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172 8. GRANULAR MEDIA FILTRATION
The SDI pads used for measurement of the silt density index of the feed and source waters are visually inspected for color, odor, and retention/accumulation of solids, and coagulant after every test. These pads are typically light creamy or off-white if the granular media filters perform well. If the pads are slightly yellow, this is typically an indication of iron and/or colloidal organics in the source water. Iron could originate from overdosing of coagulant (if ferric chloride or sulfate is used) and/or from corrosion along the path of the pretreatment system flow.
Colloidal organics could originate from the natural organic matter (NOM) in the source water, overdosing of an organic polymer or antiscalant or bio growth on the intake facilities (e.g., wells).
Reddish brown discoloration of the SDI pads used for filter effluent testing is a clear indication of overdosing of iron-based coagulant, and this challenge would need to be troubleshooted by reduction of the coagulant dosage by at least 50%. The optimum coagulant dosage, as well as the need to add flocculant and to adjust pH of the source water, should be determined based on jar testing and on the past operational experience with similar source-water quality.
Gray discoloration of the SDI pads is indicative of the presence of carbon fines and usually occurs if GAC filtration layer is incorporated into the filter media or granular activated filter is used to abate source water with high organic loading.
If solid particles are observed on the SDI pads from the filtered water test prior to the cartridge filters, check the condition of the underdrain systemdthere may be breaches of its integrity (i.e., broken nozzles, cracks on the bottom of the filter cells, or other defects that may cause small amounts of filtration media to be conveyed with the filtered water). If solid particles are observed on the SDI pads from the test of sample of filter effluent, then check the condition of the cartridge filtersdit is likely that some of the individual cartridges may have collapsed or are short- circuiting feed water and allowing particles in the filtered water to escape being captured by the cartridge filters.
If dark black discoloration is observed on the cartridge filters, this problem is most likely caused by high content of manganese impurities in the ferric chloride coagulant. To address this challenge, operators can either switch to ferric sulfate or identify alternative source of ferric chloride supply, which has lower manganese content.
8.5.4.2 Sand and Anthracite Media Intermixing
Under normal operational conditions of well-designed dual-media (anthracite and sand) downflow filters, the anthracite and sand media stay separated into two distinctive filtration layers during filter operations. If intermixing of the two media occurs (Fig. 8.7), the propagation of anthracite into sand often results in significant reduction of the length of the filtration cycle (typically from 24 h or more down to 2e6 h).
Practical experience shows that one of the most common reasons for the intermixing of the sand and anthracite media in filter cells is the use of overly heavy anthracite. In general, two types of anthracite products are available on the marketda lighter product (referred to as “Welsh type”) anthracite that has a specific gravity of 1400e1450 kg/m3 and is suitable for granular media filtration; and a heavier (“Pennsylvania type”) anthracite with specific gravity of 1600e1650 kg/m3, which is often used as construction material. While the heavier anthracite is less costly and could sometimes be selected based on lower overall media cost, when used as a top layer over a typical sand filtration media of specific gravity of 2500e2650 kg/m3, this anthracite sinks into the sand media and the two media completely intermix within several filter cycles of their installation. Media intermixing is irreversible (i.e., sand and