Bio digesters offer cheaper way to manage home waste

With land becoming scarce in urban areas, people are turning to bio digesters to save space.

According to Sammy Irungu, the founder of 47 Investment Construction Company that specialises in the installation of bio digesters, a bio digester is simply a tank that digests organic material biologically. In this case, human, animal and kitchen waste.

In his five years of bio digester installation, Irungu reveals that bio digesters are cheaper to instal and more effective in the long run compared to the traditional system that requires connection to the nearest sewage line or construction of expensive sewage treatment plants.

Sewerage systems are high maintenance because of the constant exhaust services required.

AFFORDABLE

He further reveals that bio digesters are ideal for a household of up to 10 people.

They consume a small portion of land, usually a square metre and two for commercial use, hence releasing space for other construction purposes. “With the limited amount of space in Nairobi, bio digesters are the way to go,” he adds.

With an installation cost is about Sh80,000, Irungu reveals that this is relatively affordable. Once installed, bio digesters function very well and exhausting is completely eliminated.

“Should a large entity like church and school opt for bio digesters, they are advised to have a significant land area for water recycling,” he says.

DECOMPOSITION

Bio digesters have three parts — the body (bio digester tank), the inlet and the outlet baffles. “The baffles regulate the fluids in each part”, says Irungu, adding that the main aim of bio digesters is to reduce the solid content in sludge. Sludge is the water waste from a household.

“Sludge is divided into grey and black water. Grey water is the gently used water from your bathroom sinks, showers, tubs, and washing machines. Grey water has a combination of fats, oils, grease, waxes, and detergents.

"This makes grey water chemically unstable, and therefore cannot be released into the soil. This water is taken into the grease interceptor to remove the oils. After the removal of grease, it is directed to the outlet and into the surrounding soil environment,” Kennedy Wedah, a Nairobi-based environmentalist, offers.

“Bio digester tanks are designed to be incubators to bacteria. They provide all the conditions necessary for aerobic and anaerobic decomposition of organic waste. The main process that occurs in a bio digester is known as anaerobic digestion. This is a natural process in which micro-organisms break down (eat) organic materials,” says Wedah.

BIOGAS

This leads to the release of biogas, a gas made up of methane, water vapour and carbon dioxide. A wet mixture is left when gases are released. This mixture is separated into solid matter and liquid, he adds.

“The biodegradation process that takes place in the tank makes it easier for faster digestion of waste. As a result, it is not common to have a bad odour when using the tanks, as the digestion process begins immediately the waste enters the tank. With this kind of protection, it possible to instal the tank anywhere in the compound,” Wedah advices.

The methane gas can be used as biogas, but only if it is available in large quantities. Waste water can be released into the soil, or it can be further treated for human and farming use.

Bio digester tanks apply to the displacement principle, which dictates that when waste water is flushed out of the tank, an equal amount of water from the tank to the soakage pit is replaced. This therefore ensures that the system never fills up, says Irungu.

PLASTIC

You have to be careful about the type of bio digester contractor you get, Irungu warns.

“He should be able to use plastic materials, because they prevent water from the surrounding underground area to permeate in. Plastic is more airtight and completely sealed. This makes it more advantageous for the anaerobic decomposition process,” he advices.

Irungu further adds that plastic materials such as sanitary towels or condoms should not be flushed down to the bio digester, because plastic leads to the interference of the anaerobic process.

It may lead to the destruction of bio digesters since plastics do not decompose as is the case with organic human refuse.

Irungu believes that bio digesters are the best way for both rural and urban areas. The installation time, he claims, is only five days.