It has been suggested that these inferior effluents after washing should be wasted. Such a procedure would mean wasting probably on an average two per cent of the water filtered, and a corresponding increase in the cost of filtering. Mr. Fuller[43] in his Louisville report comes to the conclusion that with adequate washing and coagulation it is unnecessary to waste any effluent, and that inferior results after washing usually indicate incomplete washing. While our experiments certainly indicate a reduction in efficiency after washing so regular and persistent as to make it doubtful whether incomplete washing can be the cause of it, it may be questioned whether or not wasting the effluent would be necessary or desirable in actual operation. At any rate the results as given in this report are not materially influenced by this factor. INFLUENCE OF AMOUNT OF SULPHATE OF ALUMINA ON BACTERIAL EFFICIENCY OF MECHANICAL FILTERS. The number of bacteria passing a mechanical filter is dependent principally upon the amount of sulphate of alumina used; and by using a larger quantity of sulphate of alumina than was actually used in the experiments the bacterial efficiency could be considerably increased. To investigate this point, the results obtained each day with each of the mechanical filters were arranged in the order of the sulphate of alumina quantities used, and averaged by classes. In this and the following tables a few abnormal results were omitted.[44] A summary of the results is as follows: SUMMARY OF RESULTS WITH WARREN MECHANICAL FILTER, ARRANGED ACCORDING TO SULPHATE OF ALUMINA QUANTITIES.