even ten yards per million gallons. On the other hand, so far as this sand regains its power of purifying sewage this amount will be reduced. If the sewage contained more or less suspended matter, correspondingly more or less new sand would be required, and if the suspended matter was first removed from the sewage by settling, we may believe that the amount of sand to be removed would be very small. Experiments are now in progress to determine this point. CHARACTER OF THE SEWAGE. The sewage used at the Experiment Station is brought through a two and one-half inch pipe about 4,300 feet long from one of the main city sewers, as described in the special report upon the Purification of Sewage and Water (pages 3 and 34). A certain amount of the insoluble matters of the sewage, especially the sand, is deposited in the pipe, which requires to be cleaned at intervals. In order to get information in regard to the matters removed from the sewage by this deposit, on Jan. 26, 1892, when the pipe was cleaned by blowing out with water from a hydrant, samples of the water passing the pipe were taken. From the analyses of these samples, and an estimate of the quantity of water used to clean the pipe, was calculated the total amount of various substances washed from the pipe in pounds, and these reckoned on the entire quantity of sewage which had passed the pipe since the previous cleaning, showed how much the analysis of the sewage was changed by passing through the pipe. The results were as follows: It thus appears that the matters retained in the pipe represented 10 per cent. of the albuminoid ammonia, and 6 per cent. of the oxygen consumed, and a much larger proportion of the fats. The average composition of the sewage as received at the station during the past four years is as follows: The strength of the sewage is slowly increasing, the average total nitrogen for 1891 being 25 per cent. greater than that for 1888. The results of the daily sewage analyses since Nov. 1, 1889, are given in the following table: The pipe leading from the Lawrence Street main sewer to the Experiment Station was cleaned out December 5. Nitrates. Nitrites. Oxygen Consumed. Bacteria per Cubic meter. Centi |