6.5 Pond & Lagoon Operations, Trickling Filters & Rotating Biological Contactors (RBCs)

Pond & Lagoon Operations, Trickling Filters & Rotating Biological Contactors (RBCs)

Many small communities, rural special sanitation districts, and industrial facilities throughout Colorado rely on wastewater lagoons and attached-growth (fixed-film) biological processes. Unlike suspended-growth activated sludge systems that require high electrical power inputs and continuous mechanical aeration, lagoons and fixed-film systems offer robust process stability, lower mechanical complexity, and high resistance to hydraulic and organic shock loads.

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1. Wastewater Lagoon Systems: Ecology & Operations

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|                        FACULTATIVE LAGOON STRATIFIED ZONES                              |
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| 1. AEROBIC SURFACE ZONE (Upper 1.5 - 3.0 ft)                                            |
|    • Algae (Chlorella, Scenedesmus) + Aerobic Bacteria in Symbiosis                     |
|    • High sunlight, dissolved oxygen generation, elevated daytime pH (8.5 - 10.0)       |
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
| 2. FACULTATIVE INTERMEDIATE ZONE (Middle 1.5 - 3.0 ft)                                  |
|    • Facultative bacteria decomposing soluble organics with or without dissolved oxygen |
+=========================================================================================+
| 3. ANAEROBIC BOTTOM BENTHIC ZONE (Bottom 1.0 - 2.0 ft)                                  |
|    • Anaerobic acid formers + methanogens decomposing settled sludge solids             |
|    • Dark, DO = 0.0 mg/L, generates CH4, CO2, organic acids, H2S                        |
+-----------------------------------------------------------------------------------------+

The Algal-Bacterial Symbiosis in Facultative Ponds

Facultative ponds operate through a delicate biological partnership between aerobic heterotrophic bacteria and microscopic green algae (Chlorella, Scenedesmus, Euglena):

1. Bacterial Decomposition: Aerobic bacteria break down complex organic carbonaceous BOD, consuming dissolved oxygen and releasing carbon dioxide ($CO_2$), water, ammonium ($NH_4^+$), and orthophosphate ($PO_4^{3-}$).
2. Algal Photosynthesis: Green algae absorb sunlight, $CO_2$, and inorganic nutrients to produce new algal biomass and release massive quantities of dissolved oxygen ($O_2$) back into the water column:

$6 CO_2 + 6 H_2O + \text{Nutrients} + \text{Sunlight} \xrightarrow{\text{Algae}} C_6H_{12}O_6 \;(\text{Algal Cells}) + 6 O_2\uparrow$

Diurnal Chemical Swings in Lagoon Ponds

Because photosynthesis is driven strictly by solar radiation, lagoon chemistry undergoes extreme 24-hour diurnal fluctuations:

• Peak Afternoon (14:00 – 16:00): Intense solar radiation maximizes photosynthesis. Dissolved oxygen surges to supersaturation ($12–20+\text{ mg/L}$). Concurrently, rapid algal consumption of dissolved $CO_2$ depletes carbonic acid ($H_2CO_3$), shifting the carbonate equilibrium and driving pH up to $8.5–10.0$.
• Pre-Dawn (04:00 – 06:00): In total darkness, photosynthesis halts while algal and bacterial respiration continues uninterrupted, consuming oxygen and generating $CO_2$. DO plummets to daily minimums ($<1.0–2.0\text{ mg/L}$ or near zero in organically overloaded lagoons), while accumulated carbonic acid lowers pH back down to $7.0–7.5$.

Lagoon Types & Functions

• Facultative Ponds: Depth $4–6\text{ ft}$, detention time $30–120\text{ days}$, organic loading $15–35\text{ lbs BOD/acre}\cdot\text{day}$.
• Aerated Lagoons: Depth $8–15\text{ ft}$, detention time $5–20\text{ days}$, equipped with mechanical surface aerators or submerged bubble diffusers providing complete or partial mixing without relying solely on algal photosynthesis.
• Polishing / Maturation Ponds: Tertiary shallow ponds ($3–4\text{ ft}$) following secondary treatment; detention time $10–15\text{ days}$; solar UV radiation and high pH provide natural disinfection ($2–4\text{ log}$ pathogen kill) and nutrient polishing.

Operational Control & Colorado Specific Challenges

• Seasonal Water Depth Adjustments: In Colorado winters, operators raise lagoon operating levels to $5.0–6.0\text{ feet}$ to provide thermal insulation under surface ice sheets and prevent bottom freezing. In early summer, levels are lowered to $3.0–4.0\text{ feet}$ to maximize sunlight penetration throughout the water column.
• Spring / Fall Seasonal Turnover (Destratification): Rapid changes in ambient temperature cause density shifts that mix anoxic, sulfide-rich bottom sludge throughout the pond, generating intense rotten-egg ($H_2S$) odors and short-term effluent BOD/TSS spikes. Mitigated by operating cells in parallel, adding sodium nitrate as an anoxic oxidant, or temporary mechanical surface aeration.
• Duckweed (Lemna) Control: Duckweed forms floating green blankets that completely block sunlight penetration, halting algal photosynthesis and turning facultative ponds anaerobic. Operators install surface floating baffles, utilize windward harvesting rakes, or apply approved aquatic herbicides.
• Dike Integrity & Burrowing Animals: Muskrats and burrowing rodents dig tunnels through earthen lagoon berms, creating catastrophic structural breach hazards. Rip-rap stone armoring on inside slopes and continuous vegetation mowing are mandatory.

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2. Trickling Filters (Attached-Growth Fixed-Film)

Trickling filters consist of a permeable bed of high-surface-area media over which settled wastewater is distributed continuously. Wastewater trickles downward over a fixed biological slime layer (zoogleal biofilm) attached to the media surfaces.

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|                        TRICKLING FILTER SYSTEM ARCHITECTURE                             |
+-----------------------------------------------------------------------------------------+
| Rotary Distributor Arms (Driven by hydraulic reaction thrust or electric motor)         |
|       │                                                                                 |
|       ▼  [ Wastewater sprayed uniformly over top surface ]                              |
| Packing Media Bed:                                                                      |
| • Rock Media:          2.5 - 4.0 inch rock; 4 - 6 ft deep; SSA = 15 - 20 sq ft / cu ft |
| • Synthetic Plastic:   Cross-flow sheets; 15 - 30 ft deep; SSA = 30 - 45 sq ft / cu ft |
|       │                                                                                 |
|       ▼  [ Heterotrophs absorb soluble BOD; outer aerobic layer, inner anoxic layer ]   |
| Underdrain System: Vitrified clay blocks sloped to center collection flume              |
|       │                                                                                 |
|       ▼  [ Sloughed biomass flushed to Secondary Clarifier ]                            |
| Clarifier ──► Effluent  |  Underflow ──► Sludge Wasting & Recirculation (Qr)            |
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Media Types: Rock vs. Synthetic Plastic

1. Rock / Slag Media: Natural crushed river stone ($2.5–4.0\text{ inch}$ diameter); shallow bed depth ($4–6\text{ ft}$); low specific surface area ($15–20\text{ ft}^2/\text{ft}^3$); low void ratio ($40%–50%$); heavy structural load; prone to plugging and ponding.
2. Structured Synthetic Plastic Media: Corrugated modular plastic sheets (PVC or polypropylene); deep bed depth ($15–30\text{ ft}$); high specific surface area ($30–45\text{ ft}^2/\text{ft}^3$); high void ratio ($90%–95%$); lightweight; superior air draft ventilation.

Biofilm Dynamics & Sloughing

As wastewater trickles over the media, heterotrophic bacteria, fungi, and protozoa absorb soluble organics and dissolved oxygen.

As the biofilm thickens ($>1.0–3.0\text{ mm}$), dissolved oxygen cannot penetrate to the deepest layer at the media surface. The inner layer becomes anaerobic, loses cellular adherence, and breaks off from the media—a natural, essential process known as sloughing. Sloughed solids must be settled and removed in a downstream secondary clarifier.

Critical Operating Parameters & Formulas

• Hydraulic Loading Rate (HLR): Total liquid volume applied per unit surface area per day, including recirculation flow ($Q_r$):

$\text{HLR (gpd/ft}^2\text{)} = \frac{Q + Q_r}{\text{Surface Area (ft}^2\text{)}}$

• Organic Loading Rate (OLR): Mass of primary effluent BOD applied per unit volume of filter media per day:

$\text{OLR (lbs BOD/1,000 ft}^3\cdot\text{day)} = \frac{\text{Primary Effluent BOD (lbs/day)}}{\text{Media Volume (1,000 ft}^3\text{)}}$

• Recirculation Ratio ($R = Q_r / Q$): Recycled clarifier effluent or underdrain flow dampens organic surges, maintains minimum wetting rates to prevent media drying during low night flows, increases hydraulic shear to thin heavy biofilm, and dilutes toxic influent spikes.

Trickling Filter Troubleshooting

• Ponding / Blinding: Standing puddles of wastewater on the media surface caused by excessive biological growth, media breakdown, or debris accumulation. Remedies: High-pressure water hosing; flooding the filter bed for 24 hours; applying a shock dose of chlorine ($2–5\text{ mg/L } Cl_2$ for several hours) to slough top biomass; or increasing the recirculation rate.
• Filter Flies (Psychoda Midges): Tiny, moth-like flies that breed in moist biological slime zones. Remedies: Flood the filter bed for 24 hours every 1–2 weeks to drown fly larvae; maintain continuous, uninterrupted distributor arm rotation to keep media surfaces continuously wet; or apply approved biological larvicides (e.g., Bacillus thuringiensis israelensis / BTI).
• Underdrain Ventilation Restrictions: Natural air drafts depend on temperature differentials between ambient air and wastewater ($\Delta T \ge 3–5^\circ\text{F}$). Underdrain air vents must maintain a minimum open area of $1.0\text{ ft}^2$ per $250\text{ ft}^2$ of filter surface area.

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3. Rotating Biological Contactors (RBCs)

Rotating Biological Contactors (RBCs) are attached-growth systems featuring a series of closely spaced, circular corrugated high-density polyethylene (HDPE) discs mounted on a horizontal steel shaft:

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|                        ROTATING BIOLOGICAL CONTACTOR (RBC)                              |
+-----------------------------------------------------------------------------------------+
| • Construction:     10 - 12 ft diameter HDPE corrugated discs mounted on 25 ft shaft    |
| • Submergence:      ~40% of media surface submerged in contoured wastewater trough       |
| • Rotation:         1.0 - 2.0 RPM (Mechanical gear drive or air-cup drive)              |
| • Biological Cycle: Submerged: Biofilm absorbs soluble BOD & nutrients                  |
|                     Exposed to Air: Biofilm absorbs atmospheric oxygen & sheds CO2      |
+-----------------------------------------------------------------------------------------+

Staging & Biomass Differentiation

RBC systems are divided into 3 to 4 sequential stages separated by physical baffles:

• Stage 1 (Carbonaceous BOD Removal): High organic loading. Biofilm is thick ($2–4\text{ mm}$), shaggy, and gray-brown, dominated by fast-growing heterotrophic bacteria that remove $70%–80%$ of soluble BOD.
• Stages 2 to 4 (Polishing & Nitrification): As soluble BOD drops below $15–20\text{ mg/L}$, slow-growing autotrophic nitrifiers (Nitrosomonas and Nitrobacter) establish a thin ($0.5–1.0\text{ mm}$), dense, golden-tan biofilm that oxidizes ammonia to nitrate.

Operational Limitations & Troubleshooting

• First-Stage Organic Overloading: First-stage loading must not exceed $2.5–3.0\text{ lbs soluble BOD / 1,000 ft}^2\text{ media surface per day}$. Severe overloading depletes DO ($<1.0\text{ mg/L}$), triggering explosive growths of the filamentous sulfur bacterium Beggiatoa. Beggiatoa forms a heavy, chalky white/gray slime that creates immense structural weight, stalls rotation, and leads to catastrophic shaft fracture or bearing failure.
• Shaft Weight & Load Cell Monitoring: Modern RBC installations incorporate electronic load cells under shaft bearing pillow blocks to track total wet biomass weight, alerting operators to excessive biofilm accumulation.
• Loping: An unbalanced, eccentric rotation caused by unequal biomass accumulation on one side of the shaft when rotation stops during power failures. Operators must manually spray wash the heavy side before restarting drive motors.

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4. Worked Fixed-Film Loading Calculation

A municipal trickling filter with a diameter of $90.0\text{ feet}$ and a rock media depth of $6.0\text{ feet}$ treats a primary effluent flow of $1.5\text{ MGD}$ with a BOD concentration of $160.0\text{ mg/L}$. The plant operates with a constant recirculation flow of $1.5\text{ MGD}$ ($R = 1.0$).

Step 1: Calculate Filter Media Surface Area and Volume

$\text{Radius } (r) = \frac{90.0\text{ ft}}{2} = 45.0\text{ ft}$

$\text{Surface Area } (A) = \pi \times r^2 = 3.14159 \times (45.0)^2 = 6,361.73\text{ ft}^2$

$\text{Media Volume } (V) = A \times \text{Depth} = 6,361.73\text{ ft}^2 \times 6.0\text{ ft} = 38,170.35\text{ ft}^3 = 38.17\text{ (in } 1,000\text{ ft}^3\text{)}$

Step 2: Calculate Total Hydraulic Loading Rate (HLR)

$\text{Total Flow Applied } (Q_{\text{total}}) = Q_{\text{influent}} + Q_{\text{recirculation}} = 1.5\text{ MGD} + 1.5\text{ MGD} = 3.0\text{ MGD} = 3,000,000\text{ gpd}$

$\text{HLR} = \frac{3,000,000\text{ gpd}}{6,361.73\text{ ft}^2} = 471.57\text{ gpd/ft}^2$

Step 3: Calculate Organic Loading Rate (OLR)

(Note: Recirculated flow is not counted in organic loading calculations as it does not add new primary organic mass)

$\text{Primary BOD Mass Applied (lbs/day)} = 1.5\text{ MGD} \times 160.0\text{ mg/L} \times 8.34 = 2,001.60\text{ lbs BOD/day}$

$\text{OLR} = \frac{2,001.60\text{ lbs BOD/day}}{38.17\text{ (in } 1,000\text{ ft}^3\text{)}} = 52.44\text{ lbs BOD / 1,000 ft}^3\cdot\text{day}$

Process Assessment: An OLR of $52.44\text{ lbs BOD / 1,000 ft}^3\cdot\text{day}$ and an HLR of $471.57\text{ gpd/ft}^2$ fall within the standard operating range for a high-rate rock-media trickling filter.