• Fri. Sep 29th, 2023

Which is Better; Bio-digester or Septic tank?

UGANDA, Kampala | Real Muloodi News | When building a home, you should have a clear picture of what type of sewerage management system you will need.

The size of your household and your budget are important considerations when choosing between a bio-digester or septic tank. This article discusses what you need to know to help you make an informed choice. 

What is a bio-digester?

A bio-digester is a waste and sewerage management and treatment system based on biodegrading processes. A bio-digester breaks down organic waste matter into carbon dioxide, methane gas, and water. The bacteria in the tank feed on the effluent and transform it into water and gas. The decomposition process takes about two weeks.

The methane gas produced is otherwise known as Biogas. It is possible to harness the gas from large bio-digesters and use it for cooking, however, in most cases, the gas is released to the atmosphere.

After the water treatment is complete, it leaves through the outlet for recycling or left to soak into the soil. The water is very rich in nitrogen and is therefore great for irrigation purposes. It can also be used for flushing the toilets. However, the water is not safe for human or animal consumption without further treatment.

Advantages of Bio-digesters

The Size

When Sarah Kiyemba, a resident of Gayaza in the Wakiso district, moved into her new house, she spent several months using a very inconvenient pit latrine. She had not saved the USh 5 million needed to construct a septic tank that would allow her to use the flush toilets.

“It was during that time when a friend introduced me to the bio digester system, an economical and sustainable substitute for a septic tank,” she says.

One of the many advantages of a bio-digester is its small size, making it a better choice for small spaces. “With a bio digester, you do not need to spend a fortune on building a septic tank or soak pit,” she added.

Chrissy Ethel Namono, a landscaper with Iconic Hedges located in Ntinda, agrees with Kiyemba. She says that oftentimes, a small compound is taken up by a septic tank in one corner and soak pits in another, giving no room for a lush beautiful garden.

Easy Maintenance

Biodigesters are easy to maintain, so long as no plastic or other solid material is thrown into the toilets, which might block the connection to the bio-digester. Further, a bio-digester typically does not need to be emptied on a regular basis, but may occasionally need to be emptied and cleaned.

A bio-digester interior has two chambers. Namely: the inlet and outlet. The inlet chamber has naturally occurring bacteria that work in the presence of oxygen. The outlet chamber is devoid of oxygen and contains anaerobic bacteria.

“When human waste reaches the bio-digester, the aerobic bacteria break it down into water. Water and waste are separated using two sewer lines so that the faecal matter and soft tissue go to the bio-digester for processing by the aerobic bacteria, to turn it into water before it flows to the soak pit through the outlet. The bio-digester is not emptied like a septic tank,” said Mr Rukundo, a resident in Sentema that just completed his home. 

A bio-digester does not give off any foul smell like a septic tank does. Further, the water soaks into the surrounding soil, thereby increasing the amount of underground water.

In addition, there is no wastewater discharge in the streets, making it cleaner and most importantly safe. It can work with a depth of 1.5 metres.

Relatively Cheaper and Convenient

Installing a bio-digester is cheaper than constructing a traditional septic tank. A bio-digester requires half the materials used to build a traditional septic tank. Further, you only have to excavate a hole and drainage trenches if water will not be recycled.

A bio-digester will cost about USh 2.5 million compared to a septic tank. In addition, it saves money on emptying which is now not necessary.

Disadvantages of Biodigesters

Bio-digesters also have shortcomings, for example, they can only process soft tissue like toilet paper.

Julius Kirumira, the managing director of Home Contractors Limited, a company that builds bio-digesters, advises, “If you do not caution the users about the tissue to use with a bio-digester, the system will get blocked and you may end up replacing and concluding that it is poor technology,” he cautions.

He explains that the usage of a bio-digester depends on the number of people that will use it. For example, a cubic metre will serve 21 people and cost 2.5 million within Kampala, while the cost will be 3 million outside Kampala.

Advantages of Septic Tanks

John Harrison, a FINISH Mondial Technical Advisor, says that a septic tank is a tried and tested, simple solution, ideal for African conditions if designed and installed correctly. He believes bio-digesters only make sense if one intends on harvesting the gas. However, in order to do so, one needs an extensive system which takes in additional biomass as fuel. “If the gas is not being harvested, then a [bio-digester] does not make any sense as you are adding significant complexity,” he adds.

Disadvantages of Septic Tanks


Jackie Ekayu planned to build a house on her 50*50 ft plot in Kitende, Entebbe. Choosing a septic tank might have been costly.

“I was given price quotations for a septic tank and a bio-digester for comparison. The cost of building one septic tank can put up two or three bio-digesters for a big house. I consulted two different engineers and my conclusion on the sewerage system to use was based on low cost and the amount of space required,” Ekayu said.

Ibrahim Kajjoba, a civil engineer, explains that to build a septic tank, you will need 2000 bricks, equivalent to 600,000 Ugandan shillings. You will need 25 bags of cement and two trips of sand.

According to Kajjobba, a strong septic tank will cost you about 10 million Uganda shillings. He also notes that a septic tank will require a lot of cement and water-resistant clay bricks to be effective.


Homeowner, Petit Najjemba, woke up one morning to a foul smell in her house. At first, she thought it was the garbage truck that normally passes by every Saturday morning. But after 30 minutes, the smell had not gone away.

“I asked one of the house helps to go out and look for the source of the smell. After moving around the compound, she found that the septic tank had filled and its contents spilled into the compound.”

Frank Tumushabe, a plumber,  says exceeding the capacity of the septic tank system can cause surface discharges. These discharges can create a health hazard on your property and pollute nearby water sources.

He adds that septic tanks need pumping to reduce sludge and scum build-up, therefore it is important to measure sludge and scum depths, or routinely pump out the septic tank  – at least once every year if you own a plastic septic tank.

So, a septic tank gives off a smell and is challenging to maintain.



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