Patent Application
20100264083
None.
This invention relates generally to the field of wastewater treatment and more particularly to an activated sludge treatment process in which waste activated sludge is treated in an improved manner.
Wastewater has long been treated using activated sludge processes in which the influent is biologically treated in a basin to produce a mixed liquor. Clarification or another technique is used to separate the mixed liquor into a clear liquid effluent and a solid biomass which takes the form of activated sludge. The effluent is discharged, whereas part of the activated sludge is returned to the activated sludge basin in order to maintain a sufficient bacteria concentration for effective treatment of the influent.
The part of the activated sludge that is not return activated sludge is referred to as waste activated sludge. The waste activated sludge must be removed from the system and disposed of by incineration, deposit in a landfill, or in some other way such as use as fertilizer. Handling and disposal of waste activated sludge is a significant problem that makes it highly desirable to minimize the quantity of the waste activated sludge that is generated in a treatment plant. While various processes have been developed attempting to reduce the amount of waste activated sludge, they have not been wholly satisfactory.
Such processes are generally either a form of biological digestion or a mechanical process in which the cell tissue is physically ruptured. Mechanical methods have included high frequency sonic cell disruption and high pressure/shear cell destruction. Both of these methods require high capital expenditures and they are both subject to high energy requirements.
The most common biological process that attempts to achieve a low waste activated sludge yield involves use of a reactor known as an interchange reactor. The interchange reactor modifies the biological population spectrum from the activated sludge basin so that different organisms predominate. Treated sludge from the interchange reactor is added to the influent and introduced back into the activated sludge basin. Even though processes using interchange reactors can achieve improvement, there are significant drawbacks including the need for additional equipment, plumbing, pumps and instrumentation that call into question whether the benefits outweigh the added cost and complexity.
In accordance with the present invention, both aerobic and anaerobic digestion techniques are used in a unique process that reduces the quantity of waste activated sludge by increasing the solids destruction compared to what is achieved by either aerobic or anaerobic methods alone. The waste activated sludge may be screened, alone or together with the return activated sludge. Aeration of the waste activated sludge is carried out in a digester and may be interrupted periodically for solids settling and the decanting of clear liquid.
Either in the same zone or a different zone or zones of the digester, subsequent anoxic and anaerobic conditions are cycled through in sequence, optionally followed by aeration. The anoxic treatment denitrifies nitrate that might be present due to the aerobic treatment. The anaerobic treatment results in liberation of biodegradable intracellular material and ammonia nitrogen as well as moderate acidification. Some of the aerobic cell tissue is converted to anaerobic organisms which yield a much reduced mass to further decrease the net amount of sludge. The aerobic and anaerobic treatments can be carefully managed using oxidation reduction potential measurements to create the optimum conditions for achieving minimum sludge quantities and other desired benefits.
Other and further objects of the invention, together with the features of novelty appurtenant thereto, will appear in the course of the following description.
In the accompanying drawings, which form a part of the specification and are to be read in conjunction therewith in which like reference numerals are used to indicate like or similar parts in the various views:
Referring now to the drawings in more detail and initially to
The sludge on line 18 may be passed through a screen 20 having screen openings that may range from 0.02 inch to 0.08 inch. The screen material can be wedge wire, woven mesh, perforated plate or any other suitable material. Larger solid materials that are removed by the screen 20 are discharged as trash on line 22. The sludge that passes through the screen 20 is separated into return activated sludge and waste activated sludge. The return activated sludge is delivered on line 24 to the influent line 12 and is thus recycled into the activated sludge basin 10 to maintain the proper concentration of bacteria needed for the activated sludge process. The waste activated sludge is delivered on line 26 to a digester 28. It should be noted that only the waste activated sludge may be screened, although it is usually preferred that both the return sludge and the waste sludge be passed through the screen 20.
The digester 28 may be divided into two or three stages such as zones 30 and 32 which may be separated by a partition 34 or in some other manner. The waste activated sludge that is delivered to the digester 28 on line 26 is first treated aerobically in zone 30. Conventional aerators (not shown) may operate in zone 30 to provide the aeration. Preferably, the dissolved oxygen concentration in zone 30 is maintained at a level equal to or greater than 0.5 mg/l for the majority of the time during the aeration stage of the process. The aeration in zone 30 can be interrupted periodically to allow for solids settling for thickening and concentration of the sludge, along with decanting. Clear supernatant water that is decanted in zone 30 may be delivered on line 36 to the influent line 12 and then into the activated sludge basin 10. After an appropriate period of time (which may range from a few hours to several days) of retention time in the aerobic stage 30, the sludge is then transferred into zone 32 of the digester 28. This transfer of the waste activated sludge may be carried out either on a batch basis or continuously.
In zone 32, the waste activated sludge is subjected sequentially to treatment under anoxic conditions, then under anaerobic conditions, and then optionally under aerobic conditions. The principal purpose of the anoxic cycle of treatment is to denitrify any nitrate that may be transferred from zone 30 into zone 32. During the anoxic period of treatment, the oxidation reduction potential is maintained in a range of about +50 MV to about −200 MV. The anoxic portion of treatment can be a separate zone.
In the subsequent anaerobic treatment cycle, the oxidation reduction potential is allowed to drop below about −200 MV and is preferably maintained in the range of approximately −200 MV to approximately −400 MV. During the anaerobic treatment, lyses and hydrolysis of cell tissue occurs, liberating biodegradable intracellular material. These reactions also liberate ammonia nitrogen. Although moderate acidification also occurs, no significant reduced sulfur compounds are produced. Some of the aerobic cell tissue is converted to anaerobic organisms which have a much lower mass yield in order to further reduce the net quantity of sludge. In addition, the anaerobic zone promotes phosphorus release by phosphorus accumulating organisms (PAOs) and encourages subsequent luxury phosphorus uptake upon return to the aerobic zone of the digester, resulting in a supernatant low in phosphorus, which is important for biological nutrient removal (BNR) processes. After the oxidation reduction potential reaches the selected lower limit (−400 MV, for example), the sludge in zone 32 may be aerated to minimize the production of objectionable odors.
A recycle line 38 is provided to recycle sludge from zone 32 back to zone 30. The recycled sludge which passes through line 38 returns soluble biodegradable cell material and ammonia nitrogen to the aerobic zone 30 where nitrification and further degradation take place. Sludge may be removed from the digester 28 and discharged from the treatment facility from either zone 30 or 32, such as along line 40.
Control of the aerobic/anoxic/anaerobic conditions can be either manual or automatic. The infeed, settling, decanting, intradigester sludge transfer and waste sludge discharge can be controlled either manually or automatically as well. Along with effective screening of the sludge, the amount of waste activated sludge that must be handled and disposed of can be reduced by 50% to 80% compared to conventional practice.
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From the foregoing it will be seen that this invention is one well adapted to attain all ends and objects hereinabove set forth together with the other advantages which are obvious and which are inherent to the structure.
It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.
Since many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative, and not in a limiting sense.
