Kjeldahl nitrogen in sewage refers to the total nitrogen content in sewage that can be determined using the Kjeldahl method. The Kjeldahl method is a widely used technique for the determination of nitrogen content in organic and inorganic substances, including sewage.

The correct answer is option 'C', which states that Kjeldahl nitrogen in sewage is the sum of ammonia nitrogen and organic nitrogen. Let's understand why this is the correct answer:

1. Kjeldahl Method:
The Kjeldahl method involves the digestion of the sample with concentrated sulfuric acid, which converts all the nitrogen present in the sample into ammonium sulfate. The ammonium sulfate is then distilled and the ammonia released is captured in a receiving solution. The amount of ammonia collected is then determined by titration with a standardized acid.

2. Ammonia Nitrogen:
Ammonia nitrogen refers to the nitrogen content in the form of ammonia (NH3) or ammonium ions (NH4+). In sewage, ammonia nitrogen is primarily derived from organic nitrogen compounds present in urine, feces, and other organic matter.

3. Organic Nitrogen:
Organic nitrogen refers to the nitrogen content present in organic compounds, such as proteins, amino acids, and other nitrogen-containing organic molecules. In sewage, organic nitrogen is derived from various organic sources, including human waste, food waste, and other organic materials present in the sewage.

4. Calculation of Kjeldahl Nitrogen:
During the Kjeldahl analysis of sewage, both ammonia nitrogen and organic nitrogen are converted into ammonium ions. Therefore, the total nitrogen content determined by the Kjeldahl method includes the nitrogen present in both forms.

Hence, the correct answer is option 'C', which states that Kjeldahl nitrogen in sewage is the sum of ammonia nitrogen and organic nitrogen. This is because the Kjeldahl method measures the total nitrogen content in sewage, including nitrogen present in the form of ammonia and organic compounds.