**3. Limits of CALM for drinking water by U.S-EPA, WHO, BOBS**

To provide guidance in resolving the contamination of drinking water by microorganisms and heavy metals, WHO has developed guidelines and water safety plans


#### **Table 1.**

*Permissible limits of CALM in drinking water as prescribed by USEPA, WHO, and BOBS [2, 3, 62].*

for drinking water quality [2]. The guidelines highlight that the proposed permissible threshold values are designated as provisional, given the practical difficulties of treating drinking water up to these required standards [2]. However, water authorities are encouraged to make an effort to maintain pollutants' concentrations below the guideline values.

**Table 1** summarizes the maximum contamination level (MCL) for Cd, As, Pb, and Hg as proposed by BOBS, WHO, and the US EPA [2, 3, 63]. Control measures, such as pre-treatment, coagulation, flocculation, sedimentation, filtration, and disinfection, are highlighted as crucial steps during drinking water treatment processes [2, 64]. The piping system of the treatment plant and of the distribution network must be taken into consideration to avoid the unintended input of residual heavy metals through corrosion and leaching of plumbing networks. Proper maintenance and regulation or survey procedures of the treatment plant to prevent contamination help in determining the presence of unwanted HMs and the eventual removal of internal accumulation of deposits.
