Aerated Lagoon Problems: Causes, Troubleshooting and Fixes
Aerated Lagoon Problems: Common Causes, Troubleshooting Steps and Fixes
Aerated lagoons are popular because they are simple, but simple does not mean trouble-free. Operators regularly face high effluent solids, ammonia violations, odors, aerators that fail in winter, dead zones and bottoms filled with sludge. Most of these problems have a small number of root causes, and once you know what to look for, they are much easier to diagnose.
This guide covers the most common aerated lagoon problems: what they look like, why they happen, what to check, and how to fix them, including when it is time to upgrade rather than keep patching.
This article is educational. Troubleshooting should be done with your engineer or supplier and in line with your permit and state requirements.
For background, read aerated lagoon design and aerated lagoon vs activated sludge.
Quick Troubleshooting Table
| Symptom | Most likely causes | First checks |
|---|---|---|
| High effluent TSS | Algae, no sludge return, short-circuiting, sludge buildup, undersized settling | Microscope check, settling basin condition, sludge depth, flow pattern |
| High effluent BOD | Overloading, low DO, cold water, toxic shock, lost volume | DO profile, influent load, temperature, sludge survey |
| High effluent ammonia | Short sludge age, cold, low DO or alkalinity | Nitrate, alkalinity, pH, temperature (see lagoon ammonia removal) |
| Odors | Low DO, sludge decomposition, overloading, septic influent | DO, sludge depth, influent sulfide/septicity, aerator coverage |
| Low DO | Aerator failure or fouling, overloading, ice | Aerator operation, power, load, DO at several points |
| Dead zones, uneven mixing | Poor aerator layout, short-circuiting | DO and solids mapping, tracer test |
| Floating scum, foam, duckweed | Grease, algae, calm surface, sunlight and nutrients | Grease sources, surface coverage, wind and mixing |
| Winter ice and aerator damage | Cold, surface aerators, spray | Ice pattern, aerator condition and spray, DO under ice |
| Liner or embankment damage | Erosion, burrowing animals, wave action, aging | Visual inspection, groundwater monitoring, splash zones |
1. High Effluent TSS (Solids and Algae)
What it looks like: cloudy or green effluent, TSS above the permit limit, worse in warm months.
Why it happens: an aerated lagoon has no sludge return, so biological solids leave with the effluent. In warm, sunny months, algae grow in the lagoon and settling basin and add to the solids. Short-circuiting, accumulated sludge and an undersized or poorly arranged settling basin make it worse.
What to do:
- Check the effluent under a microscope: algae, bacterial floc or both
- Survey sludge depth in the settling basin and lagoon, and remove sludge if it has reduced volume
- Add baffles to the settling basin to improve flow pattern
- Evaluate polishing (for example filtration) for stricter limits
- If limits keep tightening, consider adding a clarifier and sludge return (see how to upgrade an aerated lagoon)
2. High Effluent BOD
What it looks like: BOD above the limit, often alongside high TSS (because solids carry BOD).
Why it happens: loads above the design capacity, low dissolved oxygen, cold temperatures, toxic or shock loads, and reduced treatment volume from sludge buildup.
What to do:
- Compare current flow and load with the design values
- Check the DO profile and aerator performance
- Look for industrial or seasonal discharges and fats, oils and grease
- Survey sludge depth and consider removal
- Check for short-circuiting
3. High Effluent Ammonia
What it looks like: ammonia exceedances, especially in winter and early spring.
Why it happens: the sludge age in a lagoon equals the detention time, which is too short for slow-growing nitrifying bacteria, especially in cold water. Low DO and low alkalinity make it worse.
What to do: optimize DO and alkalinity, remove sludge, and consider adding biomass retention or conversion. Read the full guide on lagoon ammonia removal and the details of lagoon to activated sludge conversion.
4. Odors
What it looks like: rotten-egg or musty smells, complaints from neighbors, often in spring or after upsets.
Why it happens: low dissolved oxygen, decomposing sludge on the bottom, organic overloading, septic influent from long force mains, or inadequate aerator coverage.
What to do:
- Check DO at several locations and depths
- Look for dark, gray or black water and bubbling from the bottom
- Remove accumulated sludge and reduce loading peaks if possible
- Improve aeration coverage and distribution
- Check upstream for septic conditions and consider pre-aeration at the headworks
5. Low Dissolved Oxygen and Aerator Problems
What it looks like: DO below target, aerators tripping, uneven aeration pattern, higher energy use with poor results.
Why it happens: worn or fouled aerators, motor or power problems, ice, undersized aeration for the current load, or poor layout.
What to do:
- Inspect and service aerators and electrical supply on a schedule
- Provide spare units and basic DO monitoring or alarms
- Re-check oxygen demand vs aeration capacity for the current load, including nitrification
- Consider efficient fine-bubble aeration to improve oxygen transfer. See BIOWORKS’ OXIWORKS® aeration system
6. Dead Zones, Short-Circuiting and Poor Mixing
What it looks like: areas of floating solids or low DO, effluent quality worse than expected from the nominal detention time, patches of different color in the lagoon.
Why it happens: aerators clustered in one area, inlet and outlet too close together, no baffles, wind-driven flow, or sludge mounds that change the flow path.
What to do:
- Map DO, temperature and solids at multiple points
- Perform a tracer test to see the real flow pattern
- Add baffle curtains to create plug-flow paths or cells in series
- Redistribute or add aeration where it is needed
7. Sludge Accumulation
What it looks like: shallower water over time, shorter effective detention time, more odors, more ammonia and BOD in the effluent.
Why it happens: settleable solids accumulate on the bottom of lagoons and settling basins over the years, and many lagoons have never been cleaned.
Illustrative impact: a lagoon of about 167,000 ft³ (1.25 million gallons) with a surface area of about 13,900 ft² and 5 days of detention loses about 12 percent of its volume if sludge is spread 1.5 ft deep over the whole floor (13,900 × 1.5 = about 20,900 ft³). Detention time falls from 5 days to about 4.4 days, which can be enough to tip a marginal system into violation. Illustrative only; sludge depth is rarely uniform.
What to do: survey sludge depth, plan removal where needed (dredging, pumping or dewatering), and follow federal and state biosolids rules (for example 40 CFR Part 503) for disposal or beneficial use.
8. Floating Scum, Foam and Duckweed
What it looks like: surface scum, grease balls, foam near aerators, green carpets of duckweed in calm cells.
Why it happens: grease and detergents in the influent, algae and nutrients, low circulation and still surface conditions in settling basins or polishing cells.
What to do:
- Control fats, oils and grease at the source and at the headworks
- Improve surface circulation and use baffles or floating barriers to concentrate and remove surface material
- Remove duckweed physically as needed, since it blocks light and reduces oxygen exchange
9. Algae Blooms and pH Swings
What it looks like: green water, high pH in the afternoon, greater TSS in summer.
Why it happens: sunlight, warm temperatures and nutrients favor algae, which raise pH during photosynthesis. High pH increases the toxic fraction of ammonia in the discharge and algae add to effluent solids.
What to do: manage nutrients and flow patterns where possible, improve settling and polishing, and check effluent pH and ammonia together. Persistent algae problems are a sign that a basic lagoon may need additional treatment steps.
10. Winter Ice and Cold-Weather Problems
What it looks like: ice build-up on surface aerators, spray freezing on equipment and embankments, reduced aeration performance, higher ammonia and BOD.
Why it happens: large surface area, spray from surface aerators, low air temperatures and lower biological activity.
What to do:
- Inspect equipment for ice damage and adjust aerator operation in cold weather
- Consider submerged or fine-bubble aeration that creates less spray
- Design and operate for the coldest condition, not the average
- Review ammonia and BOD results seasonally to decide whether an upgrade is needed
11. Liner, Embankment and Structural Problems
What it looks like: erosion at the water line or near aerators, burrowing animals, cracked or exposed liner, settling embankments, seepage or changes in groundwater monitoring results.
Why it happens: wave action, aerator splash and turbulence, unprotected slopes, vegetation and animals, age and weathering.
What to do:
- Inspect embankments and liner on a schedule and after storms
- Protect liner and slopes under and around aerators (for example with splash pads or rock protection)
- Control vegetation and burrowing animals
- Monitor groundwater where required and report changes promptly
12. Hydraulic Overload and Inflow
What it looks like: high flows during rain or snowmelt, shorter detention time, solids washout, bypassing.
Why it happens: infiltration and inflow into the sewer system, growth beyond the design flow, or poor flow control.
What to do: quantify flows, address sewer defects, add flow equalization or storage where appropriate, and check whether current flows already exceed the design basis.
13. Disinfection Problems
What it looks like: bacteria limits missed even though disinfection equipment is running.
Why it happens: high effluent TSS and algae shield bacteria and reduce UV transmittance, and chlorine demand increases.
What to do: fix solids first, then review disinfection dose, contact time and equipment condition.
How to Diagnose Systematically
- Collect at least a year of data on flow, temperature, BOD, TSS, ammonia, nitrate, alkalinity, pH and DO
- Look at seasonality: problems in winter point to temperature and sludge age; problems in summer point to algae and solids
- Map the lagoon: DO, temperature and solids at several points and depths
- Survey sludge in the lagoon and settling basin
- Run a tracer test if short-circuiting is suspected
- Inspect equipment and structures: aerators, power, controls, liner and embankments
- Compare current loads and flows with the design basis
Preventive Maintenance Checklist
| Frequency | Tasks |
|---|---|
| Daily or weekly | Check aerators and alarms, DO readings, scum and floating material, odors, flow records |
| Monthly | Sample and review BOD, TSS, ammonia, pH and alkalinity trends; inspect embankments and inlet/outlet structures |
| Seasonally | Prepare for ice and cold weather; check for algae and duckweed in warm months; review compliance trends |
| Annually | Service aerators and electrical equipment; survey sludge depth; inspect liner and embankments; review capacity and permit changes |
When to Stop Patching and Upgrade
Some signs mean the lagoon itself has reached its limits:
- Ammonia or TSS violations return every winter or summer despite good operation
- Permit limits have tightened beyond what a lagoon can reliably achieve
- Flows or loads have grown beyond the design basis
- Aerators are repeatedly failing and are no longer efficient
- Sludge removal is needed repeatedly to keep the lagoon compliant
In these cases, a planned upgrade is usually more reliable than repeated fixes. Compare aerated lagoon vs activated sludge, review how to upgrade an aerated lagoon, and see lagoon to activated sludge conversion.
How BIOWORKS Can Help
BIOWORKS provides process design services, floating fine-bubble aeration, Integrated Clarifier solutions, a process guarantee where BIOWORKS supplies or approves the key treatment components, field service and support for the life of the project. BIOWORKS reports that its extended aeration process, installed in 120+ plants in 25 countries, can achieve effluent below 5 mg/L BOD, 10 mg/L TSS and 1 mg/L ammonia-N depending on site and influent conditions.
Learn more on our Aerated Lagoon page, and explore our Municipal Solutions, Industrial Solutions and case studies.
Frequently Asked Questions
What are the most common aerated lagoon problems?
The most common are high effluent TSS (including algae), ammonia violations in cold weather, odors, low dissolved oxygen and aerator failures, dead zones and short-circuiting, sludge accumulation, ice problems and liner or embankment damage.
Why does my aerated lagoon smell?
Odors usually come from low dissolved oxygen, decomposing sludge on the bottom, organic overloading, or septic influent. Check DO at several points, survey sludge depth and look at aeration coverage.
Why is my lagoon effluent TSS so high?
Because an aerated lagoon has no sludge return, biological solids and algae leave with the effluent. Short-circuiting, accumulated sludge and an undersized settling basin make it worse, especially in warm months.
How often should a lagoon be cleaned of sludge?
There is no fixed interval. It depends on loading, design and operation. Survey sludge depth regularly and plan removal when it reduces the effective volume enough to affect performance.
When should I upgrade my lagoon instead of fixing it?
When problems keep returning despite good operation, limits have tightened beyond what a lagoon can achieve, flows have outgrown the design, or equipment is repeatedly failing. An upgrade such as fine-bubble aeration with a clarifier and sludge return is then usually more reliable.
Talk to a Lagoon Expert
Is your lagoon struggling? Contact BIOWORKS North America with your permit limits, recent results and basin details, and our process engineers will help you diagnose the problem and choose the right fix.
Notes
This article is general information for troubleshooting and planning purposes. Causes, fixes and requirements vary by site, state and reference; confirm with your engineer and regulator.
