7.4 Fixed-Film & Hybrid Biological Processes

Principles of Attached-Growth (Fixed-Film) Treatment

Unlike suspended-growth activated sludge systems where microorganisms are kept suspended in liquid mixed liquor, attached-growth (fixed-film) processes cultivate microbial populations anchored as a sessile biological slime layer (biofilm or zoogloeal mass) onto stationary or moving solid media.

                  WASTEWATER FLOWING OVER BIOFILM LAYER
  ┌────────────────────────────────────────────────────────────────────────┐
  │ Dissolved Organics (BOD) + Dissolved Oxygen (DO) Diffuse Inward        │
  ├────────────────────────────────────────────────────────────────────────┤
  │ 1. OUTER AEROBIC LAYER (0.1 - 0.2 mm): Active Heterotrophs & Nitrifiers│
  ├────────────────────────────────────────────────────────────────────────┤
  │ 2. INNER ANAEROBIC LAYER: Endogenous Decay, H2S & CH4 Gas Production    │
  ├────────────────────────────────────────────────────────────────────────┤
  │ 3. INERT SOLID SUPPORT MEDIA (Rock / Structured Plastic / HDPE Carrier)│
  └────────────────────────────────────────────────────────────────────────┘
                                     │
                                     ▼ (Hydraulic Shear + Anaerobic Gas Lifting)
                  SLOUGHING (Detached Biomass Humus to Clarifier)

Biofilm Dynamics & Sloughing Kinetics

1. Substrate Diffusion: Dissolved organics ($\text{BOD}_5$), ammonia ($\text{NH}_4^+$), and dissolved oxygen ($O_2$) diffuse from the liquid film into the outer 0.1 to 0.2 mm of the biofilm where active aerobic metabolism occurs.
2. Anaerobic Interfacial Zone: Because oxygen cannot penetrate deeper than 0.2 mm, bacteria adjacent to the media surface enter an anaerobic, endogenous decay state. They produce anaerobic gases ($CO_2, CH_4, H_2S$) and organic acids that break down the cellular slime bond holding the biofilm to the media.
3. Sloughing Cycle: As biofilm thickness exceeds 1.0 to 3.0 mm, the combined effect of weakened adhesion and surface hydraulic shear causes sheets of biomass to detach (sloughing). The sloughed solids (humus) are swept into underdrains and separated in downstream secondary humus clarifiers.

---

Trickling Filters: Rock vs. Plastic Media Comparison

Trickling filters consist of a packed bed of inert media over which settled primary effluent is continuously distributed via rotating distributor arms.

┌────────────────────────────────────────────────────────────────────────┐
│               Trickling Filter Media Design Comparison                 │
├──────────────────────────┬─────────────────────┬───────────────────────┤
│ Design Parameter         │ Rock / Slag Media   │ Structured Plastic    │
│                          │ (Standard Rate)     │ Media (High Rate)     │
├──────────────────────────┼─────────────────────┼───────────────────────┤
│ Bed Depth                │ 3 to 8 feet         │ 15 to 30 feet         │
│ Specific Surface Area    │ 15 – 20 ft²/ft³     │ 30 – 45 ft²/ft³       │
│ Void Space Ratio         │ 40% – 50%           │ > 95%                 │
│ Hydraulic Loading Rate   │ 1.0 – 2.0 gpm/ft²   │ 2.0 – 8.0+ gpm/ft²    │
│ Organic Loading Rate     │ 5 – 25 lbs BOD/     │ 30 – 100+ lbs BOD/    │
│                          │ 1,000 cu ft/day     │ 1,000 cu ft/day       │
│ Ventilation Mechanism    │ Natural draft       │ Enhanced draft/forced │
│ Ponding Vulnerability    │ High (low voids)    │ Extremely low (>95%)  │
└──────────────────────────┴─────────────────────┴───────────────────────┘

1. Rock Media Filters

• Media Construction: Graded crushed granite, basalt, or blast-furnace slag (2.5 to 4.0 inch diameter).
• Limitations: Shallow depth (3 to 8 ft) due to massive structural weight; low void space (40%–50%) restricts natural ventilation and makes the bed highly vulnerable to plugging and surface ponding.

2. Structured Plastic Cross-Flow Media

• Media Construction: Lightweight, thermoformed corrugated sheets of rigid PVC or polypropylene configured in cross-flow or vertical-flow modules.
• Advantages: Deep vertical towers (15 to 30 feet), 2 to 3 times greater specific surface area ($30\text{ to }45\text{ ft}^2/\text{ft}^3$), and >95% void space, eliminating ponding risks, maximizing natural chimney draft aeration, and supporting 3 to 5 times higher organic loading rates.

---

Hydraulics, Recirculation & Dosing Mechanics (SK Value)

Settled wastewater is applied to the filter bed using a 2-arm or 4-arm rotary distributor assembly propelled by the reaction force of water discharging from orifice nozzles (or variable-speed electric motors).

Primary Effluent (Q) ──► ┌────────────────────┐ ──► [ Rotary Distributor Arms ]
                         │ Recirculation Well │                 │
Recycle Flow (Q_r)   ──► └────────────────────┘                 ▼
                                  ▲                  [ Packed Biofilm Bed ]
                                  │                             │
                                  │                             ▼
                                  │                  [ Underdrain System ]
                                  │                             │
                                  │                             ▼
                                  └─────────────── [ Secondary Clarifier ] ──► Effluent
                                                   (Humus Sludge to Waste)

1. Recirculation Ratio ($R$)

Recirculation involves pumping a portion of the filter effluent (or secondary clarifier effluent) back to the filter influent well:

$R = \frac{Q_r}{Q}$

$\text{Total Applied Flow } (Q_{\text{total}}) = Q + Q_r = Q \times (1 + R)$

• Operational Purposes of Recirculation:
  1. Dilution: Dilutes high-strength influent BOD and dampens toxic industrial shock loads.
  2. Minimum Wetting Rate: Maintains continuous hydraulic wetting ($\ge 0.15\text{ to }0.20\text{ gpm/ft}^2$) during low night flows, preventing the biofilm from drying out and dying.
  3. Reseeding: Constantly reseeds the media bed with active acclimated organisms and dissolved oxygen.
  4. Scouring: Increases downward hydraulic shear to continuously purge old sloughed biomass.

2. Dosing Rate & Spülkraft (SK) Flushing Value

Biofilm thickness on trickling filter media must be strictly controlled to prevent void clogging and surface ponding. The German Spülkraft (SK) value—meaning flushing intensity—quantifies the instantaneous liquid volume discharged per single pass of a distributor arm:

$\text{SK Value (mm/pass)} = \frac{\text{Total Hydraulic Loading Rate (gpd/ft}^2\text{)} \times 0.0407}{\text{Rotational Speed (rpm)} \times \text{Number of Arms}}$

• Operational Control:
  • Fast Arm Rotation (Low SK): Delivers a thin, continuous drizzle that allows thick, heavy biofilm to accumulate unchecked, promoting rock clogging.
  • Slow, Controlled Arm Rotation (High SK: 100 to 300 mm/pass): Delivers a powerful, pulsing hydraulic wave with each pass that physically shears excess biomass, dislodges snail shells, and flushes void spaces clean. Operators adjust distributor reverse-thrust jet nozzles or VFD drive motors to maintain the target SK value.

---

Troubleshooting: Ponding, Filter Flies & Snail Infestations

┌────────────────────────────────────────────────────────────────────────┐
│                Trickling Filter Troubleshooting Guide                  │
├──────────────────┬─────────────────────┬───────────────────────────────┤
│ Symptom          │ Probable Cause      │ Operator Corrective Action    │
├──────────────────┼─────────────────────┼───────────────────────────────┤
│ Surface Ponding  │ Excessive organic   │ Increase recirculation (SK    │
│ (Standing water  │ loading; biomass    │ value); high-pressure spray;  │
│ on rock media)   │ accumulation; snails│ flood bed 24 hr; dose Cl2.    │
├──────────────────┼─────────────────────┼───────────────────────────────┤
│ Filter Flies     │ Damp, unflooded     │ Continuous wetting; flood bed │
│ (Psychoda) swarms│ breeding zones      │ for 24 hr; low-dose Cl2 spray.│
├──────────────────┼─────────────────────┼───────────────────────────────┤
│ Snail Overgrowth │ Prolific Physa snail│ Chlorination shocks; high SK  │
│ (Clogged drains) │ grazing on biofilm  │ flushing; underdrain flushing.│
├──────────────────┼─────────────────────┼───────────────────────────────┤
│ Foul Odors       │ Anaerobic pockets;  │ Increase recirculation DO;    │
│ (H2S generation) │ plugged underdrains │ clear underdrains; unclog air.│
└──────────────────┴─────────────────────┴───────────────────────────────┘

1. Ponding Remediation

• High-Rate Hydraulic Flushing: Increase recirculation pumping rate to maximum capacity while slowing arm speed to maximize the SK flushing value.
• Surface Mechanical Washing: Use high-pressure fire hoses or rakes to break up surface algae and dense slime mats.
• Low-Dose Chlorination: Dose chlorine into the filter influent at 1.0 to 2.0 mg/L free residual for 4 to 8 hours (or a slug dose of 5–10 mg/L at night) to oxidize the surface slime layer without destroying deeper biofilm.
• Filter Flooding: Close underdrain effluent gates and flood the filter bed for 24 hours to drown larvae and loosen biomass.

2. Filter Flies (Psychoda alternata)

• Tiny moth-like flies that breed in damp, stagnant biofilm voids. Controlled by maintaining continuous wetting (eliminating dry zones), 24-hour flooding, or periodic light chlorination.

3. Snail Infestations (Physa / Lymnaea)

• Aquatic snails graze heavily on biofilm, drastically reducing BOD removal efficiency. Millions of calcified snail shells slough into underdrains, plugging vitrified clay channels and jamming sludge pumps. Controlled through high SK flushing, periodic salinity/chlorine shocks, and mechanical underdrain flushing.

---

Rotating Biological Contactors (RBC)

Rotating Biological Contactors (RBCs) consist of a series of closely spaced, large-diameter (12 ft / 3.7 m) corrugated High-Density Polyethylene (HDPE) discs mounted horizontally along a heavy central steel shaft (typically 25 to 27 feet long).

┌────────────────────────────────────────────────────────────────────────┐
│                     RBC Mechanical Staging Layout                      │
├────────────────────────────────────────────────────────────────────────┤
│  Primary Effluent ──► [ Stage 1 ] ──► [ Stage 2 ] ──► [ Stage 3 ] ──► [ Stage 4 ] ──► Clarifier
│                       (Heavy BOD)     (BOD Polishing) (Nitrification) (Nitrification)
│
│  • Discs: 12 ft Diameter Corrugated HDPE Sheets (100,000 - 150,000 sq ft)
│  • Submergence: Exactly 40% Submerged in Wastewater Tank
│  • Rotation: 1.0 to 2.0 rpm (Peripheral tip speed ~60 ft/min)
│  • Drive: Mechanical Gearbox Drive or Peripheral Air Cup Drive
└────────────────────────────────────────────────────────────────────────┘

1. Staging & Microbiology

RBC basins are divided into 3 to 4 stages in series separated by baffling walls:

• Stages 1 & 2 (Carbonaceous BOD Removal): High organic loading ($>6\text{ to }8\text{ lbs BOD/1,000 sq ft/day}$). Fast-growing heterotrophic bacteria dominate, creating a thick (1.5 to 3.0 mm) gray/brown biofilm that removes 80% to 90% of soluble BOD.
• Stages 3 & 4 (Nitrification): Soluble BOD drops below 15 mg/L. Slow-growing autotrophic nitrifying bacteria (Nitrosomonas and Nitrobacter) establish on the thin (0.2 to 0.5 mm) reddish-brown biofilm, converting ammonia-nitrogen ($\text{NH}_4^+$) to nitrate ($\text{NO}_3^-$).

2. Critical Mechanical Failure Modes & "Loping"

• Shaft Fatigue & Deflection: Extreme organic overloading on Stage 1 causes massive biofilm growth. The excess biomass weight imposes intense cyclic rotational cantilever stresses on the central steel shaft, leading to catastrophic shaft fatigue failure.
• Rotational Imbalance ("Loping"): If an RBC unit stops rotating during a power outage or motor failure, the upper 60% of exposed media dries out while the bottom 40% remains submerged and wet. When restarted, the massive weight disparity creates an eccentric flywheel imbalance ("loping"), causing surging, gearbox destruction, and motor burnout. Operators must manually rotate stalled shafts with auxiliary drives during power outages.

---

Hybrid Biofilm Systems: MBBR & IFAS

Modern wastewater engineering combines the process resilience of fixed-film systems with the high treatment intensity of activated sludge.

┌────────────────────────────────────────────────────────────────────────┐
│                 MBBR vs. IFAS Hybrid System Comparison                 │
├──────────────────────────┬─────────────────────┬───────────────────────┤
│ Feature                  │ MBBR (Moving Bed)   │ IFAS (Integrated)     │
├──────────────────────────┼─────────────────────┼───────────────────────┤
│ Media Type               │ Free-floating HDPE  │ Free-floating HDPE    │
│                          │ carriers (Kaldnes)  │ carriers (Kaldnes)    │
│ Tank Carrier Fill Rate   │ 30% to 65% of volume│ 30% to 50% of volume  │
│ RAS Pumping Network      │ NONE (No RAS!)      │ YES (Full RAS Loop)   │
│ Biomass Mode             │ 100% Fixed Biofilm  │ Dual: Fixed Biofilm + │
│                          │ on carriers         │ Suspended MLSS Floc   │
│ Sludge Blanket Bulking   │ Zero risk (No MLSS) │ Possible in clarifier │
│ Primary Retrofit Goal    │ Simple, compact BOD/│ Upgrades capacity 2-3x│
│                          │ Nitrogen removal    │ in existing tankage   │
└──────────────────────────┴─────────────────────┴───────────────────────┘

1. Moving Bed Biofilm Reactor (MBBR)

• Operating Principle: Thousands of specialized high-density polyethylene carriers (e.g., Kaldnes K1, K3, BioChip with specific surface area 500 to 1,200 m²/m³) are freely suspended in aeration basins. Coarse bubble diffused aeration or submersible mixers keep the carriers continuously fluidized throughout the tank volume.
• Sieve Screens: Perforated stainless steel retention screens mounted on the basin outlet keep the plastic carriers inside the reactor while treated water flows out.
• Pure Fixed-Film Operation: Operates without any Return Activated Sludge (RAS) recycling. Sludge bulking, SVI management, and clarifier blanket washouts are completely eliminated.

2. Integrated Fixed-Film Activated Sludge (IFAS)

• Operating Principle: Combines suspended carrier media directly inside a conventional activated sludge aeration basin with an active Return Activated Sludge (RAS) loop.
• Dual Microbial Populations: Slow-growing nitrifiers (Nitrosomonas, Nitrobacter) establish within the protected internal channels of the plastic carrier biofilm, while fast-growing heterotrophs remain in the suspended mixed liquor floc (MLSS 2,000–3,000 mg/L).
• Facility Retrofits: Enables overloaded municipal facilities to achieve year-round complete biological nitrification and double biological treatment capacity within their existing concrete tank footprints without building new aeration basins.

---

Waste Stabilization Ponds (Lagoons)

Waste stabilization ponds (lagoons) are large, engineered earthen basins that provide natural biological wastewater treatment through long hydraulic retention times.

┌────────────────────────────────────────────────────────────────────────┐
│                     Waste Stabilization Pond Types                     │
├──────────────────────────┬─────────────────────┬───────────────────────┤
│ Lagoon Type              │ Depth & Detention   │ Primary Mechanism     │
├──────────────────────────┼─────────────────────┼───────────────────────┤
│ Anaerobic Ponds          │ 8 – 15 ft deep      │ Anaerobic digestion   │
│                          │ (HRT 1 – 5 days)    │ (Acid & Methane ferm.)│
├──────────────────────────┼─────────────────────┼───────────────────────┤
│ Facultative Ponds        │ 4 – 7 ft deep       │ Algae-Bacteria        │
│                          │ (HRT 20 – 60 days)  │ Symbiotic cycle       │
├──────────────────────────┼─────────────────────┼───────────────────────┤
│ Maturation / Polishing   │ 3 – 5 ft deep       │ Pathogen reduction &  │
│ Ponds                    │ (HRT 10 – 20 days)  │ solar UV disinfection │
└──────────────────────────┴─────────────────────┴───────────────────────┘

Symbiotic Algae-Bacteria Cycle in Facultative Lagoons

Facultative ponds are the most common municipal lagoon system, consisting of three distinct stratified biological zones:

1. Upper Aerobic Phototrophic Zone: Dominated by green algae (Chlorella, Scenedesmus, Euglena) and aerobic heterotrophs.
2. Middle Facultative Zone: Facultative bacteria thrive under fluctuating aerobic/anoxic conditions.
3. Bottom Anaerobic Sludge Layer: Accumulated solids undergo anaerobic digestion by acidogens and methanogens.

                          SUNLIGHT + SOLAR RADIATION
                                     │
                                     ▼
                      ┌──────────────────────────────┐
                      │     ALGAE PHOTOSYNTHESIS     │ ──► Dissolved Oxygen (O2)
                      │   (Chlorella, Scenedesmus)   │          │
                      └──────────────┬───────────────┘          │
                                     ▲                          ▼
              Carbon Dioxide (CO2)   │               ┌──────────────────────────────┐
              + Nutrients (NH4, PO4) └────────────── │     AEROBIC HETEROTROPHIC    │
                                                     │       BACTERIAL OXIDATION    │
                                                     └──────────────────────────────┘
                                                                    ▲
                                                                    │ (Soluble Organics)
                                                     ┌──────────────┴───────────────┐
                                                     │   ANAEROBIC BENTHIC SLUDGE   │
                                                     │       DIGESTION LAYER        │
                                                     └──────────────────────────────┘

Diurnal Chemical Swings in Facultative Ponds

• Late Afternoon (Peak Sunlight): Algae consume dissolved $CO_2$ and carbonic acid ($H_2CO_3$) rapidly through photosynthesis, shifting carbonate equilibrium. Dissolved oxygen surges to supersaturated levels (15 to 25 mg/L) and pH rises to 8.5–9.5.
• Early Morning (Pre-Dawn): Respiration by algae and bacteria consumes oxygen throughout the night while releasing $CO_2$. Dissolved oxygen drops to its diurnal minimum (0.5 to 2.0 mg/L) and pH drops to 7.0–7.4.