5.4 Trickling Filters & Rotating Biological Contactors

5.4 Trickling Filters & Rotating Biological Contactors

Fixed-film (attached growth) biological systems cultivate a sessile microbial community attached to inert media surfaces rather than suspending biological flocs in mixed liquor. Settled primary wastewater flows across this attached biological slime layer (zoogleal film), allowing heterotrophic and autotrophic microorganisms to adsorb, absorb, and biologically oxidize dissolved organic matter and nutrients. Fixed-film units offer mechanical simplicity, low operational power consumption, and high resistance to hydraulic and organic shock loads.

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1. Trickling Filter Components & Engineering Hydraulics

A trickling filter comprises a retaining structure filled with packing media, an automated rotary distributor, and an underdrain floor collection system.

Rotary Distributor Assembly

Wastewater is applied across the filter bed by a rotating distributor assembly comprising two or four radial arms mounted on a center column:

• Propulsion: Most distributors are reaction-driven, propelled by the backward jet thrust of wastewater discharging from orifice nozzles onto deflector splash plates. Motorized electric drives are used on high-rate synthetic towers to maintain constant rotational speeds (0.1 to 0.5 rpm).
• Maintenance: Operators must regularly ream orifice nozzles to clear lint and debris. Uneven nozzle clogging causes distributor arms to stall, producing dry concentric rings on the bed and local organic overloading.

Filter Media: Rock Media vs. Synthetic Plastic Media

Media properties dictate hydraulic capacity, organic loading limits, and oxygen transfer efficiency:

• Crushed Rock / Slag Media: Sized at 2.5 to 4.0 inches (65 to 100 mm), rock bed depths are restricted to 4 to 8 feet (1.2 to 2.4 m) by heavy unit weight (100–120 lbs/cu ft). Low specific surface area (15 to 20 sq ft/cu ft) and limited void space (40% to 50%) make rock beds prone to plugging and ponding when organic loading exceeds 15–20 lbs BOD/1,000 cu ft/day.
• Synthetic Plastic Media: Modular corrugated sheets of rigid PVC or polypropylene are installed in deep towers 10 to 25+ feet (3.0 to 8.0 m) deep. Plastic media provides high specific surface area (30 to 45 sq ft/cu ft) and massive void space (90% to 95%), which minimizes plugging, facilitates natural air circulation, and supports volumetric loadings of 30 to 100+ lbs BOD/1,000 cu ft/day.

Underdrain System & Natural Draft Ventilation

The filter floor features vitrified clay underdrain blocks sloping at 1% to 2% toward a central drainage channel:

• Functions: Underdrain blocks collect treated effluent and sloughed biomass while acting as an air plenum to ventilate the bed.
• Natural Draft Mechanics: Aeration is driven by the temperature differential ($\Delta T$) between ambient air and wastewater. In winter ($T_{\text{water}} > T_{\text{air}}$), warm air in media voids rises, drawing fresh air into bottom underdrains (upward draft). In summer ($T_{\text{air}} > T_{\text{water}}$), cool air inside the bed sinks, exiting through underdrains and drawing ambient air down through the top (downward draft). When $\Delta T < 3^\circ\text{F}$, air movement stalls, requiring forced draft fans. Underdrain channels should never flow more than 50% full to maintain free air space.

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2. Biofilm Dynamics & Biological Sloughing

The zoogleal biofilm consists of bacteria, fungi, protozoa (stalked ciliates), rotifers, and macroscopic worms:

• Stratification: The outermost 0.1 to 0.2 mm of biofilm receives dissolved oxygen and soluble nutrients directly from the trickling wastewater, supporting rapid aerobic metabolism. Deeper into the slime, oxygen is depleted, rendering the inner layer at the media surface completely anaerobic.
• Sloughing Mechanism: As the biofilm thickens beyond 0.2 to 2.0 mm, organic substrates cannot diffuse to basal bacteria. These inner organisms enter endogenous respiration, consuming cell protoplasm and degrading binding polymers. Weakened by starvation, the slime loses adhesion. Hydraulic shear from trickling wastewater sweeps the detached biomass out of the bed—a process called sloughing.
• Sloughed biomass ("humus") flows through underdrains to secondary clarifiers for gravitational settling. Large sloughing events ("spring unloading") occur annually as rising temperatures accelerate basal decay.

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3. Recirculation Systems & Process Control

Recirculation involves pumping clarified filter or secondary effluent back to the filter influent. The Recirculation Ratio ($R$) is defined as $R = Q_r / Q$, typically ranging from 0.5 to 2.0:

1. Wetting Rate Maintenance: Keeps distributor arms turning and prevents biofilm drying during low night flows (minimum wetting rate 0.15 to 0.30 gpm/sq ft for rock).
2. Shock Load Dilation: Blends recycled effluent with incoming wastewater to dilute high-strength organic spikes or industrial toxics.
3. Continuous Re-Seeding: Returns acclimated microorganisms to re-inoculate the bed.
4. BOD Removal Efficiency: Multiple passes over the media increase contact time, lifting BOD removal from 60–75% up to 85–90%.
5. Odor Suppression: Freshens stale primary effluent with oxygenated recycled flow.

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4. Operational Troubleshooting: Ponding, Filter Flies & Odors

Filter Ponding (Pooling)

Ponding occurs when water pools on rock surfaces due to biomass overgrowth, rock disintegration, or debris accumulation:

• Remedies: Maximize recirculation flow to shear excess biomass; apply a chlorine shock dose (2 to 5 mg/L continuous or 10 to 20 mg/L shock for 4–8 hours) to lyse surface slime; flood the bed for 24 hours; or mechanically agitate rock surfaces with high-pressure fire hoses.

Filter Flies (Psychoda)

Filter flies (Psychoda alternata) breed in damp, decaying zoogleal slime in upper rock voids. Millions can emerge to nuisance nearby facilities:

• Remedies: Seal underdrains and flood the filter several inches above the media for 24 hours every 7 to 14 days during breeding season to drown larvae and pupae; apply a continuous low-dose chlorine residual (0.5–1.0 mg/L); or dose biological larvicides like Bacillus thuringiensis israelensis (Bti).

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5. Rotating Biological Contactors (RBCs)

Rotating Biological Contactors (RBCs) mount closely spaced corrugated high-density polyethylene (HDPE) discs on a horizontal steel shaft rotated slowly inside a contoured basin.

Mechanical Architecture & Kinetics

• Discs & Submergence: Discs are standard at 10 to 12 feet (3.0 to 3.7 m) in diameter, submerged approximately 40% in wastewater. The shaft rotates at 1.0 to 2.0 rpm via electric gear drives or perimeter air cups driven by blowers.
• Oxygenation Cycle: Rotation alternately submerges biofilm in wastewater (absorbing dissolved BOD and ammonia) and lifts it into ambient air (absorbing atmospheric oxygen). Rotation also shears excess biomass from discs.

Multi-Stage Basin Staging

RBC installations are divided into 3 to 4 sequential stages via internal baffles:

• Stages 1 & 2 (Carbonaceous BOD Removal): Fast-growing heterotrophs dominate the thick (1.5–3.0 mm) dark brown biofilm, removing 80% to 90% of soluble BOD.
• Stages 3 & 4 (Nitrification): Once soluble BOD drops below 15 mg/L, slow-growing autotrophic nitrifiers (Nitrosomonas and Nitrobacter) colonize discs, converting ammonium to nitrate ($NO_3^-$) in a thin (0.2–0.5 mm) golden-brown biofilm.

Operational Hazards & Monitoring

• Shaft Fatigue & Load Cells: Because biological slime contains ~95% water, heavy growth adds tons of weight. Uneven biomass during power outages creates eccentric loads ("loping"), causing shaft deflection and structural fatigue breakage. Operators monitor assembly weight via load cells under shaft bearings.
• Organic Loading Limit: Stage 1 loading must remain below 4.0 to 6.0 lbs total BOD / 1,000 sq ft / day to prevent anoxia.
• Beggiatoa Biomass (White Biofilm): Appearance of a white, milky biofilm indicates colonization by sulfur-oxidizing Beggiatoa bacteria. Beggiatoa thrives in the presence of hydrogen sulfide under low dissolved oxygen, signaling severe first-stage organic overloading, septic influent, or drive failures. Corrective actions include step-feeding influent to subsequent stages or providing supplemental diffused aeration.