5.2 Sedimentation, Clarification & Media Filtration Systems

3.3 Sedimentation, Clarification & Media Filtration Systems

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Sedimentation Mechanics & Four Functional Zones

Sedimentation is the separation of suspended chemical flocs from water by gravitational settling. Conventional horizontal-flow rectangular basins are hydraulically partitioned into four operational zones:

+-----------------------------------------------------------------------------+
|                FOUR FUNCTIONAL ZONES OF A CLARIFIER BASIN                   |
+-----------------------------------------------------------------------------+
|  INLET ZONE   |               SETTLING ZONE                 |  OUTLET ZONE  |
|               |                                             |               |
|  Diffuser     |   --------> Flow Streamline -------->       |  Effluent     |
|  Baffle       |       \                                     |  Launders &   |
|  Dissipates   |        \ Particle Trajectory (vs)           |  V-Notch      |
|  Inlet Kinetic|         \                                   |  Weirs        |
|  Energy       |          v                                  |  (WOR Limit)  |
+---------------+---------------------------------------------+---------------+
|                            SLUDGE ZONE                              |
|   - Thickened solids blanket                                                |
|   - Mechanical flight-and-chain scrapers or suction headers                 |
+-----------------------------------------------------------------------------+

1. Inlet Zone: Receives water from the flocculator and uniformly distributes flow across the basin cross-section while dissipating inlet velocity. Uses perforated diffuser walls or slotted target baffles (head loss $0.05-0.10 \text{ ft}$) to establish calm, non-turbulent plug-flow conditions.
2. Settling Zone: The central, quiescent volume of the tank where gravitational settling occurs. Water flows horizontally at low velocity ($< 0.5-1.0 \text{ ft/min}$) while particles settle downward at terminal velocity $v_s$.
3. Sludge Zone: The basin floor where settled floc accumulates and thickens. Solids are scraped continuously or intermittently toward a central sludge hopper via motorized flight-and-chain scrapers, traveling bridge scrapers, or submersible vacuum headers.
4. Outlet Zone: Collects clarified supernatant and channels it toward the filters. Consists of effluent launders fitted with adjustable V-notch weirs or submerged orifices. Uniform collection across the entire weir length is critical to avoid high-velocity localized upwelling that sweeps settled floc out of the basin.

Settling Regimes in Water Treatment

• Type I (Discrete Settling): Gravitational settling of unhindered, non-flocculent particles (e.g., raw sand, silt, and grit) governed strictly by Stokes' Law: $$v_s = \frac{g(\rho_p - \rho)d^2}{18\mu}$$ where $g$ is gravitational acceleration, $\rho_p$ is particle density, $\rho$ is fluid density, $d$ is particle diameter, and $\mu$ is dynamic viscosity.
• Type II (Flocculent Settling): Particles coalesce and aggregate during settling, increasing in mass and altering their drag coefficient. Settling velocity accelerates as particles sink through the tank.
• Type III (Hindered / Zone Settling): Occurs at high solids concentrations where particles are so crowded that inter-particle forces inhibit settling. Particles settle as a collective mass or blanket with a distinct liquid-solids interface.

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Clarifier Configurations & High-Rate Clarification

High-Rate Tube Settlers & Lamella Plates

Conventional settling basins require massive surface areas because particles must fall $10-15 \text{ feet}$ to reach the sludge floor. Tube settlers and inclined lamella plates drastically alter this hydraulic dynamic:

• Modules comprise PVC tubes or corrugated plates inclined at a $60^\circ$ angle.
• Flow enters the bottom and travels upward in laminar flow (Reynolds number $Re < 500$).
• The vertical settling distance is reduced from $12 \text{ feet}$ to only $2 \text{ to } 3 \text{ inches}$.
• Particles settle onto the bottom surface of the inclined tube within minutes, agglomerate, and slide downward by gravity against upward flow, dropping into the sludge zone below.
• Operational Benefit: Tube settlers increase the hydraulic throughput of an existing basin by $200\% \text{ to } 400\%$ or decrease the footprint of a new facility by $75\%$.

Dissolved Air Flotation (DAF)

In cold, low-turbidity Pacific Northwest reservoirs subject to seasonal cyanobacterial blooms (e.g., coastal and Willamette Valley impoundments), alum floc is light and buoyant (specific gravity $\approx 1.002-1.010$), settling sluggishly. Dissolved Air Flotation (DAF) inverts the conventional clarification process. A recycle stream ($8-12\%$ of plant flow) is saturated with air in a pressure vessel at $60-85 \text{ psi}$. When injected into the contact zone at atmospheric pressure, the air precipitates millions of microscopic bubbles ($30-100 \ \mu\text{m}$ diameter). The bubbles attach to chemical flocs and float them to the basin surface as a dense blanket ("float"), which is mechanically skimmed off. Clarified effluent is drawn from the bottom.

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Hydraulic Operational Parameters & Equations

Plant operators must continuously evaluate three key hydraulic parameters to ensure clarifier efficiency:

1. Surface Overflow Rate (SOR)

The volume of water applied per unit of clarifier surface area per day:

$$\text{SOR} = \frac{\text{Flow Rate (gpd)}}{\text{Basin Surface Area (ft}^2\text{)}}$$

• Conventional Basins: $500 \text{ to } 1000 \text{ gpd/ft}^2$ (equivalent to $0.35 - 0.70 \text{ gpm/ft}^2$).
• Tube Settler Clarifiers: $2000 \text{ to } 3000 \text{ gpd/ft}^2$ (equivalent to $1.4 - 2.1 \text{ gpm/ft}^2$).
• Operational Significance: Any particle with a settling velocity ($v_s$) greater than the SOR will be completely captured. If plant flow is pushed beyond design SOR, rising upward currents exceed $v_s$, causing floc carryover onto the filters.

2. Weir Overflow Rate (WOR)

The volume of clarified water exiting over each linear foot of effluent weir per day:

$$\text{WOR} = \frac{\text{Flow Rate (gpd)}}{\text{Total Active Weir Length (ft)}}$$

• Standard Design Threshold: $\le 20,000 \text{ gpd/ft}$ (or $\le 10,000-15,000 \text{ gpd/ft}$ for light flocs).
• Operational Significance: Excessive WOR generates localized high-velocity approach currents ("drawdown vortices") near the launders that lift settled floc up and over the weir crest.

3. Hydraulic Detention Time ($DT$)

The theoretical time water resides within the basin volume:

$$DT = \frac{\text{Basin Volume (gal)}}{\text{Flow Rate (gpm)}} \quad \text{or} \quad DT = \frac{\text{Volume (gal)} \times 24}{\text{Flow Rate (gpd)}} \quad (\text{in hours})$$

• Conventional rectangular clarifiers require $2.0 \text{ to } 4.0 \text{ hours}$ of detention time.

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Media Filtration Physics & Transport Mechanics

Granular media filtration is not a simple physical sieve. Uncoagulated water passed through fine silica sand will show negligible removal of submicron viruses, bacteria, and colloids because the pore spaces between sand grains are $50 \text{ to } 200 \text{ times}$ larger than colloidal particles. Removal depends upon transport and attachment:

+-----------------------------------------------------------------------------+
|                     FIVE MECHANISMS OF MEDIA FILTRATION                     |
+-----------------------------------------------------------------------------+
|  [1] Mechanical Straining:  Particle larger than media pore throat          |
|  [2] Sedimentation:         Particle settles onto grain within pore channel |
|  [3] Interception:          Streamline passes within 1 particle radius      |
|  [4] Impaction:             Inertia causes particle to cross streamlines    |
|  [5] Adsorption:            Van der Waals & electrostatic attachment        |
|                             (Requires prior chemical coagulation!)          |
+-----------------------------------------------------------------------------+

1. Mechanical Straining: Particles larger than the inter-granular pore throat become physically trapped at the media boundary.
2. Sedimentation: Micro-particles flowing through interstitial pore channels settle gravitationally onto media surfaces.
3. Interception: A particle following fluid streamlines contacts a media collector grain because its radius exceeds streamline distance.
4. Inertial Impaction: Dense particles diverge from curved fluid streamlines due to momentum and impact media surfaces.
5. Physicochemical Adsorption: Once transported to within nanometer proximity, destabilized particles attach to the media surface via short-range van der Waals attractive forces and electrostatic bonding. Coagulation is mandatory: without charge neutralization, electrostatic repulsion prevents particles from attaching to media grains.

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Media Configurations: Dual-Media vs. Rapid Sand

+-----------------------------------------------------------------------------+
|                   RAPID SAND vs. DUAL-MEDIA PROFILES                        |
+-----------------------------------+-----------------------------------------+
|        RAPID SAND FILTER          |           DUAL-MEDIA FILTER             |
|                                   |                                         |
|   [Fine Sand Layer (Top)]         |   [Coarse Anthracite Coal (Top)]        |
|   * Size: 0.45 - 0.55 mm          |   * Size: 0.9 - 1.1 mm                  |
|   * Sp. Gr. = 2.65                |   * Sp. Gr. = 1.4 - 1.6 (Depth: 18-24") |
|   * Surface blinding / rapid loss |   * Traps heavy floc in deep pores      |
|                                   |                                         |
|   [Coarse Sand Layer (Bottom)]    |   [Fine Silica Sand (Bottom)]           |
|   * Under-utilized media depth    |   * Size: 0.45 - 0.55 mm                |
|   * Short filter runs (12-24 hrs) |   * Sp. Gr. = 2.65 (Depth: 8-12")       |
|                                   |   * Polishes fine turbidity             |
|                                   |   * Long runs (36-72 hrs)               |
+-----------------------------------+-----------------------------------------+

The Rapid Sand Limitation

Traditional single-medium rapid sand filters contain $24-30 \text{ inches}$ of silica sand (effective size $0.45-0.55 \text{ mm}$). When backwashed with water, hydraulic fluidization stratifies the media: fine grains rise to the top while coarse grains sink to the bottom. Consequently, incoming clarified water encounters the finest pores at the very top surface. The top $2 \text{ inches}$ become blinded with solids, causing rapid head loss accumulation while the underlying bed remains completely unutilized.

The Dual-Media Solution

Dual-media filters eliminate surface blinding by pairing coarse anthracite coal over fine silica sand:

• Anthracite Coal (Top Layer): Effective size $0.9 \text{ to } 1.1 \text{ mm}$, depth $18-24 \text{ inches}$, specific gravity $1.4 \text{ to } 1.6$.
• Silica Sand (Bottom Layer): Effective size $0.45 \text{ to } 0.55 \text{ mm}$, depth $8-12 \text{ inches}$, specific gravity $2.65$.
• Hydraulic Behavior: During backwashing, the entire bed expands and fluidizes. Because anthracite has a significantly lower specific gravity ($1.4-1.6$) than silica sand ($2.65$), it settles more slowly upon flow cessation. The coarser anthracite always resettles on top of the finer sand. This creates true depth filtration: coarse floc is captured throughout the open pore depth of the anthracite, allowing the dense sand below to capture fine particles without premature surface head loss.
• Multi-Media (Tri-Media): Adds a bottom $3-6 \text{ inch}$ polishing layer of dense garnet or ilmenite (effective size $0.2-0.3 \text{ mm}$, specific gravity $4.2$).
• Granular Activated Carbon (GAC) Filter-Adsorbers: Replaces anthracite with virgin GAC to provide dual functionality: physical particulate filtration and simultaneous adsorption of dissolved taste/odor compounds (geosmin, MIB), pesticides, and DBP precursors.

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Filter Operation, Ripening & Backwashing Dynamics

Operating Parameters

• Filtration Rate: Conventional gravity rapid rate is $2.0 \text{ to } 6.0 \text{ gpm/ft}^2$.
• Filter Run Termination: A filter run concludes when any one of three endpoints is reached:
  1. Terminal Head Loss: Head loss through the bed accumulates to $6.0 \text{ to } 9.0 \text{ feet}$ of water column.
  2. Turbidity Breakthrough: Effluent turbidity rises above the plant baseline ($> 0.10 \text{ NTU}$).
  3. Maximum Time Limit: Elapsed run time reaches $48 \text{ to } 72 \text{ hours}$ (preventing media compaction and anaerobic slime growth).

The Ripening Cycle & Filter-to-Waste

Immediately following a backwash, clean media grains lack the "collector sheath" of attached floc that aids particle capture. During the first $15 \text{ to } 60 \text{ minutes}$ of operation, a transient spike in effluent turbidity occurs. This ripening period is one of the most vulnerable phases of filtration, because pathogens can pass while particle removal is still poor. Many plants use filter-to-waste, routing the first water to waste until individual filter turbidity reaches the plant's goal (often 0.10 NTU or less), or use extended terminal subfluidization or slow-start techniques. These are optimization practices, not a specific OHA filter-to-waste mandate.

Backwashing Hydraulics & Mudball Prevention

Backwashing cleans granular media by fluidizing the bed and dislodging captured solids:

• Bed Expansion: The upward flow of clean treated water must expand the media bed by $20\% \text{ to } 50\%$.
• Upflow Rate: Typically $15 \text{ to } 20 \text{ gpm/ft}^2$. Because cold water has higher dynamic viscosity and density than warm water, it exerts greater hydraulic drag. An upflow velocity that achieves $30\%$ expansion in $5^\circ\text{C}$ winter water will under-expand the bed in $20^\circ\text{C}$ summer water; conversely, a summer backwash rate applied to winter water will wash media out into the waste troughs.
• Auxiliary Scour: Water backwashing alone cannot sheer sticky chemical floc off media grains. Facilities utilize surface wash sweeps or subsurface air scour ($3-5 \text{ scfm/ft}^2$ for $3-5 \text{ minutes}$) prior to fluidization. Auxiliary scour is essential to prevent mudballs—dense agglomerations of clay, polymer, and sand that grow to baseball size, sink to the gravel underdrain, cause localized jetting, and disrupt flow distribution.

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Regulatory Compliance: EPA & OHA Surface Water Treatment Rules

Under 40 CFR 141 and OAR 333-061-0032, conventional surface water treatment plants must adhere to strict Combined Filter Effluent (CFE) and Individual Filter Effluent (IFE) turbidimetric performance standards:

1. Combined Filter Effluent (CFE) Standards

• 95th Percentile Limit: CFE turbidity must remain $\le 0.3 \text{ NTU}$ in at least $95\%$ of measurements taken each calendar month (recorded at least every four hours or continuously).
• Maximum Instantaneous Limit: CFE turbidity must never exceed $1.0 \text{ NTU}$ at any time. Under OAR 333-061-0040, a filtered-water turbidity above $1.0 \text{ NTU}$ must be reported to OHA as soon as possible, and no later than the end of the next business day; consult OHA about public notice.

2. Individual Filter Effluent (IFE) Continuous Monitoring

Each filter is monitored continuously, with readings recorded at least every 15 minutes. IFE results are kept three years and reported monthly when a trigger occurs. For systems serving 10,000 or more people, OAR 333-061-0040 sets these triggers:

Smaller systems follow the parallel provisions in the same rule: report exceedances over 1 NTU, perform a self-assessment after three consecutive months, and complete a CPE within 60 days (report within 120 days) after two consecutive months over 2 NTU.