8.5 Attached Growth Systems: Trickling Filters, RBCs & Fixed-Film Processes

8.5 Attached Growth Systems: Trickling Filters, RBCs & Fixed-Film Processes

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1. The fixed-film principle

In an attached growth process, the microorganisms live in a biofilm on fixed media while wastewater passes over them. Oxygen reaches the film from air (trickling filters, RBCs) or from aerated water (submerged systems). The film stratifies: the outer layer is aerobic and does the carbonaceous oxidation and nitrification; the inner layer near the media goes anaerobic as the film thickens; and when the anaerobic layer loses its grip, the film sloughs and washes downstream.

Because sloughing is normal, every attached growth process needs a secondary clarifier to capture the solids.

Regulatory note. 08G .0102(26) defines submerged fixed growth as a biological system in which wastewater is treated by contact with growth fixed to submerged support media and expressly includes rotating biological contactors and sequencing batch reactors. Under .0302(b)–(d), submerged fixed growth systems are graded by permitted flow exactly like activated sludge: Grade II at 0.5 MGD or less, Grade III above 0.5 through 2.5 MGD, and Grade IV above 2.5 MGD.

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2. Trickling filters

   Influent + recirculation
            |
      [ ROTARY DISTRIBUTOR ]
            |    |    |
   ~~~~~~~~ v ~~ v ~~ v ~~~~~~~   media (rock or plastic)
   |  biofilm-coated media     |  air moves by natural draft
   |                           |
   =============================  underdrain -> clarifier

Media. Rock media (3–5 inch) is heavy, limits depth to roughly 6–8 feet, and offers less surface area. Plastic or synthetic media allows towers 20–40 feet deep with far more surface area per volume and better airflow.

Recirculation returns filter effluent (or clarifier underflow) to the filter influent. It keeps media continuously wetted, dilutes strong influent, improves distribution, reduces odor, and helps flush ponding. The recirculation ratio is recirculated flow divided by influent flow, commonly 0.5:1 to 4:1.

Ventilation. Natural draft depends on the temperature difference between air and wastewater. On mild days airflow can stall, oxygen runs out, and the filter turns anaerobic and odorous — which is why underdrains, vent stacks, and the plenum must be kept clear and why some towers use forced ventilation.

Trickling filter troubleshooting

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3. Rotating biological contactors

An RBC is a shaft of closely spaced plastic discs, roughly 40 percent submerged, rotating at about 1.5 revolutions per minute through a contour tank. Each rotation lifts the biofilm out for oxygen and returns it to the wastewater for substrate. Units are staged, with the first stage carrying the heaviest load and later stages nitrifying.

Operational and mechanical issues:

• Excess biofilm weight — the classic RBC failure. Heavy growth overloads bearings and can bend or break shafts. Controlled stripping by rotation speed change, air stripping, or chemical addition is used to manage it.
• Bearing and drive failures from continuous heavy load; monitor amperage, temperature, and vibration.
• Uneven growth causes imbalance; white biofilm on the first stage indicates Beggiatoa thriving on sulfide from a septic influent.
• Covers protect the media from sunlight (which degrades plastic and grows algae) and retain heat in winter.
• Air-drive RBCs use diffused air in cups to rotate the shaft and can also strip excess growth.

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4. Modern submerged and hybrid processes

Why operators like fixed film: stable against hydraulic and toxic shocks, low energy compared with mechanical aeration, no sludge return to balance, and simple day-to-day control. Why they watch it closely: limited process control levers — recirculation rate and loading are about all you have — and mechanical failures that stop treatment immediately.

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5. Loading calculations

• Hydraulic loading (gpd/ft²) = (influent + recirculated flow) ÷ filter surface area.
• Organic loading (lb BOD/day per 1,000 ft³) = BOD applied ÷ media volume.
• Recirculation ratio = recirculated flow ÷ influent flow.
• RBC loading (lb BOD/day per 1,000 ft²) uses total media surface area — first-stage loading is the number that governs whether the leading shaft will overload.

[!NOTE] Winter nitrification. Fixed-film nitrification is temperature sensitive. As wastewater cools, nitrifier activity drops sharply, and a trickling filter or RBC that nitrifies well in August may not in January. Plants with year-round ammonia limits plan for that seasonal loss rather than being surprised by it.