5.3 Waste Stabilization Ponds & Facultative Lagoons

5.3 Waste Stabilization Ponds & Facultative Lagoons

Waste stabilization ponds and lagoons provide robust, economical wastewater treatment throughout rural and semi-rural Illinois. Codified under 35 Ill. Adm. Code Part 380 as Class 4 wastewater facilities, lagoons utilize natural biological, physical, and solar mechanisms rather than energy-intensive mechanical machinery. They offer large buffering volumes against hydraulic surges and organic shock loads. However, maintaining compliance requires understanding pond zonation, algal-bacterial symbiosis, diurnal chemical fluctuations, and seasonal turnover phenomena.

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1. Pond Classifications & Hydraulic Operating Profiles

Wastewater stabilization ponds are divided into four distinct engineering classifications based on water depth, oxygenation mechanism, and biological stratification.

Facultative Ponds (Most Common in Illinois)

Facultative lagoons are the standard design for small Illinois municipalities. Operating at water depths of 3 to 6 feet (0.9 to 1.8 m), they maintain three simultaneous biological zones:

1. Upper Aerobic Zone: The top 6 to 18 inches of the water column, saturated with dissolved oxygen from active microalgae photosynthesis and surface reaeration.
2. Middle Facultative Zone: The intermediate layer populated by facultative bacteria that switch between aerobic respiration and anaerobic pathways (denitrification, acidogenesis) depending on local DO levels.
3. Bottom Anaerobic Sludge Layer: The benthic floor zone where settled organic solids undergo anaerobic digestion, generating organic acids, carbon dioxide, methane, and ammonia.

Aerobic Ponds (High-Rate Algae Ponds)

Aerobic ponds are shallow basins (1 to 3 feet / 0.3 to 0.9 m) allowing solar radiation to penetrate entirely to the pond floor. Photosynthetic algae thrive throughout all depths, maintaining aerobic conditions 24 hours a day. Producing high algal suspended solids, they are primarily used to polish secondary effluents.

Aerated Lagoons

Aerated lagoons operate at depths of 8 to 15 feet (2.4 to 4.5 m). Because sunlight cannot reach these depths, oxygen is supplied by floating mechanical surface aerators or submerged diffused bubble aeration grids. Artificial aeration maintains partial or complete suspended mixing, allowing significantly higher organic loading rates per acre than passive facultative ponds.

Anaerobic Ponds

Anaerobic ponds are deep impoundments (8 to 20 feet / 2.5 to 6.0 m) receiving high volumetric organic loads ($>15\text{ to }30\text{ lbs BOD}_5/1,000\text{ cu ft/day}$) without dissolved oxygen. Anaerobic fermentation decomposes complex wastes. Emitting volatile sulfide odors ($H_2S$), municipal plants rarely use them; they are restricted to agricultural or industrial pretreatment (e.g., meatpacking) and often fitted with floating covers for biogas recovery.

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2. The Algal-Bacterial Symbiotic Cycle

The biological engine driving facultative lagoons is the mutualistic algal-bacterial symbiotic cycle operating in the photic zone:

$\text{Organic BOD} + \text{O}_2 \xrightarrow{\text{Bacteria}} \text{CO}_2 + \text{H}_2\text{O} + \text{NH}_4^+ + \text{PO}_4^{3-} + \text{Bacterial Cells}$

$\text{CO}_2 + \text{H}_2\text{O} + \text{Nutrients} + \text{Sunlight} \xrightarrow{\text{Algae}} \text{Algal Biomass} + \text{O}_2\uparrow$

• Bacterial Role: Aerobic and facultative heterotrophic bacteria decompose dissolved and colloidal organic matter ($BOD_5$), consuming dissolved oxygen and releasing metabolic end-products: carbon dioxide ($CO_2$), ammonium ($NH_4^+$), and orthophosphate ($PO_4^{3-}$).
• Algal Role: Unicellular green algae (Chlorella, Scenedesmus, Chlamydomonas) absorb sunlight via chlorophyll pigments, using bacterial $CO_2$ and mineral nutrients ($N, P$) to synthesize cell mass while releasing dissolved oxygen ($O_2$) back into the liquid.
• Visual Indicator: A healthy facultative lagoon displays a vibrant dark green or pea-soup emerald color. If a lagoon turns dull gray or black, symbiosis has broken down due to organic overloading, toxic industrial inputs, or extended sunlight deprivation.

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3. Diurnal Cycles of Dissolved Oxygen & pH

Because photosynthesis is driven by sunlight while bacterial respiration occurs continuously, facultative lagoons exhibit dramatic 24-hour diurnal swings in DO and pH.

Late Afternoon Peaks (3:00 PM – 5:00 PM)

Peak solar irradiance produces maximum photosynthetic activity:

• Dissolved Oxygen: DO reaches supersaturation levels of 10 to 15+ mg/L (often >200% saturation in warm weather).
• pH Rise: Photosynthesis rapidly consumes dissolved carbon dioxide ($CO_2$) and carbonic acid ($H_2CO_3$). To obtain carbon, algae extract bicarbonate ions: $2\text{HCO}_3^- \rightleftharpoons \text{CO}_3^{2-} + \text{H}_2\text{O} + \text{CO}_2\uparrow$ This equilibrium shift leaves carbonate ($\text{CO}_3^{2-}$) and hydroxide ($\text{OH}^-$) ions in solution, driving pH up to 8.5–9.5 (and occasionally >10.0).

Pre-Dawn Valleys (5:00 AM – 6:30 AM)

At night, darkness halts photosynthesis, but algae and bacteria continue cellular respiration, consuming oxygen:

• Dissolved Oxygen: DO plummets to its daily minimum of <1.0 to 2.0 mg/L just before sunrise.
• pH Drop: Respiration releases $CO_2$, generating carbonic acid and lowering pH to 7.0–7.5.

Compliance Note: Illinois NPDES permits set pH limits (6.0 to 9.0) and minimum DO standards (typically $\ge 5.0\text{ mg/L}$). Operators must log sampling times, as afternoon samples risk pH exceedances ($>9.0$) while dawn samples risk failing minimum DO limits.

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4. Seasonal Operational Dynamics & Cold-Weather Challenges

Midwestern temperature extremes create distinct seasonal operational challenges for Illinois lagoon operators.

Spring and Fall Thermal Turnovers

Water achieves maximum physical density at 3.98°C (~4°C). In autumn, cooling surface water approaches 4°C, becomes denser than bottom water, and sinks. Wind induces fall turnover, abruptly inverting the water column:

• Turnover sweeps anoxic, sulfide-laden benthic sludge to the surface, depleting DO and releasing foul hydrogen sulfide ($H_2S$) rotten-egg odors that persist for 1 to 2 weeks.
• Remedies: Operators apply sodium nitrate ($NaNO_3$, 10–20 lbs/acre) to supply bound oxygen for denitrifying microbes, suppressing sulfide volatilization.

Winter Ice Cover & Spring Recovery

Thick ice and snow cover in midwinter block sunlight and eliminate atmospheric reaeration. Bacterial metabolism slows below 4°C, causing ponds to turn entirely anaerobic under the ice. When ice thaws in March, warming water accelerates bacterial digestion before algae colonies recover, creating a 2- to 3-week window of low DO and spring odor complaints until green algal blooms re-establish.

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5. Physical Maintenance & Troubleshooting

Duckweed (Lemna) & Weed Control

Duckweed is a floating vascular plant forming dense green mats. Unlike microalgae, duckweed blocks solar penetration and stops atmospheric reaeration, turning underlying water anaerobic. Operators deploy floating booms to corral duckweed for mechanical skimming, or apply approved herbicides (diquat, fluridone). Cattails and shoreline weeds must be removed to prevent mosquito breeding and rodent sheltering.

Dike Integrity & Burrowing Animals

Muskrats and beavers excavate extensive tunnel networks into earthen embankments, causing internal seepage and catastrophic dike failure. Operators must trap burrowing animals under Illinois Department of Natural Resources (IDNR) permits, repair dens with compacted bentonite clay, and maintain crushed stone rip-rap (6–12 inch rock) along waterlines to armor dikes against waves and rodents.

Controlled Batch Discharges

Many Illinois Class 4 permits authorize seasonal batch discharges. Wastewater is stored during freezing winter months and summer low flows, then released during spring and late autumn high stream flows. Operators isolate the discharge cell for 10 to 14 days, test grab samples for CBOD5, TSS (confirming algae have settled), pH, and ammonia, and verify adequate stream dilution before opening discharge gates.