Bio-digesters and managing animal waste


Jane Wanjiku’s farmhand Peter Kamau explains how biogas is generated.

Photo credit: Sammy Waweru | Nation Media Group

What you need to know:

  • A practical method for managing human and animal waste is using bio-digesters.
  • Bio-digesters use anaerobic and aerobic bacteria to digest waste producing gases.

If not properly managed, human and animal waste contributes significantly to greenhouse gas emissions.

Additionally, poor waste management is the leading cause of hygiene-related deadly illnesses like cholera and typhoid.

The situation worsens when the waste is discharged in rivers, lakes and other water bodies through sewer lines.

Approximately 4.5 billion people have no access to proper and adequate sanitation worldwide, often relying on pit latrines.

Methane, a potent greenhouse gas emitted from human and animal waste, accounts for more than 20 per cent of global warming.

Agriculture has been hit hard by climate change. Mitigating climate change effects requires a collective approach, including adopting green energy and green waste management solutions.

Green energy is derived from renewable sources like solar, wind, geothermal, biomass and hydroelectric power. These are environmentally friendly alternatives to fossil fuels and are often readily available.

Waste disposal methods

A practical method for managing human and animal waste is using bio-digesters.

These kits offer a hygienic and cost-effective alternative to traditional waste disposal methods like exhausters.

“Using exhausters to dispose of waste is costly and unhygienic,” says Katra Abdinasir, a resident of Njiru in Nairobi County who installed a bio-digester at her residence in 2021.

She now saves up to 90 per cent waste management costs. Unlike septic tanks, bio-digesters emit no odour.

“The installation is once and the kit requires small space,” says Japhet Mwangi Kariuki of Bio Digesters Kenya.

The system comprises pipes connected from the waste source to chambers and a soak pit, which releases liquid into the soil.

It operates efficiently and does not require emptying since the water input is balanced by the discharge.


Japhet Mwangi Kariuki from Bio Digester Kenya with Katra Abdinasir in Njiru, Nairobi County.

Photo credit: Sammy Waweru | Nation Media Group

Bio-digesters use anaerobic and aerobic bacteria to digest waste (enzymes), producing gases such as methane, nitrogen and carbon dioxide. The enzyme helps break down solid waste into liquid.

The amount of water flushed into the bio-digester from the house is displaced through the outlet baffle into the soaking area, making it impossible for the system to fill up. That is why emptying is not necessary.

The system is installed underground, with the depth depending on size.

There are several types of bio-digesters. Apart from human stool, animal waste can also be used to generate biogas, which serves as a source of fuel for cooking and lighting.

Jane Wanjiku, a farmer in Ikinu, Kiambu County, has installed a bio-digester at her residence.

The cowdung processed in the bio-digester is also used as farmyard manure.

“The manure extracted after producing biogas is full of nutrients for the vegetables, fruits, maize, beans and other plants,” Wanjiku’s farmhand, Peter Kamau, tells the Seeds of Gold.

Oxygen-free environment

A bio-digester is sealed. Micro-organisms in the bio-digester break down organic material to produce biogas and fertiliser.

The anaerobic digestion creates an oxygen-free environment conducive for bacteria to consume the waste and thrive.

According to a paper published by Nature Climate Change in 2020, composting human waste produces good fertiliser for agriculture, enhancing safety, sustainability, job creation and reducing greenhouse gas emissions and waste-borne illnesses.

Established in 2011 as the continuation of Nature Reports Climate Change, Nature Climate Change is a monthly peer-reviewed scientific journal published by Nature Portfolio.

It covers all aspects of research on global warming and its effects. 

Sanitation contributes two to six per cent of the Earth’s methane emissions and one to three per cent of nitrous oxide emissions.

Effective green waste management solutions are important in combating climate change and improving public health.

By adopting technology like bio-digestion, communities can reduce their environmental footprint and enhance the quality of life.