Lagoon Ammonia Removal: Causes, Solutions and Upgrades
Lagoon Ammonia Removal: Why Lagoons Fail and How to Fix It
Ammonia is the most common reason an otherwise healthy lagoon starts failing its permit. The lagoon still removes BOD, the effluent looks fine, but the lab results show ammonia above the limit, often in winter and early spring. As regulators tighten ammonia limits to protect streams and rivers, more lagoon owners are facing the same question: how do we get ammonia out of our lagoon effluent?
This guide explains lagoon ammonia removal: how ammonia is formed and removed, why lagoons struggle, how to diagnose the problem, and the practical options from simple operating changes to full conversion to activated sludge.
For background, read aerated lagoon design, aerated lagoon vs activated sludge and how to upgrade an aerated lagoon.
Why Ammonia Limits Matter
Ammonia in effluent can be toxic to fish and aquatic life, and it consumes oxygen in the receiving stream as it converts to nitrate. Permit limits often vary by season and depend on the stream’s flow, pH and temperature, which is why winter and low-flow limits can be strict. Many lagoons built for BOD and TSS limits were never designed for them.
How Ammonia Is Formed and Removed
Nitrogen enters a lagoon mainly as organic nitrogen and ammonia (together called TKN). In the lagoon:
- Ammonification: organic nitrogen is converted to ammonia.
- Nitrification: specialized bacteria oxidize ammonia first to nitrite and then to nitrate. This requires oxygen, alkalinity and enough time.
- Other pathways: some ammonia leaves by volatilization (stripping to air when pH is high, mainly in pond-type systems in summer), some is taken up by algae and biomass, and some settles with solids.
In an aerated lagoon, nitrification is the main route to real ammonia removal. It is the step that tends to fail.
What Nitrification Needs
| Requirement | What is needed |
|---|---|
| Oxygen | About 4.57 lb of oxygen per lb of ammonia-nitrogen oxidized, and dissolved oxygen of about 2 mg/L or more |
| Alkalinity | About 7.14 lb of alkalinity (as CaCO₃) consumed per lb of ammonia-nitrogen oxidized; keep a residual (often 50 mg/L or more) |
| pH | Best in roughly the 7 to 8.5 range |
| Temperature | Rates drop sharply as water cools toward 10°C (50°F) and below |
| Sludge age | Long enough to keep slow-growing nitrifying bacteria in the system |
| No inhibition | Avoid toxic shocks, low pH and very high ammonia loads |
Why Aerated Lagoons Fail at Ammonia Removal
1. Sludge age equals detention time
A typical aerated lagoon has no sludge return, so the biomass leaves with the effluent. Nitrifying bacteria grow slowly, and when the sludge age is only a few days they are washed out faster than they can grow, especially in cold water.
2. Cold temperatures
Lagoons have large surface areas and lose heat. In winter, water temperature drops and nitrification slows or nearly stops. Ammonia then builds up and is released in effluent, often with a spike in spring.
3. Not enough oxygen or mixing
Old surface aerators may not deliver the oxygen nitrification needs, and dead zones or short-circuiting leave parts of the basin with low dissolved oxygen.
4. Alkalinity runs out
Nitrification uses a lot of alkalinity. In low-alkalinity wastewater, pH falls, and nitrification slows down further.
5. Accumulated sludge
Sludge deposits reduce usable volume and release ammonia back into the water as they decompose.
6. Ice cover and seasonal conditions
Ice limits surface oxygen transfer in some designs, and seasonal swings (for example tourism or industrial campaigns) change the load.
Step 1: Diagnose the Problem
Good data points to the right fix. Collect at least a year of results and look at seasonality.
| What you see | What it suggests |
|---|---|
| High effluent ammonia, very low nitrate | Nitrification is not happening (cold, short sludge age, low DO or inhibition) |
| Some nitrate but still high ammonia | Partial nitrification; check DO, alkalinity and loading |
| High nitrite | Incomplete nitrification, often from low DO, low pH or inhibition |
| Falling pH and alkalinity | Alkalinity is being consumed; supplementation or process change needed |
| Ammonia spikes in spring | Cold-weather accumulation and sediment release |
| Ammonia is fine in summer, fails in winter | Temperature-limited nitrification and short sludge age |
| Ammonia increases after sludge disturbance | Sediment ammonia release; consider sludge removal |
Also check influent TKN and ammonia, dissolved oxygen profile, temperature profile, sludge depth and short-circuiting (tracer testing if possible).
Step 2: Choose a Solution
Option 1: Optimize operation and aeration
- Raise and maintain DO around 2 mg/L or higher where nitrification is needed
- Fix dead zones and short-circuiting with baffles and aerator repositioning
- Remove accumulated sludge
- Check alkalinity and add supplemental alkalinity when needed
- Reduce ammonia loads from side streams or industrial sources where possible
These changes help but rarely solve strict winter limits on their own.
Option 2: Add an attached-growth nitrification stage
An attached-growth process such as a trickling filter, rotating biological contactor, moving-bed reactor or submerged attached-growth reactor gives nitrifying bacteria a surface to grow on, so they are retained regardless of lagoon detention time. These stages can work in colder water, and they can be added downstream of the lagoon. They add a separate process to operate and maintain.
Option 3: Retain biomass with a clarifier and sludge return
Adding a clarifier and return sludge pumping lets the lagoon retain biomass and decouples sludge age from detention time. This is the key step toward activated sludge-style performance.
Option 4: Convert to extended aeration activated sludge
The most reliable year-round approach is to turn the lagoon (or part of it) into an extended aeration basin with efficient fine-bubble aeration, a clarifier and sludge return. The long sludge age of extended aeration supports consistent nitrification, even in cold weather, in a much smaller treatment volume. See how extended aeration works and extended aeration design criteria. BIOWORKS’ OXIWORKS® aeration system and Integrated Clarifier are designed for this type of conversion. Learn more on our Aerated Lagoon page.
Option 5: Add nitrogen removal (denitrification)
If total nitrogen limits apply, add anoxic zones so nitrate is converted to nitrogen gas. Denitrification also returns some alkalinity, easing the alkalinity demand of nitrification.
Option 6: Physical-chemical methods
Air stripping with pH adjustment, ion exchange and breakpoint chlorination can remove ammonia, but they involve chemical handling and residuals and are generally used for polishing, special cases or short-term compliance rather than as the primary solution for lagoons.
Option 7: Operational strategies where permitted
Some permits allow controlled or seasonal discharge, holding effluent through the coldest months and discharging when stream flows or temperatures are favorable. Check with your regulator whether this is allowed.
Comparison of Options
| Option | How it removes ammonia | Strengths | Limitations |
|---|---|---|---|
| Optimize operation and aeration | Improves conditions for existing nitrifiers | Quick, uses existing assets | Limited in cold weather or with strict limits |
| Attached-growth stage | Retains nitrifiers on media | Works in colder water, targeted at ammonia | Adds a new process; solids and recycle management |
| Clarifier plus sludge return | Retains biomass, increases sludge age | Big improvement, reuses lagoon | Needs clarifier, aeration and controls |
| Conversion to extended aeration | Long sludge age and efficient aeration | Reliable year-round nitrification, scalable, reuses basin | Larger project, requires design and startup |
| Add anoxic zone | Converts nitrate to nitrogen gas | Meets total nitrogen limits, recovers alkalinity | Needs recycle and zoning |
| Physical-chemical | Strips or exchanges ammonia | Fast for special cases | Chemicals and residuals; rarely first choice |
Illustrative Example: Oxygen and Alkalinity for Nitrification
Illustrative only. Assumptions: 0.25 MGD lagoon, influent TKN 40 mg/L, effluent ammonia target of about 2 mg/L, about 35 mg/L of nitrogen to be nitrified (the rest assimilated or left as effluent nitrogen).
| Item | Calculation | Result |
|---|---|---|
| Nitrogen to be nitrified | 0.25 MGD × 35 mg/L × 8.34 | about 73 lb N/day |
| Oxygen for nitrification | 73 × 4.57 | about 334 lb O₂/day |
| Alkalinity consumed | 73 × 7.14 | about 521 lb/day as CaCO₃ (about 250 mg/L) |
For comparison, the carbonaceous oxygen demand of a lagoon of this size is on the order of 500 lb O₂ per day (see our aerated lagoon design example), so nitrification adds roughly 60 percent more oxygen demand. Many wastewaters contain less alkalinity than this example needs, so alkalinity supplementation or denitrification is often required. This is why aeration capacity and alkalinity must be checked in any ammonia upgrade.
Cold-Weather Considerations
- Design for the coldest water temperature, not the average
- Longer sludge age, deeper basins, covers, or smaller exposed surface area all help keep nitrification going
- Efficient fine-bubble aeration can reduce ice and spray problems compared with some surface aerators
- Check that aeration equipment and controls tolerate winter conditions
- Consider storage or controlled discharge where the permit allows
Monitoring Plan for Ammonia Control
| Parameter | Why it matters |
|---|---|
| Influent and effluent ammonia, TKN | Shows loading and removal |
| Nitrate and nitrite | Confirms nitrification progress |
| Alkalinity and pH | Needed to sustain nitrification |
| Dissolved oxygen profile | Reveals dead zones and oxygen limits |
| Temperature | Explains seasonal performance |
| Sludge depth | Shows lost volume and ammonia release risk |
For converted basins, see extended aeration plant startup to understand how long nitrification takes to establish.
Common Mistakes
- Adding aeration without increasing sludge age
- Designing for summer conditions only
- Forgetting alkalinity demand
- Ignoring sludge accumulation
- Treating one winter’s results as the full picture
- Choosing a short-term fix when limits are likely to tighten further
How BIOWORKS Can Help
BIOWORKS provides process design services, efficient fine-bubble aeration, integrated clarification and a process guarantee where BIOWORKS supplies or approves the key treatment components, along with 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.
Explore our Municipal Solutions, Industrial Solutions and case studies.
Frequently Asked Questions
1: Why does ammonia increase in my lagoon effluent in winter?
Cold water slows or stops nitrification, and because a typical aerated lagoon has no sludge return, the slow-growing nitrifying bacteria cannot build up. Ammonia accumulates through winter and often spikes in spring.
2: How do you remove ammonia from a lagoon?
By encouraging nitrification: provide enough oxygen and alkalinity, keep the right pH, and increase sludge age by retaining biomass. That can be done with a clarifier and sludge return, an attached-growth nitrification stage, or conversion to extended aeration activated sludge.
3: Does aeration alone remove ammonia?
Not by itself. Aeration supplies oxygen, but nitrifying bacteria must also be retained long enough, and alkalinity and temperature must be suitable. More aeration without more sludge age may not fix ammonia limits.
4: What is the best upgrade for a lagoon that cannot meet ammonia limits?
It depends on limits, climate and site, but converting the lagoon to extended aeration activated sludge is often the most reliable year-round solution because it controls sludge age independently of detention time. An attached-growth stage can be a targeted alternative.
5: How much alkalinity does nitrification use?
About 7.14 lb of alkalinity as CaCO₃ for every lb of ammonia-nitrogen oxidized. If wastewater alkalinity is low, supplementation or denitrification may be needed to keep pH stable.
Talk to a Lagoon Upgrade Expert
Is your lagoon failing ammonia limits? Contact BIOWORKS North America with your permit limits, flows and winter data, and our process engineers will help you find the right solution.
Notes
This article is general information for planning purposes. Ammonia limits, performance and design values vary by site, state and reference; confirm with your engineer and regulator.
