6.2 Waste Stabilization Ponds, Aerated Lagoons & Alabama Grade I Lagoon Operations

Waste stabilization ponds and aerated lagoons represent the primary secondary wastewater treatment technology for hundreds of small municipalities, rural communities, mobile home parks, and consolidated school districts throughout Alabama. Under the Alabama Department of Environmental Management (ADEM) Administrative Code Division 335-10-1, a dedicated Grade I Wastewater Operator Certification specifically governs the legal and technical operation of these lagoon and pond facilities.

While lagoons have minimal mechanical complexity compared to activated sludge plants, they are highly dynamic ecological bioreactors governed by limnological, physical, and biochemical equilibria that require vigilant process control.

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1. Lagoon Classifications & Operating Mechanisms

Wastewater ponds are categorized based on their depth, method of aeration, and the presence or distribution of dissolved oxygen throughout their vertical water column.

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2. Facultative Ponds: The Three-Zone Architecture & Algae-Bacteria Symbiosis

The facultative pond is the most widely deployed lagoon design. Its name derives from the facultative microorganisms that operate within its intermediate zone, capable of thriving whether oxygen is present or absent.

The Three Vertical Zones

1. Aerobic Surface Zone (Upper 1 to 2 feet): Known as the photic zone, sunlight penetrates this layer to support dense populations of single-celled planktonic green algae (Chlorella, Scenedesmus, Chlamydomonas). The water column here maintains high concentrations of dissolved oxygen throughout daylight hours.
2. Intermediate Facultative Zone (Middle 1.5 to 3.5 feet): In this transitional zone, dissolved oxygen is present during bright daylight hours due to vertical mixing, but is completely exhausted at night. Facultative heterotrophs switch smoothly between aerobic respiration and anoxic/fermentative pathways.
3. Anaerobic Bottom Zone (Bottom Sludge Layer): Settled organic particulates, dead algal cells, and bacterial debris form a benthic sludge blanket. Sunlight and dissolved oxygen never reach this benthic floor. Anaerobic acid-forming bacteria ferment complex organics into volatile organic acids (acetic, propionic), which methanogenic Archaea (Methanobacterium, Methanosarcina) convert into methane gas ($CH_4$) and carbon dioxide ($CO_2$).

The Algae-Bacteria Symbiosis

The biological engine of the aerobic zone is a perpetual, closed-loop mutualistic symbiosis between aerobic heterotrophic bacteria and photosynthetic algae:

• The Bacterial Role: Aerobic heterotrophic bacteria consume dissolved organic matter ($BOD_5$) from the influent wastewater. Using dissolved oxygen ($O_2$) as an electron acceptor, they oxidize organic carbon into carbon dioxide ($CO_2$), water ($H_2O$), and inorganic nutrients (orthophosphate and ammonium):

$\text{Organic Matter} + O_2 \rightarrow CO_2 + H_2O + NH_4^+ + PO_4^{3-} + \text{Bacterial Biomass}$

• The Algal Role: Planktonic green algae absorb the carbon dioxide, water, and mineralized nutrients released by the bacteria. Harnessing solar irradiance, algae carry out photosynthesis, synthesizing new algal biomass and releasing copious quantities of dissolved oxygen ($O_2$) directly into the water column:

$6CO_2 + 6H_2O + \text{Sunlight (Photons)} \rightarrow C_6H_{12}O_6 + 6O_2$

Neither organism can sustain the treatment cycle without the other. If sunlight is blocked, algae cease producing oxygen; bacteria deplete remaining DO, and the pond turns septic. Conversely, if organic loading drops, bacterial $CO_2$ generation declines, slowing algal growth.

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3. Diurnal Water Chemistry Swings & Alabama Seasonal Dynamics

Facultative lagoons undergo extreme diurnal (24-hour) chemical cycles that operators must understand when collecting compliance samples and interpreting operational data.

The Diurnal DO and pH Cycle

• Daytime Dynamics (Midday to Late Afternoon):
  • Solar radiation drives algal photosynthesis at maximum velocity, producing oxygen far faster than bacteria can consume it. Dissolved oxygen levels spike into extreme supersaturation, often reaching 12 to 20+ mg/L (150% to 250% saturation).
  • Rapid photosynthesis exhausts all free dissolved carbon dioxide ($CO_2$) in the water. To continue photosynthesizing, algae extract carbon from dissolved bicarbonate ions ($HCO_3^-$):

$2HCO_3^- \rightleftharpoons CO_3^{2-} + H_2O + CO_2 \quad (\text{assimilated by algae})$

• The resulting carbonate ions ($CO_3^{2-}$) hydrolyze water, generating free hydroxide ions ($OH^-$):

$CO_3^{2-} + H_2O \rightleftharpoons HCO_3^- + OH^-$

• This accumulation of hydroxide ions shifts the carbonate buffer equilibrium and drives the lagoon pH upward to 8.5, 9.5, or even > 10.0 on sunny afternoons.

• Nighttime Dynamics (Dusk to Pre-Dawn):

  • Photosynthesis ceases entirely in darkness. However, cellular respiration by both algae and bacteria continues unabated.
  • Respiration consumes dissolved oxygen and produces large quantities of carbonic acid ($CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$).
  • Consequently, dissolved oxygen plummets steadily throughout the night, reaching its lowest daily minimum immediately before sunrise (often dropping to 1.0 to 3.0 mg/L, or near zero in overloaded lagoons). Concurrently, carbonic acid lowers the lagoon pH down to 7.2 to 7.6 by dawn.

Seasonal Overturn (Thermal Stratification & Inversion)

During hot, humid Alabama summers, solar heat warms the upper 1 to 2 feet of the pond (forming a warm, less dense epilimnion). The bottom water remains colder, denser, and strictly anaerobic (hypolimnion).

In autumn (or following severe, cold convective thunderstorms in late summer), the ambient temperature drops sharply. The surface water cools, becomes denser than the bottom water, and rapidly sinks. This initiates a complete physical turnover (inversion) of the lagoon. Anaerobic, oxygen-depleted sludge water rich in dissolved hydrogen sulfide ($H_2S$), methane, and ferrous sulfide surges to the surface. Dissolved oxygen throughout the entire pond collapses to 0.0 mg/L, turning the pond a dark grey or black color and releasing severe, offensive "rotten-egg" odors across surrounding communities.

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4. Key Operating Parameters & Alabama Grade I Lagoon Operations

Depth Control Protocols (3 to 5 Feet Rule)

Common operating guidance (always follow the plant's permit and O&M manual) keeps facultative stabilization ponds about 3 to 5 feet deep (0.9 to 1.5 meters). Operating well outside this range causes problems:

• Hazard of Operating Below 3 Feet (< 3 ft): Sunlight penetrates completely through shallow water to the bottom sediment floor. This triggers the rapid germination and growth of rooted emergent aquatic vegetation (macrophytes) such as cattails (Typha), bulrushes (Scirpus), and pondweed. Rooted weeds create massive problems: they impede hydraulic flow, cause short-circuiting, trap floating debris, and create stagnant, sheltered pockets protected from wind action. These stagnant pockets provide ideal breeding conditions for mosquitoes (specifically Culex species, the primary vector for West Nile Virus and encephalitis in Alabama). Furthermore, decaying rooted vegetation adds immense internal organic loading.
• Hazard of Operating Above 5 Feet (> 5 ft): Sunlight cannot penetrate into the deep water column, severely shrinking the aerobic photic zone relative to total volume. The expanded anaerobic bottom zone generates massive quantities of volatile acids that overwhelm the buffering capacity, lowering pH, killing algae, and releasing persistent hydrogen sulfide odors.

Embankment (Dike) Maintenance & Burrowing Animal Eradication

Maintaining the structural integrity of earthfill dikes is the paramount physical responsibility of an Alabama Grade I operator:

• Burrowing Rodents (Muskrats and Nutria): Semi-aquatic burrowing pests—primarily the common muskrat (Ondatra zibethicus) and the invasive semi-aquatic nutria (Myocastor coypus) widespread across Alabama waterways—burrow into lagoon dikes. They excavate extensive subterranean dens starting approximately 6 to 12 inches below the operating water line and sloping upward into the core of the embankment. These tunnels cause internal soil erosion, leading to piping failures, embankment slump, and catastrophic levee breaches that release millions of gallons of untreated sewage into local creeks.
  • Physical Barrier Armor: Install heavy, graded stone riprap (4 to 12 inch rock) along the entire water-side embankment slope. The riprap should extend from above the high-water line to below the lowest operating level so rodents cannot dig into exposed soil at the waterline.
  • Vegetation Control: Maintain vegetative grass cover on outer slopes, but mow regularly. Overgrown brush and cattails provide food and nesting cover for rodents.
  • Eradication: Conduct trapping and removal programs that follow Alabama Department of Conservation and Natural Resources (ADCNR) wildlife rules.

Surface Scum, Duckweed & Blue-Green Algae Mitigation

• Duckweed (Lemna minor): Duckweed is a minute, free-floating flowering plant that proliferates in quiescent lagoons. Left unchecked, duckweed can blanket 100% of the pond surface in a dense green mat. This mat completely blocks sunlight from penetrating the water column, instantly halting green algal photosynthesis. The pond rapidly turns anoxic and septic. Operators must install surface perimeter booms to corral duckweed for mechanical removal, skimmer rakes, or apply approved aquatic herbicides.
• Blue-Green Algae (Cyanobacteria): Under calm, hot, high-nutrient conditions, nuisance blue-green algae (Microcystis, Anabaena) form thick, foul, paint-like surface scums. Unlike beneficial green algae, blue-green algae can fix atmospheric nitrogen, produce cyanotoxins, clog discharge weirs, and emit unpalatable earthy/musty odors (geosmin and MIB). Broken up using surface splash aerators or broken down with localized treatments of copper sulfate (applied only according to the product label and any required permit coverage).

Emergency Odor Control: Sodium Nitrate ($NaNO_3$)

When a lagoon experiences an overturn, severe organic overload, or prolonged cloudy weather that destroys dissolved oxygen, sulfate-reducing bacteria (Desulfovibrio) reduce sulfate ions ($SO_4^{2-}$) in the wastewater to foul hydrogen sulfide gas:

$SO_4^{2-} + 2\text{Organic Carbon} + 2H^+ \rightarrow H_2S \uparrow + 2CO_2 + 2H_2O$

To rapidly extinguish hydrogen sulfide odors, the operator adds sodium nitrate ($NaNO_3$) directly into the pond:

• Biochemical Mechanism: Microorganisms follow a thermodynamic redox hierarchy for electron acceptors ($O_2 > NO_3^- > SO_4^{2-} > CO_2$). When sodium nitrate is added, facultative bacteria eagerly switch to using nitrate ($NO_3^-$) as an electron acceptor for denitrification. This drives the oxidation-reduction potential (ORP) above $-100\;\text{mV}$, which biochemically blocks and shuts down the sulfate-reduction pathway. Hydrogen sulfide production ceases immediately.
• Dosage Rate: An initial emergency shock dose of 50 to 100 pounds of $NaNO_3$ per acre of water surface is broadcast across the pond on the first day, followed by 25 to 50 lb/acre/day until green algal populations recover and produce positive dissolved oxygen.