Most cities and counties have enacted requirements for industries which discharge water into municipal sewer systems. These municipalities generally require the waste water to meet certain requirements before being discharged into the sewer systems, and these requirements usually relate to the level of contaminants which interfere with the efficient operation of the municipal waste water treatment facility.
Waste water which contains materials having a high biological oxygen demand, total suspended solids and ammonia increase the treatment cost to the city or county. For purposes of this invention, the term “biological oxygen demand” (“BOD”) refers to the quantity of oxygen utilized in the bio/chemical oxidation of organic matter; and the term “total suspended solids” (“TSS”) refers to the total suspended solids which either float on the surface of or are suspended in waste water and which may be removed by filtering, skimming, etc.
In some instances, a surcharge may be levied on an industrial plant that produces waste water which does not meet the standards as established by the municipality.
A city or county also has the authority to revoke an industrial plant's waste water discharge permit if that plant continues to discharge waste water which does not meet the requirements set by that city or county. Therefore, an industrial plant must effectively treat its waste water to reduce the level of contaminants to comply with such municipal requirements.
An effective waste water treatment process should cause the solid organic and inorganic matter in the waste water to flocculate and form a sludge cake. This sludge cake accumulates to a certain thickness, is then separated from the liquid component of the waste water and then typically is sent either for disposal or to a rendering plant for further processing.
A common method of treating waste water is the dissolved air flotation method (“DAF”) which injects dissolved air into the waste water, causing the solids to rise to the top. However, the DAF method tends to produce a sludge cake with a low percentage of solids.
Another method of treating waste water is to use a clarifying or settling tank with a filter at the bottom. The waste water is chemically treated within the tank, and the solids sink to the bottom of the tank. The liquid component of the waste water is directed out of the tank, and a wet cake is left on the filter. However, this method also tends to produce a sludge cake with a low percentage of solids.
With the DAF method or the settling tank method, a belt press can be used to produce a sludge cake with a higher solids content. However, this step requires additional equipment, expense and time.
Another method of treating waste water from an industrial plant is disclosed in Stewart U.S. Pat. No. 4,981,599 (1991). This process results in the formation of a sludge cake on top of the waste water. The sludge cake is removed, and the treated waste water is discharged into the sewer system.
However, the sludge cake (also referred to as the float material) produced by the prior treatment processes is not ideally suited for subsequent treatment at a rendering plant because (1) the water content of the float material tends to be higher than desired and (2) the float material is comprised of fatty and protein components which desirably should be separated from the water component.
Hopkins et al. U.S. Pat. No. 5,658,462 (1997) discloses a process which results in the recovery of proteins, fatty and water components from a float material produced by a waste water treatment system. The protein and fatty components can be further processed for inclusion in various products, and the water component can also be further processed before discharge into a sewer system.
Therefore, there is a need for a process that effectively (a) treats industrial waste water to meet the requirements as set by a city or county so that such treated waste water can be discharged into a municipal sewer system and (b) produces a sludge cake with a high percentage of solids.