3.1 Granular Media Filtration Operations

Granular Media Filtration Operations

Granular media filtration is the foundational physical barrier in conventional surface water treatment. Downstream of coagulation, flocculation, and sedimentation, filtration removes suspended solids, colloidal particles, natural organic matter (NOM), and microbial pathogens—specifically Cryptosporidium parvum and Giardia lamblia—to ensure compliance with the Safe Drinking Water Act (SDWA).

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1. Physical & Chemical Filtration Mechanisms

Deep-bed granular filtration combines five simultaneous mechanisms to capture suspended solids:

1. Mechanical Straining: The physical arrest of particles whose diameters exceed pore openings between media grains. Straining occurs predominantly in the top 1 to 2 inches of the bed.
2. Sedimentation on Media Grains: Gravity settling of particles denser than water onto upward-facing surfaces of media grains within pore voids.
3. Inertial Impaction: Heavy particles diverge from curving streamlines around grains and collide directly with media surfaces.
4. Interception: Particles following fluid streamlines come within one particle radius of a media grain, making direct contact as they pass.
5. Adsorption (Electrostatic & Chemical Bonding): The decisive mechanism for submicron colloids. Natural colloids and media grains both carry negative zeta potentials, causing mutual electrostatic repulsion. Upstream coagulation neutralizes these charges, enabling short-range van der Waals forces, hydrogen bonding, and surface chemical coordination to attach particles to media grains. Without coagulation, fine colloids and cysts pass directly into the effluent.

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2. Filter Media Types, Configurations & Stratification

Filter designs range from single-medium beds to multimedia configurations:

• Slow Sand Filters: Single layer of silica sand (30–36 in) operating at 0.05 to 0.15 gpm/sq ft without chemical coagulation. Microbial removal occurs within a biological surface layer (schmutzdecke). Cleaned by manual scraping without backwashing.
• Rapid Sand Filters: Single medium of silica sand (24–30 in) operating at 2.0 gpm/sq ft. Hydraulic backwashing causes fine sand to settle at the top, restricting solids capture to the upper few inches and accelerating head loss.
• Dual-Media Filters: 18 to 24 inches of anthracite coal over 8 to 12 inches of silica sand, operating at 3.0 to 5.0 gpm/sq ft. Coarse anthracite traps large floc in-depth, while finer sand below polishes small particles.
• Multimedia Filters: 16 to 18 inches of anthracite over 8 to 10 inches of sand over 3 to 6 inches of garnet, operating at 4.0 to 6.0 gpm/sq ft, supported by graded gravel.

Sizing Metrics & Specific Gravity Hierarchy

Media grains are specified using standardized sieve metrics:

• Effective Size (ES or d10): Sieve opening in mm passing 10% of media by weight and retaining 90%.
• Uniformity Coefficient (UC): Ratio of d60 to d10 (UC = d60 / d10). High-rate filters require UC ≤ 1.4 to 1.7 to prevent hydraulic classification.

In multimedia filters, hydraulic fluidization is balanced by material density:

• Anthracite Coal: ES = 0.90 to 1.20 mm; Specific Gravity ≈ 1.50 to 1.60
• Silica Sand: ES = 0.45 to 0.55 mm; Specific Gravity ≈ 2.65
• Garnet Sand: ES = 0.20 to 0.30 mm; Specific Gravity ≈ 4.00 to 4.25

Settling velocity depends on buoyant density. Denser garnet (SG 4.10) settles fastest to the bottom, silica sand (SG 2.65) settles in the middle, and low-density anthracite (SG 1.55), despite its larger diameter, settles slowest at the top. This hierarchy guarantees reverse coarse-to-fine stratification after backwashing.

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3. Filter Operation, Hydraulics & Terminal Head Loss

Municipal plants primarily use open concrete gravity filters, utilizing hydrostatic water depth (5 to 9 feet) to drive flow through the bed. Conversely, pressure filters house media within steel pressure vessels operated under pump discharge (30 to 100 psi). While common in groundwater iron/manganese removal, pressure filters prevent visual inspection and risk breakthrough under pressure surges.

• Filtration Rates: Rapid sand operates at 2.0 gpm/sq ft. High-rate gravity filters operate at 2.0 to 5.0 gpm/sq ft (up to 6.0 to 8.0 gpm/sq ft permitted under IEPA permits with polymer filter aids of 0.02 to 0.10 mg/L).
• Head Loss: Clean bed head loss is 1.0 to 1.5 feet. Accumulating solids increase friction until terminal head loss (6.0 to 9.0 feet) is reached.

A filter run terminates upon reaching terminal head loss, individual effluent turbidity breakthrough (approaching 0.10 NTU), or maximum run time (24 to 72 hours) to avoid media compaction and anaerobic growth.

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4. Ripening, Filter-to-Waste & Turbidity Standards

Immediately post-backwash, clean filters experience an initial 15 to 30 minute turbidity spike called filter ripening. Freshly cleaned grains lack remnant floc conditioning, reducing particle capture. Over 90% of protozoan cysts (Cryptosporidium and Giardia) passing through filters penetrate during this ripening window.

Filter-to-Waste (Rewash)

Plants divert early effluent to waste until turbidity drops below target (≤ 0.10 to 0.30 NTU). Operators also use flow ramping (slow-start) over 15 to 30 minutes to minimize hydraulic shear.

IEPA / SDWA Turbidity Standards

• Combined Filter Effluent (CFE): Must be ≤ 0.3 NTU in at least 95% of monthly measurements; must never exceed 1.0 NTU.
• Individual Filter Effluent (IFE): Continuous online turbidimeters record turbidity every 15 minutes. Regulatory triggers require action:
  • IFE > 0.5 NTU after 4 hours of service in two consecutive readings: record cause.
  • IFE > 1.0 NTU in two consecutive readings: produce an individual filter profile within 7 days.
  • IFE > 1.0 NTU in two consecutive readings for 3 consecutive months: conduct a Comprehensive Performance Evaluation (CPE).
  • IFE > 2.0 NTU in two consecutive readings in two consecutive months: perform a facility self-assessment.

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5. Backwash Sequence, Hydraulics & Troubleshooting

Backwashing reverses flow upward to expand media, loosen solids, and wash them into waste launders.

Backwash Execution

1. Isolate the filter and lower water to 6 to 12 inches above media.
2. Apply auxiliary scour: surface wash sweeps (2.0 to 4.0 gpm/sq ft at 40–50 psi) or air scour (2 to 5 scfm/sq ft) for 2 to 3 minutes to scrub media grains.
3. Open backwash valve slowly, ramping to full rate (15 to 23 gpm/sq ft) to achieve 20% to 30% bed expansion for 8 to 15 minutes until waste water clears.
4. Close backwash, allow bed to settle, refill slowly, and initiate filter-to-waste.

Temperature Adjustments & Operational Problems

Water viscosity increases at colder temperatures (1.79 cP at 0°C vs. 1.00 cP at 20°C). Cold water exerts higher drag, requiring operators to reduce backwash pumping rates in winter to achieve 20% to 30% expansion without washing media over troughs.

• Mudball Formation: Agglomerations of coagulant, silt, and media grains that sink to the gravel layer, causing dead spots and channeling. Prevented by auxiliary scour and adequate backwash velocity.
• Filter Bed Cracking: Surface mud shrinkage fissures that allow floc to bypass the upper bed. Prevented by avoiding excessive head loss.
• Media Boiling: Localized backwash geysers caused by displaced gravel or damaged underdrains, leading to gravel inversion. Prevented by opening backwash valves gradually.
• Air Binding: Dissolved gas bubbles choking pore spaces and spiking head loss. Occurs under negative head (head loss exceeding water depth above media) or when cold water warms in the basin.