Water hyacinth (Eichhornia crassipes) is a floating plant that grew originally in the wetlands of South America. It reached Africa in the early years of the twentieth century, and within a few decades scientists were calling it the region's most troublesome water weed. After taking hold near the southern tip of the continent, it pushed steadily northwards, clogging large reservoirs and choking the flow of major rivers.
Its arrival in Kenya owed a good deal to admiration for the way it looked. Gardeners in the colonial era, charmed by its glossy leaves and pale violet flowers, set it out in the ornamental pools of their estates. From there it escaped, drifting down the Nyando channel until, by the 1990s, it reached the waters of Lake Achora. The lake gave it steady warmth and contained nothing that would feed on it, so the weed spread across the surface unchecked, closing off the routes that fishing crews relied on and creating fresh breeding grounds for the mosquitoes that spread disease.
The heaviest burden fell on the women who buy each day's catch and preserve it by smoking. Boats that had formerly landed fish by the hundred now forced their way through dense rafts of vegetation, and earnings dropped. Nor did the trouble stop at the water's edge. Smoking fish calls for a great deal of firewood, and fetching it can mean a walk of up to fourteen kilometres there and back; while the fires burn, the women stand for long stretches in grey smoke. Across Kenya, the latest health and demographic survey reports, nearly four in five households still cook over wood or charcoal, and the share is higher still beyond the towns. Fuels of this sort pollute the air inside the home, and the World Health Organization sets the yearly Kenyan death toll from such pollution, most of it traced to cooking and heating, at close to nineteen thousand.
Until recently the shore of Lake Achora was regularly heaped with piles of hyacinth that residents had dragged from the water hoping to clear it. Few of them recognised that those decaying, waxy leaves were in fact a reserve of renewable energy. The very plant that spreads so easily also stores carbon and nitrogen in a ratio that, from about 2005 onwards, began to interest energy researchers around the world, who regarded it as a promising material for fuel. As little as five kilograms of the dried weed, some of the first studies calculated, would cover the daily energy needs of a sizeable family.
A team in Tanzania reported in 2016 that the volume of burnable gas climbed sharply when the weed was processed together with poultry droppings. Kenyan researchers, coming to the question somewhat later, reached the same verdict that others had before them: mixing in animal waste made the job of turning the plant into gas considerably more efficient.
The approach reached a lakeside village named Nyakoi in 2019, offering a single cure for two distinct ills, the smothering weed and the dependence on firewood. A pair of biogas digesters, both of them donated, were set up for the community, each designed to convert a blend of water hyacinth and goat dung into gas for the kitchen.
A digester works in much the way a stomach does. Material is put in through one opening, and across roughly the following three weeks it is broken down by microbes in a process known as digestion, which releases a steady stream of gas at the other end. Burning cleanly, that gas is then carried through slim tubes to wherever it is wanted, just as ordinary household gas is supplied. The two units at Nyakoi make enough to cover close to two-thirds of the village, running cooking stoves and handling other domestic jobs such as treating water to make it drinkable and keeping newly hatched chicks warm.
What the project is really putting to the test is whether biogas can take the place of firewood and charcoal throughout rural Kenya, and the early indications are promising. The women who smoke fish say they now fall ill less often. Released from the daily task of gathering wood, which had swallowed long hours of each day, they can turn that freed effort to other work that brings in extra income for their households.
Dr Amara Otieno, an epidemiologist studying air quality at the Nairobi Institute for Public Health, expects that households giving up smoky stoves will suffer fewer diseases of the lungs. The people most at risk, she points out, are women, girls and young children, since it is they who tend the fire indoors or in the yard.
How far the new stoves will improve the community's health is something only further study can settle, says Joseph Mwangi, who directs the social enterprise Rift Biogas. Yet one point is already clear: at around 90,000 shillings each, the digesters remain out of reach for most villages unless their cost can be lowered.
For Otieno, bringing that cost down is a challenge well worth taking on. Fuels branded 'dirty', among them kerosene, charcoal and wood, carry grave risks to health and the environment, and she is convinced that cheap biogas on a wide scale 'would transform the lives of millions across Africa and beyond'.
