In water quality analysis, turbidity of raw water is primarily a measure of the concentration of suspended and colloidal solids that cause cloudiness (not acidity or biochemical oxygen demand).

Introduction / Context:
Turbidity is one of the most routinely measured parameters in water and wastewater engineering. It reflects how “cloudy” or “murky” the water looks, which directly impacts treatment design, filter performance, and public perception of drinking water quality. Understanding what turbidity actually measures helps avoid common confusions with acidity (pH) or biochemical oxygen demand (B.O.D.).

Given Data / Assumptions:

• We are dealing with raw (untreated) water entering a water-treatment system.
• Turbidity is measured with standard instruments such as nephelometers and reported typically in NTU (nephelometric turbidity units).
• Options include suspended solids, acidity, and B.O.D., which are distinct water-quality concepts.

Concept / Approach:
Turbidity quantifies the scattering of light by particles present in water. The more suspended and colloidal particles present, the higher the light scattering and thus the measured turbidity. Acidity (pH) is a chemical property indicating hydrogen ion activity, while B.O.D. is a biological test that measures oxygen consumed by microorganisms during the degradation of organic matter over a defined period (commonly 5 days).

Step-by-Step Solution:
Step 1: Identify what turbidity gauges: light scattering from particles in suspension.Step 2: Link light scattering to particulate content: more suspended/colloidal matter implies higher turbidity.Step 3: Differentiate from acidity: pH relates to hydrogen ion concentration, not particulate cloudiness.Step 4: Differentiate from B.O.D.: B.O.D. measures oxygen uptake by microbes, not optical clarity.Step 5: Conclude that turbidity is a measure of suspended solids (and fine colloids) in water.

Verification / Alternative check:
Operators often correlate turbidity spikes with events that increase particulate loads (e.g., storm runoff). Filtration steps (coagulation, flocculation, sedimentation, filtration) reduce particulates and consequently reduce turbidity, confirming the direct link with suspended matter.

Why Other Options Are Wrong:
Acidity of water: pH does not directly influence light scattering unless it alters particle chemistry; turbidity devices do not read pH.B.O.D.: A 5-day oxygen-demand test is unrelated to instant optical scattering read by turbidity meters.None of these: Incorrect because suspended solids is exactly what turbidity indicates.

Common Pitfalls:

• Confusing turbidity (optical property) with total suspended solids (gravimetric mass). They are related but not identical.
• Assuming color alone causes turbidity; dissolved color can affect absorbance but turbidity measures scattering due to particles.
• Equating low pH or high B.O.D. with high turbidity; these relationships are not direct.

Final Answer:
suspended solids