As per Bureau of Indian Standards (BIS) drinking-water specifications, what is the maximum permissible limit for total dissolved solids (TDS) when no alternate source is available?

Introduction / Context:
Total dissolved solids (TDS) reflect the concentration of inorganic salts and small amounts of organic matter dissolved in water. BIS standards specify both desirable limits and higher permissible limits in the absence of alternative sources, guiding utilities in assessing palatability and corrosion/scaling tendencies.

Given Data / Assumptions:

• BIS provides a desirable limit and a maximum permissible limit.
• When alternate sources are unavailable, a higher TDS may be accepted.
• Units are in mg/l (equivalent to ppm for dilute aqueous solutions).

Concept / Approach:
While 500 mg/l is the desirable TDS limit for aesthetics and taste, BIS allows up to 2000 mg/l as the maximum permissible value if no better source exists. This acknowledges regional variability while maintaining health and usability considerations. Above this level, taste and scaling often become unacceptable without treatment (e.g., RO, ion exchange).

Step-by-Step Solution:

Verification / Alternative check:
Publicly available BIS 10500 tables list 500 mg/l (desirable) and 2000 mg/l (permissible) for TDS, confirming the selection.

Why Other Options Are Wrong:

• 500 mg/l: Desirable, not the maximum permissible.
• 1000 or 1500 mg/l: Not the BIS-stated permissible limit.

Common Pitfalls:
Confusing hardness with TDS; they are related but not identical. RO reduces TDS but may require remineralization for taste and corrosivity control.

Final Answer:
2000 mg/l