Answer:
Okayama-City Waterworks Bureau kindly answered the question.
 
 
The ingredients of evaporation residues in tap water
The substances that remain after evaporation are called evaporation residues. These residues are either floating on the surface or dissolved in the water before being evaporated. The amount of the evaporation residue is expressed in milligrams per liter (mg/L).
  
The evaporation residues in the city water include calcium, magnesium, silicon dioxide, sodium, potassium, and so on. Most city water contains not more than 200 mg/L of evaporation residues, and it almost never exceeds 300 mg/L. (Okayama city water contains about 100 mg/L of evaporation residue)
  
According to the bylaw No.69 of the Ministry of Health and Welfare in 1992, the amount of the evaporation residues in the city water should not exceed 500 mg/L.
  
Evaporation residues are usually white. However, when the water contains iron or some organic substances, it becomes slightly brownish. Water-soluble evaporation residues are harmless even if the amount exceeds the standard. Inorganic substances in the evaporation residues affect the taste of the water regardless of the amount of the minerals.

 
 
Acknowledgement
We would like to thank Okayama-City Waterworks Bureau for their kind answer.
 
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