The Septic Process
In the septic process the raw sewage is conveyed to tanks, and allowed to stand until the solids have settled to the bottom and have been partially destroyed or liquefied by bacterial action. Two types of tanks are used in the septic process, one known as the Cameron type and the other as the Emscher or Imhoff tank.
The best constructed Cameron tanks are not less than 8 feet in depth and are usually large enough to hold about six hours’ maximum flow of sewage. The desirable time of detention depends upon the character of the sewage, both as to strength and freshness, strong and stale sewages demanding a longer period. The tanks are usually built with baffles at the entrance to retard the current and to deflect the suspended matter to the bottom which is so constructed that the sludge, after bacterial action has taken place, can be drawn off from time to time.
H. W. Clark, formerly chemist of the Massachusetts State Board of Health, has expressed the belief that the rate of flow through a septic tank should not be greater than that which will cause passage in twelve hours.
Charles G. Hyde in the California Board of Health Bulletin says that as a rule the period should not be greater than 24 hours nor less than 12 hours, except possibly with weak or stale sewages. He advocates multiple units so that the storage periods may be controlled to give optimum results.
The effluent which is turbid, putrescible and rich in organic matter cannot be discharged into streams with safety without further treatment, unless the volume of water is sufficient to complete the purification by dilution. As the solids settle a scum which forms on top of the tank, keeps out light and air and produces a condition favorable for the bacterial activity caused by minute organisms known as anaerobic bacteria. These germs thrive and functionate best in the absence of oxygen, and their chief function in sewage treatment is the conversion of the solid organic matter into a soluble form, somewhat less complex in chemical composition. The sludge is rotted and when full bacterial action has taken place is humified. In plain sedimentation the solids are simply deposited upon the bottom of the tank and are removed practically unchanged. In the septic tank, however, a part of the solids after settling are broken down or digested, thus somewhat lessening the difficulty of disposing of the sludge.
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.”