SUCCESSIVE APPLICATION OF COAGULANT.
Mr. Fuller, in his experiments at Louisville, has ascertained that when sulphate of alumina is added to extremely muddy water the sediment absorbs some of the chemical before it has time to decompose, and carries it to the bottom, and so far as this is the case, no benefit is derived from that part of the coagulant which is absorbed. In other words, it is necessary to add more coagulant than would otherwise be necessary because of this action. The data showed that different kinds of suspended matters took up very different amounts of coagulant in this way. With only moderately turbid waters the loss of chemical from this source is unimportant. Hardly any trace of it was found at Pittsburg with the Allegheny River water. At Louisville, however, it was an important factor, as shown by Mr. Fuller’s results.
To avoid this loss of chemical Mr. Fuller has suggested the removal of the greater part of the suspended matters by sedimentation, without chemicals, or with the aid of a small quantity of chemical, followed by the application of the final coagulant prior to filtration. With the worst waters encountered at Louisville the saving in coagulant to be effected in this way is very great.
Mr. Fuller states in “Water Purification at Louisville,” p. 417: “The practical conclusions to be drawn from this experience are that with preliminary coagulation, followed by subsidence for a period of about three hours, the application of coagulants may be divided to advantage, and a considerable portion of the suspended matter kept off the filter, when the total amount of required coagulant ranges from 2 to 2.5 grains or more of ordinary sulphate of alumina per gallon. In the case of a water requiring more than this amount of coagulating treatment, a proper division of the application would increase the saving of coagulants and would diminish the frequency of washing the filter.”
In his final summary and conclusions, page 441, Mr. Fuller estimates the amount of sulphate of alumina required for the clarification of the Ohio River at Louisville at 3.00 grains per gallon of water filtered if all applied at one point, or at 1.75 grains by taking advantage of subsidence to its economical limit prior to the final coagulation. The saving to be effected in this way is sufficient to justify the works necessary to allow it to be carried out. With less turbid waters, or waters highly turbid for only short intervals, the advantages of double coagulation would be less apparent.