4.2 Fixed-Film Systems: Trickling Filters & Rotating Biological Contactors (RBCs)
Key Takeaways
- Fixed-film (attached-growth) processes treat settled wastewater by passing it over inert media supporting a stationary biological slime layer (zoogloeal film) of bacteria, protozoa, and fungi.
- Synthetic plastic media provides 30 to 40 sq ft/cu ft of specific surface area with 90% to 95% void space at depths of 15 to 30 feet, whereas rock media provides only 12 to 20 sq ft/cu ft with 40% to 50% void space at depths of 3 to 8 feet.
- The recirculation ratio (R = Qr / Q, typically 0.5 to 3.0:1) maintains media wetting during low nocturnal flows, dilutes shock organic and toxic loads, thins biofilm thickness, and prevents filter flies.
- Sloughing occurs naturally when the biofilm thickens beyond oxygen penetration depth (0.1 to 0.2 mm), starving base microbes at the media interface and shearing dead biomass into the secondary clarifier as humus sludge.
- Primary operational problems include filter ponding (remediated by high recirculation, surface flushing, or 1 to 2 mg/L chlorination), Psychoda filter flies (controlled by 24-hour filter flooding), and RBC shaft loping caused by unbalanced biomass weight accumulation.
4.2 Fixed-Film Systems: Trickling Filters & Rotating Biological Contactors (RBCs)
WPI Class I Exam Focus: Fixed-film (attached-growth) processes are heavily featured on Class I certification exams. Core testing areas include the biological stratification of the zoogloeal biofilm, differences between rock and synthetic media, calculation of recirculation ratios, the sloughing cycle and secondary humus clarifiers, and troubleshooting protocols for filter ponding, Psychoda filter flies, and RBC mechanical loping.
Attached-Growth Fundamentals & The Zoogloeal Biofilm
In contrast to suspended-growth activated sludge systems where microorganisms are kept suspended in liquid mixed liquor, fixed-film (attached-growth) systems cultivate microorganisms attached to stationary or slowly rotating inert solid surfaces. Settled wastewater (primary effluent) is distributed over the media, trickling across the microbial slime layer in thin, laminar sheets.
The Zoogloeal Biofilm Architecture
The biological slime coating the media is termed the zoogloeal film. It represents a dense, complex consortium of aerobic, facultative, and anaerobic bacteria (predominantly Zoogloea ramigera, Pseudomonas, Achromobacter, Flavobacterium, and Alcaligenes), filamentous fungi, algae (on exposed top surfaces), protozoa (ciliates and amoebas), and metazoa (rotifers, nematodes, and worms) embedded in a sticky matrix of extracellular polymeric substances (EPS).
Biofilm Stratification & Sloughing Kinetics
As primary effluent flows over the biofilm, dissolved organic matter (BOD) and dissolved oxygen diffuse into the slime layer, where heterotrophic bacteria metabolize the nutrients:
- Aerobic Outer Layer (0.1 to 0.2 mm): Dissolved oxygen can diffuse only 0.1 to 0.2 mm (100 to 200 micrometers) into the biofilm. Microorganisms in this thin outer zone respire aerobically, rapidly consuming soluble organic matter and dissolved oxygen.
- Anaerobic Inner Interface: When the biofilm grows thicker than 0.2 mm (typically reaching 1.0 to 3.0 mm), oxygen cannot penetrate to the media surface. The innermost microorganisms adjacent to the inert media enter an anaerobic state. Deprived of both oxygen and fresh organic carbon, these base microbes enter endogenous decay, die, and lyse.
- The Sloughing Cycle: As the base layer of bacteria decomposes, the biofilm loses its structural adhesive bond to the media surface. The hydraulic shearing force of wastewater trickling down the bed physically shears the heavy, dead slime layer off the media. This periodic detachment of biomass is known as sloughing.
- Humus Sludge & Clarification: The detached biological solids, known as humus sludge, wash down through the underdrain system and exit the filter. Secondary clarifiers (humus tanks) are located immediately downstream to capture this sloughed biomass by gravity sedimentation. Without secondary clarification, sloughed humus would pass directly into the receiving stream, causing massive permit violations for BOD and TSS.
Trickling Filter Mechanics: Media & Hydraulics
A trickling filter consists of a bed of porous media enclosed in an open-topped cylindrical tank, equipped with a rotary distributor arm at the top and an underdrain collection system at the base.
Media Comparison: Rock vs. Synthetic Plastic
Early trickling filters utilized natural rock media, whereas modern high-rate filters incorporate engineered modular synthetic plastic media.
| Filter Parameter | Traditional Rock Media | Synthetic Plastic Media (Cross-Flow / Vertical) |
|---|---|---|
| Material Composition | Crushed river gravel, granite, field stone, blast-furnace slag | Corrugated Polyvinyl Chloride (PVC) or Polypropylene |
| Nominal Media Size | 1 to 4 inches (2.5 to 10 cm) | Engineered modular blocks |
| Bed Depth | 3 to 8 feet (due to immense structural weight) | 15 to 30 feet (lightweight tower filters) |
| Specific Surface Area | 12 to 20 sq ft per cu ft ($40 - 65\text{ m}^2/\text{m}^3$) | 30 to 40 sq ft per cu ft ($100 - 130\text{ m}^2/\text{m}^3$) |
| Void Space Ratio | 40% to 50% void space | 90% to 95% void space |
| Hydraulic Loading Rate | 25 to 100 gal/day/sq ft (Standard-rate) | 200 to 1,000+ gal/day/sq ft (High-rate) |
| Susceptibility to Clogging | High (fines, leaves, and thick biomass cause ponding) | Extremely low (wide void channels prevent clogging) |
| Construction Cost / Weight | Low material cost, but massive concrete foundations | Higher media cost, but compact footprint and light towers |
Rotary Distributor Arms
Wastewater is applied evenly across the media surface by two or four horizontal rotary distributor arms. The arms are supported on a central column and rotate via the hydraulic reaction force (jet propulsion) of wastewater discharging from directional spray nozzles. In modern installations, electric variable-speed drive motors provide positive rotation independent of flow variations.
- Clearance: Distributor arms must clear the top of the media surface by 6 to 12 inches. Excessive clearance allows wind to blow spray off target, causing localized media drying and odors. Inadequate clearance risks striking heaved media rocks during winter freeze-thaw cycles.
- Rotational Speed: Controlled between 0.1 and 2.0 rpm. Maintaining slow, uniform rotation ensures adequate wetting without hydraulic scouring that tears healthy biofilm away prematurely.
Underdrain System & Natural Draft Ventilation
The bottom of the filter bed is lined with vitrified clay blocks or precast concrete tiles featuring slotted tops that support the media weight:
- Dual Duty: The underdrains collect treated wastewater and sloughed humus, conveying them to a central effluent channel, while simultaneously acting as ventilation air ducts.
- Channel Flow Rule: Effluent collection channels should never flow more than 50% full. Submerged underdrain openings block atmospheric air circulation, suffocating the aerobic biofilm and inducing anaerobic odors.
- Natural Draft Ventilation: Air circulation through the media bed is driven by temperature differentials between the wastewater and ambient atmospheric air. When wastewater is warmer than ambient air (typical winter conditions), air warms inside the bed, becomes buoyant, and flows upward through the filter. When air is warmer than wastewater (typical summer conditions), air flows downward. A temperature differential of 3°F to 5°F (1.7°C to 2.8°C) is necessary to maintain natural draft; otherwise, mechanical ventilation fans must be operated.
Recirculation Mechanics & Calculations
Recirculation is the continuous pumping of a portion of filter effluent or secondary clarifier effluent/sludge back to the trickling filter influent.
The Recirculation Ratio Formula
- Typical Recirculation Ratios: Modern high-rate filters operate with recirculation ratios ranging from 0.5:1 to 3.0:1 (frequently 1:1 to 2:1).
Four Critical Purposes of Recirculation
- Continuous Media Wetting: During low nocturnal flow periods, incoming plant flow drops precipitously. Without recirculation, the distributor arms stall, media dries out, biofilm dehydrates and dies, and Psychoda filter flies proliferate uncontrollably. Recirculation maintains a continuous minimum hydraulic wetting rate (typically > 0.10 to 0.15 gpm/sq ft).
- Dampening Organic & Toxic Shock Loads: High-strength industrial dumps or organic shock loads are blended with recycled, treated effluent, diluting influent BOD concentrations to manageable levels.
- Controlling Biofilm Thickness: Higher hydraulic application velocities increase physical shearing, sloughing excess biomass continuously rather than allowing massive, episodic sloughing events that overwhelm secondary clarifiers.
- Re-aeration & Seeding: Recycled effluent carries dissolved oxygen and active microbial populations back to the top of the bed.
Rotating Biological Contactors (RBCs)
A Rotating Biological Contactor (RBC) is an attached-growth process consisting of closely spaced, parallel corrugated high-density polyethylene (HDPE) discs mounted on a central horizontal steel shaft.
Mechanical Configuration & Operation
- Submergence: Discs (typically 12 feet in diameter) are approximately 40% submerged in a contoured concrete basin shaped to match the curvature of the media.
- Rotational Speed: The horizontal shaft rotates slowly at 1.0 to 2.0 rpm. Shafts are driven by direct mechanical electric motors and gearboxes, or by low-pressure compressed air cups mounted along the disc perimeter (air-drive systems).
- Treatment Mechanism: As the media rotates, the submerged 40% absorbs soluble organic matter (BOD) from the wastewater. When that section rotates out of the liquid into the atmosphere, atmospheric oxygen diffuses into the thin liquid film covering the biomass, providing dissolved oxygen for aerobic respiration. Rotation also provides continuous gentle shearing that sloughs excess biomass into the tank.
Staging of RBC Units
To optimize biological kinetics, RBC installations are configured into 3 to 4 sequential stages separated by internal baffles:
- Stage 1 (Primary BOD Removal): Receives settled primary effluent with maximum organic loading. Biofilm is thick (2 to 4 mm), shaggy, and gray-brown. Heterotrophic bacteria dominate, removing up to 60% to 80% of soluble carbonaceous BOD. Dissolved oxygen in the basin liquid is typically low (1.0 to 2.0 mg/L) due to intense microbial respiration.
- Intermediate Stages 2 & 3 (BOD Polishing): Organic food becomes rate-limiting. The biofilm thins (1 mm) and turns golden brown. Heterotrophic growth declines.
- Stage 4 (Nitrification): Soluble BOD is depleted below 15 mg/L, allowing slow-growing autotrophic nitrifying bacteria (Nitrosomonas and Nitrobacter) to establish without being outcompeted by heterotrophs. The biofilm is thin, smooth, and reddish-brown or tan. Ammonia is oxidized to nitrate, and basin DO rises above 3.0 to 4.0 mg/L.
Fixed-Film Operational Troubleshooting
Operating fixed-film facilities requires distinct troubleshooting strategies compared to activated sludge.
| Problem | Root Causes | Corrective Actions |
|---|---|---|
| Trickling Filter Ponding (Pooling) | Excessive biomass growth, media fines/spalling, accumulation of leaves/debris, low hydraulic flush | 1. Increase recirculation flow rate to scour excess solids<br>2. Dose chlorine at 1 – 2 mg/L continuously (or 5 – 10 mg/L shock for 2 hrs) into influent<br>3. Flood filter bed for 24 hours to float debris<br>4. Flush surface with high-pressure fire hose |
| Filter Flies (Psychoda) | Dry crevices in media bed, decaying stagnant organic matter, un-wetted walls and rocks | 1. Flood filter for 24 – 48 hours to drown larvae and pupae<br>2. Increase recirculation to maintain continuous surface wetting<br>3. Apply dilute chlorine solution or bio-larvicides (Bacillus thuringiensis) to filter walls and distributor arms |
| Septic Odors from Filter | Anaerobic conditions inside media bed caused by submerged underdrains, plugged vents, or severe organic overloading | 1. Lower effluent channel water level to ensure underdrains flow < 50% full<br>2. Clear leaves and trash from perimeter ventilation ports<br>3. Pre-aerate primary effluent or dose chemical oxidants ($H_2O_2$, hypochlorite) |
| RBC Shaft Loping | Unbalanced biomass accumulation on one side of discs due to prolonged power outage or drive failure | 1. Manually rotate dormant shafts 180 degrees every 2 to 4 hours during power failures<br>2. Wash off dried biomass from exposed media with water spray before restarting<br>3. Engage auxiliary rotational pony motors during drive repairs |
| White Biofilm on RBC Media | Proliferation of Beggiatoa (filamentous sulfur-oxidizing bacteria) | 1. Influent wastewater is septic with high dissolved sulfides ($H_2S$)<br>2. Pre-aerate influent or add ferric chloride / hydrogen peroxide upstream to precipitate and oxidize sulfides |
The Danger of RBC Loping
If an RBC drive stops operating during an electrical outage, the submerged 40% of the media remains wet and continues to gather solids and microbial mass. The exposed 60% dries out in atmospheric air, losing weight. Within a few hours, a massive weight disparity develops between opposite sides of the shaft. When the drive motor restarts, this eccentric weight causes the shaft to "lope" (rotate unevenly with violent cyclic surges), exerting catastrophic torsional stress that shears shaft couplings, snaps drive chains, breaks center steel shafts, and burns out motors.
Exam Traps & Rules of Thumb
- Oxygen Penetration Depth: Remember that oxygen penetrates only 0.1 to 0.2 mm (100 to 200 micrometers) into zoogloeal biofilm. Anything deeper is anaerobic.
- Recirculation Ratio Definition: $R = Q_r / Q$. If influent is 1.0 MGD and recirculation is 2.0 MGD, the ratio is 2.0:1, and total applied flow is 3.0 MGD.
- Filter Flies Control: The classic, proven exam answer for controlling Psychoda filter flies is flooding the trickling filter for 24 to 48 hours to drown larvae.
- Underdrain Clearance: Collection channels must never flow more than half full (50%) to preserve ventilation airflow through the underdrain tiles.
An operator observes localized pooling and ponding of wastewater across the surface of a rock-media trickling filter. What is the fundamental operational cause of this condition, and what immediate corrective action should be executed?
A trickling filter facility receives an average primary effluent wastewater flow of 1.5 MGD and maintains a continuous recirculation flow rate of 3.0 MGD. What is the operational recirculation ratio (R), and what is the total hydraulic flow rate applied to the filter bed?
A treatment plant experiences a major electrical power outage that shuts down all Rotating Biological Contactor (RBC) drive motors for 12 hours. What mechanical danger will occur if an operator attempts to restart the RBC drives immediately without inspecting the shafts?