How to Upgrade an Aerated Lagoon: Options and Steps
How to Upgrade an Aerated Lagoon: Options, Process and Step-by-Step Plan
Many aerated lagoons were built decades ago for simpler limits and smaller populations. Today they face tighter ammonia, TSS and nutrient requirements, more flow, aging aerators and bottoms filled with sludge. Replacing a lagoon with an entirely new plant is not always necessary. In many cases the existing basin, liner and structures can be reused and upgraded.
This guide explains how to upgrade an aerated lagoon: how to tell if it needs help, how to find the real limiting factor, the main upgrade options from simple to comprehensive, how to handle accumulated sludge, how to keep the lagoon running during construction, and a step-by-step project plan.
For background, read aerated lagoon design and aerated lagoon vs activated sludge.
Signs Your Aerated Lagoon Needs an Upgrade
- Effluent ammonia violations, especially in winter
- Effluent TSS or BOD exceedances, or algae carryover in warm months
- A new or renewed permit with tighter limits (ammonia, nitrogen, phosphorus, TSS, disinfection)
- Flows or loads that have grown beyond the original design
- Aerators that are failing, hard to maintain or no longer efficient
- Odors, dead zones, floating scum or uneven mixing
- Sludge buildup that has reduced the effective volume
- Liner, embankment or erosion problems
Step 1: Find the Real Limiting Factor
Upgrades fail when they solve the wrong problem. Start with data and a site assessment.
| Assessment | What to check |
|---|---|
| Performance history | Influent and effluent BOD, TSS, ammonia, nutrients, flow and temperature by season |
| Capacity | Current flow and load vs design, peak flows, infiltration and inflow |
| Sludge survey | Depth and distribution of accumulated sludge in the lagoon and settling basin |
| Hydraulics | Short-circuiting, dead zones, baffle condition, inlet and outlet arrangement (tracer test where possible) |
| Dissolved oxygen and mixing | DO profile across the basin, aerator coverage and performance |
| Temperature | Winter water temperature and ice formation |
| Liner and embankments | Condition, leaks, erosion, freeboard and access |
| Mechanical and electrical | Aerator condition, power supply, controls and alarms |
| Permit | Current and expected limits, compliance schedule and deadlines |
The goal is to name the main problem in one sentence, for example: “Ammonia fails in winter because sludge age is too short and aeration is inefficient,” or “TSS fails because the settling basin is full of sludge.”
Step 2: Match the Problem to an Upgrade Option
| Main problem | Likely upgrade direction |
|---|---|
| Reduced volume from sludge, short-circuiting | Sludge removal, baffle curtains, inlet and outlet changes |
| Old, inefficient or failing aerators | Fine-bubble aeration retrofit with DO control |
| High effluent TSS | Improved settling basin, clarifier, or polishing step |
| Winter ammonia failure | Sludge retention (clarifier and return), added attached-growth nitrification, or full conversion to activated sludge |
| Nitrogen or phosphorus limits | Anoxic zone, chemical or biological phosphorus removal |
| Growth beyond design capacity | Conversion to extended aeration, additional cells or a parallel train |
| Liner or structural failure | Liner repair or replacement, or a different treatment approach |
Upgrade Options, from Simple to Comprehensive
Option 1: Optimize what you have
Before adding equipment, look for improvements in operation and layout:
- Re-position or re-balance aerators to remove dead zones
- Install baffle curtains to stop short-circuiting and create cells in series
- Improve influent screening and grease control
- Remove accumulated sludge to restore volume
- Add basic dissolved oxygen monitoring and control
These steps can recover performance, but they rarely solve tight ammonia or nutrient limits on their own.
Option 2: Upgrade the aeration system
Older surface aerators can be inefficient, can create ice and spray in winter and are often difficult to maintain. Replacing or supplementing them with floating fine-bubble aeration lines improves oxygen transfer, spreads air more evenly and can often be installed without draining the lagoon. Combined with automatic DO control and efficient blowers, this reduces energy use per pound of oxygen delivered. See BIOWORKS’ OXIWORKS® aeration system.
Option 3: Improve solids separation
Lagoons discharge biological solids. Options include enlarging or cleaning the settling basin, adding floating baffles, adding a clarifier, or adding polishing such as filtration where limits are strict. Chemical addition can help with TSS and phosphorus in some cases.
Option 4: Add a nitrification stage
When ammonia is the only problem, a downstream attached-growth stage (such as a trickling filter, moving-bed reactor or submerged attached-growth reactor) can provide the sludge age that the lagoon cannot. It adds a separate process to operate, so compare it carefully with a full conversion.
Option 5: Convert to extended aeration activated sludge
This is the most comprehensive and often the most effective route. A portion of the existing lagoon (or the whole basin) becomes an aeration basin with fine-bubble aeration, and a clarifier plus return sludge system retain the biomass so sludge age is controlled independently of detention time. The result is much better and more consistent performance, including year-round ammonia removal, in a smaller treatment volume. Remaining lagoon volume can be reused for equalization, storage, polishing or future capacity.
BIOWORKS specializes in this kind of conversion using floating fine-bubble aeration and the Integrated Clarifier. Learn more on our Aerated Lagoon and Extended Aeration Activated Sludge System pages, and see how extended aeration works.
Option 6: Add nutrient removal
If nitrogen or phosphorus limits apply, add anoxic zones for denitrification and chemical or biological phosphorus removal. These are easier to incorporate in a converted extended aeration system than in a basic lagoon.
Option 7: Replace or rebuild
If the liner has failed, the site is unsuitable or land is needed for other uses, a new plant (package, modular or custom) may be the better path. Our guide to package plant vs custom wastewater treatment plant can help you compare.
Illustrative Example: Capacity Headroom from a Conversion
Illustrative only. Assumptions: an existing 1.25 million gallon aerated lagoon serving 0.25 MGD (5 days detention). A converted extended aeration section is designed for 24 hours detention.
| Item | Calculation | Result |
|---|---|---|
| Volume needed for 0.25 MGD at 24 hours | 0.25 MGD × 1 day | about 0.25 million gal, or about 20 percent of the lagoon |
| Remaining lagoon volume | 1.25 − 0.25 | about 1.0 million gal, available for equalization, polishing or future capacity |
| Theoretical flow if the full volume were run as extended aeration | 1.25 million gal / 1 day | about 1.25 MGD |
The theoretical headroom is much higher than the current flow, but actual capacity is limited by aeration capacity, clarifier size, hydraulics, mixing, permit limits and the engineer’s design. The point is that an existing lagoon often holds far more treatment volume than a plant would need, which is why reuse is attractive.
Managing Accumulated Sludge
Sludge accumulates on the bottom of lagoons and settling basins for years. Before or during an upgrade:
- Measure sludge depth and volume with a survey
- Decide whether to remove it fully, partially or leave it in place in non-critical areas
- Choose a removal method (dredging, pumping, dewatering in bags or tubes) and a compliant disposal or beneficial use route
- Follow federal and state biosolids rules (for example 40 CFR Part 503) and local requirements
Sludge removal restores volume and removes a source of oxygen demand and ammonia release, so it is often a good first step even when other upgrades follow.
Keeping the Lagoon in Service During the Upgrade
A major advantage of lagoon upgrades is that treatment can usually continue during construction:
- Floating fine-bubble aeration lines can often be installed and commissioned in the operating basin without draining it
- Baffle curtains and clarifiers can be installed in stages
- Temporary bypass and flow control keep the plant compliant
- Work is scheduled around cold weather and permit sensitivities
Your engineer and supplier should prepare a phasing plan that protects compliance throughout.
Step-by-Step Upgrade Project Plan
- Collect data and assess the lagoon (performance, sludge, hydraulics, liner, equipment)
- Confirm current and expected permit limits with the regulator
- Define the main problem and goals in clear terms
- Evaluate alternatives in a preliminary engineering report, including optimization, aeration upgrade, settling, nitrification stage and conversion
- Select the option and complete design: hydraulics, aeration, clarifier, return sludge, controls, baffles and electrical
- Permitting and approvals from the state, including plan review
- Phased construction while keeping treatment in service
- Startup and biomass development. For a converted basin, see extended aeration plant startup
- Performance testing and operator training
- Monitor, optimize and plan for future limits
Common Mistakes to Avoid
- Upgrading aeration without addressing solids separation
- Ignoring sludge accumulation and lost volume
- Designing only for the summer condition and forgetting winter
- Not checking mixing and dead zones after adding new aeration
- Overlooking liner and embankment condition
- Skipping a phasing plan that keeps the system compliant
- Not planning for tighter future limits
Questions to Ask Your Supplier
- How will the upgrade be phased so the lagoon stays compliant?
- What effluent performance is guaranteed, and under what conditions?
- How will cold weather and ice be handled?
- What is the plan for existing sludge?
- How is the new system controlled, monitored and maintained?
- What training and long-term support are provided?
- Can you share reference projects with similar lagoon upgrades?
How BIOWORKS Can Help
BIOWORKS provides process design services for lagoon upgrades, floating fine-bubble aeration, integrated clarification, a process guarantee where BIOWORKS supplies or approves the key treatment components, 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.
Explore our Municipal Solutions, Industrial Solutions and case studies.
Frequently Asked Questions
1: Can I upgrade an aerated lagoon without draining it?
Often yes. Floating fine-bubble aeration lines and baffle curtains can frequently be installed in an operating lagoon, and clarifiers can be built beside it, so treatment continues during construction. Your engineer should prepare a phasing plan.
2: What is the first step in upgrading an aerated lagoon?
Collect performance data and assess the lagoon: effluent trends, sludge depth, hydraulics, dissolved oxygen, liner and equipment condition, and permit requirements. This identifies the real limiting factor before you choose an upgrade.
3: How can an aerated lagoon meet ammonia limits?
By increasing sludge age. That can be done by adding a clarifier and sludge return (converting to extended aeration activated sludge), adding an attached-growth nitrification stage, and improving aeration efficiency and alkalinity management.
4: Do I need a clarifier when upgrading a lagoon?
If you want to retain biomass and control sludge age, yes. A clarifier with return sludge is what allows a lagoon to operate like an activated sludge plant. For modest TSS problems, an improved settling basin or polishing step may be enough.
5: How long does a lagoon upgrade take?
It depends on the scope, permitting, design and equipment lead times. Simple aeration and baffle upgrades are faster than full conversions. Your engineer and supplier can provide a schedule once the scope is defined.
Talk to a Lagoon Upgrade Expert
Is your lagoon struggling with ammonia, TSS or tighter limits? Contact BIOWORKS North America with your permit limits, flows and basin details, and our process engineers will help you evaluate the best upgrade path.
Notes
This article is general information for planning purposes. Upgrade options, performance and requirements vary by site, state and reference; confirm with your engineer and regulator.
