6.4 Stabilization Ponds, Lagoons & Small-Plant Processes
Key Takeaways
- Stabilization ponds are named explicitly in 23 CCR 3701(c)(1) as Grade I wastewater exam content, so they are tested at every grade.
- Under 23 CCR 3675 a conventional treatment pond is Class I at all design flows and a modified treatment pond is Class II at all design flows.
- Facultative ponds have an aerobic surface layer sustained by algal photosynthesis and wind, a facultative middle zone, and an anaerobic benthic layer.
- Pond effluent carries algae, so total suspended solids in the effluent can exceed the influent TSS even when BOD removal is excellent.
- Typical facultative pond loading is roughly 20 to 50 pounds of BOD per acre per day with 30 to 120 days of detention, depending on climate.
Why Ponds Are on Every Wastewater Exam
Title 23 CCR section 3701(c)(1) states that Grade I examinations shall test the applicant's knowledge of ... specifics regarding the operation of stabilization ponds. Because 3701(c) is cumulative, that content carries all the way up to Grade V. Ponds are also the process class where California's classification table is most distinctive:
| Pond type | Class under 23 CCR 3675 | Design flow |
|---|---|---|
| Conventional treatment pond | Class I | All flows |
| Modified treatment pond | Class II | All flows |
Unlike primary, biofiltration, activated sludge, SBR, and tertiary processes - all of which move up in class as flow increases - ponds carry a fixed classification regardless of size. A 12 MGD conventional pond system is still Class I. That is a favorite exam item because it looks wrong until you check the table.
Pond Types
| Type | Depth | Oxygen source | Detention | Character |
|---|---|---|---|---|
| Aerobic (high-rate) pond | 1-4 ft | Photosynthesis + surface reaeration throughout | 3-50 days | Shallow, fully aerobic, heavy algae growth |
| Facultative pond | 4-8 ft | Algae + wind at the surface; anaerobic at the bottom | 30-120 days | The standard California municipal pond |
| Anaerobic pond | 8-20 ft | None; fermentation and methanogenesis | 20-50 days | High-strength pretreatment; odor risk |
| Aerated lagoon | 6-20 ft | Mechanical or diffused aeration | 3-20 days | Much smaller footprint; energy cost |
| Maturation / polishing pond | 3-5 ft | Photosynthesis | 5-20 days | Pathogen die-off and effluent polishing after another process |
| Storage / evaporation pond | Varies | - | Seasonal | Used where discharge is prohibited during dry months |
How a Facultative Pond Works
A facultative pond is a solar-powered treatment plant with three vertical zones:
- Aerobic surface layer. Algae photosynthesize during daylight, releasing oxygen; wind adds surface reaeration. Aerobic heterotrophic bacteria oxidize soluble BOD using that oxygen. This layer also delivers most of the pathogen die-off through sunlight (UV), elevated pH, and long detention.
- Facultative middle zone. Oxygen is intermittent. Facultative organisms switch between aerobic respiration and fermentation.
- Anaerobic benthic layer. Settled solids undergo anaerobic digestion, producing methane, carbon dioxide, ammonia, hydrogen sulfide, and organic acids.
The algae-bacteria symbiosis is the engine: bacteria oxidize organics and release CO₂, ammonia, and phosphate; algae consume those nutrients and release oxygen; the cycle repeats.
The Diurnal Cycle
This is central to pond operation and is heavily tested:
| Time | Photosynthesis | Dissolved oxygen | pH |
|---|---|---|---|
| Mid-afternoon | Maximum | Peak, often supersaturated at 15-20 mg/L | Peak, often 9 to 10+ because algae strip CO₂ and carbonic acid |
| Pre-dawn | Zero; algae respire | Minimum, sometimes near zero | Minimum, often 7 to 7.5 as respiration returns CO₂ |
Two operational consequences follow directly. First, a single midday DO reading tells you almost nothing - sample at dawn to find the true minimum. Second, the afternoon pH excursion above 9 converts ammonium to free ammonia, which both strips to the atmosphere (a genuine nitrogen removal mechanism in ponds) and can be toxic to fish in the receiving water, so ammonia limits in a pond system's waste discharge requirements are often tied to pH and temperature.
Loading and Design Criteria
| Parameter | Typical facultative pond value |
|---|---|
| Organic loading | 20-50 lb BOD₅ per acre per day (lower in cold climates, higher in warm) |
| Hydraulic detention time | 30-120 days |
| Depth | 4-8 ft, with 3 ft freeboard typical |
| Cells | At least 2, preferably 3, operable in series (better quality) or parallel (better load distribution) |
| BOD removal | 75-95 percent |
| Effluent TSS | Often 40-100 mg/L or more because of algae |
Worked loading example. A pond system has two cells of 5.5 acres each and receives 0.32 MGD at 210 mg/L BOD₅. Load = 0.32 x 210 x 8.34 = 560 lb BOD₅/day Area = 11.0 acres Loading = 560 / 11.0 = 51 lb BOD/acre/day - at the top of the acceptable range, so this system has little margin before it goes anaerobic and odorous in warm weather.
The Algae Problem
Ponds are excellent at removing soluble BOD and poor at producing a low-TSS effluent, because the algae that do the work leave with the effluent. An NPDES permit written with a 30 mg/L TSS limit is often unachievable from a pond without polishing. Common solutions:
- Effluent polishing: rock filters, intermittent sand filters, cloth media filters, dissolved air flotation, or a constructed wetland
- Baffling and series operation to reduce short-circuiting and improve settling
- Drawing effluent from below the surface rather than from the algae-rich top layer
- Chemical addition (alum or ferric) to coagulate algae ahead of a filter
[!TIP] A pond effluent BOD test can also read high purely because of algae - the algal cells exert oxygen demand in the bottle. Some permits allow a filtered BOD (soluble BOD) to distinguish algal solids from actual unstabilized organic load. Know the difference between total BOD and soluble BOD when troubleshooting a pond that appears to be failing.
Troubleshooting
| Symptom | Likely cause | Response |
|---|---|---|
| Odor, gray or pink color, low DO at all hours | Overloading; pond has gone anaerobic. Pink or red often indicates purple sulfur bacteria thriving on sulfide | Recirculate effluent to the head, add supplemental aeration, reduce loading, operate cells in parallel to spread the load |
| Duckweed or emergent vegetation mats | Nutrient-rich stagnant surface; the mat blocks sunlight and wind, shutting down photosynthesis and reaeration | Physically remove the mat; it is not harmless cover |
| Berm erosion, burrowing animals, tree growth | Neglected embankment maintenance | Riprap, rodent control, mow and remove woody growth - roots create seepage paths and berm failure |
| Short-circuiting | Poor inlet/outlet geometry, wind-driven currents | Baffle curtains, relocate outlet, dye or tracer study |
| Sludge accumulation reducing volume | Normal over 10-20 years | Survey depth, dredge and dewater |
| Insects and mosquitoes | Emergent vegetation providing larval habitat | Vegetation control and mosquito abatement coordination |
| Winter ice cover and spring release | Reduced light, stored load | Increase detention, plan for a spring load release |
| High effluent ammonia | Cold water, low pH, no nitrification | Longer detention, added aeration, or a downstream nitrifying process |
Conventional vs Modified Ponds
A conventional treatment pond relies solely on natural processes. A modified treatment pond adds enhancements - typically supplemental aeration, recirculation, or chemical addition - and moves the plant from Class I to Class II under 23 CCR 3675, which raises the required chief plant operator from Grade I to Grade II. Adding a blower to fix an odor problem is therefore not only an operational decision but a staffing and classification decision, and the owner must report the change so the Office of Operator Certification can reclassify the plant.
A community operates a 6 MGD conventional treatment pond system. What is the plant classification under 23 CCR 3675, and what grade must the chief plant operator hold?
An operator measures dissolved oxygen at 17 mg/L and pH at 9.6 in a facultative pond at 3 p.m. What is the correct interpretation?
A facultative pond achieves 90 percent BOD removal but its effluent total suspended solids consistently exceeds the permit limit. What is the most likely explanation?