Activated Sludge Process
Sewage treatment by aeration in the presence of sludge is the latest development in sewage disposal, and the process is attracting a great deal of attention in America. Milwaukee has constructed a plant to treat two million gallons of sewage a day. Houston, Texas, is operating a plant to treat the sewage for 160,000 persons, and Escanaba, Michigan, and Jersey City, N. J., have favored the process. Experiments are now being conducted in Milwaukee, Baltimore, Washington, Cleveland, Regina, Chicago, Lawrence, Mass., Brooklyn, New Haven, Conn., the University of Illinois and many other places. The efficiency and economy of the process as compared with others which have long been in use have not been completely established. The chief points in dispute are sludge disposal and cost, but the indications are that these questions will soon be satisfactorily answered.
The process consists of passing raw sewage through tanks from eight to twenty feet deep in which a certain amount of activated sludge is always present. To mix the sewage and the activated sludge air is forced into the bottom of the tank under low pressure of sufficient volume to keep the liquor violently disturbed. From this aerating tank the mixture passes to another or sedimentation tank where the sludge settles and from which the clear effluent passes over a weir to its final destination. In order to maintain the proper volume of activated sludge in the aerating tank a portion of the sludge is pumped back from the sedimentation tank. The balance of the sludge is pressed and used for fertilizer base. The Milwaukee experiments indicate that in order to produce a clear, non-putrescible effluent about four hours aeration is required, twenty per cent. of activated sludge maintained in the aerating tank, and about 1.75 cubic feet of free air supplied per gallon of sewage treated.
The effluent is clear, odorless and practically free from suspended matter. The sludge will begin to decompose after forty-eight hours and must be pressed and dried within that time. Chief Engineer, T. Chalkley Hatton, of the Milwaukee Sewerage Commission, estimates that the sludge can be reduced to a fertilizer basis for about $8.75 per dry ton, including overhead charges. Basing the value of the sludge produced upon a low price per unit, he finds that Milwaukee sludge is worth $12.50 per dry ton, which represents a clear profit of $3.75 a ton. From ten to twelve million gallons can be treated upon one acre of ground, which is about one-fifth the area required for sedimentation tanks and sprinkling filters. The reasons for the adoption of this process by Milwaukee after experimentation by competent engineers for more than a year are given by Mr. Hatton in a recent address before New York State city officials as follows: “It produces a better effluent than any other known process of sewage treatment except land treatment or intermittent sand filtration; it can be built upon a comparatively small area; it produces no objectionable odors or flies; it produces a sludge of sufficient value to meet the cost of its reduction to a fertilizer and therefore relieves the city of the difficult, complicated and wasteful method of sludge disposal common to all other processes; it is subject to complete and satisfactory control throughout its operation; it is not materially influenced by climatic conditions; occupying a small area, its first cost is less than any other known process from which an equal character of effluent can be obtained; its operating cost is not prohibitive.”
In a discussion before the Iowa Section of the American Waterworks Association Dr. Edward Bartow commended activated sludge for its value as a fertilizer. This has been proved, he said, by its chemical composition, by its reaction with various solids and by its effect on the growth of plants. Pot cultures and garden experiments have shown that the nitrogen is in a very available form.
E. E. Sands, City Engineer of Houston, Texas, bases this statement on results of experiments conducted for a year: “Our investigation has demonstrated that sewage can be disposed of anywhere that there is a vacant tract of land in the city without creating a nuisance and without any objectionable feature.” The total estimated cost for treatment will be about $9.14 per million gallons when the plant is run at the rate of 18,900,000 gallons per day. He estimates that the total cost for treatment by the Imhoff tanks and the sprinkling filters would be not less than $11 per million gallons.
After an extended investigation by their sanitary engineers, Armour & Company have concluded that the activated sludge method will satisfactorily purify the industrial wastes from their Packingtown factories. Assistant Superintendent, M. D. Harding, estimates that from data now available the cost per million gallons exclusive of depreciation, interest and repairs, will be $3.
When considering the applicability of this process to conditions in any city consideration should be given to the following points. The process requires competent supervision, which Mr. Hatton claims may be a blessing in disguise in view of the experiences of cities which, after having built disposal plants of various kinds, have left their operation to the kind mercies of Providence with disastrous results. This process also requires the expenditure for constant power. The cheaper the power the more adaptable the process is commercially; but if the unit is small and the power cost high, the operating cost may be too great. The sludge must be constantly treated to avoid nuisance. The process produces a high degree of purification. If the local conditions do not demand this the process might be too expensive in comparison with some other process which will produce a satisfactory effluent.
Other Processes
A few cities, including Oklahoma City and Santa Monica, Cal., have electrolysis treatment plants. The process consists in passing the sewage between a system of electrodes. The change is brought about by chemical reaction from newly formed chemical reagents produced by the decomposition of inorganic compounds already in solution. It is still regarded as an unestablished process.