During the earliest stage of decomposition of organic matter in sewage (pre-dominantly hydrolysis and initial biological breakdown), which primary product appears first in significant quantity?

Introduction / Context:
Sewage contains organic nitrogen (proteins, urea). Understanding the sequence of transformations helps in process selection and troubleshooting in wastewater treatment.

Given Data / Assumptions:

• Raw domestic sewage with typical organic nitrogen compounds.
• Initial decomposition occurs under reducing or mildly anaerobic conditions before full aeration.

Concept / Approach:
The first biochemical step is ammonification: organic nitrogen → ammonia (NH3/NH4+). Only later, under aerobic conditions, nitrifying bacteria oxidize ammonia to nitrites (NO2−) and then to nitrates (NO3−). Carbon dioxide is also formed but ammonia is the hallmark nitrogenous product of the first stage.

Step-by-Step Solution:
Step 1: Recognize ammonification as the earliest nitrogen transformation.Step 2: Identify that nitrification (to nitrite/nitrate) is a later, aerobic step.Step 3: Conclude that ammonia is the earliest significant nitrogenous product.

Verification / Alternative check:
Process flow in conventional treatment: organic N → NH4+ (primary), then NH4+ → NO2− → NO3− in aeration.

Why Other Options Are Wrong:

• Nitrites/Nitrates: Require aerobic nitrification, not first stage.
• Carbon dioxide: Produced, but question focuses on nitrogenous product; ammonia is the specific early indicator.
• Hydrogen sulphide: Related to sulfur cycle under anaerobic conditions, not the first nitrogen product.

Common Pitfalls:
Confusing nitrification with ammonification; assuming nitrates appear immediately in raw sewage.

Final Answer:
Ammonia