Lagoon Septic Systems: What they are and how they work
For people in rural or remote communities, septic tanks and subterranean discharge are the most prevalent ways of treating wastewater.
This said variations in soil, topography and proximity to rivers or water basins mean not every location allows for underground discharge. And not every home or building discharges an amount of wastewater that will fit the capacity of a common septic system.
In previous articles, we have explored various types of septic systems (aerobic, above ground systems etc). Here, we will look into a septic system alternative that is well suited in dealing with wastewater treatment: the Lagoon System.
What is a lagoon septic system?
A sewage lagoon is a large pond-like basin, into which wastewater flows for storage and treatment. Much of the treatment in a lagoon septic system occurs naturally.
Environmental factors like sun and wind provide light, warmth and oxygen to the lagoon water, which encourages bacterial growth. This bacteria is essential for breaking down the sewage and effluent.
The sun also promotes the growth of algae, which helps the bacteria do its work. The wind helps evaporate the water and creates waves that stop insects from breeding and living in the lagoon. Disease-causing mosquitoes, for instance, need still water to breed.
To speed up the treatment process or to allow existing systems to treat a larger amount of wastewater, some systems use aeration devices. As we’ve covered in our article about aerobic septic systems, these devices pump oxygen into the wastewater, which boosts bacterial growth and makes treatment more efficient.
Lagoon systems must be designed to suit the location and use. Factors to consider include the local climate, how much land is available, how deep the soil goes and the type of soil present. Other important factors include the amount and type of wastewater to be treated and the level of treatment required by regulations.
Wastewater that leaves a lagoon is usually returned to the environment. But before this happens, additional treatment (or “polishing”) may be required to remove excess nutrients or disease-causing organisms.
If needed, lagoons are lined with clay or an artificial lining material to prevent the wastewater from contaminating the groundwater below.
There are a number of different terms for lagoon systems
The terms ‘lagoon’ and ‘pond’ are both correct and often used interchangeably. A ‘sewage lagoon’ might also be called an ‘effluent pond’.
Then, there are terms like “anaerobic”, “aerobic”, “hydrograph” and “continuous or controlled discharge”, which refer to a lagoon system’s specific design or treatment method. This can get a little confusing when you’re trying to evaluate lagoon systems.
To clarify these terms and to make your evaluation easier, outlined below are some of the most common types of lagoon systems.
Anaerobic means “living, active, occurring, or existing in the absence of oxygen”. So without oxygen is exactly what you can expect the conditions to be inside this type of lagoon.
Usually 8 to 15 feet deep (which is quite deep for lagoon systems), anaerobic lagoons are designed to hold and treat wastewater over a period of 20 to 150 days. They are often just the first step in a series of lagoons because the wastewater that leaves an anaerobic lagoon will need further treatment. Why? Because the lack of oxygen means there are fewer bacteria present in the lagoon to help break down the effluent.
Anaerobic lagoons work in a very similar way to septic tanks. Wastewater flows into the lagoon and eventually separates into layers. Rising to the top are grease, scum, and other floating materials. Settling on the bottom are the solids, or “sludge”, which accumulate over time and must be pumped out periodically.
Along with being the first treatment step in systems with multiple lagoons, anaerobic lagoons are typically used to treat waste from commercial or industrial activities, and animal waste from farms.
While the smell of an anaerobic lagoon can be an issue, there are a number of solutions such as regular maintenance, recirculating pond effluent and adding sodium nitrate.
Naturally aerobic lagoons
These types of lagoons are usually much shallower than other lagoons so the wastewater is more exposed to sunlight, wind and air. The not so secret ingredient when it comes to aerobic lagoons is oxygen.
As we’ve discovered already, when oxygen is present throughout a lagoon, this promotes the growth of both bacteria and algae. Bacteria help break down the nutrients and germs in the wastewater, and algae further encourage bacterial growth.
Typically, wastewater should remain in an aerobic lagoon from 3 to 50 days to receive adequate treatment. Exact storage times are based on factors such as the lagoon’s design, the amount of wastewater to be treated, and the level of treatment desired.
Aerobic lagoons tend to be better suited to warm, sunny climates, where they are unlikely to freeze. Every now and then, the wastewater might need to be stirred to allow sunlight to reach all of the algae and to stop the algae from forming a layer impenetrable by the sun or wind.
The shallowness of aerobic lagoons means their bases often need to be paved or lined, to stop weeds from growing inside them.
Aeration is the process of circulating, mixing or dissolving air into a substance.
When it comes to septic systems, aeration allows more oxygen to be circulated through the wastewater. This speeds up bacterial growth and makes the bacteria more effective, which means less land area and shorter treatment times are required. Aerated lagoons are therefore very popular in small communities, where land (and thus, lagoon size) is not sufficient for the amount of wastewater that needs or be treated.
There are different types of aerated lagoons, distinguished by the extent of aeration: partial-mix and complete-mix. ‘Partial-mix aerated lagoons’ usually refers to anaerobic lagoons that have been altered and upgraded to be able to treat more wastewater.
While most aerators need energy to operate, these energy costs are often found to be much less than costs for other mechanical treatment systems. This is, as you may have guessed because aerated lagoons require less land and offer a faster and more effective treatment.
Lagoon Systems Are Distinguished by Design
To throw some more lagoon lingo into the mix, lagoon systems can be distinguished by a particular design feature: how they discharge water.
Complete retention lagoons
These types of lagoons never release wastewater. Instead, the wastewater is left to evaporate. Consequently, complete retention lagoons are only practical in very dry climates where the water is more likely to evaporate than be drenched in a downpour.
Hydrograph controlled release lagoons
This design includes devices that measure the depth and quality of the wastewater inside the lagoon, as well as the speed of the water, is received. All these factors are calculated and help the system determine when to release the wastewater. This design can reduce or eliminate the need for further treatment and is often used for lagoons that empty directly into rivers and streams (or “surface water”).
Continuous Discharge Lagoons
These lagoons are designed to release wastewater continuously, at a rate where the amount of water entering the lagoon equals the water being discharged. The hydraulic flow of these lagoons is set to ensure the incoming wastewater stays in the lagoon long enough to receive treatment before it reaches the outlet.
Controlled Discharge Lagoons
These lagoons release water in controlled amounts, usually just once or twice each year. This design is common in cold climates where the treated wastewater is released after the spring thaw and again in fall.
The British Columbia zero discharge lagoon
Unique to our region is the British Columbia (BC) zero discharge lagoon. This lagoon disperses effluent by infiltration into the soil as well as evaporation from the pond surface.
A BC zero discharge lagoon system is made up of one or more large excavated cells surrounded by a berm. This is the “lagoon”. If topography allows, gravity directs sewage either directly or from a septic tank into the lagoon. Otherwise, the effluent is moved to the lagoon by a pump, syphon or some other dosing device.
The berm is made of excavated clay material and serves to prevent surface water from entering the lagoon and to provide reserve capacity. For safety and security (and to prevent animals from entering the lagoon), fencing is installed on or immediately adjacent to the berm.
A BC zero discharge lagoon works best in situations where the volume of wastewater is small due to low water use and the annual evaporation greatly exceeds precipitation.
Key considerations when it comes to the design of these lagoons include preventing odour, mosquito breeding, disease transmission by insects, exposure to animals and managing vegetation over the long term (tree roots, for instance, can cause leakage).
The more lagoons, the better
Lagoon systems that use two or three smaller cells rather than one large cell tend to provide more effective treatment.
In brief, this is because each cell can have a slightly different function and design, purifying the wastewater to an increasingly better quality as the water moves along the series. When lagoon cells are designed to operate in series, there is more time for the solid material in the wastewater (grit, sand, food scraps, algae etc.) to settle, before the effluent is released.
Sometimes, a serial design is the best or only way to ensure that effluent from the lagoon septic systems can meet local requirements. Another consideration is algae growth: some lagoon systems use more cells during the summer months when algae growth is highest.
Lagoons in a series are a common design for community systems because they usually require more land than an individual household has to spare. More land allows for greater wastewater capacity, more cells and treatment that meets regulated standards.
Advantages of lagoon systems
Low-cost construction – Lagoon systems can be cost-effective to build in areas where land is available and inexpensive.
Minimum operating cost – These systems use less energy than most other septic systems.
Reduced labour – Lagoon systems are easy to operate and maintain. This means they require less labour than other methods and are therefore a great option in areas where labour is limited.
Can handle shock loads – Lagoon systems can handle sporadic use better than many other systems, which makes them a great option for seasonal properties like campgrounds and hotels.
Eliminate pathogens – Lagoon systems are very effective at removing disease-causing organisms (pathogens) from wastewater.
Suitable for irrigation – Because of its high-nutrient and low pathogen content, the effluent from lagoon systems is often suitable for irrigation.
Disadvantages of lagoon systems
Less effective in cold climates – Lagoon systems are less effective in cold climates and may require extra land or a longer retention time period in these regions.
Ineffective treatment of heavy metals – Lagoon systems are not effective at removing heavy metals from wastewater.
Need for additional treatment – The effluent from some types of lagoons contains algae and may require additional treatment or “polishing” to meet local discharge standards.
Bad Odor – Even a properly functioning anaerobic lagoon can produce unpleasant smells. Odor from lagoons can get particularly bad during algae blooms, spring thaw in cold climates, or with anaerobic lagoons and lagoons that aren’t well maintained.
Size – Lagoon systems require much more land than other wastewater treatment methods.
May form mosquito habitat – Unless they are properly maintained, lagoons can provide a breeding area for mosquitoes and other insects.