The Imhoff Tank
In an effort to overcome the defects in the Cameron tank, the Imhoff or Emscher tank was developed and this now seems to have the preference among cities making new installations. The tank consists of two compartments, one above the other. It has a smaller area than the ordinary septic tank, but is much deeper. The sewage passes at a low velocity through the upper chamber, which is comparatively shallow and V-shaped, the sides being sufficiently steep to allow the solids to be deposited at the bottom of the V which is equipped with slots. Through these the solids pass into the second chamber below which is much deeper than the other. The inclined partition wall must be cleaned frequently with hose or squeegee in such a way as not to clog the slots. The floating pieces of wood and cork must be skimmed off, but the greater part of the suspended matter that floats will generally sink after a time. Dr. Karl Imhoff, the inventor of the tank, advises spraying with a hose to expedite the sinking. Care must be taken to keep the sides clean and the sludge in the lower tank below the slot level. If neglected suspended matters will rise to the surface behind as well as in front of the scum boards. Dr. Imhoff advises the reversal of the flow of sewage about every three weeks after skimming off the floating matter when one sedimentation chamber feeds more than one sludge chamber. The rate of flow in the upper chamber is sufficiently rapid to prevent any septic action, yet slow enough to allow much of the suspended matter to settle.
The effluent in a comparatively fresh condition passes out of the tank for further treatment or for discharge into water courses. It therefore does not become stale nor does it come in contact with decomposing sludge, thus eliminating in part the objections advanced by authorities against the Cameron tank.
In the lower tank the sludge, after passing through the slots is slowly digested through septic and other actions without any disturbance by the flow of the liquid sewage, above. Before the tank can deliver good, well digested sludge—that is, a black alkaline odorless sludge—it must be inoculated with a proper amount of good sludge, or the raw sludge must be permitted to “ripen.” Dr. Imhoff has found that even without inoculation a tank will discharge good sludge from the beginning if ripe sludge is emptied into the system from cesspools which have been in use a long time.
In some instances cities have had considerable trouble with acid decomposition during the ripening period. This produces a sludge of objectionable odor and one not easily dried. It decomposes very slowly and may rise in a mass to the surface of the sludge chamber. Various remedies have been suggested, among them the addition of lime. “I cannot advise such addition,” Dr. Imhoff has written. “All plants which are known to me and in which acid decomposition has occurred have sooner or later adjusted themselves of their own accord.”
When properly inoculated the particles of sludge rise and fall constantly in the process of giving off the gases. The fresh sludge particles entering the chamber through the slot are covered so that the entire mass becomes thoroughly mixed and the untreated sludge in a short time is inoculated with the proper organisms. The decomposed sludge is discharged from time to time through pipes leading from the bottom of the tank to drying beds.
Dr. Imhoff has advocated the discharge of sludge from each sludge chamber once every two to six weeks, that the optimum of the sludge level should be about three feet below the slot level and if it is desired to promote the early incidence of proper decomposition the sludge should not be allowed to remain quiet at the bottom of the sludge chamber. He advocates constant stirring and a uniform introduction of fresh organic matter and the discharge of the decomposed matter. The scum layer, he says, must be agitated frequently by a jet of water or otherwise and the sludge at the bottom of the chamber should be agitated by a water stirring system. As a substitute, he suggests that the whole body of sludge be pumped out and returned. To determine the elevation of the sludge surface, he advises lowering into the sludge chamber a very thin piece of sheet iron one foot square in area held in a horizontal position. If the level is too high, there will be gas bubbles on the surface of the settling chamber above the slot or there will be floating sludge and in extreme cases foaming sludge. As compared with other tank processes the experience of cities indicates that the Imhoff type has many advantages. Certain inherent difficulties, however, have been pointed out in several reports. Gilbert J. Fowler has expressed the belief that “the comparative short time of settlement means that variations in the character of the sewage must be quickly reflected in the character of the tank effluence and that the filters (when they are used for further treatment) must be called upon rapidly to accommodate themselves to fluctuating conditions.” He believes that this is not conducive to the development of the most efficient bacterial activity. Storm water above moderate dilution, he says, will have to receive separate treatment and he is of the opinion that ordinary stand-by tanks will still be necessary for this purpose, the sludge from which will have to be dealt with. From the results of the operation of an experimental plant in Worcester, Massachusetts, Matthew Gault, Superintendent of Sewers, draws these conclusions: “It appears to be perfectly feasible to treat Worcester sewage by means of Imhoff tanks and sprinkling filters. The results of experimental treatment of the effluent from chemical precipitation tanks indicated that the advantages gained by chemical precipitation as a preliminary treatment were not commensurate with the cost. The Imhoff tank was quite as efficient in sludge digestion as experimental septic tanks have been and much more efficient so far as sedimentation of the sewage is concerned. It was operated without the production of the offensive odors characteristic of the septic tank and the sludge itself was disposed of without creating a nuisance. The effluent from the Imhoff tank was normally as fresh in appearance and odor as the sewage flowing into the tank.”
The experience of the New Jersey State Board of Health with Imhoff tanks has been that if properly designed, constructed and operated, they are a valuable means of sewage clarification. The observation of its engineers has shown that under these conditions the tanks overcome a great deal of trouble due to odors and greatly simplify the sludge problem. “However, their proper operation is a considerable problem,” reads one of its reports. “And the cost of keeping them in working order is several times greater than for septic or sedimentation tanks.” In view of the initial cost of this form of tank as compared with the older single story types the New Jersey engineers believe that “in cases where the works are far removed from a populous community, so that the odor problem is not serious, it is doubtful whether the Imhoff tank has any material advantage over a properly constructed, well baffled sedimentation tank of the old type.”
The Cleveland Sewage Testing Station reports that the most consistent results were obtained from the operation of the Imhoff tank, an average suspended matter removal of 50 per cent. being secured. A recent city report says: “In general it may be said that a detention period of thirty minutes accomplished a removal of suspended matter from 40 to 45 per cent. as compared with a 50 per cent. removal effected by a detention period of two hours and fifteen minutes.”
In a bulletin of the California Board of Health, Charles G. Hyde sums up the importance of the septic process thus: “The septic process as carried out either in the Cameron or Imhoff type, but especially in the latter, has at present two distinct fields of usefulness; first, it constitutes an effective means of preparation for any final process which can be better conducted with a sewage from which the suspended solids are more or less completely removed; secondly, it may be employed when disposal by dilution is permissible if the source of unsightly sludge and scum is removed.” Another advantage may be added, the Imhoff tank produces a sludge that can be disposed of easily.