6.1 Trickling Filters & Rotating Biological Contactors (RBCs)

Fixed-film systems—commonly termed attached-growth processes—represent one of the oldest and most mechanically resilient methodologies for secondary biological wastewater treatment. Unlike suspended-growth systems (such as conventional activated sludge) where microorganisms are suspended in mixed liquor, fixed-film processes cultivate a stationary or rotating sessile biofilm (slime layer) on an inert solid support medium. Wastewater flows across this biological slime, allowing dissolved organic pollutants ($BOD_5$) and nutrients to be absorbed and metabolized by the microbial consortium.

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1. Principles of Attached Growth & Biofilm Mechanics

The fundamental engine of fixed-film treatment is the active biological slime layer. This biofilm is not a uniform homogeneous film, but rather a structured, gelatinous matrix comprised of heterotrophic bacteria, nitrifying autotrophs, protozoa, fungi, and an extracellular polymeric substance (EPS) matrix that anchors the microorganisms to the media surface.

Biofilm Stratification & Mass Transport

Mass transfer of dissolved organic carbon, inorganic nutrients ($NH_4^+$, $PO_4^{3-}$), and dissolved oxygen ($DO$) occurs via molecular diffusion from the bulk wastewater flowing over the media surface through a stagnant liquid boundary layer into the biofilm:

1. Aerobic Outer Slime Layer: The outermost 0.1 to 0.2 mm (100 to 200 µm) of the biofilm maintains positive dissolved oxygen concentrations. In this aerobic boundary, active heterotrophic bacteria rapidly oxidize soluble carbonaceous biochemical oxygen demand ($CBOD$):

$\text{Organic Matter} + O_2 + \text{Nutrients} \rightarrow CO_2 + H_2O + \text{New Biofilm Biomass}$

1. Anaerobic Inner Slime Layer: Because dissolved oxygen is consumed by heterotrophs faster than it can diffuse into deeper layers, the biofilm region beyond 0.2 mm depth becomes strictly anaerobic and anoxic. Microorganisms adjacent to the inert media surface are deprived of both oxygen and soluble nutrients, forcing them into endogenous respiration (auto-oxidation of cellular mass).

2. Biomass Sloughing (Humus Production): As the cells in the deep anaerobic layer starve and die, they lose their ability to cling to the media surface. The structural integrity of the EPS matrix weakens. The hydraulic shear force of wastewater trickling or splashing over the biofilm shears the dead and excess biomass off the media. This periodic or continuous detachment is termed sloughing, and the discharged solids are known as humus. Humus solids are carried out in the filter effluent and must be removed in secondary clarifiers (humus tanks) to prevent elevated Total Suspended Solids (TSS) and particulate BOD violations in plant discharge.

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2. Trickling Filter Engineering, Anatomy & Hydraulics

A trickling filter consists of a bed of highly permeable media through which primary-settled wastewater percolates downward. The primary components of a modern trickling filter include the rotary distributor, media bed, underdrain collection system, and natural/forced draft ventilation.

Media Selection: Rock vs. Structured Plastic

Historically, trickling filters utilized crushed rock, field stone, or metallurgical slag. Modern designs and retrofits predominantly utilize engineered structured plastic packing. The physical properties of these media drastically dictate loading capacity:

• Rock / Slag Media:
  • Specific surface area: 12 to 18 $\text{ft}^2/\text{ft}^3$ (40 to 60 $\text{m}^2/\text{m}^3$).
  • Void ratio: 40% to 50% void space.
  • Typical bed depth: Shallow, 6 to 8 feet (limited by the structural weight of rock, approx. 90 to 100 $\text{lb/ft}^3$, and poor natural ventilation in deep rock beds).
  • Clogging potential: High; small void passages are easily bridged by excessive biomass, causing surface ponding.
• Structured Cross-Flow Plastic Media:
  • Specific surface area: 30 to 42 $\text{ft}^2/\text{ft}^3$ (100 to 140 $\text{m}^2/\text{m}^3$).
  • Void ratio: 90% to 95% void space.
  • Typical bed depth: Deep towers, 15 to 30 feet (low structural weight, approx. 3 to 5 $\text{lb/ft}^3$).
  • Clogging potential: Very low; wide, interlocking cross-flow channels maximize convective air transfer and prevent biological bridging.

Natural Draft Ventilation Mechanics

Trickling filters rely on passive convective airflow driven by the temperature differential (ΔT) between the ambient outdoor air and the wastewater trickling through the media bed:

• Winter Operation (Wastewater Warmer than Ambient Air): The wastewater warms the air inside the media voids. This warm air becomes less dense than the cold ambient air and rises upward through the filter bed like a chimney (updraft), drawing fresh cold air into the bottom underdrains.
• Summer Operation (Wastewater Cooler than Ambient Air): The wastewater cools the ambient air entering the top of the bed. This cool air becomes denser and flows downward through the media bed (downdraft), discharging out through the underdrains.
• Stagnant Inversion Conditions (ΔT ≈ 0): When ambient air temperature equals wastewater temperature, natural draft halts entirely. To prevent oxygen depletion in the biofilm, the underdrain channels must never run more than 50% full of water, ensuring a continuous open airway. In heavy industrial or high-rate installations, low-pressure forced draft ventilation fans are installed to guarantee continuous airflow.

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3. Trickling Filter Process Classifications & Loading Rates

Trickling filters are classified based on their hydraulic loading rate (HLR) and organic loading rate (OLR). Recirculation of secondary clarifier effluent back to the filter influent is a vital design feature distinguishing high-rate from low-rate systems.

The Operational Role of Recirculation

In high-rate trickling filters, effluent recirculation (recycling secondary effluent or filter effluent back to the influent distributor) serves several critical operational objectives:

1. Maintains Minimum Wetting Rate: Prevents media drying during diurnal low-flow periods (e.g., 2:00 AM to 5:00 AM), which protects the sessile microbial population from desiccation.
2. Dilutes High-Strength Influent: Dampens organic shock loads and dilutes toxic industrial spikes before they hit the biological slime.
3. Enhances Hydraulic Shear: Sustains a steady liquid wash rate across the media, causing continuous, controlled sloughing of excess biomass rather than catastrophic, massive seasonal sloughing that overloads secondary clarifiers.
4. Improves Dissolved Oxygen Transfer: Re-aerates wastewater passing through the system multiple times, suppressing anaerobic odor generation.

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4. Operational Troubleshooting: Ponding, Filter Flies & Odors

Filter Ponding (Standing Water on Media Surface)

Filter ponding occurs when the void spaces within the media bed become clogged with excessive biological slime, accumulated primary solids, or degraded/crushed rock media. Puddles of wastewater form on top of the filter, cutting off natural ventilation and inducing rapid septic conditions.

• Corrective Remediation Protocol:
  1. High-Pressure Hosing: Direct a high-pressure fire hose stream directly into ponded surface areas to physically break up and dislodge surface slime mats.
  2. Bed Flooding: Cap the underdrain outlets and flood the filter bed with wastewater for 24 hours. The resulting anaerobic conditions deep in the media cause the attached biomass to loosen its grip and slough off rapidly upon draining.
  3. Chlorination Shock: Add chlorine to the filter influent to achieve a 5 to 10 mg/L free chlorine residual for 4 to 8 hours (or 1 to 2 mg/L continuous residual). Chlorine oxidizes the superficial biological slime layer without penetrating deep enough to sterilize the entire bed.
  4. Increase Recirculation: Maximize recirculation pump rates to increase the hydraulic flushing intensity ($Spülkraft$), washing loosened solids down into the underdrain.

Filter Fly Control (Psychoda alternata)

Filter flies (Psychoda) are minute moth-like flies that breed in the damp, decaying organic slime of trickling filters. While larvae aid in consuming excess biofilm, adult flies can multiply into millions, escaping the filter perimeter and creating severe public health nuisances and community complaints.

• Ecology: Larvae thrive in moist, partially exposed filter zones where wastewater spray is intermittent, allowing them to breathe atmospheric air without being washed away.
• Control Measures:
  1. Flooding the Filter: Plug underdrain gates and flood the media bed completely with wastewater for 24 hours once every 7 to 14 days during breeding season. This drowns fly larvae and pupae without permanently damaging the treatment biofilm.
  2. Continuous Wetting: Adjust distributor arm rotation speed or install back-spray spreader nozzles to ensure the entire media surface and retaining walls remain continuously drenched.
  3. Chemical / Biological Larvicides: Apply Bacillus thuringiensis israelensis (BTI) or approved insect growth regulators directly to the filter influent. Spray adult resting surfaces on outer filter retaining walls with an approved residual insecticide.

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5. Rotating Biological Contactors (RBCs)

Rotating Biological Contactors (RBCs) are attached-growth treatment units consisting of large-diameter (typically 10 to 12 feet), closely packed corrugated high-density polyethylene (HDPE) disks mounted horizontally on a massive central steel shaft (up to 25 to 27 feet in length). The shaft and disks are submerged to approximately 40% of their diameter in a contoured concrete wastewater trough.

Operational Mechanics & Submergence Dynamics

The shaft rotates slowly at 1 to 2 revolutions per minute (rpm), corresponding to a peripheral disk rim speed of approximately 60 ft/min (0.3 m/s). As the media rotates:

• Submerged Arc (40%): The attached biofilm absorbs soluble organic matter ($BOD_5$) and nutrients from the wastewater trough.
• Exposed Arc (60%): The rotation lifts a thin film of wastewater into the atmosphere, where molecular oxygen diffuses into both the liquid film and the underlying biological slime layer.
• Mixing and Shearing: Disk rotation creates gentle hydraulic mixing in the basin and imparts shear forces that strip sloughed biomass from the media, keeping it in suspension until discharged to the secondary clarifier.

Staging and Biological Succession

RBC systems are segmented into 3 to 4 staged compartments in series, separated by structural baffles. Staging is vital to foster natural ecological succession:

1. First Stage (Carbonaceous Removal): Subjected to high organic loading ($> 2.5\;\text{lb soluble } BOD_5/1,000\;\text{ft}^2/\text{day}$). Heterotrophic bacteria dominate, rapidly assimilating soluble sugars, organic acids, and proteins. Biofilm is thick (1/8 to 1/4 inch), shaggy, and dark brown.
2. Intermediate Stages: Heterotrophs continue to strip remaining soluble organics, reducing $BOD_5$ below 20 mg/L.
3. Downstream Stages (Nitrification): Once soluble $BOD_5$ drops below 15 mg/L, slow-growing autotrophic nitrifiers (Nitrosomonas and Nitrobacter) can successfully compete for media surface area without being smothered by heterotrophs. Biofilm in nitrifying stages is thin (1/16 inch), dense, and golden-tan to reddish-brown in color.

Shaft Load Monitoring & Filamentous Sulfur Bacteria (Beggiatoa)

RBC shafts are subject to extreme cyclical torsional stress and fatigue failure. A critical operator duty is continuous monitoring of shaft bearing load cells, which weigh the disk media assembly:

• Excess Biomass Weight: If excessive biofilm accumulates, total shaft weight can increase from a clean weight of 10,000 lbs to over 40,000 lbs, inducing severe shaft deflection, bearing failure, or catastrophic structural fracture.
• Beggiatoa Overgrowth: Under conditions of severe organic overloading in the first stage, accompanied by septic influent containing high concentrations of dissolved hydrogen sulfide ($H_2S > 2\;\text{mg/L}$) and depressed dissolved oxygen ($DO < 1.0\;\text{mg/L}$), filamentous sulfur-oxidizing bacteria (Beggiatoa) outcompete normal heterotrophs.
  • Physical Appearance: Beggiatoa forms a distinct, chalky-white or silvery-grey shaggy mat across the entire disk surface.
  • Operational Impact: Beggiatoa consumes dissolved oxygen aggressively, yields negligible BOD removal, adds massive dead weight to the shaft, and releases pungent odors.
  • Remediation: Step-feed influent wastewater to distribute the organic load across stages 1 and 2; add supplemental aeration diffusers directly beneath the first-stage RBC media; or chlorinate influent at low dosages to oxidize sulfides.