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The Difference between Aerobic and Anaerobic Septic Systems

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Difference Between Aerobic & Anaerobic Septic Systems

Anaerobic septic systems are more typical among home and business properties in the US. Although recently, aerobic septic systems have gained more popularity and are being installed in properties more and more. There are, of course, advantages and disadvantages to both types of systems.

The basic difference between aerobic and anaerobic septic systems is the presence or absence of oxygen, and thus, the bacteria and quality of waste breakdown. Traditional anaerobic septic systems operate in the absence of oxygen.

This means that the bacteria that break down sewage must be able to thrive without oxygen. On the other hand, while most aerobic septic tanks are likewise located underground (similar to anaerobic), aerobic systems depend on aerators – mechanisms that inject oxygen into the tank. Aerobic septic systems are thus able to support aerobic bacteria which are believed to be more efficient in digestion of waste.

If your property’s situation is more suited for an on-site waste treatment system rather than the municipal centralized sewer, you’ll be faced with deciding between either an anaerobic or aerobic septic system. Let’s now take a look at how both of these systems work, their advantages, disadvantages, and other factors to consider.


With every flush of a toilet and draining of a sink, wastewater and solid waste matter travel to your septic tank. Here, anaerobic bacteria break down the solid matter while oxygen is absent, treating the wastewater and preparing it for its next destination which is your drainfield.

In an anaerobic system, you have a septic tank built with two main pipes. One of which is connected to your house or facility, while the other heads out into your yard. That main pipe eventually splits into several pipes that sit just beneath the surface of your lawn.

Inside the septic tank, solid waste matter settles at the base and is eaten away by the anaerobic bacteria. Liquid waste, on the other hand, floats to the top. Effluent from the tank travels out to the smaller pipes in the drainfield, which have holes at their ends. The wastewater then filters (or “percolates”) out into your groundsoil.

This anaerobic digestion transforms solid waste into renewable energy. This type of septic system is typically used for industrial wastewater with high concentrations of biodegradable organic matter and this method is known to be energy-efficient as it requires fewer chemicals and costs less.

Aerobic septic systems would involve three tanks. Waste enters the first tank and settles into layers, similar to anaerobic systems. In the second tank, or the treatment tank, an aerator moves oxygen bubbles through the waste. This allows aerobic bacteria to digest the waste. From there, wastewater moves to a pump tank where it undergoes further treatment.

Once wastewater leaves the septic tank, it flows to the drainfield where aerobic (oxygen-loving) bacteria acts on it. As the effluent is pulled downward by gravity, aerobic bacteria in the soil kill toxic components in the effluent before it joins clean groundwater. This is why your soil needs to be healthy with the correct pH level.

Aerobic treatment systems (ATS) include fixed film systems, continuous flow systems, retrofit systems and composting toilets.

Fixed film systems (FFS) are biological treatment processes that utilize a medium such as rock, wood, plastic or other natural or synthetic solid material that can support biomass on its surface and within its ideally porous structure. The treatment process employs rotating disks that move within the wastewater and is referred to as rotating biological contactor (RBC).

Typical Continuous Flow systems are usually preceded by a septic tank and followed by a subsurface wastewater infiltration system (SWIS). For surface water discharge processes, the system must be followed by disinfection to consistently meet discharge standards.

Retrofitting your existing septic tank will turn into a highly efficient septic system that transforms your sewage, effluent, and organic waste into reusable liquid fertilizer that is distributed underground throughout your property.

A composting toilet treats human excretion by a biological process called composting. This process decomposes organic matter and turns human excreta into compost. This process is facilitated by microorganisms (mainly bacteria and fungi) under controlled aerobic conditions. Most composting toilets do not require water for flushing and are thus known as “dry toilets”.

What mainly distinguishes an ATS from a traditional septic system is that Aerobic Treatment Systems produce a higher quality effluent and will require smaller drainfields.

The Difference

Bacteria. Aerobic bacteria are generally believed as better than their non-oxygen-loving counterparts (anaerobic) when it comes to septic treatment. According to the UN’s Food and Agriculture Organization, aerobic bacteria are better at digesting human waste and are less vulnerable to household chemicals than anaerobic bacteria can and are. However, aerobic bacteria are less efficient at breaking down inorganic solids and this is why aerobic systems may be more susceptible to clogs than anaerobic systems are. Aerobic bacteria also require constant aeration in order to thrive, and so power outages, especially in long durations, can harm the bacterial colonies in aerobic systems.

Drainfield. Your drainfield receives partially-treated effluent from your septic tank and evenly distributes it through pipes to the soil for further treatment. Because aerobic bacteria are able to treat wastewater more extensively, aerobic septic systems typically require smaller drainfields than anaerobic septic systems do. Aerobic systems are thus the more space-efficient option, making it suitable for properties with smaller spaces or soil quality that is not optimal for drainfields.

Alarm System. While most aerobic septic systems are built with an alarm that alerts a property owner to any issues within the system, anaerobic septic systems typically do not have this feature. This is a crucial advantage for aerobic systems, because septic problems that go undetected can easily and quickly get aggravated and extensive (underground) septic repairs can be quite costly. Septic systems that have been left un-monitored and without maintenance can even fail totally and need to be replaced. Expensive. Built-in alarm systems in aerobic septic systems definitely help in avoiding expensive repairs and septic system failure.

Cost to Operate. Typically, anaerobic septic system installation is a lot simpler and therefore less costly. Aerobic septic systems, on the other hand are more expensive to install, not to mention maintain than anaerobic septic systems. Because aerobic septic systems are built with more moving mechanical parts, such as the aerator, they are more prone to mechanical malfunction and will typically require routine maintenance more frequently. Also, aerobic septic systems require electricity to operate, and so they can become a strain on one’s monthly electricity bill.

    Aerobic Anaerobic
Conventional Systems
Assessments and Permits $200 to $400 $250 to $1,000, depending on location
System and Installation $10,000 to $20,000 $2,000 to $5,000
Pumping and Maintenance $200 every 1 to 3 years $200 to $600 every 1 to 3 years
Septic Tank $500 to $2,000 $500 to $2,000
Others Aerobic systems may need motor & timer replacements over time. Motor replacements may cost between $500 & $600; timers average $100 Piping and related materials have an average cost between $100 and $200

 Installation Costs: Aerobic vs. Anaerobic

Aerobic vs Anaerobic: Advantages and Disadvantages

Aerobic septic systems typically have three separate tanks. Waste enters the first tank and settles into layers, then moves into the second tank (the treatment tank) where an aerator pumps bubbles of oxygen through the waste. This is where the aerobic bacteria digests the solid matter. Lastly, it moves to the third pump tank for final treatment. Of course, there are benefits and disadvantages to aerobic septic systems.

Aerobic Septic System
Advantages Disadvantages
Design: Aerobic septic systems offer a range of designs suitable for various set-ups. Cost: Installation, operation, and maintenance is almost twice that of anaerobic.
Environmental Pollution: multiple treatments yield less groundwater Weather: Extremely low temperatures adversely affect quality of treatment.
Space Requirements: Very little space is required for aerobic septic systems. Ideal for properties with limited space. Maintenance: More maintenance is required. If the system if poorly-maintained, treatment quality suffers or, worse, the system may fail.

Anaerobic septic systems are simpler than their aerobic counterparts. They consist of a septic tank and two main pipes: one that goes to the facility and one that goes into the yard. This second one splits into several smaller pipes and are situated just below the surface of the lawn. While solid waste settles in the tank and is eaten by anaerobic bacteria, liquid waste floats to the top, moves to the smaller pipes under the surface of the soil, and then filters out into the soil. Likewise, there are advantages and disadvantages to anaerobic systems.

Anaerobic Septic System
Advantages Disadvantages  
Availability: Aerobic systems are more commonly installed, thus more readily available. Pumping: Lesser tanks and so may require more frequent pumping.  
Cost: Less complex set-up means a lower price tag. Resale Value: Still a lot of people are not used to maintaining on-site septic systems.  
Environmentally-Friendly: No chemicals are required in cleaning the system. Solid Waste: With only one tank and pipe system, anaerobic systems are more sensitive to solid waste.  

Why You Should Have an Aerobic Septic System

Aerobic septic systems introduce oxygen throughout the sewage waste that leaves your facility and enters your system. Oxygen encourages the growth of aerobic (oxygen-thriving) bacteria which is a totally different type of bacteria present in a conventional septic system. Conventional septic systems operate with anaerobic bacteria which do not need oxygen to survive, unlike their aerobic counterparts. Top reasons why an aerobic septic system is the more popular option nowadays is because:

  1. Aerobic bacteria are known to break down solid matter more rapidly than anaerobic bacteria
  2. The treated water (effluent) that flows out of an aerobic system is cleaner and safer for the environment, making an aerobic system ideal for properties that have high water tables or other environmental limitations
  3. Aerobic septic systems require smaller drainfields, making it ideal for small yards

There are certain conditions in which one might actually require an anaerobic system. First of all, for an anaerobic system to function, the soil in the property must be able to allow for percolation (gradual filtration). When you’re deciding on a type of septic system to install, contract a septic professional to come and conduct a percolation test to see how quickly water is absorbed into your soil.

If water doesn’t percolate through the soil quickly enough, this is an indication that a traditional sub-surface system won’t work. In cases like this, an aerobic septic system may be more efficient, or might actually be your only option.

Consider an aerobic septic system, too, if your land is notably “flat”. Anaerobic (or sub-surface) systems require at least a subtle slope or tilt to help wastewater move along out into your drainfield.

Outside of such situations, it is more common for homes to be built with conventional anaerobic septic systems. Anaerobic systems are relatively more cost effective and less complex, and this is why it has been a popular choice.

Anaerobic systems function with a material known as “biomat” that breaks down sewage. This material consists of bacteria that naturally break down organic waste. The lack of oxygen in anaerobic systems, however, can eventually cause a heavy accumulation of bacterial by-products that form slimy sludge within the biomat. This sludge in the biomat usually causes the biomat to grow within the drainfield and actually deter the natural organic breakdown process.

Anaerobic septic systems that have become clogged with sludge potentially leads to septic tank problems. The problems all begin when the septic system is left with little or no room to process new wastewater because of this sludge build-up. This causes problems such as back-ups, overflows, pooling, and foul odors from your septic system. When the drainfield becomes filled with the biomat and sludge, wastewater either backs up into your facility or into your yard.

This is hardly a problem with aerobic septic systems. Aerobic bacteria has the ability to break down the sludge in the biomat. With an aerobic system, expensive though they may be, you can expect a cleaner, more evenly-flowing septic system, considerably saving you time and money in the long run. Aerobic bacteria is more effective in preventing the build-up of the biomat and sludge, keeping the septic system functioning properly for longer. This extends the overall life of the whole system.

The naturally-occurring aerobic bacteria in a septic tank system is relatively simple to grow, and does not require modifications to the septic tank nor other methods such as introducing additives or other chemical agents into the system. Anaerobic systems are designed to naturally break down organic sewage and waste, but are not as efficient as aerobic septic systems.

Do Aerobic Septic Systems Need Pumping?

There are several reasons why your aerobic septic system needs to be pumped as part of regular and periodic maintenance:

  • The more sludge builds up inside the septic tank, the less room there is for treated wastewater.
  • High solid waste build-up in the pump tank may cause premature pump failure.
  • Solid wastes may potentially cause sludge to clog up the spray heads and spray lines. This poses a potential health hazard as it may happen that sludge gets sprayed onto the lawn through the spray heads

High sludge levels are an indication that your aerobic septic system is due for pumping.  Sludge tests are conducted in the septic tank as part of aerobic systems’ routine servicing (inspection and maintenance). When the sludge reaches approximately 50% to 60% of the septic tank, it is recommended that the tank be pumped out. If you notice surges of water in your system, this is an indication that sludge is accumulating in the pump tank and likewise requires immediate pumping.

If your septic system is not pumped regularly, it can eventually lead to septic problems requiring expensive repairs. The frequency of pumping across aerobic septic systems may vary, but a good average is once between one and five years. Factors that determine the required frequency of pumping is usually dependent on the following factors:

  • The size of the aerobic system’s tanks: the trash tank, aerobic tank, and pump tank
  • The number of occupants in the home or business facility
  • Items typically flushed down the toilets and allowed in sink drains
  • Habitual and excessive use of chemical cleaners
  • Hydraulic loads the septic system is subjected to

Take note, too, that if your system alarm comes on, you should call your trusted septic professional to conduct an inspection as soon as possible.

How Long Does an Aerobic Septic System Last?

Septic systems (including the tank and absorption system) will not fail immediately if they are not pumped. However, a poorly-maintained septic tank can encumber your drainfield with wayward solid waste. In other words, a neglected septic tank shortens the drainfield’s life and may result in system failure.

A steel septic tank that’s been around for more than 15 or 20 years is likely to already have rusted-out baffles and perhaps base. A septic tank made of plastic or fiberglass may last a bit longer, with an average lifespan of 30 to 40 years. Finally, a concrete septic tank can last 40 years to nearly indefinitely, though it’s a lot shorter if poor quality concrete was used or there is acidic ground water that hastens the deterioration of baffles and other tank components. Remember that a septic tank is designed to digest and filter solid wastes prior to the effluent traveling to the absorption field and store residual solids. But the tank is only one part of an entire system. Taking care of the septic tank will certainly extend the life of the costly second part of the system: the drainfield.

A conventional septic drainfield’s lifespan may vary, depending heavily on size, soil percolation rate, and usage level. It is possible, for instance, that a large septic drainfield situated in healthy soil with a well-maintained septic tank to last for more than 50 years. On the other extreme end, however, a conventional septic drainfield could actually fail within 24 hours of first use with a new system when piping has been poorly installed.

Aerobic septic systems are a little more complex than conventional septic systems, yes, but certainly manageable.  Aerobic systems need occasional maintenance to keep them optimally functional. In order to help extend the life of your aerobic septic system, it is important to take note of the following:

Be conscientious about what you allow into your system. Do not just put anything down your drains or toilets. Keep in mind that whatever is flushed or drained will go into your septic system and will be undergoing treatment. Be mindful of this fact and allow only substances that can be organically digested or broken down. Aerobic septic systems are designed to treat wastewater, not to act as your garbage disposal unit.

Keep a regular schedule for inspection and pumping of your septic tank. Once every one to three years between pumpings should be ideal. Annual inspections are wise, too. Though, do have your tank pumped if your maintenance provider indicates that it needs to be done outside of the regular schedule you are keeping. Routine inspection and pumping of the septic tank and system are both crucial in keeping your system optimized and will definitely help avert potentially costly repairs or replacements.  

Be mindful of the alarm on your septic system. Promptly call a licensed septic company when your alarm sounds off or the light turns on. Also, reduce non-essential water usage immediately. Although, you do not need to get above-board frantic when you hear or observe the septic alarm. Systems should be designed with a buffer room so you can still use the bathroom or do dishes. But do hold off on laundry as it uses a lot of water. The point is, never ignore your septic alarm. It went off for a reason and the longer you wait to address it, the more costly it can potentially become.

Regularly check your aerator mechanism for ants. The septic aerator pump is the mechanism that breathes air into a holding chamber in your septic tank. This allows for the growth of oxygen-loving (aerobic) bacteria. Check on your aerator regularly, especially if what you have is installed above-ground or where ants can easily access it. Ants are drawn to this part of the system and can cause damage.  

Schedule your laundry, spacing loads apart. Do so, especially if you have a large family or several occupants. Doing several loads in one day will overload the system with water. Space them out. Planning a load in the morning and then another one at night is significantly better than doing, say 10 in one day. Minimizing the volume of water that goes into your septic system by scheduling laundry and/or dish washing over a number of days, instead of washing in volume, helps ensure that your system is not overwhelmed with excessive water levels. Furthermore, cutting down on shower times and the use of efficient-flow showerheads and fixtures will help maintain optimum water flows in and out of your septic system. Wasteful water usage forces your septic tank to hold more water than it has the capacity for. Excess water levels hamper the breakdown of solid wastes likewise forces unprocessed solids to pass out of the tank and into the distribution lines that are built to handle only liquids. When this happens, you end up with blocked pipes or lines, which is never good for any septic system, let alone an aerobic septic system.

Avoid excessive use of detergents and other harsh chemicals. The phosphate content of detergent products acts as fertilizer stimulating the growth of algae that has the potential to block the perforations within your distribution pipes. The problem with excessive detergent use when you do your laundry is that there is no guarantee that your washing machine will be able to rinse away all the detergent residue and adequately dilute it. This is why the overuse of such products, especially the non-septic-safe types, can be harmful to your septic pipes and tank. Shift to dishwashing and laundry soap in liquid or gel form as these will likely not contain phosphates, unlike those in powdered form. Also, chemical products such as solvents, paint thinners, pesticides, motor fluid, and the like are highly toxic. Introducing them into your septic system will disperse them into the surrounding soil, causing pollution and impeding plant growth. Additionally, these harsh chemicals will kill off the helpful bacteria in your tank, diminishing their ability to break down waste that enters it. Never use your septic system as way to dispose of toxic materials like these as they won’t be eliminated or processed – instead, they will simply be distributed to your property’s soil. Also, commercial synthetic products that claim to “unclog” or clear septic systems would typically contain toxic or hazardous chemicals like lye that can be just as harmful to the environment.

Making the all-important choice between aerobic and anaerobic septic systems virtually comes down to personal preference. Before you finalize your decision, ask yourself these questions: What is the budget? If you’re on a limited budget, a traditional anaerobic system might be considered due to its relative simplicity and lower cost to install and maintain. How big is my property’s lot/yard? For very small lots, you might need to shell out extra for an aerobic system (for installation and maintenance). How vital is environmental impact? If you’re someone who is environmentally aware and conscientious about safety, an aerobic system might be worth the extra cost because of the better quality (cleaner) wastewater it produces. On the other hand, if your immediate environment is not faced with challenges or not easily susceptible to disturbance, an anaerobic system might be suitable. What are the local laws in your area? Some jurisdictions prohibit aerobic systems for residential properties. Or the contrary. Am I able to stick to a maintenance schedule? Aerobic systems may be more efficient and longer-lasting, but these benefits will perish without proper and timely maintenance. The bottomline is aerobic septic systems require more attention and resources than anaerobic systems, though promise better results altogether.

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