receive 155 X. MECHANICAL FILTERS 159 Providence Experiments 159 Louisville_Experiments 161 Lorain Tests 161 Pittsburg Experiments 162 Wasting Effluent After Washing Filters 163 Influence of Amount of Sulphate of Alumina on Bacterial Efficiency of Mechanical Filters 165 Influence of Degree of Turbidity upon Bacterial Efficiency of Mechanical Filters 167 Average Results Obtained with Various Quantities of Sulphate of Alumina 171 Types of Mechanical Filters 172 Efficiency of Mechanical Filters 179 Pressure Filters 180 XI. OTHER METHODS OF FILTRATION 181 Worms Tile System 181 The Use of Asbestos 181 Filters Using High Rates of Filtration Without Coagulants 182 Household Filters 183 XII. REMOVAL OF IRON FROM GROUND-WATERS 186 Amount of Iron Required to Render Water Objectionable 186 Cause of Iron in Ground-waters 187 Treatment of Iron-containing Waters 189 Iron-removal Plants in Operation 192 XIII. TREATMENT OF WATERS 197 Cost of Filtration 200 What Waters Require Filtration 207 XIV. WATER-SUPPLY AND DISEASE—CONCLUSIONS 210 Appendix I. RULES OF THE GERMAN GOVERNMENT IN REGARD TO THE FILTRATION OF SURFACE-WATERS USED FOR PUBLIC WATER-SUPPLIES 221 II. EXTRACTS FROM “BERICHT DES MEDICINAL-INSPECTORATS DES HAMBURGISCHEN STAATES FÜR DAS JAHR 1892” 226 III. METHODS OF SAND-ANALYSIS 233 IV. FILTER STATISTICS 241 Statistics of Operation of Sand Filters 241 Partial List of Cities Using Sand Filters 244 List of Cities and Towns Using Mechanical Filters 247 Notes Regarding Sand Filters in the United States 251