The rate of filtration recommended and used has been gradually reduced during the past thirty years. In 1866 Kirkwood found that 12 vertical feet per day, or 3.90 million gallons per acre daily, was recommended by the best engineers, and was commonly followed as an average rate. In 1868 the London filters averaged a yield of 2.18 million gallons[8] per acre daily, including areas temporarily out of use, while in 1885 the quantity had been reduced to 1.61. Since that time the rate has apparently been slightly increased.

The Berlin filters at Stralau constructed in 1874 were built to filter at a rate of 3.21 million gallons per acre daily. The first filters at Tegel were built for a corresponding rate, but have been used only at a rate of 2.57, while the more recent filters were calculated for this rate. The new Hamburg filters, 1892-3, were only intended to filter at a rate of 1.60 million gallons per acre daily. These in each case (except the London figures) are the standard rates for the filter-beds actually in service.

In practice the area of filters is larger than is calculated from these figures, as filters must be built to meet maximum instead of average daily consumptions, and a portion of the filtering area usually estimated at from 5 to 15 per cent, but in extreme cases reaching 50 per cent, is usually being cleaned, and so is for the time out of service. In some works also the rate of filtration on starting a filter is kept lower than the standard rate for a day or two, or the first portion of the effluent, supposed to be of inferior quality, is

wasted, the amount so lost reaching in an extreme case 9 to 14 per cent of the total quantity of water filtered.[9] In many of the older works also, there is not storage capacity enough for filtered water to balance the hourly fluctuations in consumption, and the filters must be large enough to meet the maximum hourly as well as the maximum daily requirements. For these reasons the actual quantity of water filtered in a year is only from 50 to 75 per cent of what would be the case if the entire area of the filters worked constantly at the full rate. A statement of the actual yields of a number of filter plants is given in Appendix IV. The figures for the average annual yields can be taken as quite reliable. The figures given for rate, in many cases, have little value, owing to the different ways in which they are calculated at different places. In addition most of the old works have no adequate means of determining what the rate at any particular time and for a single filter really is, and statements of average rates have only limited value. The filters at Hamburg are not allowed to filter faster than 1.60 or those at Berlin faster than 2.57 million gallons per acre daily, and adequate means are provided to secure this condition. Other German works aim to keep within the latter limit. Beyond this, unless detailed information in regard to methods is presented, statements of rate must be taken with some allowance.