9.4 Fixed-Film Processes & Waste Stabilization Ponds

9.4 Fixed-Film Processes & Waste Stabilization Ponds

While suspended-growth activated sludge systems rely on microorganisms suspended in liquid mixed liquor, attached-growth (fixed-film) systems cultivate biological communities attached to fixed, inert media surfaces. Wastewater flows in thin sheets over this biological film, allowing dissolved organics and oxygen to diffuse into the biomass.

In addition to fixed-film bioreactors, waste stabilization ponds (lagoons) provide cost-effective, low-maintenance biological wastewater treatment for hundreds of small communities and rural sewer districts throughout Missouri. Certified operators must understand the design, hydraulics, biological mechanisms, and operational troubleshooting for trickling filters, Rotating Biological Contactors (RBCs), and municipal wastewater lagoons.

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Principles of Attached-Growth (Fixed-Film) Systems

In fixed-film processes, bacteria, fungi, protozoa, and rotifers form an active biological slime layer—termed a zoogleal film ($0.1 - 2.0\text{ mm}$ thick)—on the media surface.

  Wastewater Flow (Thin Liquid Film with Dissolved Organics & DO)
  ─────────────────────────────────────────────────────────────────────────────►
  ┌───────────────────────────────────────────────────────────────────────────┐
  │ Aerobic Biofilm Zone (Heterotrophic BOD Oxidation & Nitrification)        │  ◄── DO Diffuses In
  ├───────────────────────────────────────────────────────────────────────────┤
  │ Anaerobic Biofilm Zone (Endogenous Decay & Acid Fermentation)             │  ◄── Oxygen Depleted
  └───────────────────────────────────────────────────────────────────────────┘
  ═════════════════════════════════════════════════════════════════════════════
                         SOLID INERT MEDIA SURFACE

The Biological Sloughing Mechanism

1. Biofilm Growth: As heterotrophs absorb dissolved organic matter and oxygen from passing wastewater, the outer biofilm layer flourishes aerobically.
2. Anaerobic Starvation: As the biofilm thickens beyond $0.2 - 0.5\text{ mm}$, dissolved oxygen cannot penetrate through to the inner media-attached layer. Microorganisms at the media interface enter anaerobic endogenous decay, losing cellular adherence.
3. Sloughing: Hydraulic shear from passing wastewater washes the loosened biomass off the media (sloughing). Sloughed solids pass out of the filter into a secondary clarifier (historically called a humus tank) for gravity settling.

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Trickling Filters: Media, Hydraulics & Operation

A trickling filter consists of a circular bed of coarse media over which settled wastewater is distributed continuously by a motorized or hydraulically driven rotary distributor arm.

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|                                TRICKLING FILTER MEDIA TYPES COMPARISON                                  |
+---------------------------------------------------------------------------------------------------------+
| 1. CRUSHED ROCK / STONE MEDIA:                                                                          |
|    - Material: Washed granite, limestone, or slag (diameter: 2.5 to 4.0 inches).                        |
|    - Bed Depth: Shallow beds, typically 6 to 8 feet deep (limited by rock weight).                      |
|    - Specific Surface Area: Low (15 to 20 sq ft per cu ft of media).                                    |
|    - Void Ratio: Low (40% to 50% void space).                                                           |
|    - Limitations: Heavy structural load; prone to air blockage, pooling, and media ponding.             |
+---------------------------------------------------------------------------------------------------------+
| 2. SYNTHETIC STRUCTURED / CROSS-FLOW PLASTIC MEDIA:                                                     |
|    - Material: Corrugated PVC or polypropylene modules stacked in cross-flow sheets.                    |
|    - Bed Depth: Deep towers, typically 15 to 30+ feet deep.                                             |
|    - Specific Surface Area: High (30 to 45 sq ft per cu ft of media).                                   |
|    - Void Ratio: Extremely high (90% to 95% open void space).                                           |
|    - Advantages: Lightweight; superior natural draft ventilation; handles 3x - 5x higher organic loading|
|      without plugging or ponding; enhanced nitrification.                                               |
+---------------------------------------------------------------------------------------------------------+

Ventilation & Underdrain Systems

Trickling filters rely on natural draft ventilation through vitrified clay or precast concrete underdrain blocks at the floor of the filter. Air moves through the media bed due to temperature differentials between ambient air and wastewater (chimney effect):

• Wastewater warmer than ambient air: Air flows upward through the filter bed.
• Wastewater cooler than ambient air: Air flows downward through the filter bed.
• If the temperature differential is $< 3^\circ - 5^\circ\text{F}$, air movement stalls, requiring forced-air draft blowers.

Recirculation Ratio ($R/Q$)

Recirculating secondary clarifier effluent or filter underflow back to the filter influent distributor is a core operational strategy:

• Dilution: Dilutes high-strength organic shock loads and toxic influent spikes.
• Wetting Rate: Maintains a minimum hydraulic wetting rate ($0.15 - 0.25\text{ gpm/sq ft}$) during low night-time flows to keep biofilm moist and active.
• Hydraulic Shearing: Flushes excess biological growth to prevent media clogging and ponding.

$\text{Recirculation Ratio } (R) = \frac{\text{Recirculated Flow Rate } (Q_r)}{\text{Raw Influent Flow Rate } (Q)}$

Trickling Filter Operational Troubleshooting

+-----------------------------------------------------------------------------------------+
|                     TRICKLING FILTER PROBLEM DIAGNOSIS & REMEDIES                       |
+-----------------------------------------------------------------------------------------+
| 1. Filter Media Ponding (Surface Pooling):                                              |
|    - Cause: Excessive zoogleal slime growth, accumulation of snail shells, or media     |
|      breakdown clogging surface void spaces.                                            |
|    - Remedies:                                                                          |
|      a. High-pressure water jetting of the media surface.                               |
|      b. Flood the filter bed for 24 hours to promote biomass sloughing.                 |
|      c. Dose chlorine (5 - 10 mg/L free chlorine) to influent for 2 - 4 hours.          |
|      d. Increase the recirculation rate to increase hydraulic flushing velocity.         |
+-----------------------------------------------------------------------------------------+
| 2. Psychoda Filter Fly Infestations:                                                    |
|    - Cause: Proliferation of tiny Psychoda moth flies breeding in moist, decaying       |
|      biofilm in dry or poorly wetted zones.                                             |
|    - Remedies:                                                                          |
|      a. Flood the filter for 24 hours to drown larvae and pupae.                        |
|      b. Increase the hydraulic wetting rate to wash larvae into underdrains.             |
|      c. Apply periodic low-dose chlorination (1 - 2 mg/L) or biological larvicide.       |
+-----------------------------------------------------------------------------------------+
| 3. Severe Odor Emissions:                                                               |
|    - Cause: Anaerobic conditions within the media caused by clogged underdrains,        |
|      inadequate natural ventilation, or septic influent wastewater.                    |
|    - Remedies: Clear blocked underdrain openings; flush drainage floor; pre-aerate      |
|      influent; increase recirculation flow.                                             |
+-----------------------------------------------------------------------------------------+

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Rotating Biological Contactors (RBCs)

A Rotating Biological Contactor (RBC) consists of a series of closely spaced, large-diameter ($10 - 12\text{ ft}$) corrugated high-density polyethylene (HDPE) discs mounted horizontally on a rotating steel shaft ($25 - 27\text{ ft}$ long).

Operational Parameters & Staging Mechanics

• Submergence: Discs are approximately $40%$ submerged in a contoured concrete trough.
• Rotational Velocity: The shaft rotates slowly at $1.0 - 1.6\text{ rpm}$ (peripheral speed $\approx 60\text{ ft/min}$), driven by mechanical gear drives or air cups.
• Aeration & Shearing: As the media rotates out of wastewater into the air, it lifts a thin film of water that absorbs atmospheric oxygen. Rotation provides gentle hydraulic shear to slough excess biomass and keeps wastewater mixed in the trough.
• Staging: RBC systems are divided into 3 to 4 sequential stages in series separated by baffles:
  • Stage 1: Receives high organic loading; dominated by thick, grey/white heterotrophic biofilm removing soluble carbonaceous $\text{BOD}_5$.
  • Intermediate Stages: Polishes remaining carbonaceous BOD ($< 15 - 20\text{ mg/L}$). Biofilm turns brown and thins.
  • Final Stages: Autotrophic nitrifiers (Nitrosomonas, Nitrobacter) establish a thin, reddish-brown/gold biofilm, achieving advanced ammonia nitrification.

RBC Maintenance & Mechanical Failure Prevention

• First-Stage Overloading: Excessive organic loading ($> 4 - 6\text{ lb BOD}/1,000\text{ sq ft}\cdot\text{day}$) on Stage 1 promotes massive filamentous growth (Beggiatoa, Sphaerotilus), adding thousands of pounds of dead biomass weight.
• Shaft Deflection & "Loping": Excessive biomass weight leads to structural shaft deflection, bearing failure, and loping (unbalanced, jerky rotation where the heaviest section swings down). Operators must monitor drive motor amperage, install load cells under bearings, and use supplemental air drives to assist rotation.

Waste Stabilization Ponds (Lagoons) in Missouri

Waste stabilization ponds (lagoons) are engineered earthen basins designed to treat municipal wastewater through natural physical, chemical, and biological interactions. Across Missouri, small communities rely extensively on lagoons due to their low capital cost, minimal power requirements, and ability to absorb seasonal hydraulic surges.

Classifications of Wastewater Lagoons

1. Facultative Lagoons (Most Common in Missouri):
   • Operating depth: $3 - 6\text{ feet}$ ($0.9 - 1.8\text{ m}$).
   • Features three stratified biological zones working concurrently:
     • Aerobic Upper Zone (Top $1 - 2\text{ ft}$): Powered by the mutual symbiosis between algae and aerobic bacteria.
     • Facultative Middle Zone: Contains facultative microorganisms that operate aerobically during daylight (when DO is present) and switch to anaerobic fermentation at night.
     • Anaerobic Bottom Zone: Settled solids decompose anaerobically via acid-forming bacteria and methane-producing archaea.
2. Aerated Lagoons:
   • Operating depth: $8 - 15\text{ feet}$.
   • Supplemental oxygen is supplied by floating surface aerators or submerged diffused aeration tubing. Capable of handling $5\times - 10\times$ higher BOD loading per acre than natural facultative ponds.
3. Anaerobic Lagoons:
   • Operating depth: $8 - 20\text{ feet}$.
   • Un-aerated, heavily loaded primary basins used for high-strength industrial/agricultural wastes. A thick floating grease/scum blanket forms on top, sealing oxygen out and trapping odors.
4. Hydrograph Controlled Release (HCR) Lagoons:
   • Designed with massive storage capacity to hold wastewater during low-flow summer stream conditions, discharging only when receiving river flows are sufficiently high to provide required dilution without violating Missouri Department of Natural Resources (MoDNR) water quality standards.

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The Algae-Bacteria Symbiotic Cycle

The biological engine of a facultative lagoon is the reciprocal symbiotic relationship between photosynthetic green algae (Chlorella, Scenedesmus, Euglena) and aerobic heterotrophic bacteria:

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|                         ALGAE - BACTERIA SYMBIOTIC RELATIONSHIP                         |
+-----------------------------------------------------------------------------------------+
| 1. Photosynthetic Algae:                                                                |
|    - Use solar energy, water, bacterial carbon dioxide (CO2), and inorganic nutrients    |
|      (ammonia, phosphate) to synthesize new algal cells and release DISSOLVED OXYGEN (O2):|
|      6 CO2 + 6 H2O + Sunlight  ──►  C6H12O6 + 6 O2↑                                     |
+-----------------------------------------------------------------------------------------+
| 2. Aerobic Heterotrophic Bacteria:                                                      |
|    - Utilize the dissolved oxygen produced by algae to biochemically oxidize organic    |
|      matter (BOD), releasing CARBON DIOXIDE (CO2), water, and nutrients:                |
|      Organics + 6 O2  ──►  6 CO2↑ + 6 H2O + Nutrients (NH4+, PO4(3-))                   |
+-----------------------------------------------------------------------------------------+

Diurnal (Day / Night) Fluctuations in Lagoons

• Daytime (Peak Sunlight): Algal photosynthesis drastically outpaces respiration. Dissolved oxygen rises rapidly, often reaching supersaturation ($> 15 - 25\text{ mg/L}$). Algae consume dissolved carbon dioxide, shifting the carbonate equilibrium and driving pH upward to $\text{pH } 8.5 - 10.0$.
• Nighttime (Darkness): Photosynthesis ceases entirely. Both algae and bacteria consume dissolved oxygen through respiration, causing DO to drop to its diurnal minimum (often $< 2.0\text{ mg/L}$ just before sunrise) and releasing $\text{CO}_2$, which lowers pH.

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Missouri Lagoon Operational Challenges & Management

1. Spring Overturn & Odor Control

During winter, ice cover prevents surface aeration and cold temperatures retard algal growth, causing lagoons to turn anaerobic. In early spring, as surface water warms to $4^\circ\text{C}$ ($39.2^\circ\text{F}$, maximum density of water), thermal stratification breaks down (spring overturn). Wind action mixes anaerobic bottom waters containing hydrogen sulfide ($\text{H}_2\text{S}$) to the surface, generating severe rotten-egg odors.

• Corrective Action: Dose sodium nitrate ($\text{NaNO}_3$) across the lagoon surface ($100 - 200\text{ lbs/acre}\cdot\text{day}$). Nitrate provides an alternate electron acceptor, preventing sulfate reduction to sulfide while algae re-establish.

2. Blue-Green Algae (Cyanobacteria) Blooms

During calm, hot summer weather with high phosphorus levels, noxious blue-green algae (Microcystis, Anabaena) form thick, foul-smelling, paint-like scum blankets on the surface. Blue-green algae block sunlight, produce toxins, and elevate effluent Total Suspended Solids (TSS).

• Corrective Action: Apply copper sulfate ($\text{CuSO}_4\cdot 5\text{H}_2\text{O}$) at low concentrations ($0.5 - 1.0\text{ mg/L}$) targeted directly to the surface scum layer on sunny mornings.