CONTENTS. PAGE CHAPTER I. INTRODUCTION. 1 II. CONTINUOUS FILTERS AND THEIR CONSTRUCTION 5 Sedimentation-basins 8 Size of Filter-beds 10 Covers for Filters 12 III. FILTERING-MATERIALS 20 Sand 20 Gravel 35 Underdrains 39 Depth of Water on Filters 45 IV. RATE OF FILTRATION AND LOSS OF HEAD 47 Rate of Filtration 47 Loss of Head and Apparatus for regulating it 52 Limit to the Loss of Head 60 V. CLEANING FILTERS 68 Scraping 68 Frequency of Scraping 72 Sand-washing 76 VI. THEORY AND EFFICIENCY OF FILTRATION 83 Bacterial Examination of Waters 93 VII. INTERMITTENT FILTRATION 97 The Lawrence Filter 100 The Chemnitz Filter 107 VIII. TURBIDITY AND COLOR, AND THE EFFECT OF MUD UPON SAND FILTERS 113 Color 114 Turbidity 117 Preliminary Processes to remove Mud 133 Effect of Mud upon Sand Filters 137 IX. COAGULATION OF WATERS 144 Substances used for Coagulation 145 Amount of Coagulant required to remove Turbidity 150 Amount of Coagulant required to remove Color 153 Successive Applications of Coagulant 154 Amount of Coagulant which Waters will receive 155 X. MECHANICAL FILTERS 159 Influence of Amount of Coagulant on Bacterial Efficiency 165 Types of Mechanical Filters 172 XI. OTHER METHODS OF FILTRATION 181 XII. REMOVAL OF IRON FROM GROUND-WATERS 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 XIV. WATER-SUPPLY AND DISEASE 210 APPENDIX I. GERMAN OFFICIAL REGULATION IN