3.3 Attached Growth Systems & Advanced Biological Processes

3.3 Attached Growth Systems & Advanced Biological Processes

Fundamentals of Attached-Growth (Fixed-Film) Processes

Unlike suspended-growth activated sludge systems where microorganisms are kept floating in liquid suspension, attached-growth (fixed-film) systems cultivate microorganisms on a stationary or rotating solid support medium. As primary clarified wastewater flows over the media surfaces, dissolved organic matter and oxygen diffuse into the biological slime layer (biofilm).

               Wastewater Flow Film ────────►
               ──────────────────────────────
               Aerobic Layer (0.1–0.2 mm)    ◄── Oxygen & BOD Diffuse Inward
               ──────────────────────────────
               Anaerobic Layer               ◄── Fermentation / Endogenous Decay
               ──────────────────────────────
               Solid Media Support Surface   ◄── Biofilm Weakens & Sloughs Off

Biofilm Kinetics & The Sloughing Phenomenon

1. Aerobic Zone (Outer Layer): The outer 0.1 to 0.2 mm of the biofilm receives dissolved oxygen from overlying liquid and air. Heterotrophic bacteria rapidly oxidize soluble BOD.
2. Anaerobic Zone (Inner Layer): As the biofilm thickens beyond 0.2 to 2.0 mm, oxygen is completely consumed before it can penetrate to the media face. Bacteria at the media boundary enter anaerobic endogenous decay, losing their ability to cling to the support surface.
3. Sloughing: Hydrodynamic shear forces from trickling water strip the weakened, decaying biomass from the media. This periodic detachment of accumulated biomass is known as sloughing. Sloughed solids must be settled and removed in a downstream secondary (humus) clarifier.

---

Trickling Filters

A trickling filter consists of a bed of highly permeable media over which wastewater is continuously sprayed by a motorized or hydraulically driven rotary distributor arm.

                     Rotary Distributor Arm (Rotates 0.5–2.0 rpm)
                             ▼                 ▼
Raw / Recycled Feed ──► ┌───────────────────────────┐
                        │ ▼  ▼  ▼  ▼  ▼  ▼  ▼  ▼  ▼ │
                        │   Fixed Biological Media  │
                        │ (Rock or Structured PVC)  │
                        │   Slime Biofilm Layer     │
                        └─────────────┬─────────────┘
                                      │ Underdrain Tiles
                                      ▼
                  Effluent & Sloughed Biofilm to Humus Clarifier

Media Comparison & Operational Parameters

| Media Parameter | Rock / Crushed Slag Media | Modern Structured Plastic / PVC Media | |:---|:---|:---|:---| | Bed Depth | 4 – 8 feet (1.2–2.4 m) | 15 – 30 feet (4.5–9.0 m) | | Specific Surface Area | 12 – 20 sq ft / cu ft (40–65 m²/m³) | 25 – 45 sq ft / cu ft (80–150 m²/m³) | | Void Ratio | 40% – 50% | 90% – 95% | | Hydraulic Loading Rate | 25 – 100 gpd/sq ft | 200 – 1,500 gpd/sq ft | | Organic Loading Rate | 5 – 25 lb BOD/1,000 cu ft/day | 30 – 100 lb BOD/1,000 cu ft/day | | Plugging & Ponding Risk | High (low void space, prone to pooling) | Very Low (open vertical flutes) |

Recirculation Hydraulics

Recirculating secondary clarifier effluent or filter effluent back to the trickling filter influent (recirculation ratio $R = Q_r / Q = 0.5\text{ to }3.0$) is critical for:

• Maintaining a continuous minimum wetting rate to prevent drying of the biofilm during low night flows.
• Diluting high-strength industrial organic shock loads.
• Providing continuous hydraulic shear to prevent excessive biofilm accumulation and media ponding.

Trickling Filter Troubleshooting

• Filter Ponding (Pooling): Caused by excessive biomass buildup, media breakdown, or low hydraulic flush. Corrective Actions: Increase recirculation flow to generate higher hydraulic flush shear; dose chlorine solution (1–2 mg/L $\text{Cl}_2$) to burn surface slime; physically loosen surface rocks; or flood the filter for several hours.
• Filter Flies (Psychoda): Tiny nuisance gnats breeding in moist, decaying biofilm. Corrective Actions: Flood the filter bed for 24 hours to drown larvae; apply low-dose periodic chlorination (0.5–1.0 mg/L); or adjust the distributor arm speed to maintain a continuous, heavy water flush curtain.

Rotating Biological Contactors (RBCs)

An RBC consists of large circular corrugated plastic sheets (typically high-density polyethylene, 10 to 12 feet in diameter) mounted closely spaced along a horizontal steel shaft up to 25 feet long. The shaft rotates at 1.0 to 2.0 rpm (peripheral tip speed ~60 ft/min), with the media discs approximately 40% submerged in a semi-circular concrete trough.

                    Rotates 1.0–2.0 rpm
                             ┌─┐
                         ┌───┘ └───┐  ◄── Disc Exposed to Air (Absorbs Oxygen)
                       ┌─┘         └─┐
                      ┌┘   ● Shaft   └┐
  Wastewater Level ───┼───────────────┼─── (~40% Submerged in Basin)
                      └┐   Biofilm   ┌┘
                       └─┐ Layer   ┌─┘  ◄── Disc Submerged (Absorbs Soluble BOD)
                         └───┐ ┌───┘
                             └─┘

Staging Dynamics

RBC systems are segmented into 3 to 4 stages in series separated by baffle walls:

• Stage 1 (Carbonaceous Oxidation): Receives primary effluent with high soluble BOD. Biofilm is thick (1/8 to 1/4 inch), heavy, and greyish-brown. Dissolved oxygen in the trough is low (<1.0 mg/L) due to intense heterotrophic respiration.
• Stages 3 & 4 (Nitrification & Polish): As BOD is reduced below 15–20 mg/L in early stages, autotrophic nitrifiers colonize the discs. Biofilm becomes very thin (1/16 inch), velvety golden-brown, and dissolved oxygen rises to 2.0–4.0 mg/L.

RBC Mechanical & Operational Failure Modes

1. First-Stage Organic Overload & Loping: If first-stage organic loading exceeds 4.0–6.0 lb soluble BOD/1,000 sq ft/day, filamentous bacteria (Beggiatoa) proliferate, and the biofilm grows excessively thick (>1/4 inch). The immense wet biomass weight places severe rotational torque on the center shaft. When rotation stops momentarily, biomass dries unevenly, creating an eccentric weight distribution known as "loping", which snaps steel drive shafts and shears drive chains.
2. Bearing & Drive Maintenance: Operators must perform routine lubrication of pillow block bearings and verify motor current draw to detect mechanical drag from unbalanced disc loading.

---

Hybrid Biofilm-Activated Sludge Systems

Modern facilities upgrade existing aeration basins to hybrid processes that combine attached and suspended growth within a single reactor volume:

• Integrated Fixed-Film Activated Sludge (IFAS): Fixed media modules or hanging rope meshes are installed directly into conventional suspended-growth aeration basins with continuous RAS recycling. IFAS doubles the effective biomass inventory without increasing solids loading on secondary clarifiers.
• Moving Bed Biofilm Reactor (MBBR): Thousands of high-density plastic carrier elements (e.g., Kaldnes K1/K3 carriers, specific surface area 500–800 m²/m³) freely circulate in the aeration basin, suspended by aeration bubbles or mechanical mixers. Perforated sieves on basin outlets retain the carriers. MBBR operates without any RAS recycle stream; all biomass in the basin is permanently attached to the moving carriers.

---

Advanced Suspended-Growth & Membrane Configurations

Sequencing Batch Reactor (SBR) 5-Phase Automated Cycle (Single Basin):

┌──────────────┐  ┌──────────────┐  ┌──────────────┐  ┌──────────────┐  ┌──────────────┐
│ 1. FILL      │  │ 2. REACT     │  │ 3. SETTLE    │  │ 4. DECANT    │  │ 5. IDLE      │
│ Influent In  │  │ Aerate/Mix   │  │ Quiescent    │  │ Clear Effl.  │  │ Waste Sludge │
│ Anoxic/Aerat │  │ BOD/Nitrify  │  │ Zero Flow    │  │ Drawn Off    │  │ Rest/Cycle   │
└──────────────┘  └──────────────┘  └──────────────┘  └──────────────┘  └──────────────┘

1. Sequencing Batch Reactors (SBRs)

An SBR is a fill-and-draw activated sludge system where all process steps—equalization, biological oxidation, clarification, and decanting—occur sequentially in the same tank across a timed 5-phase cycle (typically 4 to 6 hours total):

1. Fill: Wastewater fills the tank. Can be Static Fill (anaerobic), Mixed Fill (anoxic denitrification), or Aerated Fill.
2. React: Dedicated aeration and mixing period to complete carbonaceous BOD oxidation and autotrophic nitrification.
3. Settle: All mixing and aeration cease. The basin acts as a completely quiescent secondary clarifier with zero forward flow velocity, achieving exceptional solids separation.
4. Decant: A motorized floating or submersible decanter lowers into the clear supernatant, drawing off clarified effluent without disturbing the settled sludge blanket.
5. Idle / Sludge Wasting: Small volume of WAS is pumped out to solids handling before the next cycle begins.

2. Membrane Bioreactors (MBR)

MBRs replace gravity secondary clarifiers with submerged microfiltration (0.1–0.4 µm) or ultrafiltration (0.04–0.08 µm) membrane cassettes (hollow fiber or flat sheet) immersed directly in the biological aeration basin or an adjacent membrane tank.

• Elevated Biomass Operation: Because membranes provide positive physical liquid-solids separation independent of gravity settling, MBRs operate at an MLSS of 8,000 to 12,000 mg/L (3 to 4 times higher than conventional activated sludge), dramatically reducing required tank footprint.
• Effluent Quality: Consistently produces effluent turbidity <0.1 NTU, TSS <1.0 mg/L, complete removal of protozoan cysts (Giardia, Cryptosporidium), and log-4 reduction of bacteria.
• Membrane Maintenance: Continuous coarse-bubble air scouring agitates fibers to prevent cake layer accumulation. Periodic backpulsing and chemical Clean-In-Place (CIP) cycles use sodium hypochlorite (to remove organic biofouling) and citric acid (to dissolve inorganic mineral scaling).

3. Oxidation Ditches

Oxidation ditches utilize a continuous oval racetrack channel equipped with horizontal brush rotors or disc aerators. Operating in extended aeration mode with an HRT of 18 to 36 hours and MCRT of 15 to 30 days, the system provides immense buffer capacity against organic shocks and achieves full nitrification.