2.4 Granular Media Filtration, Head Loss & Backwash Protocols

2.4 Granular Media Filtration, Head Loss & Backwash Protocols

Quick Answer: Granular media filtration is the primary physical barrier that removes suspended solids, coagulated flocs, clay, silt, and chlorine-resistant pathogenic protozoa (Giardia lamblia cysts and Cryptosporidium oocysts). Dual-media filters stratify lightweight, coarse anthracite coal over dense, fine silica sand, enabling depth filtration where particulates are captured throughout the entire bed profile. A filter run terminates upon reaching terminal head loss ($6\text{ to }9\text{ feet}$), turbidity breakthrough ($> 0.10\text{ to }0.30\text{ NTU}$), or maximum run time ($60\text{ to }100\text{ hours}$). Backwashing fluidizes the bed by $20\%\text{ to }30\%$, followed by a mandatory filter-to-waste cycle to manage the post-wash ripening spike.

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Granular Media Filtration Mechanics

Filtration does not function as a simple sieve. Particulates several orders of magnitude smaller than the media pore spaces are effectively removed through a combination of physical and chemical transport-and-attachment mechanisms:

1. Mechanical Straining: Particles larger than the pore openings are physically captured between adjacent grains. Dominates primarily in the upper $1\text{ to }2\text{ inches}$ of single-media sand filters.
2. Sedimentation: Microscopic flocs deviate from fluid streamlines and settle by gravity onto the upper surfaces of individual media grains within interstitial void spaces.
3. Inertial Impaction & Interception: Particles possessing mass and momentum cross diverging streamlines as flow curves around media grains, colliding directly with the grain collector.
4. Chemical Adsorption: Destabilized flocs carrying residual cationic charges adhere to media surfaces coated with aluminum or iron hydroxide collector coatings via electrostatic attraction and van der Waals forces.
5. Biological Degradation: In biological filters (e.g., non-chlorinated granular activated carbon or anthracite), fixed-film biofilms metabolize dissolved organic carbon (AOC) and oxidize ammonia.

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Filter Media Types & Depth Stratification

+------------------------------------------------------------------------+
|                   DUAL-MEDIA FILTER PROFILE & STRATIFICATION           |
|                                                                        |
|   Water Flow (Downward in Filtration)                                  |
|   ==================================================================   |
|   ANTHRACITE COAL LAYER: Coarse & Low Density                          |
|   - Depth: 18 - 24 inches                                              |
|   - Effective Size (ES): 0.9 - 1.2 mm | Specific Gravity (SG): 1.5 - 1.6|
|   - Captures 70-80% of macro-flocs (True Depth Filtration)             |
|   ------------------------------------------------------------------   |
|   SILICA SAND LAYER: Fine & High Density                               |
|   - Depth: 8 - 12 inches                                               |
|   - Effective Size (ES): 0.45 - 0.55 mm | Specific Gravity (SG): 2.65 |
|   - Polishes fine pin-floc and turbidity colloids                      |
|   ==================================================================   |
|   GRADED GRAVEL SUPPORT LAYER: 12 - 18 inches (3/4" down to 1/8")      |
|   ------------------------------------------------------------------   |
|   UNDERDRAIN LATERALS / LEOPOLD BLOCKS (Filtered Effluent Collection)  |
+------------------------------------------------------------------------+

Single-Media vs. Dual-Media vs. Mixed-Media

• Single-Media Sand Filter: Contains $24\text{ to }30\text{ inches}$ of silica sand ($\text{SG} = 2.65$, $\text{Effective Size } 0.45-0.55\text{ mm}$). During backwash, hydraulic fluidization classifies the sand: fine grains settle on top, coarse grains on the bottom. Consequently, all solids are trapped at the very top surface, causing rapid surface blinding, high head loss, and short filter runs ($12\text{ to }24\text{ hours}$).
• Dual-Media Filter: Consists of $18\text{ to }24\text{ inches}$ of coarse anthracite coal resting on top of $8\text{ to }12\text{ inches}$ of fine silica sand. Because anthracite is significantly lighter ($\text{SG} \approx 1.50\text{ to }1.60$) than silica sand ($\text{SG} \approx 2.65$), it settles on top during backwash despite its larger particle size ($0.9\text{ to }1.2\text{ mm}$). The upper anthracite captures heavy flocs throughout its depth, while the fine sand polishes residual turbidity, extending filter runs to $36\text{ to }72+\text{ hours}$.
• Mixed-Media (Tri-Media) Filter: Adds a bottom $3\text{-inch}$ layer of ultra-dense, ultra-fine garnet or ilmenite ($\text{SG} = 3.8\text{ to }4.2$, $\text{Effective Size } 0.20-0.30\text{ mm}$) beneath the sand. Provides the highest particulate capture efficiency and maximum resistance to turbidity breakthrough.

Media Specifications: Effective Size & Uniformity Coefficient

Filter media quality is defined by laboratory sieve analysis curves:

• Effective Size ($d_{10}$ or $ES$): The sieve opening size (in millimeters) that permits $10\%$ by weight of the media sample to pass, while retaining $90\%$. It governs the hydraulic resistance and particle trapping capability.
• Uniformity Coefficient ($UC$): A dimensionless ratio indicating the variation in grain sizes: $UC = \frac{d_{60}}{d_{10}}$ where $d_{60}$ is the sieve opening passing $60\%$ of the sample by weight. Design standards require a low $UC$ (typically $< 1.5\text{ to }1.7$). A low $UC$ indicates uniformly sized grains, which prevents fine sand from segregating and forming impermeable lenses.

Underdrain Systems & Support Gravel

The media bed is supported by an underdrain system that collects filtered water during filtration and distributes high-pressure water and air scour during backwash:

• Graded Gravel Layer: $12\text{ to }18\text{ inches}$ of rounded river gravel placed in $4\text{ to }5$ graded layers (ranging from $1.5\text{ inch}$ at the bottom to $1/8\text{ inch}$ on top). Prevents fine sand from entering underdrain orifices.
• Underdrain Types: Perforated pipe laterals, Wheeler false bottoms (concrete spheres seated in hoppers), vitrified clay/plastic dual-parallel lateral blocks (e.g., Leopold blocks), and direct-retention porous polyethylene plates (gravel-less underdrains).

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Filtration Hydraulics, Loading Rates & Head Loss

Filtration Loading Rate

The volume of water processed per minute per square foot of filter surface area:

$\text{Filtration Rate (gpm/ft}^2) = \frac{\text{Total Flow Rate (gpm)}}{\text{Filter Surface Area (ft}^2)}$

• Slow Sand Filters: $0.05\text{ to }0.15\text{ gpm/ft}^2$ (uses biological Schmutzdecke; no chemical coagulation).
• Conventional Rapid Sand Filters: $2.0\text{ to }3.0\text{ gpm/ft}^2$.
• High-Rate Dual/Mixed-Media Filters: $3.0\text{ to }6.0+\text{ gpm/ft}^2$.

Head Loss Development & Negative Head (Air Binding)

Head Loss is the loss of energy (expressed in feet of water column) resulting from frictional drag as water forces its way through the pore channels of the media bed.

• Clean Bed Head Loss: The baseline resistance of clean media immediately after backwash ($1.0\text{ to }2.0\text{ feet}$).
• Terminal Head Loss: The maximum allowable head loss before backwashing (typically $6.0\text{ to }9.0\text{ feet}$).

Critical Hazard — Negative Head and Air Binding: As solids clog media pore spaces, head loss increases. If head loss through the bed exceeds the static water depth above the media, the hydraulic grade line drops below the elevation of the filter bed, creating a partial vacuum (negative head) in the lower layers.

This pressure drop pulls dissolved gases (nitrogen and oxygen) out of solution, forming gas bubbles trapped inside the pore channels. This phenomenon is called Air Binding. Trapped air severely restricts filtration area, creates localized high-velocity channeling, cracks the media bed, and causes turbidity breakthrough.

+------------------------------------------------------------------------+
|               DEVELOPMENT OF NEGATIVE HEAD & AIR BINDING               |
|                                                                        |
|   Water Surface (Static Head Elevation)                                |
|   |                                                                    |
|   v  [ Clean Water Layer Above Media ]                                 |
|   --------------------------------------- Top of Filter Media          |
|   |  Frictional head loss accumulates                                  |
|   |  as solids clog pores                                              |
|   v                                                                    |
|   --------------------------------------- Level where Head Loss        |
|   |                                       Exceeds Static Head          |
|   |  PRESSURE DROPS BELOW ATMOSPHERIC                                  |
|   |  --> Partial Vacuum Developed (NEGATIVE HEAD ZONE)                 |
|   |  --> Dissolved Gases (N2, O2) Evolve as Gas Bubbles                |
|   |  --> Pores Blocked: AIR BINDING & CRACKING OCCURS                  |
|   --------------------------------------- Underdrain Floor             |
+------------------------------------------------------------------------+

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Filter Run Termination Triggers

An operator must immediately terminate a filter run and initiate backwashing when any one of three conditions is met:

1. Terminal Head Loss: Operating head loss reaches the design ceiling (typically $6.0\text{ to }9.0\text{ feet}$), indicating pore channels are fully loaded.
2. Turbidity Breakthrough: Effluent turbidity rises toward $0.10\text{ to }0.30\text{ NTU}$, signaling that shear forces are dislodging previously captured flocs.
3. Maximum Allowable Run Time: The filter reaches a pre-set maximum run time (typically $60\text{ to }100\text{ hours}$, commonly set at $72\text{ hours}$) regardless of head loss or turbidity. Permitting a filter to run beyond 100 hours promotes deep solids cementation, anaerobic biodegradation, and mudball formation.

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Backwash Hydrodynamics, Auxiliary Scour & Step-by-Step SOP

Backwashing is the reversal of flow through the filter bed using clean, treated water pumped upward through the underdrains at high velocity to flush out accumulated solids.

Bed Fluidization & Expansion

High-rate backwash water lifts the media grains, expanding the bed volume by $20\%\text{ to }30\%$ (e.g., a $30\text{-inch}$ media bed expands to $36\text{ to }39\text{ inches}$):

• High-Rate Wash Velocity: $15\text{ to }23\text{ gpm/ft}^2$ ($24\text{ to }36\text{ inches/minute}$ vertical rise rate).
• Temperature Dependency: Because cold water is denser and more viscous, it exerts greater hydraulic drag on media grains. Colder water in winter requires lower backwash flow rates to achieve $25\%$ expansion; warmer summer water requires higher flow rates.

Auxiliary Scour Systems

Fluidized media grains oscillate along hydraulic streamlines without colliding vigorously enough to dislodge sticky chemical flocs. Auxiliary scouring is necessary:

• Surface Wash Agitators: High-pressure rotating spray arms ($50\text{ to }75\text{ psi}$) situated $2\text{ inches}$ above the unexpanded bed spray jets of water into the media to shatter the tough surface crust.
• Air Scour Systems: Compressed air injected through the underdrain manifold at $2\text{ to }5\text{ scfm/ft}^2$ before or during low-rate water wash. Air bubbles produce intense grain-on-grain abrasion that scrubs flocs free.

Standard Backwash Standard Operating Procedure (SOP)

1. Isolate Influent: Close the filter influent valve. Allow water to filter down to roughly $6\text{ inches}$ above the media surface.
2. Isolate Effluent: Close the filter effluent valve.
3. Open Waste Gate: Open the backwash waste (drain) valve to the washwater recovery basin or waste sump.
4. Initiate Auxiliary Scour: Start the air scour blower ($2\text{ to }4\text{ minutes}$) or start surface wash pumps.
5. Low-Rate Water Wash: Open the backwash supply valve to low-rate wash ($5\text{ to }8\text{ gpm/ft}^2$) to fluidize the lower sand and purge air.
6. Shut Down Auxiliary Scour: Stop the air scour or surface wash before high-rate wash to prevent blowing media into the waste troughs.
7. Ramp Up High-Rate Backwash: Slowly open the backwash valve over $30\text{ to }60\text{ seconds}$ to high-rate wash ($15\text{ to }22\text{ gpm/ft}^2$, achieving $20-30\%$ expansion). Maintain for $5\text{ to }10\text{ minutes}$ until waste water clears.
8. Slow Deceleration: Gradually close the backwash valve over $60\text{ seconds}$ to permit hydraulic re-stratification (heavy sand settles first, light anthracite settles on top).
9. Close Waste Valve & Refill: Close the waste gate. Slowly open the influent valve to refill the filter bay without disturbing the media surface.
10. Filter-to-Waste (Rewash): Direct initial effluent to waste for $10\text{ to }30\text{ minutes}$ until turbidity drops below $0.10\text{ NTU}$ before placing the unit into service.

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Filter Ripening & Regulatory Compliance

The Filter Ripening Phenomenon

Immediately after backwashing, even the cleanest filter exhibits a transient spike in effluent turbidity—frequently exceeding $0.3\text{ to }0.5\text{ NTU}$ for the first $15\text{ to }45\text{ minutes}$ of operation. This is called the filter ripening period.

Causes:

• Freshly washed media grains lack the active adhesive coating of destabilized flocs that act as chemical collectors.
• Remnant backwash water containing detached micro-particles remains in the underdrain conduits and gravel support layer.

Filter-to-Waste (Rewash)

To prevent the ripening spike from entering the clearwell and violating drinking water regulations, plants employ Filter-to-Waste (Rewash) valves. The initial filtered water is diverted to the plant sewer or backwash recovery impoundment for $10\text{ to }30\text{ minutes}$ until turbidity stabilizes below $0.10\text{ NTU}$.

EPA Surface Water Treatment Rules (SWTR, IESWTR, LT2ESWTR)

Federal drinking water regulations mandate strict turbidity limits to ensure removal of Cryptosporidium oocysts and Giardia cysts:

• Combined Filter Effluent (CFE): Must be $\le 0.3\text{ NTU}$ in at least $95\%$ of measurements taken each calendar month.
• Maximum CFE Limit: Must never exceed $1.0\text{ NTU}$ at any time.
• Individual Filter Effluent (IFE): Each filter's turbidity must be monitored continuously and recorded at least every 15 minutes (ADEM Rule 335-7-10-.03 also requires each filter every 15 minutes or continuously). Follow-up is triggered by two consecutive 15-minute readings above $1.0\text{ NTU}$ at any time, or above $0.5\text{ NTU}$ at the end of the first 4 hours of operation after backwash; the system reports the event and produces a filter profile or explains the cause. Repeated exceedances over several months trigger a filter self-assessment or a comprehensive performance evaluation (Section 10.1).
• Alabama reporting: ADEM must be notified within 24 hours whenever filtered surface water exceeds 1.0 NTU (ADEM Admin. Code r. 335-7-10-.06(4)).

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Troubleshooting Common Filter Bed Defects