alexa Hey, pig manure offers clean biogas, but don’t use the slurry to grow food crops - Daily Nation

Hey, pig manure offers clean biogas, but don’t use the slurry to grow food crops

Saturday September 3 2016

James Wokabi watches as the pigs are fed in his Nyeri farm.

James Wokabi watches as the pigs are fed in his Nyeri farm where he keeps some 200 pigs and further grows tea, coffee and eucalyptus trees. PHOTO | CAROLINE WAMBUI | NATION MEDIA GROUP 

More by this Author

Seated behind his desk in his farm office, Julius Wokabi scratches his head several times as he works out some figures in a book.

The 52-year-old father of two, who works as an accountant in Nairobi, farms in Othaya, Nyeri, where he keeps some 200 pigs and further grows tea, coffee and eucalyptus trees.

However, it is the pig venture on his 12-acre farm christened Jidlaph that make it tick. Despite keeping the many animals, Wokabi has managed to maintain high standards of hygiene, with nothing going to waste as he makes biogas from the droppings.

It all starts with the way the farmer has constructed the pigsties. Each shed is subdivided into two to hold a sleeping and a dunging area as pigs should not sleep on a dirty environment, despite the perception that they love dirt.

The watering system in the shed is also automated to avoid wastage and uses displacement method where the inflow is equal to the amount consumed.

Waste from the dunging area finds its way to a collecting point through pipes and gravity, where it is mixed in a ratio of 1:1 and the slurry is added to the bio-digester for biogas generation.


“These pigs have come as a blessing ever since I started keeping them in October last year. I used to spend up to Sh320,000 on fertiliser alone, Sh80,000 on tea which is on two acres and Sh240,000 on coffee that is on six acres. The two cash crops require constant fertiliser, which becomes expensive, but this is now in the past,” offers Wokabi, noting he went into pig farming because of their high breeding rate, which makes them profitable.

To start, Wokabi consulted experts from Farmers Choice and was advised on housing and feeds before he invested into 20 mature pigs, with the venture costing him about Sh1.2 million in total.

“One thing I was warned about is sub-standard feeds. Even if you have a genetically right breed, substandard feeds only lead to wastage in terms of time, money and labour as the animal will not attain the required weight on time and the female may not go on heat.”

One of Wokabi's employees cooks using the
One of Wokabi's employees cooks using the biogas he generates from his pigs' manure. PHOTO | CAROLINE WAMBUI | NATION MEDIA GROUP

Once his piglets are born, he clips the teeth in the first 15 minutes to prevent them from biting and inflicting pain on the sow while suckling leading to infection of the udder.

John Kihumba, an extension officer with the Ministry of Agriculture in Othaya, explains that in the first two weeks, if the piglets are more than six, they should be separated from their mother to suckle in turns so that the sow may not trample on them.

“The piglets should also be castrated within 21 days to avoid inbreeding and hygiene be maintained to keep diseases at bay,” Kihumba adds.
Wokabi weans the piglets at six weeks, feeding them with 0.75g of concentrate per pig, per day so that they can add about 400g daily.


In the seventh week, he adds the concentrate to 1kg per pig, per day and by the fourteenth week, the piglets now referred to as pokers are fed up to 1.7kg, which is in three equal portions, at 7am, at noon and at 4pm to add about a kilo a day.

At 22 weeks, during the beconer stage, the pigs are fed 2.5-2.7kg and are ready to be served in seven months where they will be fed 25 per cent of their body weight. To supplement the commercial concentrate, Wokabi also grows vegetable and sweet potato vines.

“A pig if properly fed should weigh 100kg by the fifth month,” Kihumba says.

Wokabi sells his pigs to Farmers Choice at an average of Sh18,000 for a 100kg animal. “I sell about 20 animals a month though there are high and low seasons. To be contracted as a supplier of Farmers Choice, volumes matter most,” he offers.

Wokabi constructed the biogas plant, which cost him Sh600,000, and the pig shed at the same time even before installing the pigs, therefore, he did not have any challenge with pig waste, which experts warn should not be used on food crops.

“I produce a minimum of 25 and a maximum of 32 cubic metres of gas per day, with the pig offering about 500kg of poo a day. I use the gas for cooking, lighting and warming the pigsties using heaters. I plan to install a gas generator that runs on biogas to produce electricity,” says Wokabi, noting he manages to save around Sh20,000, which would have gone on fuel.

Edward Mugambi, an engineer with Green Energy Revolution, says that pig waste produces more gas than cow’s as a kilo generates up to 60 litres of biogas as compared to cow dung that offers 40 litres.

Wokabi mixing the biogas making slurry. PHOTO |
Wokabi mixing the biogas making slurry. PHOTO | CAROLINE WAMBUI | MATION MEDIA GROUP

Pig waste, according to Mugambi, has a high total solid waste than cow dung, which is more watery.

However, for the initial feeding of pig waste to the digester, a farmer can incorporate cow dung to balance the nitrogen level, which is quite high in pig waste.

Mugambi further adds that, “Biogas production can be optimised by mixing different types of substrates to ensure that the optimum concentration of nitrogen is achieved for production of quality biogas.”


Waste from goats, rabbits and chickens can also be used for biogas production as long as they are first sorted to remove sawdust and inorganic matter before being fed into the bio-digester.

Dr Jane Maina, a veterinarian from the Faculty of Veterinary Medicine and Surgery, Egerton University, notes that pig waste, whether in slurry or raw form should not be used on food crops like vegetables.

“For trees, coffee and tea, that is fine, but for food crops, unless proper precautions are followed, pig manure can be harmful to human health. The composting process that can kill organisms harmful to man requires high temperatures and special composting conditions not available locally.”

“The primary hazard present in pig manure is parasites such as roundworms and tapeworms that can be passed on to man. Some resistant bacteria such as listeria and staphylococcus found in pig may also survive the composting process and have been responsible for disease outbreaks in agricultural and livestock workers,” she adds.



  • Biogas production is influenced by the carbon/nitrogen (C/N) ratio among other factors like the PH, temp and hydraulic retention time.
  • C/N ratio varies for different organic matter with agricultural residues having a high C/N ratio as compared to animal waste.
  • The ideal C/N ratio that optimises biogas production is in the range of 20-30.
  • A higher C/N ratio outside the above range causes low biogas production as nitrogen is rapidly consumed by the methane thus not reacting on surplus carbon in the material, a thing that can be corrected by adding nitrogen in animal urine effect of the same causes reduced biogas production.