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What Is a Biodigester Waste System and How Does It Work?

Alexander is a professional engineer who specializes in the construction of affordable houses and structures using recycled materials.

Everything you need about biodigesters: what they are, how they work, and possible features

Everything you need about biodigesters: what they are, how they work, and possible features

Septic tanks are a common wastewater management solution that have been in use for a long time. People invested in various tanks to cater to their homes, apartments, and other real estate developments. Luckily, as the years went by, innovations have come up in the field of science that have yielded ideas such as the biodigester.

So what is it? Let's find out.


A biodigester is a mechanized toilet that uses decomposition and sedimentation as its main mode of operation. Instead of rotting the waste until the exhauster comes to suction it away, this is a bio-toilet where the human waste decomposes in a digestive tank using certain high-graded bacteria, which further converts into biogas or water.

This new innovation is pocket-friendly because you don't have to schedule an exhauster clean-up every now and then. It's very low in maintenance and doesn't require a sewage system.

The kind of bio-toilet technology used with the digester is anaerobic in a place where degradation occurs by the use of unique and specific microbial processes that work under high temperatures to turn it into inflammable gas.

So, from a low temperature of about -20°F, the final product is a colorless, odorless, and inflammable gas containing 50–70% methane. The cold-active bacteria is what allows the biodigester to earn its name for working biologically to recycle waste into gas.

Anaerobic digestate

Anaerobic digestate

Septic Tank Active Bacteria

How Does It Work?

Now that we have the definition out of the way, let's have a look at how it works. Biodigesters work with a digestive process called anaerobic digestion. The process entails waste moving in three chambers where the input is waste, processed using gas and bacteria, and the output is inflammable biogas.

The biodigesters have three anaerobic chambers: the grease interceptor, a biological digester, and a soak pit. These three segments treat the waste differently and effectively so they don't need any cleaning nor emptying. Its systematic arrangement follows a step-by-step process where the waste finally separates into grey and black waters.

To start off, the waste will get into the first chamber, where all the outlets from the household gather to dispose of waste. The solid waste (along with liquid sewage) will combine in one place and, after some time, the solid matter will settle at the bottom of the tank. Because of how the biodigester is designed, the lack of air, as well as pre-residing bacteria, will begin to eat away at the waste—which is referred to as anaerobic digestion.

The bacteria will decompose the organic water as well as all other pathogens in the wastewater. Once the chamber arrives at its capacity, the water will then overflow to the second chamber, where the liquid matter will be treated.

Up to that point, you now understand the first segment of the biodigester. All the solid waste, also known as sludge, will be decomposed and treated from the first chamber. As the waste continues to fill up from the household, the water will eventually reach the brink and overflow to the second chamber.

The Cleaning Section

Once the water has overflown to the second chamber, the process of sedimentation begins. Here's where smaller solid particles will continue to drop to the bottom of the tank, allowing the water to clean itself off the waste and sludge.

Again, once it has overflown again, it will move to the third chamber. By now, the water is much cleaner than when it was in the first chamber. By the time it exits the biodigester, the cleansing process will have ended and the water can now be distributed for irrigation and other purposes.

Since the process of biodigestion occurs due to bacteria, the water from the process can only be used for agricultural purposes. This includes farming, irrigation, and gardening. The water may be 98% clear, but it's not yet safe for human consumption. If you'd like to consume the water, it is recommended that you boil the water to get rid of bacteria and harmful pathogens.

Terms Used in the Biodigesting Process

You have read about the meaning of a bi-digester, and you know about how it works. Now let's take a look at the different terms used while talking about this system.

  • Blackwater: This contains solid waste from the toilets and grey waste that's collected from the kitchen, hand washbasins, and laundry.
  • Greywater: This is a combination of fats, oils, greases, waxes, and detergents, which makes it chemically unstable and, therefore, not fit for the biodigester.

Because the greywater can be hazardous to include in the biodigester, the blackwater is what is taken to decompose to produce water and gases for use around the home. After the toilet has been flushed, the blackwater will drop to the tank through a piping system and into the first chamber as elaborated above.

As for the grey matter, you can't really avoid grease and other hazardous materials from entering the tank. Hence, the grey waste will instead pass through a grease interceptor to get rid of fats, which can be harmful to the environment once emitted.

After the chemicals, fats, and other harmful substances have been sieved from waste, it then goes directly to the soak pit, which is the last chamber of the biodigester. The digester will release the water to sink through the stones and into the ground for irrigation and other gases.

Biodigester Technology

Human waste disposal in innocuous forms has become an environmental hazard. It's causing various problems, including aesthetic nuisance, the threat of organic pollution, and several infectious diseases caused by contaminated water and the environment. Like many other countries, people without proper homes tend to pollute the environment, which is very risky.

Luckily with biodigester technology, many people can now change that. With less pollution, gas and water have aided homes to live better with access to biogas for cooking and other applications. Modern shipping container homes are also using these, as they take up less space—hence a better solution than the previous septic tanks.

With the threat untreated waste has brought about—like being responsible for several diseases such as dysentery, diarrhea, amoebiasis, viral hepatitis, cholera, and typhoid—this biotechnology comes at the best time to save the environment.


The popularity of biodigesters has grown because of their many benefits. Below are some of the benefits they exhibit and why you should think about incorporating one into your next housing project.

  • No pungent smells in toilets from the tanks: Previously, you could catch a smell in the loo, especially if the tank is full and hasn't been emptied.
  • Fecal matter in the tank is not visible: The tanks feature enclosed anaerobic environments.
  • No infestation of cockroaches and flies: Houseflies are accustomed to environments such as the sewage tank. Because the biodigester is enclosed, no smells can attract these insects.
  • No clogging of digester: The systematic process allows for fast and effective movement and treatment of waste.
  • Reduction in pathogens: By the third chamber, the pathogens will have greatly reduced, allowing the water to be used in the soil.
  • Reduction in organic matter: The waste is treated to become water and gases that can be recycled.
  • No maintenance required: Very low maintenance compared to septic tanks.
  • No requirement of adding bacteria or enzymes to decompose the waste.
  • No need for removal of solid waste: Again with low cost, you won't need to call out to exhausts.

If you doubted the benefits shown by the biodigester, now you know. These salient features allow for them to be a leading wastewater treatment plan.

The neck of a biodigester

The neck of a biodigester

Available Models

Biodigesters can come in different models to fit each and every one of your real estate developments. Below are some of the most common models available:

  • Microbial inoculum (cold-active): Isolated, screened, selected, and enriched through acclimatization and bio-augmentation. It can withstand freezing and thawing, as well as inactivate the intestinal pathogens.
  • High-altitude model: Made from metal/FRP of cylindrical shape and works at low temperatures as well as high temperatures.
  • Glacier model: Works under high temperatures, and it has an insulation and heating arrangement with the solar system.
  • Plain area model: It's suitable for existing or independent toilets and can be used with top-mounted toilets.
  • Island model: Made with FRP, it involves a longer path for treatment and is suitable for high water usage and areas with a high water table.
  • Bio-tank model: Very low on cost and a simple design for on-site construction. Can be used with multiple housing projects

What Are the Advantages of Biodigester?

Against other sewage and wastewater treatment schemes, the biodigester has exhibited greater advantages over most of them. Let's take a look at some of these advantages:

  • It digests organic solids in an ecological way. Just like shipping container homes, biodigesters are ecological as well.
  • It prevents human waste and untreated water from contaminating groundwater. Before being exposed to the ground, the water has been treated to remove over 99% of pathogens.
  • It offers an alternative to the dumping of wastes into rivers, lakes, and fields in rural and semi-rural areas where there are no sewage systems.
  • The effluent can be used to water plants in the form of irrigation.
  • The effluent is cleaner, more effective, and easier to use than a septic tank, because it doesn’t need to be emptied.
  • The effluent exhumes odorless, non-obnoxious, colorless air, as compared to the end products of the toilets being used these days.
  • It doesn’t require the work and energy involved in relocating composting toilets every year.
  • It requires no further wastewater management or treatment.

These biodigesters have overtaken the old and traditional methods of waste management. They are more efficient and effective in how they work.

Another biodigester under construction

Another biodigester under construction

Maintenance Tips

From the text above, you can learn that biodigesters require little to no maintenance. They are long-lasting and very suitable for just about any home development plan. As long as the chambers are well aligned in a systemic manner, the digester will process as usual.

Besides that, there are other things you could do to ensure the longevity of the tanks is extended. This includes:

  • No plastic or other solid material is to be disposed of into the toilets. They may block the connection to the biodigester, causing unnecessary repairs and other costly problems.
  • Ensure the smooth flow of the process in terms of water and gas production. It should be used often in order to keep the anaerobic process equilibrated and to stop the gas from going through the water outline pipes.
  • The solid waste should be dislodged once every two to three years. An excessive leaking of water is a sign that this operation is needed. For a biodigester built for schools, with an average use of 50 students per toilet per day, it is necessary to dislodge the solid part once every two to three years.

Tank Sizes

Besides the models you've read about above, here are three different types of tanks available. They have been divided by sizes.

  • Standard: This is the smallest size and it can handle between 18–21 people (continuous users). This is the best size for residential properties and small commercial establishments.
  • Jumbo: The jumbo sizecan easily handle 100 people (continuous users) and is therefore ideal for mid-size establishments, apartment blocks, hotels, farms, and small gated communities.
  • Jumbo Deluxe: This is the biggest size available and it can handle up to 400 continuous users. This is great for much bigger establishments, such as estates, shopping malls, hotels, schools, hospitals, and blocks of flats.

Apart from these three standard sizes, different sizes can be created based on the need and the size of the project.

Protecting the Environment

With the rate of global warming experienced in major parts of the world, having an ecological way to dispose of, clean, and recycle is the way to go. With the biodigesters, you're assured of protecting the environment as well as smooth wastewater management.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2020 Alexander Okelo