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.”
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.