2.3 Filtration Technologies & Media Management

Granular Media Filtration Mechanics

Filtration serves as the principal physical barrier in drinking water treatment for the removal of suspended particles, coagulated floc carryover, asbestos fibers, and protozoan parasites (Cryptosporidium oocysts $3-5,\mu\text{m}$ and Giardia lamblia cysts $8-12,\mu\text{m}$). In a conventional surface water treatment plant, filtration follows coagulation, flocculation, and sedimentation to produce polished, low-turbidity effluent ready for final disinfection.

Physical Particle Removal Mechanisms

Granular media filtration operates through two sequential steps: Transport (moving the suspended particle from the fluid streamline into contact with the media grain) and Attachment (bonding the particle to the media grain surface).

  1. Mechanical Straining    2. Gravitational Settling   3. Interception           4. Brownian Diffusion
     [Particle > Pore]          [Density > Water]           [Follows Streamline]      [Submicron Particles]
     
        |   |   |                  |   |                       \       /                \     /
        | P |   |                  | P |                        \  P  /                  (P)->*
       [Media Pore]               [Media Grain]                [Media Grain]           [Media Grain]

1. Mechanical Straining: Entrapment of particles whose physical dimensions are larger than the pore spaces between media grains. In single-medium sand filters, straining occurs almost entirely within the top $1-2\text{ inches}$ ($2.5-5\text{ cm}$) of the bed, causing rapid surface blinding.
2. Gravitational Sedimentation: Heavy particles deviate downward from fluid streamlines due to gravity and settle onto the top surfaces of media grains.
3. Interception: A particle following a fluid streamline comes within one particle radius of a media grain and collides with it.
4. Inertial Impaction: Particles with substantial mass and inertia cross fluid streamlines when the flow curves around a media grain, impacting the media directly.
5. Brownian Diffusion: Extremely small submicron particles ($< 1,\mu\text{m}$) collide randomly with water molecules, vibrating across streamlines until contacting media grains.
6. Attachment Mechanisms: Once transport brings a particle into contact with a media grain, attachment is governed by electrostatic forces, van der Waals attraction, and chemical bridging with residual coagulant polymers. Chemical pre-treatment (coagulation) is mandatory for attachment to occur; un-coagulated particles carry negative charges identical to clean silica sand grains, resulting in electrostatic repulsion and zero attachment.

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Filter Media Classifications & Stratification Dynamics

Filter media quality and hydraulic performance are defined by standard sieve analysis metrics:

• Effective Size ($d_{10}$): The sieve opening size in millimeters that passes exactly $10%$ of the media sample by weight and retains $90%$. It represents the smallest grain size in the bulk media and controls clean-bed head loss and particle penetration.
• $60%$ Passing Size ($d_{60}$): The sieve opening size that passes exactly $60%$ of the media sample by weight.
• Uniformity Coefficient ($UC$): The numerical ratio of $d_{60}$ to $d_{10}$:

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

A low $UC$ ($< 1.5\text{ to }1.7$) signifies highly uniform media grains. Higher $UC$ values mean a wide disparity in grain sizes, which leads to dense packing, reduced pore volume, and accelerated head loss.

Media Configurations: Single, Dual, and Multimedia

   SINGLE-MEDIUM (Sand)             DUAL-MEDIA (Anthracite/Sand)        MULTIMEDIA (Tri-Media)
   ---------------------            ----------------------------        ----------------------
   Fine Sand at Top                 [Coarse Anthracite Coal]            [Coarse Anthracite Coal]
   (Surface Straining Only)         d10 = 0.8-1.2 mm, SG = 1.5          d10 = 1.0-1.4 mm, SG = 1.5
   Depth: 24-30 in                  Depth: 18-24 in                     Depth: 18 in
                                    ----------------------------        ----------------------
   Coarse Sand at Bottom            [Fine Silica Sand]                  [Fine Silica Sand]
   d10 = 0.45-0.55 mm               d10 = 0.45-0.55 mm, SG = 2.65       d10 = 0.45-0.55 mm, SG = 2.65
   SG = 2.65                        Depth: 8-12 in                      Depth: 9-12 in
                                                                        ----------------------
                                                                        [Very Fine Garnet/Ilmenite]
                                                                        d10 = 0.20-0.35 mm, SG = 4.0
                                                                        Depth: 3-6 in

Why Dual-Media Works: During upflow water backwash fluidization, settling velocity is governed by Stokes' Law ($v_s \propto d^2(\rho_p - \rho_w)$). Because anthracite coal is significantly lighter ($SG = 1.5$) than silica sand ($SG = 2.65$), the coarse anthracite grains re-stratify cleanly on top of the finer, denser sand grains upon backwash cessation. This creates a true coarse-to-fine pore gradient in the direction of downward filtration flow.

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Regulatory Turbidity Standards & Monitoring Triggers

Under the EPA Surface Water Treatment Rules (IESWTR, LT1ESWTR, and LT2ESWTR), turbidity is regulated as a direct performance indicator of microbial pathogen barrier integrity.

  CFE Regulatory Limits:  [ <= 0.30 NTU in >= 95% of Monthly Measurements ]  |  [ Max Ever <= 1.0 NTU ]

  IFE 15-Min Monitoring Follow-Up Ladder (IESWTR):
  -------------------------------------------------------------------------------------------------
  Trigger 1: IFE > 1.0 NTU, 2 consecutive readings                  ===> Report to state w/ the cause
  Trigger 2: IFE > 0.5 NTU, 2 consecutive readings at end of the
             first 4 hours of operation after backwash              ===> Report to state w/ the cause
  Trigger 3: IFE > 1.0 NTU, 2 consecutive readings, in 3
             CONSECUTIVE MONTHS                                     ===> FILTER SELF-ASSESSMENT
  Trigger 4: IFE > 2.0 NTU, 2 consecutive readings, in 2
             CONSECUTIVE MONTHS                                     ===> COMPREHENSIVE PERFORMANCE
                                                                          EVALUATION (third party/state)

Combined Filter Effluent (CFE) Mandates

1. 95th Percentile Limit: Combined filter effluent turbidity must be $\le 0.3\text{ NTU}$ in at least $95%$ of all compliance measurements recorded each calendar month.
2. Maximum Allowable Limit: Combined filter effluent turbidity must never exceed $1.0\text{ NTU}$ at any time.

Individual Filter Effluent (IFE) Continuous Monitoring Rules

Waterworks must monitor each individual filter's effluent continuously using on-line turbidimeters, recording values every 15 minutes:

• Trigger 1 - report: $\text{IFE} > 1.0\text{ NTU}$ in two consecutive 15-minute readings. The system reports the exceedance to the primacy agency (VDH) with the cause, if known, in the monthly report.
• Trigger 2 - report: $\text{IFE} > 0.5\text{ NTU}$ in two consecutive 15-minute readings taken at the end of the first four hours of continuous filter operation after a backwash. Reported the same way.
• Trigger 3 - filter self-assessment: $\text{IFE} > 1.0\text{ NTU}$ in two consecutive 15-minute readings in three consecutive months. The system must conduct a filter self-assessment on the affected filter, covering assessment of filter performance, development of a filter profile, identification and prioritization of factors limiting performance, assessment of the applicability of corrections, and preparation of a corrective action plan.
• Trigger 4 - comprehensive performance evaluation: $\text{IFE} > 2.0\text{ NTU}$ in two consecutive 15-minute readings in two consecutive months. The system must arrange a comprehensive performance evaluation (CPE) by the state or a third party approved by the state.

Read the ladder carefully. The escalation is driven by repetition across months, not by a single event. A single reading above 2.0 NTU is a reportable exceedance and an obvious reason to take the filter out of service operationally, but the CPE obligation attaches only when the 2.0 NTU condition recurs in two consecutive months.

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Filter Backwash Sequence, Hydraulics & Media Care

Filters operate continuously until reaching one of three operational termination criteria:

1. Terminal Head Loss: Head loss across the bed reaches $6.0\text{ to }9.0\text{ feet}$ ($1.8-2.7\text{ m}$), indicating pore space exhaustion.
2. Turbidity Breakthrough: Effluent turbidity begins to rise above $0.15-0.20\text{ NTU}$, indicating particle detachment.
3. Maximum Time Limit: Filter run duration reaches $48\text{ to }72\text{ hours}$ (to prevent anaerobic biological growth and media compaction).

  STEP 1: ISOLATE & DRAIN DOWN       STEP 2: AUXILIARY SCOUR           STEP 3: FLUIDIZED UPFLOW
  - Close Influent Valve             - Surface Wash (50-100 psi)       - Upflow Backwash (15-22 gpm/sq ft)
  - Drain water to 6 in above bed    - OR Air Scour (3-5 scfm/sq ft)   - Bed Expands 20% to 30%
                                     - Time: 2 to 5 minutes            - Time: 8 to 15 minutes
                                     
  STEP 4: SETTLE & STRATIFY          STEP 5: REFILL BASIN              STEP 6: FILTER-TO-WASTE (RIPENING)
  - Slow backwash valve closure      - Open Influent Valve slowly      - Divert to waste for 5-30 min
  - Coal settles on top of sand      - Fill basin above wash troughs   - Return to service when NTU < 0.10

Step-by-Step Backwash Protocol

1. Isolation & Drawdown: Close the influent valve. Allow water to filter down through the bed until the surface is approximately $6\text{ inches}$ above the top of the media. Close the effluent valve.
2. Auxiliary Scour (Surface Wash or Air Scour):
   • Surface Wash: Fixed nozzles or rotating arms deliver high-pressure water jets ($50-100\text{ psi}$, $0.5-2.0\text{ gpm/ft}^2$) directly at the anthracite surface for $2-5\text{ minutes}$ to scrub sticky coagulant coatings and break surface crusts.
   • Air Scour: Blower systems inject oil-free compressed air ($3-5\text{ scfm/ft}^2$ at $5-10\text{ psi}$) through the underdrains to vigorously agitate media grains against one another throughout the full bed depth.
3. Fluidized Upflow Backwash: Open the backwash supply valve to pump filtered water upward through the underdrains at $15\text{ to }22\text{ gpm/ft}^2$, fluidizing the media bed to achieve $20%\text{ to }30%$ bed expansion.
   • Water Viscosity Correction: Cold water is more dense and viscous, generating greater drag on media grains. An operator must reduce upflow backwash velocity in winter to prevent media loss, and increase velocity in summer warm water to maintain $20-30%$ expansion.
4. Deceleration & Stratification: Close the backwash valve slowly over $1-2\text{ minutes}$. Controlled deceleration allows media to re-stratify by specific gravity without gravel underdrain disruption.
5. Filter Ripening & Filter-to-Waste: When raw water is reintroduced, newly washed media grains possess zero captured particles and exhibit lower attachment efficiency. A temporary turbidity spike occurs during the first $15-30\text{ minutes}$ of operation (Filter Ripening Period). To prevent pathogen breakthrough into the clearwell, the effluent must be diverted to waste (Filter-to-Waste) until effluent turbidity stabilizes below $0.10\text{ NTU}$ before entering the distribution header.

Operational Pathologies of Granular Media

• Mudballs: Compact spherical masses of clay, floc, and media ($0.25\text{ in to }> 2\text{ in}$) that form when wash agitation is inadequate. Mudballs sink to the gravel interface, creating dead zones and channeling.
• Media Boiling & Gravel Displacement: Localized excessive upflow backwash velocities rupture the supporting gravel layer, causing gravel mounds and media loss down the underdrain.
• Air Binding: Occurs when negative head pressure develops in the lower media layers (effluent valve pulled too far open while bed is clogged). Dissolved gases come out of solution and form air pockets, choking pores and fracturing the filter bed.

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Membrane Filtration Technologies (MF, UF, NF, RO)

Membrane processes replace granular media with synthetic semi-permeable membranes, utilizing pressure-driven physical size exclusion.

  0.1 - 1.0 um:  MICROFILTRATION (MF)     ===> Retains Clay, Bacteria, Giardia, Cryptosporidium
  0.01 - 0.1 um: ULTRAFILTRATION (UF)     ===> Retains All MF Constituents + Viruses (>= 4-log)
  0.001 - 0.01 um: NANOFILTRATION (NF)    ===> Retains Divalent Ions (Hardness Ca/Mg), TOC, Color
  < 0.0001 um:   REVERSE OSMOSIS (RO)     ===> Retains Monovalent Salts (Na/Cl), PFAS, All Organics

Operating Hydraulics & Performance Metrics

• Transmembrane Pressure ($TMP$): The net driving pressure pushing water across the membrane barrier:

$\text{TMP} = \left(\frac{P_{\text{feed}} + P_{\text{concentrate}}}{2}\right) - P_{\text{permeate}} \quad \left(\text{or } P_{\text{feed}} - P_{\text{permeate}} \text{ in dead-end mode}\right)$

• Membrane Flux ($J$): The volumetric flow rate of filtered water (permeate) produced per unit of active membrane surface area:

$J = \frac{Q_{\text{permeate}}}{A_{\text{membrane}}} = \frac{\text{gallons/day}}{\text{ft}^2} = \text{gfd} \quad \left(\text{typical UF flux: } 20-50\text{ gfd}\right)$

• Specific Flux: Flux normalized by transmembrane pressure ($J / \text{TMP}$), temperature-corrected to $20^\circ\text{C}$. A decline in specific flux indicates membrane fouling.

Integrity Verification & Cleaning Protocols

1. Direct Integrity Testing (Pressure Decay Test - PDT): Conducted daily on each membrane skid. The filtrate side is pressurized with clean air to $15-30\text{ psi}$, the skid is isolated, and the rate of air pressure decay is monitored. A decay rate $\le 0.05-0.10\text{ psi/min}$ proves that no hollow fibers are broken, validating $4\text{-log (}99.99%\text{)}$ Cryptosporidium removal credit.
2. Routine Maintenance (Backpulse & Air Scrub): Automated every $15-45\text{ minutes}$; filtered permeate is pumped in reverse with concurrent air bubbling to strip surface cake.
3. Clean-in-Place (CIP): Intensive chemical cleaning cycle when TMP rises by $10-15\text{ psi}$:
   • Acid Wash (Citric Acid or $HCl$ at $pH,2.0-2.5$): Dissolves inorganic mineral scale ($CaCO_3$, iron/manganese precipitates).
   • Alkaline / Oxidant Wash (Sodium Hydroxide $NaOH$ + Sodium Hypochlorite $NaOCl$ at $pH,11.0-12.0$): Breaks down biofilms, natural organic matter (NOM), and extracellular polymeric substances.