10.3 Stabilization Ponds and Aerated Lagoons
Key Takeaways
Facultative ponds have an aerobic upper layer, where algae supply oxygen to bacteria, over an anaerobic sludge layer, so dissolved oxygen and pH peak in the afternoon and fall by dawn.
Organic loading is pounds of BOD per acre per day; detention time is pond volume divided by flow, with 1 acre-foot equal to about 325,851 gallons.
Bright green water is healthy; blue-green suggests cyanobacteria; gray or black means septic, overloaded conditions; pink or red indicates purple sulfur bacteria and anaerobic stress.
Keep operating depth high enough to block emergent weeds, control burrowing animals, and maintain dikes, inlets and outlets to prevent short-circuiting.
Algae can push effluent suspended solids above secondary limits, so federal rules let states set alternative TSS limits for small stabilization ponds.
Where Lagoons Fit
Waste stabilization ponds are shallow earthen basins that treat wastewater with sunlight, algae and bacteria and very little equipment. In DEQ's Table A, a non-aerated lagoon is Class I at any flow, and an aerated lagoon is Class I up to 1 MGD and Class II above it. Many Oregon systems store lagoon effluent in winter and irrigate in summer under WPCF permits, or discharge only during high-flow seasons under NPDES permits.
Types of Ponds
| Type | How it works | Typical depth | Notes |
|---|---|---|---|
| Facultative | Aerobic upper zone (algae and bacteria), facultative middle, anaerobic bottom sludge | about 3-6 ft | The most common small-town pond; long detention times |
| Aerated (partial mix) | Mechanical or diffused aeration supplies oxygen; algae still present | 8-15 ft | Smaller footprint, more power |
| Aerated (complete mix) | Enough mixing to keep solids suspended | 10-15 ft | Needs a settling cell afterward |
| Anaerobic | Heavily loaded, little oxygen | deep | Used for strong industrial wastes; odor risk |
| Polishing or maturation | Low-load final cells | shallow | Lowers BOD, TSS and pathogens before discharge |
The Algae-Bacteria Partnership
Bacteria break down organic matter and release carbon dioxide and nutrients. Algae use CO2, sunlight and nutrients to grow and release oxygen, which bacteria then use. That creates a strong daily (diurnal) cycle:
- Afternoon: photosynthesis peaks. Dissolved oxygen may be supersaturated, and pH can rise above 9 as algae strip CO2.
- Night to dawn: respiration exceeds photosynthesis. DO and pH reach their lowest levels just before sunrise.
The swing is normal. A pond that stays at low DO through the afternoon is overloaded or short of algae. Seasonal turnover in spring and fall can mix bottom sludge upward and cause temporary odors.
Loading and Detention Math
Organic loading
Facultative ponds are commonly designed for roughly 15-35 lb BOD per acre per day, depending on climate.
Example. A town sends 0.25 MGD at 200 mg/L BOD to 15 acres of primary ponds.
- BOD load = 0.25 × 200 × 8.34 = 417 lb/day
- Loading = 417 ÷ 15 = 27.8 lb BOD/acre/day
Detention time
1 acre = 43,560 sq ft, and 1 acre-foot = 43,560 cu ft × 7.48 ≈ 325,851 gallons.
Example. Fifteen acres at 5 ft depth holds 75 acre-feet, or about 24.4 MG. At 0.25 MGD the detention time is 24.4 ÷ 0.25 ≈ 98 days.
Reading a Pond
| Observation | Likely meaning | Response |
|---|---|---|
| Bright green, DO high in afternoon | Healthy algae population | Keep routine monitoring |
| Blue-green scum, paint-like streaks | Cyanobacteria bloom | Check for toxins near recreational or drinking water use; review nutrients and detention |
| Gray or black, sewage or H2S odor | Septic and overloaded | Reduce load, switch cell order, add aeration or recirculation |
| Pink or red | Purple sulfur bacteria, anaerobic | Investigate overloading and sludge depth |
| Brown with floating mats | Sludge rising, turnover | Break up mats; plan sludge removal |
| Clear water with little algae | Toxic slug, grazing zooplankton (daphnia) or very cold weather | Check influent, DO and pH trends |
Routine Operation and Maintenance
- Depth control: keep water deep enough (often at least 2 to 3 feet) so cattails and tules cannot root. Emergent weeds create mosquito habitat and short-circuit flow.
- Dike care: mow or graze grass, repair erosion and wave-cut damage, use riprap where needed, and trap muskrats, nutria and other burrowing animals that tunnel through dikes.
- Inlets and outlets: clear debris, adjust weirs and change cell sequence (series or parallel) by season to balance load.
- Sludge: measure the sludge blanket with a sludge judge or core sampler. Plan removal before sludge cuts volume enough to lower detention time and raise odors.
- Seepage and liners: watch for unexplained level losses. Seepage can violate groundwater protection requirements.
- Records: pond levels, DO, pH, temperature, flows, color observations and maintenance.
Effluent Challenges
- TSS from algae: algae cells leaving the last cell count as suspended solids. Federal rules (40 CFR 133.103) let states set adjusted TSS limits for small waste stabilization ponds. Operators also use draw-off below the surface, intermittent sand filters, rock filters or chemical treatment.
- Ammonia and nutrients: nitrification slows sharply in cold water, so winter ammonia removal is limited.
- Disinfection: algae shield bacteria from UV and exert chlorine demand. Lagoon effluent often needs filtration or higher doses, and dechlorination afterward.
- Seasonal discharge or reuse: storage lagoons let a town avoid summer discharges. Stored water used for irrigation must meet the applicable recycled water class.
Sludge Depth Checks
Settled solids build up slowly in pond bottoms, especially near the inlet. Measure sludge depth at least yearly at a grid of points from a boat, using a sludge judge or a white towel on a pole that shows the blanket line. Plot the results. Sludge accumulating near the inlet, a rising blanket or shrinking clear-water depth means removal is coming. Plan removal, by dredging or by draining and drying a cell, before solids cut detention time and start showing up as effluent TSS. Removed pond sludge is a biosolid and is managed under the Oregon biosolids rules.
A facultative lagoon's dissolved oxygen is 14 mg/L and its pH is 9.6 at 3 p.m., but DO is 1 mg/L at 6 a.m. What does this pattern indicate?
A toxic discharge has killed off the pond's bacteria
Severe organic overloading that requires an immediate bypass
A normal day-night cycle of algal photosynthesis and respiration
Faulty meters, because DO can never exceed saturation in a pond
A lagoon system receives 0.18 MGD of wastewater at 240 mg/L BOD and has 12 acres of primary cells. What is the organic loading?
36 lb/acre/day
15 lb/acre/day
360 lb/acre/day
30 lb/acre/day
An operator sees cattails rooting along the shallow edges of a lagoon cell. What is the best corrective strategy?
Raise the depth if possible and remove the plants, which breed mosquitoes
Leave the plants in place, because they add oxygen to the pond water
Lower the water level so the plants dry out and die back on their own
Add chlorine along the shoreline to kill the roots of the plants
Sections you finish are checked off in the contents.