Method for Treating Sewage to Produce a Fuel

I. Field of the Invention

The present invention relates to a method for treating sewage and, more particularly, to a method for separating the combustible components of the sewage from the sewage scum.

II. Description of the Prior Art

As used herein, the term "sewage" refers not only to raw sewage but also to byproducts obtained during treatment of the sewage. Similarly, the term "scum" as used herein means the portion of the sewage which floats on top of the raw sewage and is conventionally skimmed from the top of the sewage.

Sewage scum is primarily composed of fats, oils, greases and the like, hereinafter sometimes collectively referred to as oils, intermixed with both water and assorted solid debris. Due to the water and assorted solid debris in the sewage scum, the sewage scum has a low combustibility and previously has been disposed of by grinding, landfill disposal, incineration and other methods. Such disposal of the scum, however, is not only expensive but is also a waste of a valuable energy source.

The present invention provides a method for treating sewage scum to produce a valuable primary fuel.

The method according to the present invention comprises the initial step of screening large solid materials from the influent sewage. This screening step is typically accomplished by passing the influent sewage through bar racks or screens which remove solid particles greater than about four-six inches from the sewage scum.

The screened sewage is then collected in a sedimentation tank where the scum is skimmed off. The skimmed scum is then transferred to a separatory tank where it is allowed to stand in a quiescent and substantially nonagitated state for a period of time of at least twelve hours. After this time, the scum is rendered separable.

During the quiescent period within the separatory tank, a phase separation of the oils from the water occurs by biochemical and biophysical reactions of the scum. This phase separation of the oils from the water can be accelerated by chemical accelerators, heat, periodic aeration, and the like.

Following the quiescent period, the scum is conveyed to a processing unit where the oils are separated from the water and solid particles in the scum. This can be accomplished in a plurality of different fashions, for example, by centrifuging, filtering, distilling, vibrating and/or the like. Elevated temperatures, preferably above 100.degree. F., are employed to effect a more complete and efficient separation.

At the end of the processing stage, the scum has been separated into three distinct components, i.e., the water, the solid debris with some absorbed water, and the oils. The upgraded water is disposed of by returning it to the sewage treatment plant, while the debris or solid material is incinerated or disposed of in a landfill. The resulting oil, which has a viscosity and heat value substantially the same as number six fuel oil, can then be stored for use as a primary fuel.

In sewage treatment plants, the raw sewage undergoes a series of different processes designed not only to kill harmful bacteria within the sewage but also to process the sewage to a state where it can be easily and safely disposed of. One such step during the sewage process is commonly known as "skimming" in which animal fat, oils, greases and other materials which float on the sewage, (hereinafter the "oils") are skimmed from the top of the sewage along with some solid debris and water. The material that is skimmed from the top of the sewage is commonly known as "scum" and is a liquid with a very high viscosity.

According to the method of the present invention, the influent sewage is first screened in order to remove large solid material, e.g., solid material greater than about four to six inches. This screening step can be achieved in any conventional fashion such as by screening bars, screens, or the like.

The screened sewage is then conveyed to a sedimentation tank where the scum is then skimmed and collected. This is accomplished by decreasing the flow rate of the sewage. This decrease in the flow rate of the sewage allows the scum to raise to the top of the sewage, which permits it to be then skimmed from the surface.

The scum, (after collection from the sedimentation tank) is transferred to and maintained in a separatory tank for at least twelve hours in a quiescent and substantially nonagitated state. During this period, the scum undergoes biochemical and biophysical reactions which result in the phase separation of the oils from the water which renders the oils separable from the water. This phase separation can be further enhanced by the addition of certain chemical accelerators, such as aluminum hydroxide or other polyvalent cations, periodic aeration and/or the addition of heat. Following the quiescent period, the scum contains approximately 20% moisture, 20% entrapped solid materials, which are commonly referred to as debris, and approximately 60% oils.

The oils have a lower density than water so that these oils rise to the top of the scum within the separatory tank while the water descends to the bottom of the separatory tank. Preferably, the separatory tank is equipped for drainage of the water from the bottom of the tank and also for decanting of the oil from the top of the tank.

Following the phase separation of the oils from the water in the separatory tank, the decanted scum is conveyed to a processing unit for final dewatering and debris removal by any conventional means, such as by pumping. The processing unit then separates the scum into three basic components, i.e., the water, the debris with some absorbed water and the oils. In the processing unit the scum is maintained at an elevated temperature, e.g., 100.degree. F., to achieve a more complete and efficient separation of the scum components. Examples of suitable processing units are vacuum filters, distillation units, vibrating or rotating screens, and centrifuges.

The water separated from the scum is then disposed of by returning it to the sewage treatment plant or to any sanitary sewer. The debris, which has the characteristic of a high grade coal (approximately 12,000 BTU/lb) can be either incinerated or disposed of in a landfill.

The separated oils form the desired product from the method of the present invention. The product has the physical characteristics of No. 6 fuel oil such as viscosity, sulfur content and air pollutants, with a heat value of approximately 16,500 BTU/lb. This product can then be burned in any conventional burner that is equipped to handle No. 6 fuel oil.

From the foregoing, it can be seen that the present invention provides a unique method for producing a valuable energy source from sewage. The present invention transforms a previously costly and wasteful practice into an efficient and profitable production of a valuable energy source.

Although the preferred embodiment has been described, many modifications thereto will become apparent to those skilled in the art to which it pertains without deviation from the spirit of the invention as defined by the scope of the appended claims.