Seawater and RO focused facility maintenance managers often sample the water at both the inlet and outlet of an RO system, getting an understanding of the operations efficiency and performance. The data provided from these tests (SDI and turbidity) help tell a story, one that influences the longevity of the RO system and its components. Fouling is the key concern for RO systems; creating an environment that promotes fouling will quickly plug your RO membranes, yielding increased maintenance and operational costs, reduced membrane life, increased RO failure and routine plant shutdown.
Seawater Reverse Osmosis Fouling: RO fouling includes; biological fouling, particle fouling, colloidal fouling, organic fouling, mineral fouling and oxidant fouling. Fouling is a general term referring to the deposit and accumulation of scale, algae, suspended solids and insoluble salts on a surface. Such suspended particles, approximately 5 microns in size, are more likely to clog your membrane system, leading to fouling. Like scale, fouling will also cause rapid declines in efficiency levels and membrane life-cycle.
Silt Water Density Index (SDI): Silt is composed of suspended particles accumulating on the surface of the membrane. Common sources of silt include organic colloids, iron corrosion products, precipitated iron hydroxide, algae, and fine particulate. SDI testing is an accepted method for estimating the rate at which fouling will occur in water purification systems, especially those using reverse osmosis or nanofiltration membrane systems.
According to the ASTM Standard D4189, SDI measures the time required to filter a fixed volume of water through a standard 0.45µm pore size microfiltration membrane with a constant given pressure of 30 psi. The difference between the initial time and the time of a second measurement after normally 15 minutes (after silt-built up) represents the SDI value. So what do these values mean you might ask? Generally speaking, and this differs with each application; an SDI level below 1 can yield several years without fouling, a level below 3 can yield several months, between 3 to 5 you are experiencing frequent cleaning with fouling being a big concern, and finally any level above 5 is extremely concerning and immediate pretreatment is a must.
Learn how high efficiency cross-flow microsand technology yielded levels of SDI below 2 with an average of 2.4! The study was done by an independent North American engineering firm, Fortex Experts.
Turbidity: The cloudiness or haziness of a fluid caused by large numbers of individual fine particles that are generally invisible to the naked eye. The measurement of turbidity is one of many water quality tests. Turbidity solely measures the amount of suspended solids in a sampling of water, and just confirms that particles are present. It is important to note that turbidity and levels of SDI are not the same, and there is no correlation between the two measurements.
The most common measurement for turbidity is the Nephelometric Turbidity Units (NTU). There are several ways you can measure levels of turbidity in water, one being measuring the strength of a light source as it passes through a water sample.