In municipal wastewater engineering, approximately what percentage of sewage is water by content? Select the most appropriate value from the following, keeping in mind that domestic sewage contains only a very small fraction of suspended and dissolved solids.

Introduction / Context:
Domestic sewage is predominantly water with a relatively tiny fraction of solids (both suspended and dissolved). Recognizing the typical water content helps in sizing pumps, clarifiers, sludge handling units, and understanding why large flow volumes carry comparatively little solid matter.

Given Data / Assumptions:

• Typical municipal (domestic) sewage composition is required.
• Solids fraction is small compared to the liquid fraction.
• No industrial-strength concentrations are implied.

Concept / Approach:

Standard design texts and practice indicate that sewage generally contains about 0.1% solids by weight; the remainder is water. Hence, water content is of the order of 99% (and often cited even closer to 99.9% for purely domestic sewage). Among the provided choices, 99% best represents this fact.

Step-by-Step Solution:

Verification / Alternative check:

Plant operations experience and standard handbooks consistently list water content ~99% (sometimes more), reinforcing this selection for conceptual questions.

Why Other Options Are Wrong:

90% or 95% imply unrealistically high solids; 9.9% is off by an order of magnitude; 98% is closer but still lower than commonly reported values.

Common Pitfalls:

Confusing total dissolved solids (TDS) with total solids concentration; assuming industrial discharges without context.

Final Answer:

99%