Making biogas and slurry faster and in huge amounts on the farm

Compared to the commonly used method of using underground digesters, the plastic above-ground tank produces biogas and slurry faster. File Photo.

What you need to know:

Farms produce a lot of waste such as animal droppings, which can be used as a source of energy and as fertiliser. There are various ways of doing it, this is one of them.

An efficient and faster way of making biogas for energy use as well as bio-slurry on farm using PVC tanks is available for farmers in Uganda.

The plastic tank as a digester is unlike the more common method of cemented digester that are placed underground.

One advantage is that this one can be transferred from one place to another for convenience.

Manish Kalla, the general manager, Crestanks Ltd, who are the dealers in Uganda, says that the bio-slurry and biogas solution is meant to improve lives while providing gas for energy and a natural fertiliser, as a by product.

David Bukenkene, the biogas technician, says that for a farmer to gain from this bioslurry technology, he or she needs to dig a pit first, depending on the size of the tank acquired.

“If the tank is 6,200 litres (or Large Farm Model (LFM), biggest size, dig a hole or pit, 2.5 metres long, 2.5 metres wide and 2.5 metres deep. And if it is a Small Household Model (SHM), medium size, the pit should be 1.5 metres long, 1.5 metres wide and 1.5 meters deep,” he explains.

Bukenkene adds that the pit should be lined with polythene sheeting. Then start collecting cow dung in it and add clean water at the rate of about 20 percent to keep the dung moist.

“When the pit is filled with cow dung, mix in about 300 litres of water, cover it with polythene sheeting and leave it for 10 days. The polythene is meant as a cover to protect it from rain and keep it from drying up,” he says.

After the PVC tank is stationed, the farmer should mix uncontaminated cow dung with water in a vessel and mix through until everything is dissolved.

Later, the mixture is poured into the digester until the tank is full. The 6,200-litre tank takes 1,500 kilogrammes of uncontaminated cow dung and 1,500 litres of water while a 3,300-litre tank takes 750 kilogrammes of uncontaminated cow dung and 750 litres of water.

“Feed the 6,200-litre tank twice a day for better output. In the morning, mix in 40 kilogrammes of cow dung and 40 litres of water and also do the same in the evening. This yields 110 to 130 kilogrammes of bio-slurry every day. If it’s a 3,300-litre tank, feed it with 20 kilogrammes of cow dung and 20 litres of water for a daily output of 56 to 66 kilogrammes of bio-slurry,” he says.

Good source
The more the gas is used, the more bio-slurry is produced. Each of the gas stoves consumes biogas at the rate of 350 to 450 litres per hour of use.

“We strongly recommend the use of only standardised and approved biogas stoves because the cooking hours are computed based on the use of stoves,” Bukenkene cautions.

The technician notes that the effluent, bio-slurry, is a product of controlled fermentation of manure, in absence of oxygen, to produce methane rich gas (biogas).

This effluent or bio-slurry is a good source of nutrients for crops. It is well decomposed and can be safely used on forage such as Napier grass, and on food crops. It releases nutrients to plants much faster than fresh or decomposed manure.
Using the bio-slurry tank

1. Collect 1,500kgs of cow dung and 1,500 litres of water or urine for the initial filling of the biogas unit. Then, mix the cow dung with the water or urine in a separate container beside the unit until it attains a porridge-like texture. After mixing, a the volume will be about 3,000 litres. Note that the amount of water or urine used depends on texture of the dung, therefore keep adding water or urine until the mixture is neither too thick nor too watery.

2. Close the outlets of the digester and gas holder and then pour the mixture from the inlet on the side. There will be a discharge from the outlet on the opposite side once the digester is full to capacity. Then open the outlets for the air to exit from the gasholder completely. Close the outlets when the gasholder has descended.

3. Leave the unit outdoors for 12 hours and the bacteria in the digester will start to work on the mixture. As it ferments, gas (methane) is produced and trapped in the gas holder. This will cause the gas holder to rise gradually until it covers 17 segments marked on the gas holder from the exterior. This should happen within 24hours or more, depending on the texture of the cow dung.

4. Connect a burner or lamp from the outlet at the base of the digester to tap the gas, a blue flame will be emitted ready to be used for cooking and lighting on ignition. As you start using the gas, set more material to feed into the digester. Get 40 kg of dung and 40 litres of water and mix thoroughly and continue feeding as the gas begins to run out. Do not allow the mix simply to run out by fast feeding. Recommended feeding is 80-litre mixture for morning and evening.

5. As feeding is effected, say, 80 litres, it will emit 60 litres of the used cow dung-water/urine mixture from the outlet on the opposite side of the digester as bio-slurry (effluent) is released. Each time the unit is fed, it is rejuvenated hence the continued buildup of gas. The bio-slurry is used as a fertiliser in the garden to boost crop production or to be used as a food supplement for chicken, fish and pigs.

6. Note that feed material can be any organic decomposing matter like animal dung, plant residue and sewage. Other than cow dung, droppings from other animals like pigs, goats and chicken can be used. Also, agricultural and food waste include over-ripe or rotten fruit pulps, crushed peelings or vegetables, spoilt fruit juice and vegetables from processing factories, finely crushed non-edible and wild fruits, sugarcane juice, finely chopped sugar cane and molasses, rotten or waste grain, bran of wheat, rice and other cereal, spoilt potatoes, cassava, sorghum, maize, millet, powdered oil cake from edible and non-edible oil seeds like cotton, castor and jatropha.

Caution: With agricultural waste and food remains, it is a two-stage process. The first stage being fermentation, which takes place in a separate vessel, and then the second stage will be feeding of the fermented mass into the digester. This is done to allow the anaerobic bacteria digest the waste.

“Here the material has to be chopped into tiny pieces. Later, the chopped material together with water is poured into a separate vessel. The fermented mass is then poured in the digester,” explains David Bukenkene, a biogas technician.