IMPACTS OF SOURCE WATER AND MARINE MICROBIOLOGY ON BIOFOULING OF SEAWATER REVERSE OSMOSIS MEMBRANES DURING SEAWATER DESALINATION

Reverse osmosis desalination (SWRO) is the most energy efficient method of desalting seawater. However, membrane biofouling is the most difficult operational problem that increases water treatment cost. Conventional intake types that use surface seawater from channels or various types of pipes contain high concentrations of organic matter that tends to accumulate on the surfaces of the membranes as a biofilm. Despite complex pretreatment to remove particulate organic matter and some marine bacteria, most dissolved forms of organic carbon pass through the pretreatment process. Therefore, the membrane biofouling problem is not cured but perhaps slowed to a degree.

One effective method for removal of key organic compounds, like transparent exopolymer particles (TEP), is to use subsurface intake types (wells or galleries). Aquifer treatment causes significant reduction in total organic carbon, dissolved organic carbon, TEP, and the larger molecular weight organic compounds (biopolymers and humic substances). A significant percentage of marine bacteria are also removed. It has been demonstrated that the rate of membrane biofouling is slowed significantly using aquifer pretreatment methods compared to any conventional intake type. Membrane cleaning time is reduced from as great as 6 times per year to as little as once per year.

One mystery in SWRO water treatment is the passage of marine bacteria through aquifer treatment, extensive pretreatment including membrane filtration, and sometimes through the actual SWRO marines. Many engineers have suggested that bacterial passage through membranes is caused by failure of welds in the seams of the membranes. This issue is of great concern to the regulatory community because of the potential of pathogenic bacteria moving into the treated water downstream of the membrane process. We report for the first time that ultramicrobacteria with a size range from 0.02 to 0.1 µm are the likely problem because they will actually pass through the pores of the membranes. A new technique using flow cytometry has allowed a histogram of size of marine bacteria to be reported. The use of wells (aquifer treatment) does remove a high percentage of these tiny bacteria, thereby reducing the potential for passage through the membrane during treatment.