11.2 Fixed-Film Treatment: Trickling Filters, RBCs, MBBR and IFAS

How Attached Growth Works

In fixed-film reactors, microorganisms grow as a biofilm (slime layer) on a surface. Wastewater flows over the film, organics and ammonia diffuse in, and oxygen diffuses from air or aerated water. The film thickens until the inner layers starve and lose their grip. Pieces then slough off and are captured in a downstream clarifier. Because the biomass stays put, fixed-film units handle shock loads well and need no return activated sludge control. In exchange, the operator has less direct control over the biology.

Trickling Filters

A rotary distributor spreads wastewater over a bed of rock (3-8 ft deep) or plastic media (often 15-40 ft tall). The liquid trickles down while air moves up or down through the voids by natural draft, driven by the temperature difference between air and wastewater, or by fans.

Loading formulas

$$\text{Hydraulic loading (gpd/sq ft)} = \frac{\text{Plant flow} + \text{Recirculated flow (gpd)}}{\text{Filter surface area (sq ft)}}$$ $$\text{Organic loading (lb BOD/1,000 cu ft/day)} = \frac{\text{Flow (MGD)} \times \text{BOD applied (mg/L)} \times 8.34}{\text{Media volume (cu ft)} \div 1,000}$$ $$\text{Recirculation ratio} = \frac{Q_{\text{recirculated}}}{Q_{\text{plant}}}$$

Example. An 80-ft-diameter rock filter has 6 ft of media. It receives 1.2 MGD of primary effluent at 140 mg/L BOD, with 1.2 MGD recirculated (R = 1.0).

• Area = 0.785 × 80² = 5,024 sq ft; volume = 5,024 × 6 = 30,144 cu ft
• Hydraulic loading = 2,400,000 ÷ 5,024 = 478 gpd/sq ft
• BOD applied = 1.2 × 140 × 8.34 = 1,401 lb/day; organic loading = 1,401 ÷ 30.144 = 46.5 lb/1,000 cu ft/day

Recirculation benefits

Recirculation dilutes strong or toxic influent, keeps the media wetted during low night flows (preventing dry spots and fly breeding), helps flush excess growth, and evens out the load reaching the biofilm.

Troubleshooting trickling filters

Rotating Biological Contactors (RBCs)

An RBC is a shaft of closely spaced plastic disks, about 40 percent submerged, turning slowly (about 1 to 1.6 rpm) in a tank. As each disk rotates, the biofilm alternately absorbs organics from the water and oxygen from the air. Units are arranged in stages: the first stages remove BOD, and later stages can nitrify. Covers protect the biofilm from cold, rain and sunlight.

Fixes for first-stage overload include removing baffles to spread load over more stages, step-feeding, supplemental air, or pretreating to remove sulfides.

Moving Bed Biofilm Reactors (MBBR) and IFAS

• MBBR: small plastic carriers with high protected surface area float freely in an aerated (or mixed anoxic) tank, typically filling 25 to 65 percent of the volume. Screens at the outlet keep carriers in the tank. No sludge return is needed; a downstream clarifier or DAF removes sloughed solids.
• IFAS (integrated fixed-film activated sludge): carriers or fixed media are added to an activated sludge basin. Slow-growing nitrifiers live on the media while the suspended biomass handles BOD. That allows nitrification in less basin volume or at a lower suspended sludge age, which is useful when upgrading an existing plant for ammonia limits.

Operating concerns include screen blinding by rags or foam, carrier loss through damaged screens, uneven carrier mixing, and enough dissolved oxygen (often 3 to 4 mg/L) to penetrate the biofilm. DEQ's Table A rates SBR, MBBR, integrated fixed film and activated sludge with separate fixed film as Class III up to 5 MGD ADWF and Class IV above.