176. A starch containing wastewater sample with high BOD, CaCl2 and NH4NO3 was subjected to aerobic
oxidation using a designed bacterial consortium. The oxidised product(s) will have:
(1) Cl2, N2, O2, and CO2
(2) Cl2, N2, and O2
(3) O2 and CO2
(4) CO2

Aerobic Oxidation of Starch-Containing Wastewater: Why CO₂ Is the Main End Product

When dealing with starch-containing wastewater, especially one with a high Biological Oxygen Demand (BOD), aerobic biological treatment is often the go-to method. A properly designed bacterial consortium can efficiently break down complex organic materials such as starch. But what exactly are the end products of this process?

What Happens During Aerobic Oxidation?

Aerobic oxidation involves the breakdown of organic compounds by bacteria in the presence of oxygen. In the case of starch (a carbohydrate), the general reaction is:

C₆H₁₀O₅ (starch units) + O₂ → CO₂ + H₂O + energy (ATP)

So, the primary end product of starch oxidation is carbon dioxide (CO₂), along with water.

Understanding the Wastewater Composition

The wastewater in question contains:

• Starch – a complex carbohydrate that contributes to high BOD.

• CaCl₂ – calcium chloride, an inorganic salt.

• NH₄NO₃ – ammonium nitrate, a nitrogen source.

CaCl₂ and NH₄NO₃ are not significantly oxidized in the biological process, and their transformation doesn’t yield Cl₂, N₂, or O₂ in significant amounts under aerobic conditions.

Why the Other Options Are Incorrect

• Option (1), (2): Suggests Cl₂ and N₂ as products, but under aerobic microbial degradation, chlorine and nitrogen gas are not typical end products.

• Option (3): Lists O₂ as a product, but O₂ is consumed, not produced, in aerobic processes.

Key Role of Bacteria

The designed bacterial consortium uses aerobic respiration to metabolize starch:

• Converts carbon to CO₂

• Nitrogen and other minerals may be assimilated or remain unchanged

• No harmful gases or chlorine compounds are produced in significant quantities

Final Outcome

So, the main oxidized product after aerobic treatment of starch-containing wastewater is:

✅ CO₂ (Carbon Dioxide)

This product is environmentally benign, making aerobic treatment a sustainable solution for starch-rich effluents in food processing and other industries.