Question

Cow dung and water are mixed and fed into a biogas plant to allow digestion of biowastes. The person performing this process says that there is no need to provide an inoculum.
Do you agree with him? Justify your answer.
What happens to the biowaste inside the digester ?
(c) Name the useful by-products obtained from this process and mention how they are used.

Hint:

Cattle eat large amounts of cellulose-rich grass, so their stomachs are packed with methanogens to help break it down. Because these bacteria are already abundant in fresh cow dung, the raw material acts as its own active starter culture.

Answer 1

Step 1: Understanding the Question:
The question presents a scenario involving a anaerobic biogas plant. We need to evaluate whether adding an external microbial starter (inoculum) is necessary, explain the breakdown of waste inside the digester, and identify the resulting useful by-products and their applications.

Step 2: Key Formula or Approach:
Biogas generation is a multi-step anaerobic process driven by methane-producing bacteria called methanogens, notably Methanobacterium. These bacteria naturally live inside the digestive tracts of cattle.

Step 3: Detailed Explanation:
(a) Evaluation of the Statement: Yes, I agree with the person's statement .
Justification:There is no need to add an artificial starter or external microbial inoculum to the mixture. This is because methanogenic bacteria (specifically Methanobacterium) are already naturally present in large numbers inside the rumen (stomach) of cattle. When cow dung is collected and mixed into the plant, these native bacteria are carried along with it, automatically serving as an active biological starter culture to begin the fermentation process.
(b) Process Inside the Digester: Inside the sealed, oxygen-free digester tank, anaerobic microbes break down the organic biowaste through a multi-stage fermentation process:
1. Solubilization Hydrolysis: Complex polymers in the dung (such as cellulose, hemicellulose, and proteins) are broken down into simpler soluble monomers.
2. Acidogenesis: Acid-producing bacteria convert these monomers into volatile fatty acids (like acetic acid), hydrogen, and carbon dioxide.
3. Methanogenesis: Finally, the strictly anaerobic methanogenic bacteria consume these organic acids and gases, converting them into a mixture of methane ($\text{CH}_4$), carbon dioxide ($\text{CO}_2$), and trace hydrogen sulfide ($\text{H}_2\text{S}$). This gas mixture is harvested as biogas.
(c) By-products and Their Uses:
1. Biogas: Collected from the top gas outlet of the digester tank. It is an excellent clean-burning fuel used directly for cooking and rural household lighting.
2. Spent Slurry Residual Sludge: The leftover digested material is discharged through the outlet chamber. This sludge is rich in essential plant nutrients like nitrogen, phosphorus, and potassium. It is dried and used as a highly effective organic manure in agricultural fields to improve soil structure and boost crop yields.

Step 4: Final Answer:
(a) Yes, I agree, because methanogens are naturally present in cow dung, removing the need for an external inoculum .
(b) Inside the digester, anaerobic bacteria ferment and break down complex organic compounds into a mixture of methane and carbon dioxide gases .
(c) The useful by-products are biogas (used as a clean fuel for cooking and lighting) and spent slurry (used as a nutrient-rich organic manure for farming).