Hardness of water is expressed as milli-equivalent per litre (mEq/L), one unit of which is equivalent to which one of the following -

Hardness of water us expressed in milliequivalents/L. One mEq/L is equal to 50mg of CaCO3(50 ppm) in one litre. Park's Textbook of Preventive and Social Medicine, 25th edition, Page No. 786