An Above Ground Septic System Offers Alternatives To Challenging Sites
An above ground septic system, also known as a sand mound septic system, is used for the on-site treatment of sewage when site conditions are not suitable for installing a conventional septic system due to the increased risk of the system failing. This article looks at the primary conditions that can make a septic system vulnerable to failure, the consequences of such a failure, and how an above-ground septic system can offer a solution to these problems.
Every home that is not connected to a sewage network needs some method of treating the sewage generated by the people who live there. Septic systems are the most commonly used on-site sewage treatment system. These typically consist of an underground concrete, plastic or fibreglass chamber with two compartments. The first compartment separates the sludge and scum from the effluent before it moves into the second compartment. The solids are treated anaerobically inside the chambers, while the remaining effluent flows via gravity into a drain field, where it is filtered naturally by microorganisms in the soil before draining into the groundwater below. However, there are several scenarios where it simply isn’t practical or safe to employ this method, and an alternative solution is needed.
There are three naturally occurring conditions that make it unfeasible to install a conventional underground septic tank system discharging to a regular drain field:
- Soil permeability (when the percolation rate of the soil is too slow or too fast): In highly permeable soils (such as very sandy soils) with a fast percolation rate, effluent will reach the groundwater without being adequately filtered or treated by microorganisms in the soil. On the other hand, in soils with a low permeability (such as clay soils) that have a low percolation rate, water may not drain away fast enough. As a result, the surrounding soil may become waterlogged, leading to ponding on the surface, which poses a health hazard. Considering that the primary objective of a drainage field is to allow the unsaturated flow of effluent to the groundwater, as this exposes the wastewater to microorganisms in the soil that break down the waste and destroy pathogens, in order for treatment to be effective, it is vitally important that the soil does not become too saturated (this is explained in more detail below).
- Restricted layer: When there is a restricted layer, typically a shallow layer of soil lying over a layer of clay, porous bedrock, or perched groundwater sandwiched between rock and soil, that reduces the effectiveness of effluent treatment as it filters through the soil.
- High water table: When the distance between the drainage field and the water table is insufficient to effectively filter and treat the effluent before it reaches the water table, the groundwater can become contaminated, potentially threatening drinking water sources. Should the groundwater become contaminated, these pollutants can be transported offsite with lateral groundwater movement to contaminate other freshwater systems.
If a site has one or more of the above conditions, a conventional septic system will not be suitable as the septic system is more likely to fail, posing both an environmental and human health risk. In cases such as these, above ground septic systems, which allow more favourable conditions to be created above ground level by artificially increasing the height of the filtration bed, offer a more appropriate sewage treatment solution.
Protecting Environmental & Human Health
Household wastewater consists of grey water (from sources such as bathing/showering, laundry, dishwashing, etc.) and black water (human waste). This effluent can contain a variety of chemical pollutants and pathogenic organisms that can be harmful to our health if not treated effectively. Human waste can harbour disease-causing microorganisms that can lead to outbreaks of typhoid, cholera, dysentery, giardiasis and other gastrointestinal infections, which in severe cases can be fatal.
Poorly treated human waste can pollute soils, groundwater and surface water systems, posing a health risk to anyone who comes into contact with it directly or indirectly, for example by drinking contaminated water, eating contaminated food (e.g. fruits, vegetables, shellfish) or swimming in contaminated rivers, lakes or coastal bays. In addition, because untreated sewage is both unsightly and smelly, it is definitely not something you want to have pooled up on your back lawn. However, flies, rats, and even the family dog don’t seem to share these sentiments, but rather tend to be drawn to it like bees to a honeypot, spreading it around, together with any pathogens that may be lurking in there.
In addition, household detergents released with wastewater typically contain high levels of nitrates and phosphates, which can pose both an environmental and health threat when they accumulate in high concentrations in freshwater systems. Infants who are exposed to nitrates in their drinking water can suffer from blue baby syndrome, where the blood’s ability to carry oxygen is inhibited, which can be fatal. Because nitrates and phosphates are also plant nutrients, they can stimulate the growth of algae in rivers, lakes and coastal bays, resulting in extensive algal blooms that can be toxic to wildlife, pets and humans.
A key element to keep these types of systems functioning properly over long periods of time have a look at our maintenance plan for above ground septic systems.
How do Sand Mound Septic Systems Work?
Wastewater flows from the home to a septic tank, then via gravity to a pumping tank which pumps it to a sand mound located above ground level, where it is evenly distributed throughout the drain field. Once the effluent is discharged into the drain field, it percolates through the sand and gravel bed where microbes in the soil remove any nasty pathogenic bacteria that may be present.
Removal of Pathogens
The vertical separation between the septic system and the groundwater is the first line of defence for preventing the spread of disease. It is therefore vitally important that there is sufficient depth between the two so that the community of microorganisms (good bacteria, fungi, protozoans and nematode worms) living within the soil can do their job. Unlike the anaerobic bacteria living inside the septic tank, these microbes need oxygen to survive and do their work. Consequently, in order to ensure aerobic microbial activity within the soil is not hindered, it is important that the soil does not become saturated. It is also important to note that these microbes live on the surface of the soil particles, removing contaminants from the effluent as it flows past them through the air spaces between the grains of soil.
This soil matrix — consisting of the soil granules, the biological communities living on the soil granules, air spaces and moisture — essentially serves as a biological sewage treatment plant where waste is broken down and pathogens are destroyed. The microorganisms that have taken up residence are the blue-collar workers that keep the system functioning smoothly. And if we want to keep our workforce happy and productive, we need to ensure that their working conditions are favourable, providing an unsaturated working environment where they can breathe easily.
Removal of Nutrients
Planting vegetation on sand mound septic systems doesn’t only make a garden look aesthetically more pleasing to the eye, it also improves the overall functioning of the septic system. The plant roots will absorb effluent water from the soil, which serves two purposes: 1) it helps reduce the risk of the soil becoming waterlogged; and 2) the plants will also absorb the dissolved nutrients (the nitrates and phosphates responsible for blue baby syndrome and algal blooms) and utilize these for their own growth. However, it is important to choose vegetation wisely, opting for grass and/or small plants with shallow root systems. Trees and other plants that have deep root systems are not suitable as they are likely to damage a sand mound septic system’s drainage pipes, which could be expensive to rectify.
Key Components of an Above Ground Septic System
Above ground septic systems consist of three main components: 1) a septic tank (pretreatment unit), 2) a dosing chamber that houses the pump, and 3) the above ground sand mound which serves as the drainage bed.
The Septic Tank
The septic tank (where pretreatment separation takes place) is a large concrete, polyethene or fibreglass chamber that is buried underground. Household wastewater flows into the septic tank. Once in the tank, the wastewater separates into three layers: the heavier solid matter (sludge) sinks to the bottom, the middle layer consists of liquid effluent, while the lighter solids (scum) float to the surface. The sludge at the bottom of the septic tank is partially decomposed by anaerobic bacteria that thrive in these oxygen-free conditions. The liquid effluent then flows from the septic tank via a pipe into the dosing chamber, aided by gravity.
The Dosing Chamber
The dosing chamber, or pump chamber, which collects the effluent that flows from the septic tank, is also constructed of concrete, polyethene or fibreglass, and is fitted with a pump, pump control floats, as well as a high-water alarm float. Once the effluent in the tank reaches the level of the control float it triggers the pump to turn on and pump a predetermined amount of effluent up to the sand mound located above ground level. Once the level of the effluent in the dosing chamber is reduced to the level of the ‘off’ float, the pump automatically switches off and stops pumping. The control floats housed in the pump chamber can be adjusted to deliver the most appropriate volume of effluent that the mound can process. In addition, to prevent system failure, most pump chambers also contain an ‘alarm’ float that activates a warning alarm should there be any problems with the pump or the system. This is set to trigger if the effluent levels rise above the ‘on’ float without triggering the pump. The alarm usually consists of a buzzer and a warning light and is typically housed in a garage or outbuilding nearby. It is a good idea to ensure that the pump can be removed without too much sweat should things go pear-shaped. The pump discharge pipe should be fitted with a quick release coupler to allow easy access to the pump. A length of nylon rope attached to the pump will make it easier to haul it out the chamber should it fail.
The Sand Mound
The sand mound is a drain field that has been artificially created by forming a raised drainage bed above the natural surface of the ground. It consists of a bed of sand and gravel containing a distribution system made up of perforated pipes. Effluent in the dosing chamber is pumped up to the mound under low pressure to ensure even distribution throughout the drainage bed. The effluent then trickles out of the small perforations in the pipes, down through the bed of gravel into the sand mound below. Aerobic bacteria in the sand treat the effluent, removing pathogens before it filters through the soil to the groundwater.
Things to Consider When Designing Sand Mound Septic Systems
Before installing an above ground septic system it is important that you understand the soil characteristics of the site. It is best to get a professional soil assessment done by a certified soil analyst. Depending on where you live, this may be legally required, but even if it is not, it is still a good idea to do so in order to establish the flow rate of the soil so that the septic system can be designed and sized for optimal performance according to the specific site conditions. Another thing to consider is allocating space in your garden for a replacement mound should the existing mound need to be repaired or expanded. Some areas require a designated area be set aside for a replacement mound — it is always good to have a plan B…
Conclusion: Advantages of an Above Ground Septic System
Above ground septic system can offer an alternative onsite sewage treatment solution on sites where soil conditions are unsuitable for conventional in-ground sewage treatment systems.
In addition to protecting the groundwater by artificially increasing the vertical separation layer, sand mound septic systems do not discharge directly to a surface water body such as a ditch, stream or river, limiting the potential for polluting surrounding freshwater systems.
Since very little excavation is needed for the construction of a sand mound drain field, construction damage is usually minimal if undertaken with care. Above ground septic systems are suitable for all climates, including areas that receive high rainfall, they are the ideal choice. So if you are restricted from installing a conventional septic system due to your site conditions and are in a quandary as to how to treat your household sewage, an above-ground septic system or sand mound septic system offers the perfect solution to your situation.
For some insight into the construction of an above-ground septic system check out this video
