Recycling cow dung for green energy

Many livestock farmers in Uganda are struggling with how to deal with cow dung. Most cowsheds have heaps of unattended cow dung swarming with houseflies.

In most rural areas, some farmers still use cow dung to pave floors, while others use it as manure. Yet there is so much more cow dung can do.

Dr Moses Mwesigwa, the programme leader in dairy research at the National Livestock Resources Research Institute (NaLIRRI) in Nakyesasa, says it is possible for livestock farmers to have a complete dairy value chain with zero waste.

To address this issue and recycle cow dung, various means have been implemented bringing into focus traditional composting to produce fertiliser and modern anaerobic digestion to extract renewable energy and so much more.

Farmers need to look at the cow waste as a useful thing and be able mitigate the methane emissions into the environment. Experts say the methane found in cow manure is equivalent to the amount produced by a gas-powered vehicle.

“Cows consistently make manure every day. All this goes into making milk and a lot of waste. Taking care of cow dung is a responsible way of farming for a sustainable industry,” Dr Mwesigwa says.

But before reaping the benefits of cow manure, it must first be composted to get rid of weed seeds, ammonia gas and pathogens.

Proper harvesting

By using dairy digester processes, dairy manure is made more useful while methane gas is extracted.

“Harvesting gas helps reducing methane escaping into the environment and you have a useful fuel,” Dr Mwesigwa says.

He says manure thickening is required to produce homogenised feedstock with consistent solids content to improve methane potential value.

The process also helps to capture unwanted native cellulosic biomass that is resistant to degradation.

The pre-digester manure platform at NaLIRRI is targeted to produce close to two tonnes of cow dung every day.

The harvested manure is covered up with a flexible membrane that has no oxygen. The bacteria eat the remaining calories thinking they are still in the cow’s stomach and emit methane gases that are captured to be used as fuel.

Domestic Bio-gas production

Electricity is a big part of everyday life for many of us, from home life to your workplace and everything in between, the use of energy has become a necessity.

With increased electricity usage comes an increased need for the use of renewable sources as energy to become more environmentally friendly – and biogas is one of those sources.

NaLIRRI has two bio-digesters each requiring 900 kg of cow dung every day. This produces 100 Kg of Domestic gas every day. A bio-digester system utilises organic waste to produce fertiliser and biogas.

Biogas is generally used in the same way as natural gas for lighting and cooking.

Relatedly, NaLIRRI produces biogas that runs a 75 KVA generator.

According scientists, 100 cows produce 6.25 tonnes of waste every day. However, the biogas from cow dung has high water content. What the scientists are doing is to use cow dung as feedstock for the digester to convert 20-30 percent of volatile compounds into biogas. Bio-gas, a combination of methane and carbon dioxide, is the fourth largest source of energy supply in the world.

This is in line with the Ministry of Energy and Mineral Development whose national biogas program aims to have 13,600 biogas plants constructed in households to curb indoor air pollution. In addition, they aim to reduce the rate of deforestation attributed to firewood harvesting. Uganda also issued its first energy policy in 2002 and created a renewable energy policy five years later.

Liquid slurry manure

NaLIRRI is currently producing 10,000 litres per week of liquid slurry fertiliser.

In crop production, it is used as quality organic fertilizer (root and foliar) and pesticide for the control of especially insect pests. It can be used to build healthy fertile soils for since it contains readily available plant nutrients (NPK) which contributes to among others, increased crop yield and improved crop quality or decrease in use of artificial pesticides.

Clean materials (cow dung, water and urine) are harvested and mixed appropriately.

He says that too much dung leads to production of smelly bio-slurry and inadequate feeding leads to failure of slurry expulsion from the expansion chamber and hence production of maggots.

“If one intends to make their own bio-slurry, proper mixing ratios which yield results with porridge-like mixture, are preferred. A thick mixture leads to production of smelly bio-slurry and a thin mixture lead to low gas production,” he warns.

Bio-slurry can be applied and utilised on almost all crops including fruits such as mangoes, oranges and paw paws as well as maize, sugarcane and rice. It can also be applied on fodder such as napier (elephant grass).

He says that bio-slurry can be sold as either liquid or composted manure to crop, livestock and fish farmers who do not own biogas plants.

At the post-digester slurry manure platform, NaLIRRI can produce 8000 litres of manure every day.

Carbonic acid production

Experts contend that carbonised cow dung could be used as a high-performance anode in bio-electrochemical systems, which could convert cow dung waste into an economical and environmentally friendly electrode material. Currently, practical applications of this technology are being tested at NaLIRRI.

According to Dr Mwesigwa this platform cleans biogas by ‘scrubbing’ carbon-dioxide for use in locomotive batteries. The same carbonic acid is going to be availed for use as a preservative in drinks.

Animal feeds

The experts are also harvesting solar-dried slurry by extracting solids which can be used in the production of aquaculture, poultry and piggery feed with crude protein of 7 per cent compared to maize which has a crude protein of 7.5 percent.

Dr Mwesigwa explains that this dry manure which can be packed and transported easily.

NaLIRRI is also working on a process to produce organic bio-pesticides from liquid Slurry.

Apart from getting fertiliser, slurry is a promising natural pesticide.

“We can confirm the notion that bio-slurry could replace the synthetic fertilisers and support organic farming,” Dr Mwesigwa says.

Bio-liquid soap production

In many countries, soap can be in short supply and without it contamination can be a key issue in health emergencies.

Dr Mwesigwa explains that a solution to avail cheap liquid soap to people lies in harnessing the potential of cow dung.

He says that a team is already making soap from cow dung and cow urine as ingredients.

The institute’s platform is producing various liquid and solid detergents, shampoos, shower gels from slurry as the base ingredient. A 5-litre jerrycan of liquid soap is sold at Shs10,000.

Process

How it works

The manure and other organic matter are flushed out of the barns into the tanks. The material is covered with a membrane that enables the bacteria to eat the remaining calories. This helps produce methane that is used as fuel.

Once the methane is harvested, the remaining manure is stored in lagoons for manure production and other uses.