Sludge Disposal and Value
Authorities are generally agreed that the sludge problem is the center of the entire sewage problem, because it causes more trouble and is the most expensive part of the treatment. The method of handling it is just as important as the treatment of the sewage.
Wet sludge can be pumped out on land or into shallow places or it can be sent to sea in ships and allowed to sink. If pumped on land it must be spread out in very thin layers. If discharged into trenches it is ploughed into the ground after it has dried. In either case a large area of land is necessary and odors cannot be eliminated. Only cities located on or near the seashore can send their sludge to sea, and then the cost of disposal is rather high.
Sludge can be dried by pressing, in centrifugal drying machines, by mixing with some dry matter or by discharging upon drying beds. The cost of pressing is high, depending upon the amount of lime added, the kind of sludge pressed, and the size of the works. George S. Webster states that the average cost in large cities is ten cents per ton of wet sludge. It is especially applicable to chemical precipitation works as it must first be treated with lime or coal powder. When dried in machines the liquid contains much organic matter and is objectionable. The simplest method is to discharge the sludge upon drying beds of porous material and underdrained. The time for drying depends upon sewage treatment. Imhoff tank sludge will dry in less than a week, septic tank sludge in two weeks or more, and sludge from plain sedimentation will require about two months in summer and almost five months in winter. Cleveland, in order to overcome weather conditions at its experimental plant, built a covered sludge bed, modeled after standard greenhouse construction. The report from the Testing Station is that during summer the period of drying is approximately the same as or possibly a little longer than with open beds. Eliminating the three winter months, the station report says, it is possible to operate beds of this type so that one square foot of surface will dry 0.8 cubic feet of sludge per year. Francis E. Daniels suggests that sludge can be handled faster by drying a small portion at one time and removing it from the bed before the next portion is drained off.
Dry sludge can be used for fertilizer or for filling low lands or it can be incinerated. Its fertilizing value is disputed except when produced by the activated sludge method. The filling in method is economical. Authorities advise the consideration of incineration by cities which burn their garbage.
Dr. Imhoff’s recommendations are the use of sludge for agricultural purposes and for filling in low land. “In both cases,” he says, “the sludge must first be dried and this is best effected upon a drying bed after the sludge has been decomposed in an inoffensive, odorless manner, in a separate tank through which sewage does not flow.”
Many unsuccessful efforts have been made to extract the valuable ingredients from sewage, but to date the experience has been that they have been more costly to recover than they are worth. Dr. McLean Wilson, Sanitary Inspector of the West Riding of Yorkshire Rivers Board, believes that the valuable ingredients of sewage will ultimately be recovered and used since many capable experimenters are at work on the problem. H. W. Clark, Chemist of the Massachusetts State Board of Health, is of the opinion that sludge has some value and that “it seems inevitable that as the processes of drying, pressing and fat separation are improved and as nitrogen advances in price sewage sludge will become of greater agricultural value than at present.” Experiments have been made at the Philadelphia Sewage Testing Station by burning dry sludge and wet sludge mixed with fine coal. The results were unsuccessful. Experiments have also been made at the Cleveland station where it was found that the sewage sludge contained about one-half as much nitrogen and one-third as much phosphates as does the garbage tankage.