4.1 Conventional Rapid Sand & Dual/Multi-Media Filtration

Granular Media Filtration Fundamentals

In conventional surface water treatment, granular media filtration serves as the final physical barrier against suspended particulates, colloidal matter, precipitated metal flocs, and microbial pathogens—specifically chlorine-resistant protozoans such as Cryptosporidium oocysts ($4\text{ to }6\text{ }\mu\text{m}$) and Giardia lamblia cysts ($8\text{ to }14\text{ }\mu\text{m}$). Filtration follows coagulation, flocculation, and sedimentation to polish clarified water to regulatory turbidity standards.

Filtration does not function merely as a mechanical sieve. While coarse debris is sieved at the surface, the vast majority of microscopic flocs and pathogens are removed within the depth of the bed through a two-step physical-chemical sequence: particle transport (moving the particle close to a media grain) followed by particle attachment (adhering the particle to the grain surface).

Influent Flocs ---> [1. Hydrodynamic Transport] ---> [2. Surface Collision] ---> [3. Chemical/Adsorptive Attachment]

Particle Transport and Capture Mechanisms

Granular filter beds remove particulate matter through five distinct hydrodynamic and physical-chemical mechanisms:

1. Mechanical Straining: Occurs when particles are physically larger than the interstitial pore spaces between adjacent media grains. Straining dominates at the top 1 to 2 inches of single-medium sand filters, forming a surface mat (schmutzdecke in slow sand; surface cake in rapid sand) that rapidly increases head loss.
2. Sedimentation (Gravitational Settling): As water meanders through interstitial pore channels, localized fluid velocity decreases within pore enlargements. Suspended particles with specific gravities greater than water settle by gravity onto the upper surfaces of underlying sand and anthracite grains.
3. Interception: Occurs when a particle following a fluid streamline approaches within a distance equal to its own radius ($r_p$) from a media grain. The particle collides directly with the grain surface and is retained.
4. Diffusion (Brownian Movement): Dominates for sub-micron colloidal particles ($< 1.0\text{ }\mu\text{m}$). Random collisions with thermal water molecules cause sub-micron particles to deviate from fluid streamlines, wandering across boundary layers into direct contact with media surfaces.
5. Electrostatic and Chemical Adsorption: Once transport brings a particle within nanometer proximity of a media grain, attachment is governed by surface chemistry. Van der Waals attractive forces, hydrogen bonding, and electrostatic attraction between positively charged coagulant microflocs ($Al(OH)_3$ or cationic polymers) and negatively charged silica/anthracite surfaces secure the particle to the grain.

Media Characteristics and Sizing Parameters

Filter performance depends heavily on the physical properties of the granular media. Media is characterized through standard sieve analysis (ASTM C136) to determine grain size distribution, density, and chemical durability.

1. Effective Size ($d_{10}$)

The effective size ($d_{10}$) is defined as the sieve opening size (in millimeters) that permits exactly $10%$ of the media sample by weight to pass, while retaining the remaining $90%$. The effective size dictates the minimum pore opening dimension and controls hydraulic head loss development across clean media.

• Sand effective size: $0.45\text{ to }0.55\text{ mm}$
• Anthracite coal effective size: $0.80\text{ to }1.20\text{ mm}$
• Garnet effective size: $0.20\text{ to }0.35\text{ mm}$

2. Uniformity Coefficient ($UC$)

The uniformity coefficient ($UC$) is a dimensionless numerical ratio that quantifies the variation in grain size within a media batch: $\text{Uniformity Coefficient (UC)} = \frac{d_{60}}{d_{10}}$ Where:

• $d_{60}$ = Sieve opening size (mm) passing $60%$ of the media by weight.
• $d_{10}$ = Sieve opening size (mm) passing $10%$ of the media by weight (effective size).

A perfectly uniform media (all grains exactly identical in size) would yield a $UC = 1.0$. In municipal drinking water practice, a low uniformity coefficient ($UC \le 1.4\text{ to }1.6$) is required. High UC values ($> 1.7$) indicate a wide spread of grain sizes; during hydraulic backwashing, excessive segregation occurs, causing tiny sand grains to accumulate at the bed surface where they create high hydraulic resistance and premature head loss.

3. Specific Gravity ($SG$) and Durability

The specific gravity ($SG$) represents the density of the dry media mineral relative to water ($1.0\text{ g/cm}^3$):

[!NOTE] Chemical Durability: Drinking water filter media must possess an acid solubility of $< 5%$ in $40%\text{ HCl}$ (preventing dissolution during acid cleanses) and exhibit minimal attrition loss ($< 0.5%\text{ per year}$) under aggressive backwashing.

Multi-Media Hydraulic Stratification & Bed Configurations

Traditional single-medium rapid sand filters suffer from a severe hydraulic drawback. Following backwash fluidization, sand grains settle strictly by size; fine sand grains settle slowest and collect at the top surface, while coarse sand grains settle fastest to the bottom. Consequently, water passes through a fine-to-coarse pore structure. Floc is immediately trapped at the upper $2\text{ inches}$, blinding the top layer and generating rapid head loss while the bottom $80%$ of the sand bed remains unutilized.

Dual-Media and Multi-Media (Deep-Bed) Filtration

To overcome surface blinding, modern treatment plants utilize dual-media (anthracite over sand) or multi-media / mixed-media (anthracite over sand over garnet) configurations to establish a true coarse-to-fine pore progression in the direction of downward filtration flow:

1. Coarse Anthracite (Top): Because anthracite coal has a very low specific gravity ($1.4\text{ to }1.6$), its large grains ($0.9\text{ to }1.1\text{ mm}$) remain at the top of the bed after backwash fluidization. This coarse layer provides high void volume ($50%\text{ to }55%$ porosity), trapping $70%\text{ to }80%$ of incoming floc mass deep within its pore matrix without generating excessive head loss.
2. Intermediate Silica Sand (Middle): Possessing intermediate specific gravity ($2.65$) and smaller grain size ($0.45\text{ to }0.55\text{ mm}$), silica sand settles directly beneath the anthracite, filtering intermediate microflocs.
3. High-Density Garnet (Bottom): With a high specific gravity ($3.8\text{ to }4.2$) and very fine grain size ($0.20\text{ to }0.35\text{ mm}$), garnet settles to the bottom. It forms a dense, fine-pore polishing barrier that arrests remaining fine colloidal particles and protozoan oocysts prior to the underdrain.

Hydraulic Settling Velocity Principle

During backwash fluidization, the terminal settling velocity ($v_s$) of media grains is governed by Stokes' and Newton's settling formulations: $v_s \propto d^2 (\rho_s - \rho_w)$ Where $d$ is grain diameter, $\rho_s$ is particle density, and $\rho_w$ is water density. By carefully selecting grain diameters such that coarse anthracite has a lower settling velocity than medium sand, and medium sand has a lower settling velocity than fine garnet, the bed automatically re-stratifies into distinct coarse-to-fine layers upon termination of backwash fluidization, with slight controlled intermixing at the media interfaces.

Underdrain Systems and Support Gravel

The underdrain system is located at the floor of the filter basin beneath the media bed. It serves two vital functions:

1. Collecting filtered effluent uniformly across the entire floor area during filtration.
2. Distributing high-rate backwash water and compressed air scour uniformly upward without creating localized high-velocity water jets ("boils") that disrupt the media layers.

Common Underdrain Designs

• Leopold Dual-Parallel Lateral Blocks: Molded high-density polyethylene (HDPE) or vitrified clay blocks with internal primary and secondary lateral compensation chambers. Equalizes pressure internally to produce perfectly uniform discharge across the filter floor.
• Wheeler Filter Bottoms: Concrete false floor embedded with inverted pyramidal hoppers containing calibrated porcelain or earthenware spheres ($1.25\text{ to }3.0\text{ in}$ diameter) that break and distribute upward water velocity.
• Nozzle / Strainer Button Underdrains: Steel or concrete plenum floor fitted with plastic or stainless steel slotted strainer nozzles ($0.25\text{ to }0.5\text{ mm}$ slot width) spaced every $6\text{ inches}$. Compatible with direct air/water simultaneous scour.
• Porous Plate Bottoms: Sintered aluminum oxide or porous plastic plates. Eliminates the need for support gravel entirely, but susceptible to chemical scaling and particulate blinding if backwash water is dirty.

Graded Gravel Support Layers

Standard underdrains (except porous plate and direct nozzle bottoms) require a graded gravel support bed ($12\text{ to }18\text{ inches}$ total depth) arranged in 4 to 5 progressively finer layers:

• Bottom layer: $1.5\text{ to }0.75\text{ in}$ gravel ($3\text{ to }4\text{ in}$ depth) directly over underdrain openings.
• Intermediate layers: $0.75\text{ to }0.5\text{ in}$ and $0.5\text{ to }0.25\text{ in}$ gravel.
• Top barrier layer: $0.25\text{ to }0.125\text{ in}$ (pea gravel / coarse sand torpedo gravel).

[!WARNING] Gravel Mounding Danger: If backwash water is introduced too abruptly, or if air scour is applied improperly in gravel-bottom filters, high-velocity hydraulic jets will displace the gravel layers ("gravel mounding"). Sand will migrate down into the gravel and underdrains, resulting in severe media loss, channelized piping, and structural underdrain failure.

Filter Ripening and Operating Run Cycles

A complete filter run progresses through three operational stages: the initial ripening period, the stable filtration phase, and run termination.

1. The Filter Ripening Period (Initial Turbidity Spike)

Immediately following backwash, the clean media bed exhibits a brief period of poor particle capture efficiency lasting $15\text{ to }45\text{ minutes}$, known as filter ripening. During ripening, effluent turbidity and particle counts exhibit an initial spike before dropping to normal low levels ($< 0.05\text{--}0.10\text{ NTU}$).

• Mechanism: Freshly washed media grains lack accumulated floc "collectors" on their surfaces. Furthermore, remnants of backwash water trapped within the underdrains contain detached micro-colloids that wash into the effluent.
• Pathogen Risk: Particle and turbidity breakthrough risk can be elevated during ripening, so restart performance must be controlled and trended under the facility procedure.

2. Stable Filtration Phase

As filtration proceeds, microflocs adhere to media grain surfaces, creating dendrites that act as secondary attachment sites. Particle capture efficiency reaches its maximum, and effluent turbidity stabilizes at baseline levels ($0.02\text{ to }0.08\text{ NTU}$). Floc accumulation gradually constricts interstitial pores, causing hydraulic head loss across the bed to increase linearly.

3. Filter Run Termination Criteria

A filter run is terminated when it reaches an approved operating limit. The following are common design or procedure examples; the actual setpoints come from the filter design, state requirements, optimization goals, and facility operating plan: