What Is a Covered Anaerobic Lagoon?
A covered anaerobic lagoon is a low-cost biogas reactor: an earthen lagoon sealed with a flexible membrane that captures gas and controls odor.
A covered anaerobic lagoon (CAL) is a wastewater treatment lagoon sealed with an impermeable flexible membrane that does two jobs at once: it captures the biogas generated by anaerobic digestion of the effluent and it controls odor and emissions. In practice it is a low-cost anaerobic digester built over an earthen excavation instead of a steel or concrete tank.
The top membrane (usually HDPE or a reinforced geosynthetic) floats on the liquid and inflates as gas accumulates, forming a dome. The biogas is drawn off through a suction point and piped to a flare, a boiler or a gas engine. The bottom and side slopes are lined with a bottom geomembrane to prevent infiltration into the soil and groundwater.
How it works
Effluent enters at one end and moves slowly toward the outlet. In the absence of oxygen, bacterial communities break down the organic matter in four stages (hydrolysis, acidogenesis, acetogenesis and methanogenesis), releasing biogas made up mainly of methane (CH4) and carbon dioxide (CO2).
- Temperature: it operates at ambient temperature (psychrophilic/mesophilic depending on climate), which is why it needs long retention times.
- Hydraulic retention time (HRT): typically 20 to 60 days, much longer than in heated reactors.
- Depth: on the order of 4 to 6 m to keep stable anaerobic conditions.
Typical reference figures
| Parameter | Indicative range |
|---|---|
| HRT | 20–60 days |
| Methane content in biogas | 55–70% CH4 |
| COD removal | 60–80% (depending on substrate and climate) |
| Lagoon depth | 4–6 m |
| HDPE membrane service life | 15–20 years (depending on thickness and UV) |
Why it matters
Compared with a tank-type digester (CSTR or UASB), the covered lagoon offers a lower upfront investment and a strong ability to buffer variable loads, at the cost of more land area and more limited process control. It is widely used for medium-strength agro-industrial effluents: dairy farms, pig operations, slaughterhouses and food processing plants, where volumes are high and effluent temperature is moderate.
The cover turns an environmental liability into an asset: an open anaerobic lagoon emits methane (a greenhouse gas) and odors; covering it captures that methane for use as energy or, at minimum, flares it to reduce its impact.
Where it fits in Tech Tank projects
Tech Tank delivers turnkey biogas and biodigester solutions, including the supply and installation of domes and covers and gas-capture membranes. To understand the equipment downstream that stores the gas, read what an industrial biodigester is; and for the role of covers in odor and emission control, see aluminum covers for odor and emission control. In Paraguay, any biogas project must comply with MADES environmental regulations (Law 294/93 on Environmental Impact Assessment); we recommend verifying the current environmental license for each case.
If you are evaluating covering an existing lagoon or designing a new one, get in touch for a technical assessment: we size the membrane, the gas-capture system and the gas utilization according to your effluent.
Frequently Asked Questions
Is a covered anaerobic lagoon the same as a biodigester?
Functionally, yes, it is a biodigester. The difference is construction: instead of a steel or concrete tank it uses a lined earthen excavation covered with a membrane. That makes it cheaper but it needs more land.
What material is used for the cover?
The most common choice is an HDPE geomembrane or a reinforced geosynthetic, designed to withstand UV radiation and contact with biogas. Typical service life is 15 to 20 years depending on thickness and exposure.
How much biogas does it produce?
It depends on the substrate, the organic load and the temperature. Biogas usually contains between 55% and 70% methane; flow is estimated from the volatile solids fed in and the retention time.
Does it work in cold climates?
It performs best in temperate and warm climates like Paraguay's, where ambient temperature sustains bacterial activity. In cold climates output drops and it may require heating or a switch to a tank-type reactor.
Does it reduce odors?
Yes. Sealing the lagoon surface with the membrane contains odorous gases (mainly H2S and volatile compounds) and channels them to the flare or gas engine instead of releasing them to the atmosphere.