8.3 Fixed-Film Technologies & Treatment Ponds

8.3 Fixed-Film Technologies & Treatment Ponds

[!NOTE] Attached Growth vs. Suspended Growth: In contrast to activated sludge systems—where biomass is maintained in continuous liquid suspension through mechanical agitation or diffused aeration—fixed-film (attached-growth) biological processes immobilize the microbial population upon solid, stationary, or rotating supporting media. Wastewater trickles or cascades over the media, bringing dissolved organic pollutants and nutrients into direct contact with a complex biological slime layer known as the zoogleal film. In natural pond and lagoon systems, wastewater treatment relies on long hydraulic retention times and passive surface interactions governed by an intricate metabolic symbiosis between aerobic bacteria and microscopic green algae.

Fixed-film technologies offer unique operational advantages for municipal and industrial wastewater facilities across Pennsylvania. Because biomass is anchored securely to solid media rather than suspended in a liquid tank, attached-growth reactors cannot experience sudden clarifier washouts during peak wet-weather storm events. Furthermore, fixed-film systems operate with significantly lower electrical power requirements than diffused-air activated sludge plants, require less complex mechanical equipment, and exhibit exceptional resilience against toxic industrial shock loads. However, fixed-film operations present distinct operational hurdles, including media clogging and ponding, psychoda fly infestations, mechanical drive fatigue on rotating shafts, and temperature-dependent efficiency drops during severe Pennsylvania winter conditions.

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Fixed-Film Fundamentals & Trickling Filter Systems

A trickling filter consists of a circular or rectangular bed of coarse, porous media through which pre-settled primary wastewater is distributed continuously or intermittently. Rotary distributor arms, propelled by hydraulic reaction thrust or electric drive motors, sweep over the top of the bed, spraying wastewater through calibrated orifices.

+----------------------------------------------------------------------------------+
|                       Trickling Filter Cross-Section                             |
+----------------------------------------------------------------------------------+
|                                                                                  |
|                     Rotary Distributor Arm (Propelled by Flow)                   |
|                    ====[   Spray Nozzles   ]====                                 |
|                          |     |     |     |                                     |
|                          v     v     v     v                                     |
|    +------------------------------------------------------------------------+    |
|    |  Biological Media Bed (Rock: 3-8 ft deep / Plastic: 15-30 ft deep)     |    |
|    |                                                                        |    |
|    |  [ Biofilm (Zoogleal Slime): Microorganisms consume dissolved BOD ]    |    |
|    |  [ Air Draft: Natural ventilation through open underdrains ]           |    |
|    +------------------------------------------------------------------------+    |
|    |                     Vitrified Clay Underdrain Blocks                   |    |
|    +------------------------------------------------------------------------+    |
|         |                                                               |        |
|         v                                                               v        |
|    Effluent Channel (To Secondary Clarifier)                     Ventilation Louver
+----------------------------------------------------------------------------------+

Biofilm Kinetics & Diffusion Gradients

As wastewater flows over the media in a thin liquid film (typically 0.1 to 0.2 mm thick), dissolved organic substrate ($CBOD$), nutrients, and dissolved oxygen diffuse through the liquid film into the active biological slime layer:

1. Aerobic Outer Zone (Depth: 0.1 to 0.2 mm): Oxygen penetrates only into the outermost layer of the biofilm. In this thin zone, aerobic heterotrophic bacteria rapidly oxidize dissolved organics into carbon dioxide, water, and new cellular mass. Autotrophic nitrifiers colonize the deeper passes of the bed where carbonaceous BOD has been depleted.
2. Anaerobic Inner Zone (Depth: > 0.2 mm to media surface): Beyond the 0.2 mm dissolved oxygen penetration limit, the biological layer is completely anaerobic. Facultative and anaerobic bacteria decompose organic matter by fermentation, producing volatile fatty acids, hydrogen sulfide ($H_2S$), and organic acids.

Traditional Rock Media vs. Modern Synthetic Plastic Media

Recirculation Dynamics & Wetting Rates

Recirculation involves returning a portion of trickling filter effluent (or secondary clarifier underflow) back to the filter distributor arm. The Recirculation Ratio ($R$) is defined as: $R = \frac{\text{Recirculated Flow Rate } (Q_r)}{\text{Plant Forward Influent Flow } (Q)}$ Recirculation fulfills four vital operational functions:

1. Maintains Minimum Hydraulic Wetting Rate: During diurnal low-flow periods (e.g., 2:00 AM to 6:00 AM), incoming wastewater drops significantly. Without recirculation, the distributor arms would stall, portions of the media would dry out, and the biological slime layer would desiccate and die. Target minimum wetting rates are typically 0.1 to 0.2 gpm per square foot for rock media and 0.2 to 0.6 gpm/sq ft for plastic media.
2. Dilutes High-Strength & Toxic Surges: Recycling treated effluent dilutes incoming raw BOD and buffers industrial pH spikes or toxic discharges.
3. Enhances Overall BOD Removal: Passing wastewater over the active biomass multiple times increases cumulative contact opportunity.
4. Flushes Excess Biomass: Provides continuous hydraulic shearing to dislodge aging biofilm, preventing media ponding.

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Biofilm Sloughing & Psychoda Fly Control

Biofilm Sloughing Dynamics

As microorganisms reproduce, the biofilm continuously thickens. Eventually, the film thickness exceeds the diffusion depth of nutrients and oxygen. Bacteria adhering directly to the media surface starve, enter endogenous decay, and lyse. Deprived of biological adhesion, the entire mat loses structural bonding to the media. The hydrodynamic shear of trickling wastewater strips the loosened biomass away from the media surface—a natural phenomenon called sloughing.

• Secondary Clarification Requirement: Unlike activated sludge (where clarified water is the primary goal), the liquid exiting a trickling filter contains high concentrations of sheared biological solids. A dedicated secondary clarifier (humus tank) is mandatory downstream to separate sloughed solids before effluent discharge.
• Seasonal Spring Sloughing: In northern climates like Pennsylvania, low winter water temperatures slow microbial metabolism, allowing thick, dense biomass to accumulate on the media without sloughing. When spring temperatures rise, microbial activity accelerates rapidly; endogenous decay at the media surface triggers massive, plant-wide sloughing ("spring turnover"), creating severe solids loads on secondary clarifiers for several weeks.

Filter Fly (Psychoda alternata) Ecology & Abatement

Trickling filter flies (moth flies, Psychoda alternata) are small (1 to 5 mm), hairy, moth-like insects that breed prolifically in the damp, decaying organic slime of trickling filters and open channels. While they do not bite, dense swarms can travel into surrounding communities, creating severe public health nuisances, entering residential homes, and causing respiratory irritation to facility personnel.

+----------------------------------------------------------------------------------+
|                       Psychoda Fly Control Strategies                            |
+----------------------------------------------------------------------------------+
|                                                                                  |
|  1. Filter Bed Flooding:                                                         |
|     - Close effluent sluice gate; fill filter bed with water for 24 hours        |
|     - Drowns larvae and pupae within the media void spaces                       |
|     - Disadvantage: Removes filter from service; potential structural stress    |
|                                                                                  |
|  2. High Hydraulic Dosing Rate:                                                  |
|     - Increase recirculation; slow distributor arm to increase flush per pass    |
|     - Hydraulically washes eggs and unhatched larvae out into underdrains        |
|                                                                                  |
|  3. Low-Dose Chemical Chlorination:                                              |
|     - Dose 1.0 to 2.0 mg/L free chlorine into filter influent for 4-8 hours      |
|     - Kills larvae in upper 1-2 feet of media without stripping deep biofilm    |
|                                                                                  |
|  4. Biological Larvicide Application:                                            |
|     - Apply Bacillus thuringiensis israelensis (BTI) bacterial toxins            |
|     - Selectively kills fly larvae with zero disruption to treatment biofilm    |
+----------------------------------------------------------------------------------+

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Rotating Biological Contactors (RBCs)

A Rotating Biological Contactor (RBC) consists of large-diameter (typically 10 to 12 feet), closely spaced, corrugated high-density polyethylene (HDPE) discs mounted horizontally on a heavy steel shaft up to 25 to 27 feet long. The plastic assemblies provide roughly 100,000 to 150,000 square feet of surface area per shaft. The discs are submerged approximately 40% of their diameter in a contoured concrete trough containing wastewater, rotated slowly at 1.0 to 2.0 revolutions per minute (rpm) by mechanical electric gear drives or supplemental air-cup drives.

+----------------------------------------------------------------------------------+
|                       RBC Mechanical Shaft & Disc Assembly                       |
+----------------------------------------------------------------------------------+
|                                                                                  |
|         Atmospheric Exposure (60%): Biofilm absorbs O2 directly from air         |
|                                      .----.                                      |
|                                   .-'      '-.                                   |
|                                 .'            '.                                 |
|     ===========================[  Central Steel ]===========================     |
|     [ Motor / Gear Drive ]=====[  Drive Shaft   ]=====[ Outboard Bearing ]       |
|                                 '.            .'                                 |
|                                   '-.      .-'                                   |
|         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~'----'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~         |
|         Wastewater Submergence (40%): Biofilm adsorbs dissolved BOD              |
|         +--------------------------------------------------------------+         |
|         |                Semi-Circular Concrete Trough                 |         |
|         +--------------------------------------------------------------+         |
+----------------------------------------------------------------------------------+

Operational Principles & Staging Dynamics

As the shaft rotates, the attached biofilm is alternately exposed to ambient air and submerged in wastewater:

• During atmospheric exposure, atmospheric oxygen diffuses into the thin liquid film adhering to the biomass.
• During submergence, microorganisms absorb dissolved organic pollutants from the wastewater.
• Shearing forces generated by rotation through the liquid bath continuously slough excess biomass, keeping the biofilm active.

RBC trains are partitioned by internal baffles into three or four separate stages in series:

• Stage 1 (Carbonaceous Oxidation): Receives raw primary effluent with maximum BOD loading. Characterized by a thick, shaggy, greyish-brown heterotrophic biofilm (2.0 to 3.0+ mm). Oxygen uptake is extraordinarily high.
• Intermediate Stages (Polishing): Soluble BOD drops below 20 to 30 mg/L; biofilm becomes distinctly thinner (0.5 to 1.0 mm).
• Final Stages (Nitrification): Carbonaceous organics are virtually exhausted ($CBOD_5 < 15 \text{ mg/L}$). In this low-organic environment, autotrophic nitrifiers outcompete heterotrophs, establishing a thin, tightly adhering, golden-brown or bronze biofilm that oxidizes ammonia to nitrate.

Mechanical Loading Challenges & Structural Failures

RBCs have a documented history of severe mechanical vulnerabilities that require vigilant operator surveillance:

1. First-Stage Organic Overloading: If first-stage loading exceeds 4.0 to 6.0 lb BOD/day per 1,000 sq ft, heterotrophs overgrow, creating an excessively thick biomass blanket (exceeding 3 to 4 mm). The physical weight of this biomass adds thousands of pounds of dead load onto the central shaft.
2. Mechanical Shaft Torque & Fatigue Shear: Excessive biological weight combined with continuous cyclic rotational bending induces metal fatigue, frequently resulting in catastrophic shearing of the central steel shaft.
3. Shaft Loping (Eccentric Rotation): If an RBC is stopped for maintenance or power failure with biomass submerged, the exposed upper biomass dries out while submerged biomass remains wet and continues growing. When restarted, the uneven mass creates a dynamic rotational imbalance ("loping"), destroying bearings and drive gears.
4. Operator Monitoring: Modern facilities utilize mechanical strain gauges, load cells mounted under shaft bearing pillow blocks, or motor amperage sensors to continuously track total shaft weight. If shaft weight exceeds manufacturer limits, operators must step-feed influent to later stages, install supplemental aeration headers beneath the discs to air-shear excess biomass, or provide chemical sloughing.

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Hybrid Technologies: MBBR & IFAS Systems

To increase biological treatment capacity without constructing new concrete tanks, modern environmental engineering utilizes hybrid fixed-film/suspended-growth processes incorporating engineered high-surface-area plastic carriers (e.g., Kaldnes K1, K3, or chip media, offering 500 to 1,200 sq m per cu m of media volume).

+----------------------------------------------------------------------------------+
|                  Hybrid Biofilm Processes: MBBR vs. IFAS                         |
+----------------------------------------------------------------------------------+
|                                                                                  |
|  MBBR (Moving Bed Biofilm Reactor):                                              |
|  - Pure Fixed-Film Process                                                       |
|  - Plastic carriers suspended throughout basin by aeration diffusers or mixers  |
|  - Wedge-wire retention sieves prevent carriers from leaving basin               |
|  - ZERO Return Activated Sludge (NO RAS line!)                                   |
|  - Secondary clarifier used ONLY for separating sloughed solids                  |
|                                                                                  |
|  IFAS (Integrated Fixed-Film Activated Sludge):                                  |
|  - Hybrid Process (Fixed-Film + Suspended Growth)                                |
|  - Plastic carriers suspended in conventional aeration basin                     |
|  - Active Return Activated Sludge (RAS) recycled continuously                    |
|  - Dual biology: MLSS treats bulk BOD; Carrier biofilm fosters slow nitrifiers   |
|  - Expands nitrification capacity in cold weather without expanding tank volume  |
+----------------------------------------------------------------------------------+

• Retention Sieves: In both MBBR and IFAS basins, heavy-duty stainless steel wedge-wire screens or perforated sieves are installed across the effluent discharge channels. These screens retain the buoyant plastic carriers within the reaction basin while allowing wastewater mixed liquor and sloughed solids to pass freely.

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Waste Stabilization Ponds & Lagoon Ecology

Waste stabilization ponds (lagoons) represent the simplest, most cost-effective secondary wastewater treatment technology for small, rural Pennsylvania communities with available land area. Relying on long hydraulic retention times (typically 30 to 120+ days) and natural biological degradation, lagoons operate without mechanical compressors or chemical coagulants.

1. Pond Classifications & Zonation

• Facultative Ponds (Depth: 3 to 6 feet): The most prevalent municipal lagoon design. The water column naturally stratifies into three distinct biological zones:
  1. Aerobic Surface Zone (Upper 1 to 2 ft): Sunlight penetration supports vigorous algal photosynthesis, supplying abundant dissolved oxygen to aerobic heterotrophs.
  2. Facultative Intermediate Zone (Middle 1 to 3 ft): Dissolved oxygen fluctuates diurnally. Facultative bacteria operate aerobically during daylight when DO is present and shift to anaerobic fermentation pathways at night when DO is exhausted.
  3. Anaerobic Bottom Benthic Zone (Bottom sludge layer): Devoid of oxygen. Heavy sewage solids settle to the benthic floor, where anaerobic acid formers and methanogens decompose organic sludge into methane ($CH_4$), carbon dioxide ($CO_2$), and water.
• Aerated Lagoons (Depth: 6 to 15 feet): Ponds equipped with floating mechanical surface aerators or submerged bottom diffusers. Oxygen is supplied mechanically rather than through algal photosynthesis, allowing higher organic loading rates and smaller land footprints.
• Polishing (Maturation) Ponds (Depth: 3 to 4 feet): Tertiary shallow basins positioned downstream of secondary treatment. Their primary objectives are final suspended solids settling, nutrient polishing, and pathogen destruction achieved through direct solar ultraviolet (UV) penetration.

2. Algae-Bacteria Symbiosis & Diurnal Swings

The biological engine of a facultative wastewater lagoon is a mutually beneficial, cyclical partnership between aerobic heterotrophic bacteria and single-celled green algae (Chlorella, Scenedesmus, Euglena):

$\text{Organic Pollutants (BOD)} + \text{O}_2 \xrightarrow{\text{Aerobic Bacteria}} \text{CO}_2 + \text{H}_2\text{O} + \text{NH}_4^+ + \text{PO}_4^{3-}$
$\text{CO}_2 + \text{H}_2\text{O} + \text{Inorganic Nutrients} + \text{Sunlight} \xrightarrow{\text{Green Algae (Photosynthesis)}} \text{New Algal Biomass} + \text{O}_2$

+----------------------------------------------------------------------------------+
|                  Facultative Lagoon Algae-Bacteria Symbiosis                     |
+----------------------------------------------------------------------------------+
|                                                                                  |
|               +----------------------- Sunlight -----------------------+         |
|               |                                                        |         |
|               v                                                        |         |
|       [  Green Algae  ] ======= Generates Dissolved Oxygen (O2) =====> |         |
|               ^                                                        v         |
|               |                                                [ Aerobic Bacteria ]
|               |                                                        |         |
|               +====== Generates Carbon Dioxide (CO2) & Nutrients ======+         |
|                                                                                  |
+----------------------------------------------------------------------------------+

Diurnal Water Quality Swings

This biological interaction causes extreme, predictable diurnal (24-hour) swings in lagoon dissolved oxygen and pH:

• Mid-Day / Early Afternoon (Peak Sunlight): Photosynthesis operates at maximum velocity, producing oxygen far faster than bacteria can consume it. Dissolved oxygen supersaturates, frequently reaching 15 to 25+ mg/L (200% to 300% saturation). Simultaneously, algae consume massive quantities of dissolved carbon dioxide ($CO_2$) and bicarbonate. The removal of carbonic acid shifts the carbonate equilibrium, driving pH upwards to 8.5 to 9.5+.
• Nighttime / Pre-Dawn (Darkness): Photosynthesis ceases completely due to lack of solar radiation. However, both algae and bacteria continue consuming oxygen through endogenous respiration. Dissolved oxygen steadily plummets, reaching diurnal minimums (often 1.0 to 3.0 mg/L, or near zero in overloaded ponds) immediately before dawn. Cellular respiration continuously releases carbon dioxide ($CO_2$), forming carbonic acid and depressing the pH back to near-neutral levels (7.0 to 7.4).

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Comparison of Fixed-Film & Lagoon Treatment Technologies