Plain Sedimentation
By allowing the sewage either to flow into properly constructed tanks or through them at a velocity low enough to allow some of the suspended matter to separate from the liquid and to be deposited on the bottom from which the sludge is removed, is another process that has been used by a number of American and European cities. The first tanks were constructed so that they could be filled with sewage and then after the suspended matter had settled the effluent was drawn off. This was known as the fill and draw plan. Later what is now known as the continuous flow principle was used. The velocity of the flowing sewage is reduced sufficiently as it enters and passes through the tank for the suspended matter to settle. The sludge which collects at the bottom of the tank must be removed frequently. The results are affected by the quantity and quality of the sewage, fresh sewage being capable of greater clarification by sedimentation than stale sewage. The range in storage period for American sewages is from four to twelve hours and the removal of suspended matter is from 45 to 75 per cent.
In some cities plain sedimentation has been used in connection with dilution and in others as an aid to filtration. The chief objection to the process is the sludge which is extremely offensive and must be treated separately. It does not dry readily, is difficult to handle and if allowed to accumulate causes serious nuisance. Because of these difficulties and the fact that the sludge from the Cameron and Imhoff tanks can be more easily disposed of the septic process has gradually forced plain sedimentation into the background.
Colloidal tanks were designed to carry the process of clarification further than plain sedimentation, but they have not come into general use. Metcalf and Eddy in their “American Sewerage Practice” say of this process: “There has been a feeling that while under some conditions a portion of the colloidal solids could be removed by such devices, the work accomplished was not likely to be sufficient to offset the expense of construction and some difficulties in operation.”
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.