Why Small Towns Still Use Lagoons

The Capital Cost Advantage

The economics of lagoons are driven by one fact: dirt is cheap and concrete, steel, and mechanical equipment are expensive. A facultative lagoon is fundamentally a lined hole in the ground with an inlet, an outlet, and a liner to prevent groundwater contamination. The major cost components are earthwork, liner (synthetic or clay), inlet and outlet structures, fencing, and the land itself. For a town of 1,000 people (about 300 connections), a facultative lagoon system costs roughly $2,000 to $5,000 per connection, or $600,000 to $1,500,000 total.

A mechanical activated sludge plant serving the same community costs $8,000 to $20,000 or more per connection, or $2,400,000 to $6,000,000 total. The difference is concrete basins, mechanical aerators or blowers, clarifiers, sludge handling equipment, a control building, SCADA systems, and all the associated electrical and plumbing. For a small rural community with a median household income of $40,000 and limited bonding capacity, the difference between a $1 million project and a $4 million project is the difference between affordable and impossible.

Operating costs show a similar gap. A lagoon system serving 1,000 people might cost $30,000 to $60,000 per year to operate: mowing, fence maintenance, sampling, reporting, and occasional sludge removal. A mechanical plant costs $100,000 to $200,000 per year: energy, chemicals, equipment maintenance, sludge disposal, and at least one full-time operator with a higher license class. The mechanical plant needs an operator on site or on call every day. The lagoon needs someone to check it a few times a week.

State revolving fund financing makes both options more affordable, but the debt service still has to be covered by user rates. A $1 million lagoon financed at 2 percent over 20 years costs about $61,000 per year in debt service. A $4 million mechanical plant costs $243,000 per year. For 300 connections, that is the difference between $17 per month per household and $67 per month per household, just for the capital cost. Add operating costs and the gap widens further. This is why small towns still build lagoons.

Loading Rates Explained

The primary design parameter for a facultative lagoon is organic loading rate, expressed in pounds of BOD per acre per day. This number determines how large the lagoon needs to be for a given wastewater flow and strength. The loading rate is driven almost entirely by climate: warmer climates support higher loading because biological activity increases with temperature, and sunlight drives algal oxygen production that supplements the treatment process.

Typical design loading rates range from 15 to 20 lbs BOD/acre/day in cold northern climates (Minnesota, Wisconsin, Montana) to 40 to 80 lbs BOD/acre/day in hot southern climates (Texas, Arizona, Florida). The tenfold range in regulatory guidance means that a lagoon in Texas can be one-quarter the size of a lagoon in North Dakota for the same population. A 1,000-person community generating 200 lbs BOD/day at a loading rate of 20 lbs/acre/day needs 10 acres of primary cell. At 60 lbs/acre/day, that drops to 3.3 acres.

When a lagoon is overloaded (too much BOD for the available surface area), the aerobic zone at the surface is overwhelmed by oxygen demand from below. The lagoon transitions from a green, healthy, aerobic-facultative system to a dark, odorous, anaerobic system. The classic sign is a purple or pink color caused by photosynthetic sulfur bacteria that thrive in anaerobic conditions. The odor is hydrogen sulfide (rotten eggs), which generates complaints and regulatory attention. Recovery requires reducing the load, which usually means diverting flow or adding supplemental aeration.

Hydraulic loading and detention time are secondary design parameters. Typical hydraulic detention times are 90 to 180 days for primary facultative cells and 30 to 90 days for polishing cells. Longer detention provides more treatment and more equalization of peak loads. Shorter detention reduces land requirements but leaves less margin for upset conditions. The combination of organic loading, hydraulic detention, and depth (typically 3 to 5 feet for facultative cells) defines the lagoon geometry.

Winter Performance: The Cold Weather Challenge

Lagoon performance degrades in winter for three reasons: biological activity slows as water temperature drops, ice cover blocks sunlight that drives algal oxygen production, and the reduced treatment capacity means effluent quality deteriorates. A lagoon producing 20 mg/L BOD effluent in July might produce 40 to 60 mg/L in January. Ammonia, which requires nitrifying bacteria that are especially sensitive to cold, can spike to 15 to 25 mg/L in winter from a summer level of 2 to 5 mg/L.

This is why lagoons in northern climates are designed 2 to 3 times larger than the same lagoon would be in Texas. The extra volume provides detention time to store winter effluent that does not meet discharge limits. Many northern lagoon systems operate on a controlled discharge basis: they store wastewater during winter months and discharge only in spring and fall when the stored effluent has been treated by rising temperatures and algal activity. This approach requires storage cells sized for 120 to 180 days of flow.

Ice cover is both a problem and a partial solution. Ice blocks wind mixing and sunlight, which reduces oxygen transfer and algal photosynthesis. But ice also provides insulation that prevents the water from dropping below 32°F, and it reduces heat loss to the atmosphere. The net effect is that water temperature under ice stabilizes at 33 to 39°F depending on the depth and climate. Treatment continues, just slowly.

Ammonia limits in winter are the most common reason northern lagoon systems face upgrades. Many states now impose year-round ammonia limits that lagoons cannot meet in cold weather. A lagoon that has operated for 30 years with a seasonal discharge permit may face a new requirement of 5 mg/L ammonia in the effluent year-round. Meeting that limit requires either adding aeration (partial upgrade), adding a polishing step (constructed wetland or intermittent sand filter), or converting to a mechanical treatment process. Each option increases cost and complexity, but may be necessary to maintain permit compliance.

When Lagoons Stop Working

Lagoons fail for four main reasons: nutrient limits they cannot meet, organic overloading from population growth, regulatory changes that require higher treatment levels, and physical deterioration of the liner or berms. Understanding which failure mode applies to your system determines the appropriate response.

Nutrient limits (nitrogen and phosphorus) are the most common regulatory driver. Facultative lagoons achieve some nitrogen removal through algal uptake and denitrification in the anaerobic zone, but they cannot consistently produce effluent below 10 mg/L total nitrogen or 1 mg/L total phosphorus. When a receiving water is designated as impaired for nutrients, the lagoon system's permit will be tightened to limits that require additional treatment. Phosphorus removal can sometimes be achieved with chemical addition (alum or ferric chloride) and a polishing step, but nitrogen removal requires biological processes that lagoons cannot reliably provide.

Organic overloading happens when a community outgrows its lagoon. If the original design was for 1,000 people and the town has grown to 1,800, the lagoon is receiving 80 percent more BOD than designed. The first signs are elevated effluent BOD and TSS, persistent algae blooms, and occasional odor events. If growth continues, the lagoon will transition to anaerobic conditions and generate complaints. The solution is either to expand (add cells) or to convert to mechanical treatment.

The typical upgrade path for a struggling lagoon follows a predictable sequence. First, add aeration to the primary cell (floating or diffused aerators) to increase oxygen transfer and treatment capacity. This is the cheapest upgrade ($200,000 to $500,000) and can extend the lagoon's useful life by 10 to 20 years. Second, add a polishing step: a constructed wetland, intermittent sand filter, or cloth media filter to reduce effluent TSS and improve ammonia removal. Cost: $300,000 to $1,000,000. Third, if nutrient limits are tight or the population has outgrown the lagoon entirely, convert to mechanical treatment. This is the most expensive option but the only one that can meet stringent nutrient limits reliably year-round.

Each step up in treatment complexity brings higher capital and operating costs, more operator skill requirements, and more regulatory reporting. The decision should be driven by permit requirements, population projections, and a realistic assessment of what the community can afford and operate. A lagoon with added aeration and a polishing wetland is still far cheaper and simpler than a full mechanical plant. Many communities can extend lagoon-based treatment for decades with incremental upgrades.

Sizing a Lagoon for Your Community

Proper lagoon sizing starts with three inputs: design population, per capita flow, and per capita BOD. Design population should be based on 20-year projections, not current census. Per capita flow for small communities is typically 80 to 120 gallons per person per day, but this varies widely with infiltration and inflow (I&I). A community with old clay sewer pipes might see effective per capita flow of 150 to 200 gpd due to groundwater infiltration. Per capita BOD is typically 0.17 to 0.20 lbs per person per day.

Worked example: a town of 800 people, projected to grow to 1,200 in 20 years. Design flow: 1,200 × 100 gpd = 120,000 gpd = 0.12 MGD. Design BOD load: 1,200 × 0.18 lbs/person/day = 216 lbs BOD/day. State design loading rate: 25 lbs BOD/acre/day (northern moderate climate). Required primary cell area: 216 ÷ 25 = 8.6 acres. With a secondary polishing cell at half the primary area (4.3 acres), total lagoon area is about 13 acres. At 4-foot operating depth, primary cell volume is about 37 acre-feet, providing roughly 90 days of hydraulic detention.

Always include a spare cell. A three-cell system (primary, secondary, and spare) allows you to take one cell offline for sludge removal, liner repair, or berm maintenance without losing treatment capacity. The spare cell does not need to be as large as the primary; half to two-thirds the size is adequate. Sludge removal is needed every 10 to 20 years depending on loading, and it requires draining the cell, which takes it out of service for weeks or months. Without a spare cell, you have nowhere to divert flow during maintenance.

Site selection is as important as sizing. The lagoon should be downwind and at least 500 to 1,000 feet from residences (check your state's setback requirements). It needs stable soils that can support berms without excessive settlement. High groundwater is the most common site problem; if the water table is within 2 feet of the lagoon bottom, you need a synthetic liner and potentially a groundwater monitoring system. The land must be accessible for sludge removal equipment (truck-mounted dredge or excavator access ramp). A site that is cheap to buy but expensive to develop (poor soils, high water table, poor access) is not a bargain.