1. Conceptual Framework & Definition

A. Definition & Role in Secondary Treatment

• Trickling Filter System (Biofilter / Fixed-Film Reactor): A secondary biological wastewater treatment unit that purifies clarified sewage by passing it over a fixed bed of coarse filter media (rocks, gravel, slag, or synthetic plastic packing) covered with a biological slime layer (biofilm or zoogloeal film).
• Attached Growth Process: Unlike the Activated Sludge Process (ASP)—which is a suspended growth system where microorganisms float freely in an aerated liquid mixture—a trickling filter is an attached growth (fixed-film) system where microbial biomass remains attached to stationary filter media while wastewater flows over it.
• Primary Objective: To oxidize dissolved and colloidal biodegradable organic matter, removing 80% to 90%+ of Biochemical Oxygen Demand (BOD) and Total Suspended Solids (TSS) from primary settled effluent.

SUSPENDED GROWTH VS. ATTACHED GROWTH BIOLOGICAL SYSTEMS

1. Suspended Growth System (e.g., Activated Sludge Process - ASP)
   - Microorganisms float freely suspended in liquid aeration tank.
   - Requires continuous mechanical air blowers / diffusers for aeration.

2. Attached Growth System (e.g., Trickling Filter System)
   - Microorganisms grow attached as a biofilm on stationary rock/plastic media.
   - Relies primarily on natural air draft ventilation through void spaces.

2. Structural Architecture & System Components

A standard trickling filter installation consists of four core structural elements: a rotating distributor arm, filter media bed, underdrain collection system, and a secondary clarifier (humus tank).

                    TRICKLING FILTER CROSS-SECTIONAL DIAGRAM

                      Central Pivot / Feed Pipe
                                 │
                   ┌─────────────┴─────────────┐
                   │  Rotating Distribution    ├──► Sprays Primary Effluent
                   │      Arm (Jet Driven)     │
                   └───┬───────────────────┬───┘
                       │                   │
             ┌─────────┴───────────────────┴─────────┐
             │ ░░░░░░░░ FILTER MEDIA ░░░░░░░░░░░░░  │ ◄── Fist-Sized Rocks /
             │ ░░░░░░ (Coarse Rock / Plastic) ░░░░░  │     Synthetic Plastic
             │ ░░░░░░ COVERED WITH BIOFILM ░░░░░░░  │     (Aerobic Slime Layer)
             │ ░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░  │
             └─────────┬───────────────────┬─────────┘
                       │ Underdrain Block  │ ◄── Ventilation Voids
                     ┌─┴───────────────────┴─┐
                     │ Underdrain Collection │
                     └───────────┬───────────┘
                                 ▼
                     To Humus Tank (Secondary Clarifier)

A. Key Components & Engineering Functions

1. Rotary Distribution Arm: A central vertical feed pipe connected to 2 or 4 horizontal hollow perforated pipes. Driven by the reaction force of water jets exiting the nozzles (or by a variable-speed electric motor), it rotates continuously to spray primary effluent uniformly across the top of the filter bed.
2. Filter Media Bed: A circular containment structure (1.5 to 3.0 meters deep for rock beds; up to 12 meters for modern plastic media beds).
   • Traditional Media: "Fist-sized" crushed granite, basalt, or field stones (50 mm to 100 mm diameter) providing 40 to 60 m²/m³ of specific surface area with 45% to 55% void space.
   • Modern Plastic Media: Structured corrugated PVC or polypropylene modules providing 80 to 200 m²/m³ of surface area with 90% to 95% void space, enabling deeper beds, higher hydraulic capacity, and resistance to clogging.
3. Underdrain System: Constructed of vitrified clay blocks or perforated concrete channels beneath the media floor. It performs a dual function:
   • Collects treated liquid effluent and sloughed biomass (humus) and directs them to the outlet.
   • Serves as a ventilation duct allowing natural air currents to rise through media voids.
4. Natural Draft Ventilation: Air circulates through media interstitial spaces due to temperature and density differentials between internal bed air and ambient atmospheric air, maintaining aerobic conditions without mechanical blowers.
5. Secondary Clarifier (Humus Tank): Settling basin downstream of the filter bed that separates sloughed biological solids (humus sludge) from clarified effluent.

3. Biological Mechanism & Biofilm Kinetics

A. Biofilm (Zoogloeal Slime) Structure

• As primary sewage trickles downward over the filter media, heterotrophic and autotrophic micro-organisms attach to the media surfaces, forming a gelatinous biological film (biofilm) typically 0.1 mm to 2.0 mm thick.
• Biofilm Microbial Ecology:
  • Bacteria: Zoogloea ramigera, Pseudomonas, Achromobacter, Flavobacterium.
  • Nitrifying Bacteria: Nitrosomonas and Nitrobacter (convert ammonia to nitrates in lower bed zones).
  • Fungi, Algae & Protozoa: Fusarium, Epistylis, Vorticella, which consume free-swimming bacteria and stabilize organic matter.
  • Macro-organisms: Worms, snails, and larvae (Psychoda) that feed on excess slime, helping regulate biofilm thickness.

                  CROSS-SECTION OF BIOFILM ON FILTER MEDIA

     Flue / Liquid Flow Zone ──► Wastewater Trickling Layer (BOD + Dissolved O₂)
  ───────────────────────────────────────────────────────────────────────────────
   Outer Aerobic Zone     ──► High O₂ & BOD Diffusion; Active Bacterial Growth
   (0.1–0.2 mm thick)         (Oxidizes Organic Waste ──► CO₂ + H₂O + Cell Mass)
  ───────────────────────────────────────────────────────────────────────────────
   Inner Anaerobic Zone   ──► No O₂ Diffusion; Endogenous Decay & Cell Death
   (Adjacent to Media)        (Loses Adhesion ──► SLOUGHING OCCURS)
  ───────────────────────────────────────────────────────────────────────────────
   Solid Media Surface    ──► Rock / Plastic Substrate

B. The Sloughing Mechanism

1. Diffusion Gradient: Oxygen and organic nutrients dissolve into the outer liquid film and diffuse into the biofilm. The outer layer (0.1 mm to 0.2 mm) remains strictly aerobic.
2. Anaerobic Layer Formation: As the biofilm thickens beyond ~0.2 mm, oxygen cannot penetrate to the inner layers adjacent to the media surface. The inner layer becomes anaerobic.
3. Endogenous Decay & Adhesion Failure: Deprived of nutrients and oxygen, inner-layer microbes enter endogenous decay, dying and producing gases (\(\text{CH}_4, \text{H}_2\text{S}\)). This weakens the biological bond attaching the slime layer to the rock/plastic media.
4. Shear Wash-off (Sloughing): The hydraulic shearing force of trickling wastewater strips the loosened slime layer off the media. This periodic or continuous shedding of detached biofilm is called sloughing.
5. Humus Separation: Sloughed solids pass into the Secondary Clarifier (Humus Tank), where they settle out as humus sludge.

4. Operational Parameters & Design Models

The performance of a trickling filter is dictated by hydraulic loading, organic loading, and effluent recirculation ratios.

OPERATIONAL LOADING PARAMETERS

1. Hydraulic Loading Rate (HLR)
   - Volume of sewage applied per unit surface area per day: HLR = Q / A (m³/m²/day)

2. Organic Loading Rate (OLR)
   - Mass of BOD applied per unit media volume per day: OLR = W / V (kg BOD/m³/day)

3. Recirculation Ratio (R/I)
   - Ratio of recycled settled effluent (R) to raw influent sewage (I): R/I or 'r'

A. The Role & Benefits of Recirculation

Recirculation involves recycling a portion of treated effluent back to the filter inlet. Recirculation provides several operational advantages:

• BOD Dilution: Dilutes high-strength incoming raw sewage, reducing shock organic loads.
• Continuous Media Wetting: Maintains uniform hydraulic wetting, preventing biofilm drying during low-flow periods.
• Shear Maintenance: Provides continuous hydraulic shearing to promote regular sloughing and prevent media clogging.
• Multi-Stage Treatment: Passes organic matter through the biofilm multiple times, increasing overall BOD removal efficiency.

B. Recirculation Factor (\(F\))

The Recirculation Factor (\(F\)) accounts for the effective number of passes through the filter:

\[F = \frac{1 + \frac{R}{I}}{\left(1 + 0.1 \cdot \frac{R}{I}\right)^2}\]

Where:

• \(R\) = Recirculated flow rate (m³/day)
• \(I\) = Influent raw sewage flow rate (m³/day)
• \(R/I\) = Recirculation ratio (typically 0.5 to 3.0)

C. NRC Design Equations (National Research Council)

The NRC Equations predict BOD removal efficiency for rock-media trickling filters:

1. First-Stage Filter Efficiency (\(E_1\)):

\[E_1 = \frac{100}{1 + 0.443 \cdot \sqrt{\frac{W_1}{V_1 \cdot F_1}}}\]

Where:

• \(E_1\) = BOD removal efficiency of first stage (%)
• \(W_1\) = Influent BOD load to first stage (kg/day)
• \(V_1\) = Media volume of first stage (m³)
• \(F_1\) = Recirculation factor for first stage

2. Second-Stage Filter Efficiency (\(E_2\)):

\[E_2 = \frac{100}{1 + \frac{0.443}{1 - E_1} \cdot \sqrt{\frac{W_2}{V_2 \cdot F_2}}}\]

Where \(W_2\) is the BOD load entering the second stage from the first-stage clarifier.

5. Classification & Operational Modes of Trickling Filters

Trickling filters are classified based on hydraulic and organic loading rates into four main categories:

6. Operational Hazards, Troubleshooting & Mitigation

TRICKLING FILTER OPERATIONAL HAZARDS & CONTROLS

1. Filter Ponding & Clogging
   - Cause: Excessive biofilm growth / fine media blocks void spaces.
   - Remedy: Dosing with Chlorine (5–10 mg/L), high-rate flushing, or replacing rock with plastic media.

2. Filter Flies (Psychoda alternata)
   - Cause: Nuisance fly breeding in moist, stagnant biological slime.
   - Remedy: Periodic bed flooding, insecticide/chlorine dosing, or maintaining high hydraulic shear.

3. Odor Nuisances
   - Cause: Anaerobic conditions from poor air ventilation or ponded sewage.
   - Remedy: Improve underdrain clearance, unblock air vents, or increase recirculation flow.

A. Filter Ponding (Surface Clogging)

• Problem: Accumulated sloughed biomass and fine debris choke media interstitial voids, causing wastewater to pool on top of the filter bed.
• Mitigation:
  • Application of heavy chlorine dosing (5 to 10 mg/L \(\text{Cl}_2\)) to strip excess surface slime.
  • Mechanical rake agitation or high-pressure water jetting.
  • Replacing fine rock media with high-void structured plastic media.

B. Filter Fly Nuisance (Psychoda alternata)

• Problem: Tiny, moth-like flies (Psychoda) breed prolifically within moist, unflooded areas of the biofilm, creating health hazards for plant operators and nearby communities.
• Mitigation:
  • Bed Flooding: Periodically filling the filter bed with sewage for several hours to drown larvae and pupae.
  • Applying bio-pesticides or maintaining continuous wetting via high-rate recirculation.

C. Odor Problems

• Problem: Anaerobic conditions caused by inadequate ventilation or heavy organic overload generate foul-smelling Hydrogen Sulfide (\(\text{H}_2\text{S}\)) gas.
• Mitigation: Unblocking underdrain vents, forced-air ventilation blowers, and increasing the recirculation ratio (\(R/I\)) to supply dissolved oxygen.

7. Comparative Evaluation: Trickling Filter vs. ASP vs. Oxidation Pond

Quick Revision

• Trickling Filter Core Concept: An attached growth secondary biological treatment system where primary effluent trickles over rock or plastic media covered with an aerobic biofilm.
• Biofilm Structure: Dual-layered slime film; outer layer (0.1–0.2 mm) is aerobic, while inner layer adjacent to media is anaerobic.
• Sloughing Process: Dying inner-layer microbes lose adhesion, causing the outer slime layer to wash off into the secondary clarifier (humus tank).
• Recirculation Role: Recycles treated effluent (\(R/I\)) to dilute BOD, maintain continuous wetting, control sloughing, and increase removal efficiency.
• Low-Rate vs. High-Rate: Low-rate filters use no recirculation and yield high nitrification; high-rate filters use recirculation (\(R/I = 1\text{--}3\)) to handle higher organic loads.
• Ponding Control: Resolved by chlorine dosing (5–10 mg/L), high hydraulic flushing, or switching to high-void plastic media.
• Fly Control (Psychoda): Controlled by periodic bed flooding to drown larvae.

Memory Map

1. Influent Entry & Distribution ──► Primary Settled Sewage ──► Rotary Distributor Arm Sprays Wastewater Uniformly over Media Bed.

2. Biofilm Biological Oxidation ──► Trickles Over Rock/Plastic Media ──► Biofilm (Bacteria/Fungi/Algae) Absorbs BOD + O₂. ──► Aerobic Outer Layer Oxidizes Waste ──► Anaerobic Inner Layer Causes Sloughing.

3. Effluent Separation & Recirculation ──► Effluent + Sloughed Biofilm ──► Secondary Clarifier (Humus Tank Settles Humus Sludge). ──► Portion Recirculated (R/I) ──► Dilutes Influent BOD & Prevents Media Drying.

4. Nuisance Control ──► Clogging/Ponding ──► Chlorine Dosing / High Flushing. ──► Filter Flies (Psychoda) ──► Periodic Bed Flooding.

Must Remember

• Attached Growth Principle: Trickling filters operate as attached growth systems, utilizing a fixed microbial biofilm on stationary media.
• Dual Biofilm Dynamics: The outer 0.1–0.2 mm layer is aerobic; the inner layer is anaerobic, leading to sloughing when inner cells enter endogenous decay.
• Recirculation Advantages: Recirculation dilutes incoming BOD, prevents media drying, controls biofilm thickness, and improves overall treatment efficiency.
• Humus Tank Function: The secondary settling tank downstream of a trickling filter separates sloughed biological solids (humus) from the treated effluent.
• Filter Fly Species: Psychoda alternata is the primary nuisance fly associated with trickling filters, controlled by bed flooding.

Key Terms

• Trickling Filter: A fixed-film biological reactor where wastewater trickles over media coated with a microbial slime layer.
• Attached Growth System: A biological treatment process where microorganisms grow attached to a solid substrate.
• Biofilm (Zoogloeal Film): A gelatinous matrix of bacteria, fungi, algae, and protozoa adhering to filter media.
• Sloughing: The periodic shedding of excess biofilm from filter media caused by inner-layer cell decay and hydraulic shear.
• Recirculation Ratio (\(R/I\)): The ratio of recycled effluent flow rate to incoming raw sewage flow rate.
• Recirculation Factor (\(F\)): A design parameter quantifying the effective number of passes through a trickling filter.
• Humus Tank: The secondary clarifier used to settle sloughed biological solids downstream of a trickling filter.
• Filter Ponding: Surface water pooling caused by excessive biofilm growth or fine media choking void spaces.

1-Line Bits / Short Points

• Trickling filters are attached growth biological treatment units that utilize fixed microbial biofilms.
• Primary effluent is sprayed over coarse filter media using a rotating distribution arm.
• Traditional trickling filter media consists of fist-sized rocks or crushed stone 50 to 100 mm in diameter.
• Modern trickling filters use synthetic plastic media providing up to 95% void space and high surface area.
• The outer 0.1 to 0.2 mm layer of biofilm is aerobic, while the inner layer is anaerobic.
• Sloughing is the process where detached, dead biofilm washes off the filter media.
• Sloughed biological solids are collected in a secondary settling tank known as a humus tank.
• Recirculation dilutes incoming BOD, maintains continuous media wetting, and improves efficiency.
• Low-rate trickling filters operate without recirculation and achieve complete nitrification.
• High-rate trickling filters utilize recirculation ratios between 1.0 and 3.0.
• Filter ponding is relieved by chlorination or high hydraulic flushing.
• The moth fly Psychoda alternata breeds in trickling filter beds and is controlled by bed flooding.
• Trickling filters consume less electrical power than Activated Sludge Systems.
• The NRC equations model BOD removal efficiency based on organic loading, media volume, and recirculation.