The rules of the Imperial Board of Health[25] require that every German filter shall be so constructed “that when an inferior effluent results it can be disconnected from the pure-water pipes and the filtrate allowed to be wasted.” The drain-pipe for removing the rejected water should be connected below the apparatus for regulating the rate and loss of head, so that the filter can be operated exactly as usual, and the effluent can be turned back to the pure-water pipes without stopping or changing the rate. The works at Berlin and at Hamburg conform to this requirement, and most of the older German works have been or are being built over to make them do so. In regard to the extent of deterioration after scraping, Piefke’s experiments have always shown much larger numbers of bacteria both of the ordinary forms and of special applied forms on the first day after scraping, the numbers frequently being many times as high as at other times. At the Lawrence Experiment Station it was found in 1892 that on an average the number of water bacteria was increased by 70 per cent (continuous filters only) for the three days following scraping, while _B. prodigiosus_ when applied was increased 140 per cent, the increase being most marked where the depth of sand was least, and with the highest rate of filtration. The same tendency was found in 1893, when the increase in the water bacteria on the first day after scraping was only 19 per cent and _B. prodigiosus_ 64 per cent, but for a portion of the year the difference was greater, averaging 132 and 262 per cent, respectively. These differences are much less than those recorded by Piefke, and with the high efficiencies regularly obtained at Lawrence they would hardly justify the expensive practice of wasting the effluent. The reduction in efficiency following scraping is much less at low rates, and if a filter is started at much less than its normal rate after scraping, and then gradually increased to the standard after the sediment layer is formed, the poor work will be largely avoided. Practically this is done at Berlin and at Hamburg. The filters are started at a fourth or less of the usual rates and are gradually increased, as past experience with bacterial results has shown it can be safely done, and the effluent is then even at first so well purified that it need not be wasted. Practically in building new filters the provision of a suitable connection for wasting the effluents into the drain which is necessary for emptying them involves no serious expense and should be provided, but it may be questioned how often it should be used for wasting the effluents. If the raw water is so bad that a good effluent cannot be obtained by careful manipulation even just after scraping, the course of the Berlin authorities in closing the Stralau works and seeking a less polluted supply would seem to be the only really safe procedure. SAND-WASHING. [Illustration: CLEANING A FILTER, EAST LONDON.]