4.1 Granular Media & Membrane Filtration
Key Takeaways
- Dual-media filters typically utilize 18–24 inches of anthracite over 10–12 inches of silica sand to achieve intermixing and depth filtration without rapid surface clogging.
- Filter backwash requires bed expansion of 20% to 50% (typically 30% to 40%) at wash rates of 15 to 23 gpm/sq ft, often preceded by air scour at 2 to 5 scfm/sq ft to break up mudballs.
- Under the Safe Drinking Water Act SWTR and SC DES Regulation 61-58, combined filter effluent turbidity must be ≤0.3 NTU in at least 95% of monthly measurements and never exceed 1.0 NTU.
- Membrane processes are classified by pore size: Microfiltration (0.1–10 µm), Ultrafiltration (0.01–0.1 µm), Nanofiltration (0.001–0.01 µm), and Reverse Osmosis (<0.001 µm / MWCO <200 Da).
- Transmembrane pressure (TMP) rises as membrane fouling occurs, requiring clean-in-place (CIP) with chemical agents (citric acid for scaling, sodium hypochlorite for organic biofouling) when TMP increases by 15–20%.
3.1 Granular Media & Membrane Filtration
Filtration is the physical and chemical process of separating suspended particles, flocculated solids, and microbiological contaminants from water by passing it through a porous medium or semi-permeable membrane. Under the Safe Drinking Water Act (SDWA) Surface Water Treatment Rule (SWTR) and South Carolina Department of Environmental Services (SC DES) Regulation 61-58, filtration serves as the primary physical barrier preventing pathogens such as Giardia lamblia cysts and Cryptosporidium oocysts from entering public drinking water supplies.
Granular Media Filter Configurations & Media Characteristics
Granular media filters are classified by their hydraulic loading rates, flow directions, and the arrangement of filter media layers. Modern surface water treatment plants in South Carolina primarily utilize rapid rate gravity filters operating at loading rates between 2 to 6 gallons per minute per square foot (gpm/ft²), and up to 8 gpm/ft² with high-rate optimization and DES approval.
Media Layer Configurations
| Filter Type | Typical Media Composition | Bed Depth | Media Effective Size ($ES$) & Uniformity Coefficient ($UC$) | Filtration Mechanism & Loading Rate |
|---|---|---|---|---|
| Single-Media (Rapid Sand) | Silica Sand | 24–30 in (600–750 mm) | $ES = 0.45\text{--}0.55\text{ mm}$, $UC \le 1.6$ | Surface filtration: Fine media at top causes rapid head loss and short filter runs. Rate: $2\text{--}3\text{ gpm/ft}^2$. |
| Dual-Media | Anthracite Coal over Silica Sand | Anthracite: 18–24 in | ||
| Sand: 10–12 in | Anthracite: $ES = 0.8\text{--}1.2\text{ mm}$, $SG = 1.5\text{--}1.6$ | |||
| Sand: $ES = 0.45\text{--}0.55\text{ mm}$, $SG = 2.65$ | Depth filtration: Coarse anthracite traps large floc; fine sand traps small particles. Rate: $3\text{--}6\text{ gpm/ft}^2$. | |||
| Multi-Media (Tri-Media) | Anthracite Coal over Silica Sand over Garnet/Ilmenite | Anthracite: 18 in | ||
| Sand: 8–10 in | ||||
| Garnet: 3–4 in | Anthracite: $ES = 1.0\text{--}1.2\text{ mm}$ | |||
| Sand: $ES = 0.45\text{--}0.55\text{ mm}$ | ||||
| Garnet: $ES = 0.2\text{--}0.4\text{ mm}$, $SG = 4.2$ | Tapered depth filtration: Progressively finer media down the bed maximizes solids storage capacity. Rate: $4\text{--}8\text{ gpm/ft}^2$. |
Hydraulic Sorting & Grain Size Characteristics
Granular filter media behavior is governed by two key physical properties:
- Effective Size ($ES$ or $D_{10}$): The sieve opening size (in millimeters) that permits 10% by weight of the media particles to pass. $ES$ determines the hydraulic resistance and particulate retention capability.
- Uniformity Coefficient ($UC$ or $D_{60}/D_{10}$): The ratio of the sieve size passing 60% of the media to the effective size ($D_{10}$). A lower $UC$ ($\le 1.5$) indicates uniform media sizing, preventing fine grains from filling interstitial voids and restricting flow.
- Specific Gravity ($SG$): Dual- and multi-media filters maintain distinct layers after backwashing due to differences in density. Anthracite ($SG \approx 1.5$) is less dense than sand ($SG \approx 2.65$), which is less dense than garnet ($SG \approx 4.2$). During backwash fluidization, the larger, lighter anthracite particles settle on top, while smaller, heavier garnet particles settle at the bottom.
Underdrain Systems & Support Gravel
The filter underdrain system lies beneath the media bed and serves two critical functions: collecting filtered water uniformly across the entire basin floor during normal filtration, and distributing backwash air and water evenly during backwash.
Common underdrain designs include:
- Wheeler Bottoms: Inverted pyramidal hoppers containing porcelain spheres that disperse backwash water.
- Leopold Dual-Lateral Blocks: High-density polyethylene or vitrified clay blocks with internal primary and secondary control lateral channels to eliminate dead zones.
- Pipe Lateral / Plastic Nozzle Systems: Stainless steel or ABS plastic strainers threaded into pipe laterals or a false floor plenum.
Beneath granular media, graded gravel support beds (typically 12 to 18 inches deep in 4 to 5 layers ranging from 1.5-inch coarse stone at the bottom to 1/16-inch fine gravel at the top) prevent media from escaping into the underdrain while ensuring uniform hydraulic distribution.
Filter Operational Dynamics: Head Loss vs. Turbidity Breakthrough
During a filter run, suspended particles accumulate within the bed voids, increasing hydraulic resistance. Operators monitor three critical parameters to determine when a filter run must be terminated and backwashed:
- Head Loss: The loss of hydraulic pressure energy across the filter bed caused by friction and void clogging. As solids collect, head loss increases from an initial clean value (1.0 to 1.5 feet) to a terminal head loss limit (typically 6.0 to 9.0 feet of head).
- Turbidity Breakthrough: The sudden increase in effluent turbidity when accumulated floc is forced through the filter bed by hydraulic shearing forces. Breakthrough often occurs rapidly if terminal head loss is exceeded or if coagulation chemistry fails.
- Maximum Filter Run Time: SC DES regulations stipulate maximum run times (typically 72 to 100 hours) regardless of head loss or turbidity to prevent biofouling, media compaction, and anaerobic conditions within the bed.
Exam Warning: Filter runs should never be driven to turbidity breakthrough. Backwash must be initiated when either terminal head loss or maximum run time is reached, or when effluent turbidity exceeds pre-set operational limits (e.g., 0.15 NTU).
Backwash Mechanics, Air Scour, & Mudball Formation
Backwashing cleans the filter bed by reversing water flow upward through the media, expanding and fluidizing the bed to shear off attached particulate matter.
Step-by-Step Backwash Procedure
- Filter Isolation & Drawdown: The influent valve is closed, and water is filtered down to 6–12 inches above the top of the media.
- Surface Wash / Air Scour: Auxiliary agitation breaks up the tough surface crust. Air scour is applied at rates of 2 to 5 standard cubic feet per minute per square foot (scfm/ft²) for 3 to 5 minutes.
- Bed Expansion & High-Rate Backwash: Washwater is pumped upward through the underdrain at rates of 15 to 23 gpm/ft², expanding the bed volume by 20% to 50% (target 30–40% expansion). Fluidization allows media grains to collide and scrub away attached solids.
- Ramp-Down & Media Resettling: Washwater flow is gradually reduced to allow hydraulic sorting of dual/multi-media layers without bed disturbance.
- Filter-to-Waste (Ripening Flush): Newly backwashed filters experience a transient surge in effluent turbidity known as the ripening period. Re-filtered water is directed to waste for 5 to 15 minutes until effluent turbidity drops below 0.10 NTU before returning the filter to service.
Mudballs & Operational Malfunctions
- Mudballs: Agglomerations of floc, mud, and media grains bound together by sticky organic matter. They form due to inadequate surface wash, low backwash rates, or premature backwash termination. Mudballs sink to the gravel interface, causing short-circuiting, localized high velocities, and bed cracking.
- Gravel Displacement: Caused by sudden surges in backwash water or air entering the gravel layer, leading to media leakage into underdrains and uneven washwater distribution.
Membrane Filtration Technologies
Membranes are engineered polymer or ceramic barriers that utilize pressure differentials to push water through microscopic pores while rejecting contaminants.
| Technology | Pore Size Range | Primary Target Contaminants | Typical Operating Pressure |
|---|---|---|---|
| Microfiltration (MF) | $0.1\text{--}10\ \mu\text{m}$ | Cryptosporidium, Giardia, bacteria, large suspended solids | 5–30 psi (0.3–2.0 bar) |
| Ultrafiltration (UF) | $0.01\text{--}0.1\ \mu\text{m}$ | All MF targets PLUS viruses, colloidal silica, macromolecules | 10–50 psi (0.7–3.5 bar) |
| Nanofiltration (NF) | $0.001\text{--}0.01\ \mu\text{m}$ (1–10 nm) | All UF targets PLUS divalent ions ($ ext{Ca}^{2+}, \text{Mg}^{2+}$), dissolved organics (NOM/TOC), color, PFAS | 50–150 psi (3.5–10 bar) |
| Reverse Osmosis (RO) | $<0.001\ \mu\text{m}$ ($<1\text{ nm}$, $\text{MWCO} < 200\text{ Da}$) | All contaminants, monovalent ions ($ ext{Na}^+, \text{Cl}^-$), heavy metals, total dissolved solids (TDS) | 150–800 psi (10–55 bar) |
Key Operating Concepts
- Flux: The rate of filtrate production per unit membrane area, expressed as gallons per square foot per day (gfd).
- Transmembrane Pressure ($ ext{TMP}$): The average pressure driving water through the membrane:
- Membrane Fouling & Clean-in-Place (CIP): Accumulation of foulants increases $\text{TMP}$ at constant flux. When $\text{TMP}$ increases by 15% to 20% above clean baseline, a chemical CIP is performed. Acids (citric or oxalic acid) remove mineral scale; oxidants/bases (sodium hypochlorite or sodium hydroxide) clean organic and biological foulants.
Regulatory Compliance Standards (SDWA & SC DES)
Under EPA SWTR and SC DES Regulation 61-58:
- Combined Filter Effluent (CFE) Turbidity: Must be $\le 0.3\text{ NTU}$ in at least 95% of monthly samples collected every 4 hours, and must never exceed 1.0 NTU at any time.
- Individual Filter Effluent (IFE) Turbidity: Continuous monitoring required every 15 minutes. If IFE exceeds 0.5 NTU after 4 hours of continuous operation, or exceeds 1.0 NTU in two consecutive 15-minute readings, specific reporting and filter profile assessments must be submitted to DES.
In a dual-media filter consisting of anthracite coal and silica sand, why does the coarse anthracite remain on top of the finer sand after backwash fluidization?
What is the primary cause of mudball formation in granular media filters, and what operational problem does it cause?
Under the Safe Drinking Water Act (SWTR) and SC DES Regulation 61-58, what are the maximum allowable Combined Filter Effluent (CFE) turbidity limits for a conventional surface water treatment plant?