Wastewater treatment process

Details Wastewater treatment process Wastewater treatment process

2001-01-23

2003-07-01

Upton, Christopher (Department: 1724)

Liquid purification or separation

Processes

Treatment by living organism

C210S624000, C210S625000, C210S626000

Reexamination Certificate

active

06585895

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to an improved process for the treatment of industrial wastewater. More specifically, the invention relates to an improved process for treating wastewater from industrial wastewater treatment plants using the biologically-activated sludge process in a manner that eliminates the need for any mechanical removal of excess solids, or the use of other conventional solids treatment units.
BACKGROUND OF THE INVENTION
The production of excess solids in conventional biologically-activated sludge wastewater treatment plants is accepted as a normal part of operations but it can be a substantial and expensive problem. For example, even a small industrial wastewater pretreatment plant that receives influent hydraulic loading at the rate of only 35,000 gallons per day from a surfactant manufacturing operation might need to dispose of 300-400 tons of excess solids per year. These relatively large amounts of excess solids must be disposed of properly and legally and most often this disposal is done in landfills.
The aerobic microbiological treatment of waste organic matter from sewage or industrial effluents often uses the conventional biologically-activated sludge process. As used herein the biologically-activated sludge process is sometimes referred to as “BAS”. A major drawback of this conventional technology is that the metabolic process of the biomass, the function that degrades and destroys the organic wastes, also produces solids growth as a normal consequence of cellular processes. Each pound of “food” (in the form of dissolved or suspended organic solids) that is consumed (converted) by the BAS process in the biomass can produce up to half a pound, or more, of inactive solids. These solids are dead and/or dying organisms of the biomass that have completed their biological cycle and are no longer effective. The accumulation of the excess solids appears in treated, wasted sludge mechanically removed from the treatment system. The routine disposal of the wasted sludge by dumping on land or at sea has the potential for adverse environmental effects and landfilling the treated sludge is increasingly more expensive, or in some extreme cases banned.
The BAS process has been used for many years for the treatment of wastewater, particularly for high-strength wastes. The BAS process maintains control over the beneficial use of the suspended biological solids in the biomass under aeration. The metabolic activity rate is determined by the presence of both sufficient dissolved oxygen and food. Aerobic bacteria, the predominant component of the biomass, feed on the dissolved organic wastes. The aerobic bacteria grow, reproduce and eventually die. The metabolic processes use dissolved oxygen and organic wastes found in wastewater to provide the fuel for cellular metabolism.
A drawback of the BAS process is that the total solids under suspension in the aeration tanks continue to increase until all the usable food is consumed, at which point the system dies, or goes septic. The only widely accepted solution to the solids buildup problem in plants using conventional BAS processes is the continual or intermittent mechanical removal of the solids from the system in order to maintain stable, optimal conditions for cell growth in the aeration tanks.
These relationship behaviors between available food and the amount of living biomass can be predicted mathematically as food/biomass ratios, referred to as F/M ratios or alternatively, as mean cell residence time's (MCRT). Using these predictive behavioral indicators, wastewater operators can better control the BAS process and stabilize solids within an optimum range.
In a BAS process what remains of the untreated, dissolved solids in the finished water from the aeration tanks can usually be settled out by gravity (sometimes with the help of polymers) in a secondary clarifier as the sludge blanket. The portion of the sludge blanket which is not returned to the aeration step in the BAS process as return activated sludge is transferred as waste-activated sludge to a solids treatment process. “Treatment” or “shot” tanks are used for staging waste-activated sludge before it is sent to a mechanical water removal and/or drying unit. In these staging tanks, strong inorganic chemicals, ferric chloride and lime, dramatically lower the wastewater's pH and aid in the shearing of the filaments of activated sludge. Usually a small amount of a flocculation polymer is added to the treated sludge to facilitate the formation of particles of sheared sludge of the proper size and shape until the point at which optimal settling out occurs.
The clear water above the thickened, polymer treated sludge layer is decanted from the tops of these tanks and directly discharged or recycled. The thickened sludge layer left in the bottom of the treatment tanks is pumped to a mechanical dewatering device, usually a filter press, where water is mechanically squeezed out leaving the modified, dewatered sludge in the form of a filter cake. This filter cake is accumulated on site and then transported to a landfill.
In conventional BAS processes, the aerobic digestion process almost never goes to completion; that is, all of the food (including the excess solids, or sludge) is consumed. When biological processes achieve 100% completion, the only byproducts are H
2
O, CO
2
, and traces of inert ash. However, to digest (biotreat) a single pound of food in a conventional BAS process as much as 0.53 lbs. of dead/dying bugs can be created. Thus, the total amount of solids under aeration slowly increases and ratio of living to dead organisms in the biomass slowly decrease. Soon total solids are so concentrated in the aeration tanks that the aerobic process is greatly inhibited. Therefore, excess solids have to be mechanically removed at that point in order for effective bioactivity to resume.
Various processes have been proposed to minimize or eliminate excess sludge in conventional BAS wastewater treatment processes. One such wastewater treating process is set forth in U.S. Pat. No. 5,356,537 to Thurmond et al. The Thurmond process teaches a process for treating wastewater including mixing wastewater with activated sludge in an aeration tank followed by clarifying in a settling tank. From 5% to 25% of the activated sludge solids from the settling tank is returned to the aerobic digester. The activated sludge solids is held in the aerobic digester from about 16 to 24 hours and then the activated sludge is transferred from the aerobic digester to the aeration tank. This process must employ both multiple cycles as well as multiple treatment units to be effective in the removal of excess solids (sludge) from the system.
In U.S. Pat. No. 3,047,492 to Gambrel there is described a modified activated sludge type of sewage treatment that eliminates the sludge beds, anaerobic digesters and costly sludge handling equipment. The Gambrel invention is said to eliminate the production of sludge and produce a clear final effluent that can be safely discharged into a creek or stream. To eliminating sludge, the Gambrel process incorporates a separate stage wherein most of the sludge from the settling tanks and approximately 25% of the daily intake of the plant is returned to the aeration tanks of the system while approximately 3% thereof is diverted to an aerobic digester. In the aerobic digester the sludge is detained for approximately 10 to 15 days and completely digested to produce an inert invisible ash and clear liquid that is then returned to the system. Once again, multiple units, as well as longer multiple cycles, would be required for the effective reduction in total sludge discharged.
Thus, there remains an unfilled need for an improved, simplified BAS process for treating wastewater to eliminate excess sludge quickly, efficiently, economically and without the use of both multiple treatment units and multiple, time-consuming, extra cycles. This improved process should also be useful in the treatment of lower-strength wastewater in other sewage treatment p

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