Drinking water biological quality standards (BIS IS 10500:2012 / WHO Guidelines):
Escherichia coli and total coliform bacteria are indicator organisms of fecal contamination.
According to Bureau of Indian Standards (BIS IS 10500) and World Health Organization (WHO) potable water guidelines:

• E. coli or thermotolerant coliform bacteria must not be detectable in any 100 ml sample of water intended for drinking (count must be 0 per 100 ml).

The presence of any coliform indicates potential presence of enteric pathogens causing waterborne diseases.

Conversion between Electrical Conductivity (EC) and Total Dissolved Solids (TDS):

• Standard Relation: In irrigation water analysis (USDA Handbook 60 / Richards, 1954), total dissolved salt concentration (TDS) is related to electrical conductivity (EC) by the empirical formula:
  $$\text{Salt concentration (mg/l)} = \text{EC (dS/m)} \times 640$$
• Calculation for $\text{EC} = 20\text{ dS/m}$:
  $$\text{TDS} = 20 \times 640 = 12,800\text{ mg/l}$$
  Converting $\text{mg/l}$ to grams per liter ($\text{g/l}$):
  $$\text{Concentration} = \frac{12,800}{1,000} = 12.8\text{ g/l}$$