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2.2 PARTICULATE FOULANTS 13 2.2 PARTICULATE FOULANTS
2.2.1 Description
Particulate foulants are organic and inorganic particles contained in the source water such as fine debris, plankton, detritus, and silt. These solids cannot pass through the RO mem- branes. All suspended solids that naturally occur in insoluble form, if not removed by pretreat- ment, would be retained on the feed side of the RO-membrane leafs and depending on the hydrodynamic conditions on the membrane surface, as well as the size and charge of these particles, and they would either migrate along the membrane leafs and ultimately exit with the concentrate, or would be trapped on the membrane surface and would begin to accumu- late there causing loss of membrane productivity over time. This type of foulants can be effec- tively removed by prefiltering of the source water prior to RO-membrane separation.
Particulate foulants in raw source saline water vary greatly in size. However, most of them, including picoplankton, are larger than 0.1mm. A well-designed and operating pretreatment system will produce water that does not have particles larger than 20 mm in size. Typically, suspended solids of size larger than 100 mm contained in saline water are settleable and can be removed by clarification of the source water prior to filtration. However, many bacteria that inhabit seawater and cause RO-membrane biofouling are smaller in size than 0.1 mm and are practically impossible to remove completely from the source seawater even if the most advanced pretreatment technologies are applied. This makes pretreatment of seawater very challenging.
2.2.2 Measurement Parameters and Methods
2.2.2.1 Turbidity
Turbidity is a water-quality parameter, which measures the content of particulate suspended solids in water. The turbidity level in the source water is indicative of the content of clay, silt, suspended organic matter, and microscopic aquatic life, such as phyto- and zooplankton. It is expressed in nephelometric turbidity units (NTUs).
The turbidity of open ocean and surface brackish waters typically varies between 0.1 and several hundred NTU, although under normal dry weather conditions, it is usually in a range between 0.5 and 2.0 NTU. Rain events, algal blooms, storms, snowmelt, river discharges, and human activity (such as wastewater discharges, ship traffic, etc.) can cause significant turbidity increases and variations.
Usually, saline water with turbidity below 0.05 NTU causes very low particulate fouling of RO membranes. Most RO-membrane manufacturers require RO-system feed water turbidity of 1.0 NTU or less in order to maintain their performance warranty, although this level is rela- tively high in practical terms. Usually, it is desirable that the turbidity of the feed water to the RO-membrane elements is lower than 0.1 NTU in order to prevent accelerated particulate RO-membrane fouling.
Although turbidity is a good measure of the overall content of suspended solid particles in the source water, on its own, it is not an adequate quantitative parameter to characterize water’s potential to cause particulate or other fouling of the downstream RO membranes. Turbidity measurement does not provide information regarding the type and size of particles