For years, waste from the slaughterhouse, based in the city´s Khor area, was either dumped in rivers or in landfills, creating pollution and stomach-churning odours. To solve the problem, city authorities teamed up with the local neighbourhood council and ´Le Partenariat´, a French NGO that has been active in Saint-Louis for thirty years. The solution came in the form of ´biodigesters´, a technology already used extensively across Asia. Biodigesters are oxygen-free vats which contain organic waste from plants or animals. The waste is broken down naturally to produce a renewable energy called biogas in a process known as ´methanisation´.
The biodigesters were set up under the close supervision of a French company called BioEco. They´ve been up and running since 2013, thanks to financing from both the Poweo foundation and Senegal´s National Biogas Programme. Working hand in hand with the NGO Le Partenariat, a local committee is currently responsible for making sure the devices run smoothly.
"Reusing organic waste lets us harness the methane that's released as it gets broken down"
In this slaughterhouse, which was originally built back in the 1970s, mainly steers and buffalos are killed. Since the project was set up, we’ve been collecting stomach waste, like entrails and excrement, in order to reuse them. Every day, a slaughterhouse worker gathers up the equivalent of five wheelbarrows of this organic waste that he then tips into a tank. The waste then needs to be ´diluted´. To do that, we use the water that´s been used to clean the slaughterhouse, turning the waste into a more watery mixture.
Then the waste is placed inside the five biodigesters, each one measuring around 10 m3. After one or two days, gas bubbles begin to form. The biogas is harnessed and stored in two tanks.
Because it isn´t pressurised, unlike the gas stored in canisters, residents use a pump to extract the gas from the tanks to use it for cooking. At the moment only five nearby households use it, but another seven should soon start taking advantage of the project.
"This biogas is far cheaper for households than conventional gas canisters"
Methane makes up 70% of the biogas. That´s pretty much the same composition that you´d find with bottled gas, so in terms of cooking time it doesn´t change anything for households. However, it is far cheaper. Roughly speaking, each household consumes around one cubic metre of gas every day in order to cook. Given that one cubic metre of biogas sells for around 200 CFA francs [Editor´s note: 0,30 euros], you´d need around 6,000 CFA francs per month [Editor´s note: roughly 9 euros]. That´s relatively little compared with how much households spend on bottled gas, which is between 30,000 and 35,000 francs per month [Editor´s note: between 46 and 53 euros].
In environmental terms, collecting this organic waste allows us to clean up the neighbourhood and also lets us harness the methane produced when it rots. That´s extremely useful, given that methane is a greenhouse gas that´s 20 times more noxious than carbon dioxide.
Solid and liquid fertilizers
Aside from the biogas, the devices also produce two types of natural fertilizer. Even though the waste gets broken down in the biodigesters, the residue builds up slowly over time. So they have to be emptied every six months when they get full. It´s very difficult because we have to use a spade and it stinks! Whenever we empty the devices, we´re basically removing the equivalent of 17 wheelbarrows of waste, which weighs 800 kg in all. We then take that residue and use it to make compost which is sold to farmers and gardeners. One wheelbarrow of the stuff sells for around 5,000 CFA francs [Editor´s note: 7.62 euros].
Moreover, we collect the blood and the water used to clean the slaughterhouse. The mixture has high levels of nitrogen, meaning that the subsequent liquid fertilizer can be used for free by gardeners to irrigate their gardens.
"We´d also like to put fish waste to good use"
All of the waste produced by the slaughterhouse is now being reused, with the sole exception being the horns that are grinded down. But we´re hoping to find a use for them soon. A similar project is now being set up to find a way to put fish waste, produced by the port in Saint-Louis, to good use.