CHAPTER XX. 211. Sanitary Improvement of Public Water-supplies 276 212. Improvement by Sedimentation 277 213. Sedimentation Aided by Chemicals 279 214. Amount of Solid Matter Removed by Sedimentation 279 215. Two Methods of Operating Sedimentation-basins 279 216. Sizes and Construction of Settling-basins 280 217. Two Methods of Filtration 281 218. Conditions Necessary for Reduction of Organic Matter 282 219. Slow Filtration through Sand—Intermittent Filtration 283 220. Removal of Bacteria in the Filter 286 221. Preliminary Treatment—Sizes of Sand Grains 286 222. Most Effective Sizes of Sand Grains 288 223. Air and Water Capacities 288 224. Bacterial Efficiency and Purification—Hygienic Efficiency 290 225. Bacterial Activity near Top of Filter 290 226. Rate of Filtration 291 227. Effective Head on Filter 291 228. Constant Rate of Filtration Necessary 292 229. Scraping of Filters 293 230. Introduction of Water to Intermittent Filters 294 231. Effect of Low Temperature 294 232. Choice of Intermittent or Continuous Filtration 294 233. Size and Arrangement of Slow Sand Filters 295 234. Design of Filter-beds 296 235. Covered Filters 299 236. Clear-water Drain-pipes of Filters 299 237. Arrangement of the Sand at Lawrence and Albany 300 238. Velocity of Flow through Sand 302 239. Frequency of Scraping and Amount Filtered between Scrapings 303 240. Cleaning the Clogged Sand 303 241. Controlling or Regulating Apparatus 305 242. Cost of Slow Sand Filters 307 243. Cost of Operation of Albany Filter 308 244. Operation and Cost of Operation of Lawrence Filter 309 245. Sanitary Results of Operation of Lawrence and Albany Filters 310 246. Rapid Filtration with Coagulants 311 247. Operation of Coagulants 312 248. Principal Parts of Mechanical Filter-plant—Coagulation and Subsidence 313 249. Amount of Coagulant—Advantageous Effect of Alum