255. Theory. 256. The Contact Bed. 257. The Trickling Filter. 258. Intermittent Sand Filter. 259. Cost of Filtration. IRRIGATION. 260. The Process. 261. Status. 262. Preparation and Operation. 263. Sanitary Aspects. 264. The Crop. 431–464 CHAPTER XVIII ACTIVATED SLUDGE 265. The Process. 266. Composition. 267. Advantages and Disadvantages. 268. Historical. 269. Aëration Tank. 270. Sedimentation Tank. 271. Reaëration Tank. 272. Air Distribution. 273. Obtaining Activated Sludge. 274. Cost. 465–479 CHAPTER XIX ACID PRECIPITATION, LIME AND ELECTRICITY, AND DISINFECTION 275. The Miles Acid Process. ELECTROLYTIC TREATMENT. 276. The Process. DISINFECTION. 277. Disinfection of Sewage. 482–493 CHAPTER XX SLUDGE 278. Methods of Disposal. 279. Lagooning. 280. Dilution. 281. Burial. 282. Drying. 495–505 CHAPTER XXI AUTOMATIC DOSING DEVICES 283. Types. 284. Operation. 285. Three Alternating Siphons. 286. Four or More Alternating Siphons. 287. Timed Siphons. 288. Multiple Alternating and Timed Siphons. 506–512 SEWERAGE AND SEWAGE TREATMENT CHAPTER I INTRODUCTION =1. Sewerage and the Sanitary Engineer.=—Present day conceptions of sanitation are based on the scientific discoveries which have resulted so much in the increased comfort and safety of human life during the past century, in the increase of our material possessions, and the extent of our knowledge. The danger to health in the accumulation of filth, the spreading of disease by various agents, the germ theory of disease, and other important principles of sanitation can be counted among the more recent scientific discoveries and pronouncements. Experience has shown, and continues to show, that the increase of population may be inhibited by accumulations of human waste in populous districts. The removal of these wastes is therefore essential to the existence of our modern cities.