In Gujarat, harnessing the value of dung to boost farmers’ income

In Gujarat, harnessing the value of dung to boost farmers’ income

Subject: Science and tech

Section: Msc

Context:

• Gujarat’s dairy cooperatives are showing the way in supplementing their farmers’ income from milk by procuring dung and converting it into Bio CNG and fertilizer.

More on news:

• Deesa-Tharad highway in Gujarat’s Banaskantha district, catering to 90-100 vehicles daily, is India’s first and only gas-filling station based on dung from cattle and buffaloes.

Dung facts

• From 40 tonnes of dung, we get 2,000 cubic meters of raw biogas containing 55-60% methane, 35-45% CO2, and 1-2% hydrogen sulfide (H2S) and moisture

About Biogas plant:

• A biogas plant requires fresh dung, which contains methane along with water. 
• The methane is produced by bovines inside their rumen (first of four stomach compartments), where the plant material they eat gets fermented or broken down by microorganisms before further digestion.
• Carbohydrate fermentation leads to production of carbon-dioxide (CO2) and hydrogen. 
• These are used by archaea (bacteria-like microbes in the rumen) to produce methane, which the animal expels either as gas or in the dung.
• The dung left to dry in the open releases both water and methane. 
• To realize its fuel value, the dung has to, therefore, be collected and delivered in fresh form at the biogas plant.

Fuel plus fertilizer:

• The raw dung unloaded at the BioCNG plant is mixed with an equal quantity of water. The resultant slurry is then pumped into an anaerobic digester.
• Anaerobic digestion is a process by which the complex organic matter in dung is broken down in the absence of oxygen to produce biogas

The digestion involves four successive stages: 

• hydrolysis (break-down of organic matter into simple molecules), 
• acidogenesis (their conversion into volatile fatty acids), 
• acetogenesis (production of acetic acid, CO2 and hydrogen) and 
• methanogenesis (biogas generation).

What is Biogas?

• It mainly comprises hydro-carbon which is combustible and can produce heat and energy when burnt.
• Biogas is produced through a biochemical process in which certain types of bacteria convert the biological wastes into useful bio-gas.
• Since the useful gas originates from a biological process, it has been termed as bio-gas.
• Methane gas is the main constituent of biogas.

Purification of Biogas:

• The raw gas is purified for removing CO2 (through vacuum pressure swing adsorption or VPSA process), H2S (using activated carbon filter) and moisture (with air dryer separator). 
• The end-product, purified (to 96-97% methane, 2-3% CO2 and below 0.1% H2S and moisture) and compressed, is stored in cascades. 
• This compressed biogas (CBG), conveyed through pipelines to the dispensers at the fuel station, is what’s being sold as BioCNG at Rs 72/kg.