2.4 Granular Media Filtration, Backwash Procedures & Turbidity Standards

Granular Media Filtration, Backwash Procedures & Turbidity Standards

Granular media filtration is the final physical barrier protecting public health from waterborne pathogens—most notably chlorine-resistant Cryptosporidium oocysts (4–6 μm) and Giardia lamblia cysts (8–14 μm). Under Colorado Regulation 11 and the Safe Drinking Water Act, filtration performance is held to uncompromising, continuously monitored standards. An operator must understand media hydraulic characteristics, filtration mechanics, backwash hydraulics, ripening sequences, and regulatory limits.

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1. Granular Filter Media Configurations

In conventional gravity filtration, water flows downward through a bed of granular materials supported by an underdrain collection system.

+-----------------------------------------------------------------------------------+
|                     GRANULAR FILTER MEDIA CONFIGURATIONS                          |
|                                                                                   |
|  RAPID SAND FILTER           DUAL-MEDIA FILTER           MULTIMEDIA (TRI-MEDIA)   |
|  [ Sand: 24-30 in ]          [ Anthracite: 18-24 in ]    [ Anthracite: 18-20 in ] |
|  ES = 0.5 mm, SG = 2.65      ES = 1.0 mm, SG = 1.5       ES = 1.0 mm, SG = 1.5    |
|                              ------------------------    ------------------------ |
|                              [ Sand: 10-12 in ]          [ Sand: 8-10 in ]        |
|                              ES = 0.5 mm, SG = 2.65      ES = 0.5 mm, SG = 2.65   |
|                                                          ------------------------ |
|                                                          [ Garnet: 3-4 in ]       |
|                                                          ES = 0.25 mm, SG = 4.2   |
+-----------------------------------------------------------------------------------+

Media Physical Characteristics

Filter media performance depends on three core physical parameters:

1. Effective Size ($d_{10}$ or $ES$): The sieve opening size (in millimeters) that permits 10% of the media grains by weight to pass and retains 90%.
2. Uniformity Coefficient ($UC$): The ratio of the sieve size that passes 60% of the media ($d_{60}$) to the effective size ($d_{10}$):

$UC = \frac{d_{60}}{d_{10}}$

A lower $UC$ ($<1.4–1.6$) indicates uniform grain sizing, which maximizes inter-grain porosity and minimizes hydraulic stratification issues. 3. Specific Gravity ($SG$): The density of the media relative to pure water (Anthracite $SG \approx 1.45–1.60$; Silica Sand $SG \approx 2.65$; Garnet/Ilmenite $SG \approx 4.0–4.3$).

Filter Types Compared

Filter Underdrain Systems

The underdrain system supports the filter media, collects filtered water uniformly across the entire floor during filtration, and distributes backwash water and air scour evenly during washing. Common configurations include:

• Perforated Lateral Pipes embedded in graded gravel layers.
• Plastic Block Underdrains (e.g., Leopold Blocks) with dual lateral compensating chambers that equalize pressure.
• Porous Plate Bottoms that eliminate the need for graded gravel layers.

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2. Mechanisms of Filtration

Granular media filtration does not function merely as a simple physical sieve. Particle removal occurs via five distinct, synergistic transport and attachment mechanisms:

   +-------------------------------------------------------------+
   |                  PARTICLE REMOVAL MECHANISMS                |
   |                                                             |
   |  1. Mechanical Straining: Particle diameter > pore size     |
   |  2. Sedimentation: Gravity settling onto grain surfaces     |
   |  3. Inertial Impaction: Inertia crosses streamlines to grain|
   |  4. Interception: Streamline passes within 1/2 particle diam|
   |  5. Physicochemical Adsorption: Van der Waals + Electrostatic|
   +-------------------------------------------------------------+

1. Mechanical Straining: Particles larger than the interstitial pore openings between media grains are physically trapped at the bed surface (dominates only for large particles $>10–20\text{ μm}$).
2. Sedimentation: Within the tiny interstitial pores, flow velocities are low ($<0.005\text{ ft/s}$), allowing fine flocs to settle by gravity onto the upper surfaces of individual media grains.
3. Inertial Impaction: Particles with significant mass and inertia cannot follow fluid streamlines as water bends around media grains, colliding directly into the grain.
4. Interception: Particles traveling along fluid streamlines contact a media grain because their path passes within one particle radius ($r_p$) of the grain surface.
5. Physicochemical Adsorption & Chemical Attachment: Once transport mechanisms bring a destabilized colloid to within nanometers of a grain surface, chemical bonding, electrokinetic attraction, and van der Waals forces lock the particle in place. Proper upstream chemical coagulation is required—without chemical charge destabilization, micro-colloids, Giardia, and Cryptosporidium carry negative charges, repel the negative sand grains, and pass completely through the filter.

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3. Operational Monitoring and Filter Run Termination

During a filter run, operators monitor three primary parameters to decide when a filter must be taken off-line for backwashing:

1. Terminal Head Loss: As solids accumulate within media pores, frictional resistance increases. Piezometer tubes or differential pressure transmitters measure this headloss. A filter is backwashed when head loss reaches $6.0\text{ to }9.0\text{ feet}$ of head (plant specific).
2. Turbidity Breakthrough: When media pore capacity is exhausted, shearing forces dislodge trapped flocs, causing effluent turbidity to rise sharply. Filtration must be terminated immediately if effluent turbidity approaches plant action levels (typically $>0.10\text{ NTU}$).
3. Maximum Allowable Run Time: Even if head loss and turbidity remain low, filters should be washed after $36\text{ to }72\text{ hours}$ of continuous operation to prevent deep floc compaction, biofouling, and mudball formation.

The Negative Head Phenomenon and Air Binding

When a filter accumulates high solids, headloss through the media bed can exceed the static head of water above the media. This creates negative head (a partial vacuum / sub-atmospheric pressure) inside the lower media layers.

• Operational Hazard: Sub-atmospheric pressure pulls dissolved air out of solution, forming millions of tiny air bubbles inside the sand and underdrain (air binding).
• Consequences: Air binding severely blocks flow, causes channeling, spikes apparent headloss, and tears pathways through the media when the air bubbles suddenly burp to the surface, causing massive turbidity breakthrough.

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4. Complete Filter Backwash and Ripening Sequence

Backwashing reverses the flow of treated water upward through the filter bed to expand the media, scour accumulated solids from grain surfaces, and flush them into washwater waste troughs.

+-----------------------------------------------------------------------------------+
|                         COMPLETE FILTER BACKWASH SEQUENCE                         |
|                                                                                   |
|  1. Drain Down  ->  2. Auxiliary Scour  ->  3. High-Rate Wash  -> 4. Media Settle|
|     Water to top      Air Scour (3-5 scfm)     Bed Expansion       Reclassifies   |
|     of troughs        or Surface Wash          20% to 50%          Layers         |
|                                                                                   |
|                                   5. Ripening (Filter-to-Waste: 5-20 min)         |
|                                      Effluent to drain until turbidity < 0.10 NTU |
+-----------------------------------------------------------------------------------+

Step-by-Step Backwash Sequence

1. Isolation and Drain Down: Close influent valve. Allow water to filter down until the water surface is approximately 6 to 12 inches above the top of the media bed.
2. Auxiliary Scour (Air Scour or Surface Wash):
   • Air Scour: Compressed air is injected through the underdrain at $3\text{ to }5\text{ scfm/sq ft}$ for 2–3 minutes. The violent bubbling induces aggressive grain-on-grain abrasion, breaking up sticky surface mats.
   • Surface Wash: High-pressure water jets (50–75 psi) from rotating arms or fixed nozzles deliver $1.5\text{ to }4.0\text{ gpm/sq ft}$ to break up hard surface crusts and prevent mudballs (clumps of coagulant, sand, and organic matter that grow and sink to the underdrain).
3. Low-Rate Washwater Initiation: Open backwash supply valve slowly. Fluidize the lower bed gently while auxiliary scour completes.
4. High-Rate Fluidized Backwash: Ramp washwater flow up to full rate ($15\text{ to }23\text{ gpm/sq ft}$, depending on water temperature). The upward water velocity expands the granular bed by $20%\text{ to }50%$ of its resting depth. Interstitial shear flushes detached solids upward into washwater collection troughs for 8 to 15 minutes until waste washwater appears clear.
5. Wash Termination and Media Re-Stratification: Slowly close the backwash valve. As upward flow ceases, gravitational settling re-stratifies the media bed by specific gravity and size: dense garnet sinks to the bottom, silica sand settles in the middle, and light anthracite coal settles cleanly on top.
6. Filter Ripening and Filter-to-Waste: When a washed filter is restarted, initial effluent exhibits an immediate turbidity spike (0.3–1.0+ NTU) lasting 5 to 20 minutes due to backwash remnants and lack of an initial particle conditioning coat (the ripening period). Operators must activate the filter-to-waste valve, routing initial effluent to the sewer/recycle basin until turbidity drops consistently below $0.10\text{ NTU}$ before opening the filter effluent to the clearwell.

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5. Regulatory Turbidity Standards (CDPHE Regulation 11 & EPA SWTR)

Turbidity is a direct surrogate measurement for microbial safety. Colorado Regulation 11 mandates strict continuous compliance for all conventional and direct filtration facilities.

Combined Filter Effluent (CFE) Standards

• Monthly 95th Percentile Limit: CFE turbidity must be $\le 0.30\text{ NTU}$ in at least 95% of all measurements taken each calendar month (measured continuously or grab sampled at least every 4 hours).
• Maximum Absolute Limit: CFE turbidity must NEVER exceed $1.0\text{ NTU}$ at any instantaneous measurement. An exceedance of 1.0 NTU is a direct Tier 2 treatment technique violation.

Individual Filter Effluent (IFE) Standards

Plants must record IFE turbidity continuously at 15-minute intervals for each individual filter. Regulation 11 establishes mandatory diagnostic triggers: