pretreatment on the nutrient supply. The AOC test provided a useful surrogate measurement for the biodegradability or biofouling potential of the RO feed water. Biofouling observed in full scale and in controlled conditions at the bench and pilot scale resulted in statistically significant correlations between AOC and the operational effects caused by biofouling. Membrane fouling rates are observed through operational changes over time such as increased differential pressure between the membrane feed and concentrate flows and decreased permeate flux through the membrane. The data showed statistically significant correlations (p < 0.01) when AOC was used as a predictor variable for the increased differential pressure (4 – 8 psi from September – December, 2012) and the 53% decrease in specific flux at a full scale treatment plant. Increased differential pressure was associated with RO membrane biological fouling when the median AOC was 50 µg/L during pilot testing. In a comparison test using 30 and 1000 µg/L AOC RO feeds, 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.
Application of antiscaling, cleaning and dechlorinating agents in most solutions and recommended doses increased AOC, and therefore the biodegradability of the seawater. AOC was also a reaction byproduct of commonly used disinfectants, such as chlorine, chlorine dioxide and ozone in antiscalant solutions or with naturally occurring organic carbon. 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 accompanied by an increase in TOC (TOC varied < 3%). These and other results indicate that 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 water quality monitoring. For other chemicals, impurities in frequently used treatment chemicals were shown to increase AOC concentrations that were undetected by TOC. Polyphosphonates and polymer-based antiscalants increased AOC by < 30 µg/L. However, phosphate-based antiscalants increased AOC levels nearly 100 µg/L AOC. Depending on the active chemical or inherent impurities, antiscalants may increase the 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. Overdosing sodium bisulfite in order to reduce ORP was observed at one of the study sites; this practice increased AOC and led to decreased specific flux and increased differential pressure on the RO membranes. 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, the most critical component of SWRO plants. 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.