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2.2 PARTICULATE FOULANTS 17
analysis of saline water completed in accordance with the standard methods for water and wastewater analysis (APHA, 2012) yields an erroneously high TSS content in the water. The higher the source water salinity and the lower its particulate content, the more inaccurate this measurement is. In order to address the challenge associated with the standard method of TSS measurement, it is recommended to wash the solids retained on the filter by spraying the filter with deionized water before drying. Unless the source water solids sample is washed before drying, the results of this sample are meaningless.
If the laboratory TSS test is completed properly and the filtered sample is well washed, this parameter usually provides much more accurate measure of the actual content of particulate solids in the source water than does turbidity, because it accounts for the actual weight of the total particulate material present in the sample. For comparison, turbidity measurement is dependent on particle size, shape, and color, and typically is not reflective of particles of very small size (i.e., particles of 0.5 mm or less), such as fine silt and picoplankton.
In fact, a change in the ratio of TSS to turbidity is a good indicator of a shift in the size of particles contained in the source water, which may be triggered by algal blooms, storms, strong winds, and other similar events, which can result in resuspension of solids from the bottom sediments into the water column.
Typically, an increase in the TSS/turbidity ratio is indicative of a shift of particulate solids toward smaller-size particles. For example, during non-algal-bloom conditions, the TSS/ turbidity ratio of an appropriately processed sample is typically in the range of 1.5e2.5, i.e., water with a turbidity of 2 NTU would have a TSS concentration of 3e5 mg/L. During heavy algal blooms dominated by small-size (pico- and micro-) algae, the TSS of the source water could increase over 10 times (e.g., to 40 mg/L), while the source water turbidity could be multiplied by 2 to 3 times onlydfor this example, it would be in a range of 4e6 NTU, with a corresponding increase in the TSS/NTU ratio from 2/1 to between 6/1 and 10/1.
2.2.2.4 Particle Distribution Profile
Particle distribution profile presents the number of solid particles in the source water for different particle size ranges. Usually, particles are classified in the following ranges: 1 mm or less;>1and 2mm;>2and 5mm;!5and 10mm;>10and 20mm;>20and 50mm;and higher than 50 mm.
Particles of sizes smaller than 1 micron (mm) are poorly removed by conventional granular media filtration and DAF clarification. Key pathogens such as Giardia and Cryptosporidium are in a range of 2e10 mm and typically 2 logs of these pathogens are removed by granular media filtration and 4e6 logs by MF or UF membrane filtration.
Particles that have sizes of higher than or equal to 20 mm are well removed by both mem- brane and granular media pretreatment filters. Typically, pretreatment system is considered to perform well if the pretreated water contains less than 100 particles per milliliter of size equal to or smaller than 2 mm and does not contain larger size particles.
2.2.3 Threshold Levels of Particulate Foulants
Table 2.1 presents a list of source water quality parameters used for characterizing partic- ulate content that are recommended to be measured when deciding upon the type of pretreat- ment needed for a given source water.