Cow Dung and Anaerobic Digestion

To get biogas, farmers are creating their own, by using cow dung and building a bio-digester. 

The end products are also a good source of fertilizer for crops. Making their own biogas saves hundreds of US$ per year in fuel costs for fuel that is used to run small water pumps.

Introduction

A biogas digester plant relies on bacterial decomposition of biomass, waste material which is biological in origin, ranging from kitchen scraps to cow dung. 

As anyone who has walked past a poorly maintained outhouse or compost pile is aware, when anaerobic conditions develop in a collection of biomass, they attract bacterial organisms which emit a number of distinctive gases.

These are most notably methane, which is produced in the process of digestion. These gases are usually viewed as a symptom of inefficiency and they are vented away for disposal, but they can actually be very useful.

Why is Cow Dung Often Used to Start Up Biogas Digesters?

Most biogas digesters use cow dung to produce biogas. There are many other organic materials as mentioned above that can be used to produce biogas. Like left over food scraps, vegetation etc.

For this reason, cow dung is commonly used to start the process of biogas production. You can change the organic material to be used to produce biogas after the production has been kicked off by cow dung.

How Much Biogas is Produced?

A one-cubic-meter digester, primed with cow dung to provide bacteria, can convert the waste generated by a four-person family into enough gas to cook all its meals and provide sludge for fertilizer. 

A model this size costs about $425 but many testimonials suggest that such a facility will pay for itself in energy savings in less than two years. 

Admittedly, that's still a high price for most Indians, even though the government recently agreed to subsidize about a third of the cost for these family-sized units.

For the equipment to produce gas, the digester is filled halfway with bio-degradable materials, like cow dung mixed with water in equal ratio. It is then refilled with smaller amount every day, or at intervals no longer than to 2 weeks. The equipment can start producing gas after seven days.

Gober Gas

Gober gas (also spelled gobar gas, from the Urdu, Punjabi and Hindi word gober for cow dung) is biogas generated from cow dung. 

A gober gas plant is an airtight circular pit made of concrete with a pipe connection. First, manure is dumped in the pit. Then, water or wastewater is added to the manure and the concoction is sealed in the airtight concrete pit with a gas pipe leading to stove unit in the kitchen serving as the only egress for gas. When the control valve on the gas pipe is opened the biogas is combusted for cooking in a largely odourless and smokeless manner.

The image above shows the fixed dome digester design often used for cow dung, chicken manure and human excreta. 

Fixed Dome Digesters

Fixed dome plants were chosen because they can last for over fifty years and they are easily insulated and scum fosrmation is less due to the digester slurry that is displaced (pushed out) by incoming feed (influent). 

A fixed dome digester is an underground biogas digester lined with brick, with a dome-shaped cover made from concrete. The cover is fixed and held in place with earth piled over the top to resist the pressure of the gas inside. A second pit, the slurry reservoir, is built above and to the side of the digester.

Final  Size of the Biogas Plant

The size of the biogas plant is to be decided based on availability of raw material. It is generally said that, average cattle yield is about 10 kg dung per day. For eg. the average gas production from dung may be taken as 40 lit/kg. of fresh dung. The total dung required for production of 3 m3 biogas is 3/0.04= 75 kgs. Hence, a minimum of 4 cattle is required to generate the required quantity of cow dung.

A one-cubic-meter digester, primed with cow dung to provide bacteria, can convert the waste generated by a four-person family into enough gas to cook all its meals and provide sludge for fertilizer. A model this size costs about $425 but will pay for itself in energy savings in less than two years. That's still a high price for most Indians, even though the government recently agreed to subsidize about a third of the cost for these family-sized units.