129. A mixture of food waste on inorganic analysis was found to contain Cl- , SO4 – , NO3 – , NH4 + ion. Successful
anaerobic digestion by a mixed microbial community will result in biogas having the following gases:
1. N2, CO2, O2, H2
2. CH4, CO2, N2, H2S
3. CH4, CO2, N2, H2S, H2
4. H2, CO2, N2, H2S, Cl2

Question

A mixture of food waste on inorganic analysis was found to contain Cl⁻, SO₄²⁻, NO₃⁻, NH₄⁺ ions. Successful anaerobic digestion by a mixed microbial community will result in biogas having the following gases:

1. N₂, CO₂, O₂, H₂

2. CH₄, CO₂, N₂, H₂S

3. CH₄, CO₂, N₂, H₂S, H₂

4. H₂, CO₂, N₂, H₂S, Cl₂

Detailed Explanation

Anaerobic digestion is a microbial process that breaks down organic matter in the absence of oxygen, typically producing biogas—a mixture mainly of:

• Methane (CH₄)

• Carbon dioxide (CO₂)

• Hydrogen sulfide (H₂S)

• Hydrogen (H₂)

• Nitrogen (N₂) (in small amounts depending on conditions)

Let’s break down the role of each ion in the context of anaerobic digestion:

1. Cl⁻ (Chloride):

• Inert under anaerobic conditions.

• Does not contribute to gas production.

• Present as a salt, with no direct volatile gas-forming impact.

2. SO₄²⁻ (Sulfate):

• Used by sulfate-reducing bacteria.

• Produces H₂S (hydrogen sulfide) during anaerobic digestion.

3. NO₃⁻ (Nitrate):

• Nitrate can be reduced to N₂ via denitrification.

• Under anaerobic conditions, denitrifying bacteria convert it to nitrogen gas (N₂).

4. NH₄⁺ (Ammonium):

• Typically remains in the aqueous phase as an ion.

• May influence pH but does not contribute to biogas.

5. Organic Matter (from food waste):

• Fermented by fermentative bacteria to volatile fatty acids (VFAs).

• Methanogens convert VFAs and CO₂ into CH₄ (methane) and more CO₂.

• Under some conditions, H₂ (hydrogen) may also be a by-product.

Gas Composition Expected

• CH₄ (Methane) → Main energy-rich component.

• CO₂ (Carbon Dioxide) → Produced from fermentation and methanogenesis.

• N₂ (Nitrogen) → From denitrification of NO₃⁻.

• H₂S (Hydrogen Sulfide) → From sulfate reduction.

• H₂ (Hydrogen) → From fermentation (sometimes transient).

❌ Why Other Options Are Incorrect

• Option 1: Includes O₂ (oxygen), which is not present in anaerobic systems.

• Option 4: Mentions Cl₂ (chlorine gas), which is not formed during anaerobic digestion of Cl⁻.

✅ Correct Answer: 3. CH₄, CO₂, N₂, H₂S, H₂

This reflects the complete spectrum of gases produced under successful anaerobic digestion with the given ions and organic matter.