Homeowners now switch to biodigesters


For many Kenyans building homes, the choice now is whether to install a septic tank or switch to a biodigester.

Waste management and scarcity of water have forced many to install biodigesters that convert waste into water that can be used in homes either for irrigating gardens, flushing toilets or washing cars.

Home waste management expert, Philip Ambaisi, the chief executive officer of Destech says choosing between the two types of waste treatment technologies depends on many factors.

Before settling on any of the two, a homeowner should look at the cost of maintenance, construction, available space, environmental safety as well as the timeline.

Mr Ambaisi explains that several homeowners nowadays favour biodigesters as opposed to a septic tanks.

This is because of the advantages that come with having one.

Less space

A biodigester requires relatively less space for construction and it can be placed anywhere in the compound as opposed to a septic tank that is usually placed close to the gate.

The septic tank takes a lot of space as it comes with at least three chambers used in the treatment of wastewater generated.

For biodigester, depending on the size of the household, one can buy from one cubic metre, two, two and a half, three, four and 7.5 cubic metres with one cubic metre one able to hold 1,000 litres of water. On the other hand, a standard septic tank is usually 10 by 10 metres.

“As a result, most city dwellers go for biodigesters because they have no luxury of space,” he says.

Construction timeline

A biodigester can be constructed in less than a day, if using prefabricated slabs or two days when using blocks. This is because the top slab has to be given time to dry before laying blocks followed by inside bedding before covering it up.

For a septic tank, it takes not less than a week to be ready for use as both the bottom and top slabs have to be given time to dry.

Then after plastering and casting, more time is spent while it cures before it is ready for use.

“The amount of work involved takes more than a week. One has to do concrete at the base and then leave it to cure before blocks are laid on it. Thereafter, you will require iron rods to cast it so that it does not sink,” says Ken Jabedo, a waste management engineer.

Construction and maintenance cost

It is more expensive to construct and maintain a septic tank compared to a biodigester. The amount of labour and materials used in setting up the former is more. Then comes additional expenses of maintenance.

“A biodigester can work for years without the need for maintenance because the waste generated is exhausted but a septic tank has to go through maintenance,” says Mr Ambaisi.

The lack of exhaustion of generated waste leads to build-up of the sludge filling up the septic tank thereby necessitating regular emptying to free up space. This costs money and the number of exhaustions increases over the years.

The same cannot be said of a biodigester where there is a rapid separation of waste from the time it is flushed with the water going to the sock pit while human waste staying to be broken into smaller particles by the bioculture freeing up space making it not to fill up quickly.

Environmental safety

A biodigester has no odour and its byproducts are environmentally friendly as they have been decomposed by bioculture process in the biodigester. What comes out is greywater which is safe for the environment.

Mr Jabedo explains that a biodigester has two types of reactions, both aerobic and anaerobic, taking place at the same time. This means that for aerobic reaction, there is the circulation of oxygen making it easier for bio-culture in the biodigester to break down the waste, in form of sludge, reducing it into smaller particles.

On the other hand, since the septic tank only allows for anaerobic reaction, less decomposition of the sludge occurs leading to build-up of the waste, which has to be dislodged for treatment before being released to the environment.

“This means that the bioculture cannot survive in a septic tank because there is no oxygen so they die off. The gas inside the biodigester helps the sludge to tilt to help the bioculture eat the sludge into small particles,” says Mr Jabedo.

Type of soil

Mr Ambaisi says that the only problem with a biodigester is that it works best in red soil areas because the soil socks up quickly water released by the sock pit.

For black cotton soil, the sock pit will have to be dug deeper till one reaches the stone.