Sewage lagoons fall within the broader field of wastewater stabilization, management, containment and treatment, and are sometimes also referred to as effluent ponds or facultative lagoons. They are essentially large, shallow, open-air ponds that exist as part of a managed wastewater flow or else stand alone, but with the purpose of holding liquid in a containment and treatment process that relies on natural biology, sometimes enhanced by mechanical and chemical additions. Although land-intensive, they are very cost-effective, useful for rural or specialized areas with limited surrounding infrastructure.
The goal is to take human and animal raw sewage, or even industrial pollutants, and employ natural processes to separate and bio-degrade the solids from the liquids. This allows a less toxic water to evaporate or to move on to further stages of treatment or purification. The biological processes include aerobic (with oxygen) and anaerobic (without oxygen) transformations, as well as from algae, and also photosynthesis from sunlight, and evaporation from wind and atmosphere.
All of this must happen without leakage or seepage, keeping the environment’s waterways, life forms and other consumable waters free from contamination. Therefore the design, fabrication and operation are all important parts of the overall working of a successful sewage lagoon. The lagoon must also account for changing seasons that speed up or slow the natural process as temperatures grow warmer or colder, as well as storm activity that can overflow the lagoon with rainfall. Done right, sewage lagoons stand out for their simplicity, cost-effectiveness, and eco-friendliness, harnessing natural processes to treat sewage.
How a Sewage Lagoon Works
An artificial basin dug into the earth, the lagoon is designed to hold sewage for a period of time, during which natural biological processes break down the waste material, resulting in treated effluent. The process is facilitated by sunlight, algae, and microorganisms present in the water, making it a green and sustainable wastewater treatment option.
The operation of sewage lagoons is largely dependent on aerobic and anaerobic processes, facilitated by the depth and design of the pond: some lagoons will be shallow with just an aerobic process, while some will be deeper and with an anaerobic process also happening at the lower depth.
In the aerobic process, oxygen from the air interacts with the water, supporting the growth of aerobic bacteria. These bacteria feed on the sewage, breaking down organic matter into carbon dioxide, water, and other less harmful substances.
In the anaerobic process in deeper sections of the lagoon where oxygen is limited or non-existent, anaerobic bacteria work to decompose the organic material in the absence of oxygen, producing methane, carbon dioxide, and other byproducts.
Algae plays a crucial role by producing oxygen through photosynthesis, which is essential for aerobic bacteria. The symbiotic relationship between algae and bacteria is central to the effectiveness of lagoons. Oxygen and wave action can be added mechanically, and biological additives can enhance the bacterial and algae activity.
This relatively simple treatment results in water that can ultimately be discharged into local water bodies or reused, subject to meeting environmental standards.
How to Build a Sewage Lagoon
Sewage lagoons are particularly advantageous for rural communities, agricultural operations, and industries with large land areas. They are cost-effective for small to medium-sized municipalities that do not have the high population density to justify more expensive, mechanized treatment plants. Industries that produce organic wastes, such as dairy, swine, and poultry farming, also find sewage lagoons an appropriate solution for their wastewater management needs. Remote accommodations such as hotels and military camps are other venues, as are temporary activities such as fairs and festivals attracting many people and in isolated (and frequently pristine) areas.
Time spent carefully planning a sewage lagoon is time well spent. As well as relevant regulations to comply with, a broken or deteriorating operation will burden the surrounding environment with contaminants it can’t absorb. Factors to calculate include the volume of sewage to be treated, the composition of the waste, local climate conditions, and the desired quality of the treated effluent.
The location must have adequate space and suitable terrain to dig the basin, and ideally with a low groundwater table to avoid contamination. For an aerobic operation, a lagoon no deeper than about three to four feet is optimal to make use of penetrating sunlight, while a basin as deep as about twenty or even thirty feet can accommodate an anaerobic process.
The banks of the basin must be gradual and shallow: too steep will risk eroding or caving in over time. Most typically, the lagoon is lined with a synthetic geomembrane liner to be impervious to seepage. Contrary to myth, concrete and clay liners ARE actually permeable, and are generally NOT suitable for toxic containment such as sewage treatment. Bare soil of course is the worst bedding for a sewage lagoon and will leak far too much effluent into the surrounding environment.
A strong and flexible geomembrane liner is typically the ideal solution for a sewage lagoon (see our lagoon liners for more). Expertise in securing the edges of the liner along the bank is important – the liner installation needs to be done right – as well as knowing how to drain the lagoon and extract the solids over time.
General water management is an important element to maintain the integrity of the lagoon. Outside of larger municipal wastewater management systems, sewage lagoons often stand alone as single-cell basins, but if storm water or other upstream waters can threaten to overflow the lagoon at any time, then an overflow pond should also be considered.
Aeration is part of the engine of the breakdown process. While many lagoons rely on natural aeration, some may need mechanical aeration systems to increase their treatment capacity and efficiency. Overgrowth of algae can become a problem too, and the lagoon should be checked over time for bad odors, which can indicate sulfur dioxide production, and mosquito breeding.
Wind and wave action will keep down mosquitoes, and for this the lagoon should be in the open, not hemmed by trees or bordered with vegetation. The lagoon should be a relatively clean-smelling place, with a light green surface color – the worse it looks and smells, the more out of balance the biological system is becoming. Finally, animals and children don’t read signs. The lagoon should be securely fenced with wildlife fencing to prevent intrusions: anything that can thrive in the lagoon is already there, and no other creature needs to explore it.
