8.6 Fixed-Film & Lagoon Treatment Systems

Fixed-Film Treatment Principles: Trickling Filters

Fixed-film (attached-growth) systems differ fundamentally from suspended-growth activated sludge. In fixed-film processes, microorganisms attach themselves to inert packing media, forming a biological slime layer (biofilm). As settled wastewater passes over the biofilm, microorganisms absorb dissolved organic matter and oxygen from surrounding air pockets.

1. Media Types & Physical Characteristics

• Crushed Rock / Blast Furnace Slag: Traditional media ($2 - 4 \text{ inch}$ rock). Bed depths range from $3 - 8 \text{ ft}$. Rock media provides low specific surface area ($15 - 20 \text{ ft}^2/\text{ft}^3$) and low void space ($45 - 55%$), making it heavy, structurally restricted, and prone to ponding.
• Synthetic Plastic Media: Modern cross-flow corrugated PVC modules or random plastic rings. Bed depths extend from $15 - 30 \text{ ft}$. Synthetic media provides high specific surface area ($30 - 45 \text{ ft}^2/\text{ft}^3$) and high void volume ($> 90%$), enabling superior oxygen transfer and significantly higher loading capacities without plugging.

2. Hydraulic & Organic Loading Rates

Trickling filter performance is governed by two key loading calculations:

$\text{Hydraulic Loading Rate (HLR)} = \frac{Q + Q_R \text{ (gpd)}}{\text{Filter Surface Area } A \text{ (ft}^2\text{)}}$

$\text{Organic Loading Rate (OLR)} = \frac{\text{lbs } \text{BOD}_5 \text{ applied/day}}{\text{Filter Media Volume } V \text{ (1,000 ft}^3\text{)}}$

3. Recirculation Ratio ($R$)

Recirculation involves pumping secondary clarifier effluent back to the trickling filter distributor arm:

$R = \frac{\text{Recirculated Flow Rate } (Q_R)}{\text{Plant Influent Flow Rate } (Q)}$

Recirculation ratios typically range from $0.5 : 1$ to $3.0 : 1$. Operating benefits include:

• Diluting high-strength or toxic influent organic spikes.
• Keeping biological media continuously moist during low night flows to prevent biofilm death.
• Providing high hydraulic shear to continuously flush excess sloughed solids.
• Deterring Psychoda filter fly breeding by maintaining wet media surface conditions.

4. Sloughing & Operational Troubleshooting

As the biofilm thickens ($1 - 3 \text{ mm}$), oxygen cannot penetrate to the inner microbial layers attached to the media. Anaerobic decomposition occurs at the media interface, weakening attachment until hydraulic flow shears the slime layer off—a process known as sloughing. Sloughed solids pass to the secondary clarifier for removal.

• Ponding Clearance: Standing water on top of media is caused by excessive biomass growth or debris plugging voids. Clear ponding by applying high-rate recirculation, shock-dosing chlorine ($5 - 10 \text{ mg/L}$ for short duration or $1 - 2 \text{ mg/L}$ continuous), hand-rodding/raking the media surface, or flooding the filter.
• Psychoda Filter Fly Control: Tiny nuisance midges breed in damp, unflooded filter media. Control flies by flooding the filter bed completely with water for 24 hours every 7 to 14 days to drown larvae, or increasing hydraulic loading to wash larvae away.

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Rotating Biological Contactors (RBCs)

An RBC consists of closely spaced, high-density polyethylene (HDPE) corrugated plastic discs (typically $12 \text{ ft}$ in diameter) mounted on a central horizontal steel shaft (up to $25 \text{ ft}$ long). Discs are submerged approximately $40%$ in a semicircular concrete trough containing wastewater.

Mechanical Operation & Aeration

The shaft rotates slowly at $1.0$ to $1.5 \text{ rpm}$. As the discs rotate, attached biofilm is alternately exposed to wastewater in the trough (absorbing organic nutrients) and atmospheric air (absorbing oxygen). Disc rotation also creates gentle hydraulic shear that maintains uniform biofilm thickness ($0.05 - 0.10 \text{ inches}$).

First-Stage Loading Limitations

First-stage RBC units process raw primary effluent containing the highest organic strength. To prevent severe operational failures, first-stage organic loading must be strictly controlled:

$\text{Maximum First-Stage OLR} \le 4.0 - 6.0 \text{ lbs BOD}_5/1,000 \text{ ft}^2 \text{ disc area/day}$

Consequences of Organic Overloading:

1. Severe dissolved oxygen depletion in RBC troughs.
2. Proliferation of Beggiatoa (filamentous white sulfur bacteria utilizing $\text{H}_2\text{S}$), creating a thick white slime layer.
3. Excessive biofilm mass accumulation, imposing massive rotational weight that causes shaft fatigue failure, mechanical drive failure, and structural collapse.

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Waste Stabilization Ponds & Lagoons

Waste stabilization ponds provide low-cost biological treatment using long hydraulic retention times (HRTs) and natural ecological interactions.

Facultative Lagoon Depth Stratification

1. Aerobic Surface Zone (Upper 1–2 ft): Contains abundant dissolved oxygen supplied by solar photosynthetic activity of green algae (Chlorella, Scenedesmus) and atmospheric wind mixing.
2. Facultative Middle Zone (Middle 2–4 ft): Contains variable DO. Facultative bacteria switch between aerobic respiration and anaerobic fermentation depending on light and mixing.
3. Anaerobic Bottom Zone (Bottom 1–3 ft): Zero DO. Settled organic sludge undergoes anaerobic digestion by methanogenic bacteria, producing methane ($\text{CH}_4$), carbon dioxide ($\text{CO}_2$), and hydrogen sulfide ($\text{H}_2\text{S}$).

Algae-Bacterial Symbiosis & Diurnal Variations

Facultative lagoons operate via a complementary symbiotic relationship:

• Heterotrophic Bacteria consume organic waste ($\text{BOD}_5$) using $\text{O}_2$, releasing $\text{CO}_2$, ammonia ($\text{NH}_4^+$), and phosphates ($\text{PO}_4^{3-}$).
• Algae utilize solar energy, $ ext{CO}_2$, and inorganic nutrients to produce new algal cells and release dissolved oxygen ($\text{O}_2$) back to bacteria.

$\text{Photosynthesis: } 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Sunlight} \xrightarrow{\text{Algae}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

$\text{Respiration: } \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \xrightarrow{\text{Bacteria/Algae}} 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy}$

Because photosynthesis requires sunlight while respiration occurs continuously day and night, facultative lagoons undergo dramatic diurnal fluctuations:

• Peak Daylight (Late Afternoon): Photosynthesis consumes free carbon dioxide ($\text{CO}_2$) and carbonic acid faster than it is produced. Removing carbonic acid shifts carbonate equilibrium and causes pH to rise up to $8.5 - 9.5$. Dissolved oxygen reaches maximum levels, frequently achieving supersaturation ($> 15 - 20 \text{ mg/L}$).
• Nighttime (Pre-Dawn): Photosynthesis ceases completely while algal and bacterial respiration continues. Carbon dioxide accumulates, forming carbonic acid ($\text{H}_2\text{CO}_3$) and causing pH to drop to $7.0 - 7.4$. Dissolved oxygen drops steadily, reaching its daily minimum ($< 1.0 - 2.0 \text{ mg/L}$) just before sunrise.

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South Carolina DES Regulation 61-67 Lagoon Standards

South Carolina regulations impose strict engineering standards on municipal lagoons to protect groundwater and surface water quality:

• Minimum Liner Permeability: All lagoon basins must be sealed with a synthetic liner or compacted clay liner achieving a maximum hydraulic conductivity $k \le 1.0 \times 10^{-7} \text{ cm/s}$.
• Minimum Retention Time: Un-aerated facultative lagoons must provide a minimum 45 to 90 days of hydraulic retention time.
• Effluent Limits: Secondary effluent standards apply ($\text{BOD}_5 \le 30 \text{ mg/L}$, $\text{TSS} \le 30 \text{ mg/L}$ 30-day average), though special permit allowances up to $90 \text{ mg/L}$ TSS may be granted for algae particles if non-toxic.