For her home construction which is expected to commence early 2022 at Kitende on Entebbe Road, Jackie Ekayu intends to use a bio-digester. Ekayu says she zeroed in on the bio-digester because it takes up less space compared to a septic tank. She is set to build on a 50x50 feet plot.
Apart from occupying less space, Ekayu's research reveals that a bio-digester also significantly saves on construction costs compared to a septic tank.
"I was given price quotations for a septic tank and a bio-digester for comparison purposes. 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 says.
Unlike Ekayu, Robert Rukundo has just completed his home in Sentema. Although he knew about the bio-digester technology, Rukundo says he stuck to having a septic tank because it is the commonly used type of sewerage system. "I almost put up a bio-digester but I changed my mind because the septic tank is what I grew up seeing people build in their homes. I understand how it works. I have never lived in a home that uses a bio-digester and I thought it wise to use what I know," Rukundo explains.
About the bio-digester
Julius Kirumira, the managing director of Home Contractors Limited, a company that builds bio-digesters, advises that before a bio-digester is installed at your home, a site visit is conducted to carry out measurements of its base. A site visit will also establish the number of users, which determines the size of the bio-digester to be installed.
"Shortly after the site visit, excavation of approximately three by three (3x3) feet is done for the smaller size that normally accommodates one to 21 users. Installation of the bio-digester takes place within two days and is ready for use on the third. A bio-digester is just delivered at the site but not built like a septic tank," Kirumira explains.
Inside the bio-digester
A bio-digester is circular. Its interior is divided into two chambers; the inlet and the outlet. When you flush, the inlet transports the waste to the bio-digester where it mixes with naturally occurring aerobic bacteria that work in presence of oxygen, whereas the outlet contains anaerobic bacteria which work in absence of oxygen.
"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," Kirumira explains.
Advantages of the bio-digester
Ibrahim Kajjoba, a civil engineer adds that a bio-digester is not only smaller but also saves on installation space compared to a septic tank that requires more space.
According to Kajjoba, a septic tank is constructed after excavating deep underground, where the depth and width is determined by the size of the home or number of users and the size of the plot. The rest of the construction materials used are the same.
For a septic tank, you will need approximately 2,000 bricks, which cost approximately Shs600,000, approximately 25 bags of cement, two trips of sand and more space unlike a bio-digester where you will need very small fractions of the above materials.
"On average, you will need approximately Shs10m or more to build a strong and durable septic tank to stand the test of time yet you can have a bio-digester installed using a quarter or half of the same budget," Kajjoba explains.
One of the similarities between the septic tank and the bio-digester is that they are both set up with soak pits.
"A bio-digester does not give off a strong smell like a septic tank. It requires less excavation of approximately one to 1.5 metre depth. It also works in all soil types. To build a septic tank in a place with considerable water concentration levels, you will need a lot of cement and other construction materials such as clay bricks that are water resistant if it is to serve you longer," Kajjoba says.
The bio-digester is built from concrete that is moulded around iron bars. The hard core stones for the soak pit are provided at the site during installation.
Just like the human digestive system, the bio-digester only receives waste that is easily processed and can decompose. This means it can only process soft paper material such as toilet paper.
"If you do not caution the users about the kind of 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," Kirumira advises.
According to Julius Kirumira, a proprietor, the cost of the bio-digester depends on the location, size and the number of users. A one-cubic metre bio-digester built for one to 21 users within Kampala costs Shs2.5m and Shs3m outside Kampala because of transportation costs. The two-cubic metre for 200 users costs between Shs5.5m to Shs6.5m depending on the location.
A bio-digester of one to 200 users is made and installed on the site because it is big while that of one to 21 users is easily transported on a truck because it is small.