Reports vary widely as to the amount of suspended matter that can be removed by the septic process. The Iowa State College bulletin says that the amount of purification does not usually exceed 25 to 40 per cent. Professor Whipple places the removal between 60 and 70 per cent., and the State Board of Health of California says it may vary between 35 per cent. and 85 per cent., averaging perhaps 50 to 60 per cent. H. W. Clark places the amount at not less than 40 per cent. and adds that it will vary according to the character of the sewage, the variations being from 30 per cent. with weak sewage to 80 per cent. with strong sewage.
All reports concur that in many cases the Cameron type of tank has failed to produce efficient results. Among the objections raised by authorities are the following:
The sludge is not thoroughly digested and is somewhat offensive. The odor is obnoxious and the effluent is too stale and is treated with difficulty by oxidation processes. Gilbert J. Fowler, Sanitary Expert of England, says the defects which have shown themselves are a nuisance both from the tank effluent and the sludge and an excessive quantity of suspended solids in the tank effluent. Charles G. Hyde believes a review of the principles and results of operation appear to justify the conclusion that “the septic effluents are only less dangerous than crude sewage to the extent of efficiency of removal of organic matter.”
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.”