7.4 Trickling Filters & Rotating Biological Contactors

7.4 Trickling Filters & Rotating Biological Contactors

Quick Answer: Trickling filters and RBCs are attached-growth (fixed-film) aerobic processes: biofilm on media oxidizes organics as wastewater passes by. Operators manage recirculation, staging, dissolved oxygen, and loading, and troubleshoot ponding and filter flies. Under TCEQ rules, a Class D wastewater license is not renewable for operators of trickling filter or RBC facilities with permitted average daily flow of 100,000 gpd or greater (and is not renewable for any activated-sludge facility).

Attached-growth processes grow microorganisms as a biofilm on solid media. Wastewater contacts the slime layer; microbes consume soluble BOD (and, under the right conditions, ammonia). Excess biofilm periodically sloughs and is removed in a downstream secondary clarifier. Trickling filters (TFs) and rotating biological contactors (RBCs) are the two classic fixed-film systems on Texas exams.

Trickling filter media, recirculation, and staging

A trickling filter distributes wastewater over a bed of media—rock in older filters, plastic modular media in many modern units. Flow trickles downward while air moves through void spaces, supplying oxygen. A rotary distributor or fixed nozzles spread the hydraulic load.

Recirculation returns a portion of filter effluent (or clarifier underflow/effluent, depending on design) back to the filter influent. Benefits include:

• Diluting strong influent BOD
• Keeping media wet during low flow
• Improving distribution and flushing of solids
• Stabilizing performance

Recirculation ratio (recycled flow ÷ influent flow) is a standard control and exam variable. Too little recirculation can leave dry spots and odor; excessive recirculation can overload hydraulics without adding treatment capacity in a useful way.

Staging means two or more filters in series (or parallel trains operated as stages). A roughing stage knocks down high BOD; a following stage polishes. Staging helps strong wastes and can support better nitrification in later stages when organic competition is lower.

Biofilm, ponding, and filter flies

Healthy biofilm is thin to moderately thick, aerobic at the surface, and actively treating. As the film thickens, the inner layer can go anaerobic, weaken attachment, and slough—normally a controlled, periodic event. Continuous thick, slimy buildup plus poor drainage points to trouble.

Ponding is wastewater standing on the media surface instead of percolating. Causes include media plugged with excess biomass, debris, or snail shells; distributor failure creating dead zones elsewhere and overload in spots; or collapsed underdrains. Ponding cuts oxygen transfer, creates odors, and tanks treatment. Responses include increasing recirculation flushing, checking distributors, media evaluation/cleaning, and verifying preliminary treatment is not sending rags and grit into the filter.

Filter flies (Psychoda and related moth flies) breed in the moist biofilm environment. They are a nuisance around the filter and nearby neighborhoods. Control strategies taught on exams include maintaining proper wetting (recirculation), periodic filter flooding where the design allows, improving housekeeping, and addressing stagnant edge areas—not random pesticide application as the first conceptual answer.

Odors from trickling filters often mean septic influent, inadequate ventilation, ponding, or a filter that is too heavily loaded for the air supply.

Rotating biological contactors

An RBC uses closely spaced circular media disks mounted on a horizontal shaft. Roughly 40% of the disk diameter is typically submerged. The shaft rotates so biofilm alternately contacts wastewater (substrate) and air (oxygen). That rotation is the aeration mechanism—no diffuser grid is required for the contactor itself.

Key RBC watch points:

• Shaft and bearing integrity — a failed shaft is a major outage.
• Media condition — broken or uneven media affects balance and treatment.
• Rotation speed / drive — too slow or stopped disks go septic; biomass weight can increase dramatically.
• DO and staging — RBC plants are often staged in series tanks; early stages see higher BOD, later stages may nitrify if load and oxygen allow.
• Secondary clarification — still required for sloughed solids.

High organic loading in the first stage can produce thick white or gray biomass and low DO in the bulk liquid. Operators may redistribute load, add stages in parallel, or improve upstream primary performance. Unlike activated sludge, you do not control an MLSS wasting rate in the same way; you control loading, staging, rotation, and clarification of sloughings.

Loading, DO, and process expectations

Both TFs and RBCs are aerobic fixed-film systems. Dissolved oxygen in the bulk liquid or free drainage/ventilation must support the biofilm. Organic loading is often expressed as pounds of BOD per day per 1,000 ft³ of media (or similar media-specific units). Hydraulic loading (gpd/ft²) matters for trickling filters because it affects wetting and contact time.

Fixed-film systems can handle shock loads differently than suspended-growth activated sludge, but they are not immune to grease blankets, toxic dumps, or freezing distributor arms. Primary clarification ahead of TFs/RBCs is common because it cuts solids and grease that foul media.

TCEQ Class D renewability note (critical for Texas)

TCEQ continuing-education and renewal rules state that Class D wastewater licenses are not renewable for operators of:

1. Any activated sludge–type facility (any size), and
2. Any trickling filter or RBC facility with a permitted average daily flow of 100,000 gallons per day or greater, and
3. Certain subsurface drip dispersal effluent disposal facilities (separate rule item).

So for TF/RBC plants at or above 100,000 gpd permitted average daily flow, a Class D license will not renew—operators need a higher class. Remember the asymmetry taught on exams: activated sludge blocks Class D renewal at any size; TF/RBC blockage is tied to the ≥100,000 gpd threshold. Provisional Class D licenses have their own non-renewable nature; do not confuse provisional rules with the process/flow renewal limits above.

Exam synthesis

When a question shows a rock or plastic tower with a rotating distributor, think trickling filter, recirculation, ponding, and filter flies. When it shows disks on a shaft, think RBC, alternate air/wastewater exposure, and mechanical shaft risk. When it asks about Texas Class D renewal at a 120,000 gpd trickling filter plant, the license is not renewable at Class D—upgrade path required. Tie biofilm health to loading and oxygen, and always send sloughed solids to a clarifier in your mental flow diagram.