Abstract:
In a wastewater treatment process involving activated sludge, the invention provides a method for improving suspended solids removal without the need for chemical coagulants. Improved solids removal is obtained through the reintroduction of return activated sludge as a natural flocculent prior to tertiary filtration. In some applications algal solids can be pre-filtered from equalization or detention ponds and lagoons by return activated sludge addition followed by mixing and high solids filtration. The inventions result in improved solids removal and longer run times for the treatment processes with reduced need for chemical coagulants.

Description:
FIELD OF THE INVENTION  
         [0001]    The inventions described below apply generally to wastewater treatment processes and systems that employ activated sludge. More specifically, the inventions are directed to the enhanced removal of suspended solids from a fluid stream occasioned by the application of a form of activated sludge during the treatment process.  
         BACKGROUND OF THE INVENTION  
         [0002]    Typical municipal and industrial wastewater streams contain solid particles in a range of sizes and densities. During conventional treatment processes, solids of larger sizes and densities are removed from the waste stream rather easily. Often, as a primary treatment step, the wastewater is detained in a basin where the heavier particles (those having a density greater than the fluid medium carrying them) settle out of the waste stream through the effects of gravity. Smaller and lighter solids, however, remain suspended in the waste stream requiring additional physical and chemical processing for removal.  
           [0003]    Following primary treatment, many treatment processes introduce activated sludge into the waste stream for secondary treatment including additional solids removal. As it is well known in the art, activated sludge is a natural beneficial biomass that interacts with the remaining solids in a manner that creates heavier settleable solids that are then more amenable to physical removal. Some suspended solids remain after the secondary treatment phase leaving a wastewater that it often still unsuitable for discharge or reuse.  
           [0004]    Subsequent tertiary treatment processes such as filtration will remove additional solids, but removal is limited by the size of the openings in the filter media. To remove particles that would otherwise pass through the filtration process, conventional wastewater treatment methods call for the introduction of chemical coagulants (such as alum, ferric chloride or organic polymers) prior to filtration. Chemical coagulants interact with suspended solids binding individual particles into larger and heavier solids which can then be removed during secondary treatment.  
           [0005]    Wastewater treatment plant operators pay a price for this chemical conditioning. The complex polymer chemicals add significantly to the cost of treatment. Additionally, the chemical sludge that is then collected on the filter can quickly clog or “blind” the filter media, requiring frequent, possibly continuous backwashing. This can occur because the combination of the coagulants with the wastewater constituents can create a viscous mat on the filter media. Excessive backwashing reduces the quantity of wastewater that can be treated resulting in either the purchase and installation of more treatment equipment or a reduction in the processes that produce the wastewater. Additionally, the solids removed from the system following chemical addition create a chemical sludge that is more expensive and more difficult to dispose of.  
           [0006]    Therefore a method and apparatus for removing suspended solids from a waste stream that reduces or eliminates the need for chemical conditioning is desirable, as well a method and apparatus that is effective at small particle removal that reduces the effect of blinding filter systems and produces a solid waste that is easier to dispose than existing chemical sludges.  
           [0007]    The inventions described below reduce or eliminate the need for chemical conditioning of wastewater by reintroducing readily available activated sludge to the treatment process after secondary treatment. An advantage of this method is that activated sludge provides a natural coagulant without the expense of chemical addition as suspended solids are adsorbed onto the activated sludge. The natural waste solids avoid the problems of heavy chemical concentrations and are thus more readily disposable.  
           [0008]    Definition of Terms  
           [0009]    The following terms are used in the claims of the patent and are intended to have their broadest meaning consistent with the requirements of law:  
           [0010]    coagulation—the agglomeration of small, dispersed solids into larger particles more amenable to settling or filtration.  
           [0011]    Where alternative meanings are possible, the broadest meaning is intended. All words in the claims are intended to be used in the normal, customary usage of grammar and the English language. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a process diagram of a conventional wastewater treatment facility that employs primary, secondary and tertiary treatment techniques.  
         [0013]    [0013]FIG. 2 is a modified process diagram according to the invention where activated sludge is introduced prior to filtration.  
         [0014]    [0014]FIG. 3 is a profile view of a filtration basin using rotating disk filters.  
         [0015]    [0015]FIG. 4 is a partial process diagram of secondary and tertiary treatment stages with an aerated return activated sludge basin.  
         [0016]    [0016]FIG. 5 is a process diagram of a pretreatment process for an equalization basin directed to the removal of algal solids.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    Set forth below is a description of what is currently believed to be the preferred embodiment or best example of the invention claimed. Future and present alternatives and modifications to this preferred embodiment are contemplated. Any alternatives or modifications which make insubstantial changes in function, in purpose, in structure or in result are intended to be covered by the claims of this patent.  
         [0018]    In a conventional wastewater treatment process that is known in the art (see FIG. 1), a wastewater influent  1  is introduced to a primary clarification basin  2 . In the primary clarification basin  2 , some of the solids that are entrained in the influent  1  settle by gravity to the bottom of the basin  2 . This solids separation process produces a primary effluent  3  that is introduced into a secondary treatment basin  4 , and a mass of settled solids  15  that are removed from the bottom of the primary basin  2  and disposed. In one method of secondary treatment known in the art, the secondary basin contains an activated sludge biomass that is aerated and suspended in the secondary basin. The primary effluent  3  mixes with the activated sludge of the secondary basin  4  creating additional settleable solids entrained in a secondary effluent  5 .  
         [0019]    The secondary effluent  5  flows into a secondary clarifier  6  where additional solids are separated from the remaining effluent. In either the secondary treatment basin  4  or the secondary clarifier  6 , chemical coagulants  14  such as alum, ferric chloride or organic polymers are added to aid in settling entrained solids from the secondary effluent  5 . Solids separation in the secondary clarifier  6  results in a secondary effluent stream  7  and a waste activated sludge stream  8 . A portion of the waste activated sludge  8  flows into a return activated sludge line  9  which is diverted to the upstream side of the secondary treatment basin  4  or reintroduced into the primary effluent  3 . The remaining solids  10  from the waste activated sludge stream  8  are collected and disposed.  
         [0020]    For further solids removal, particularly for small particle size and low density solids, the secondary effluent stream  7  flows into a tertiary treatment process such as a filtration basin  11 . In the example of FIG. 1, the filtration basin contains rotating cloth filters  12  that retain solids above a given particle size on the exterior of the filter  12  while allowing liquid and smaller entrained solids to pass through. The tertiary effluent  13  is discharged to a receiving water. Solids are removed from the filters  12  and collected as a filtered solids waste stream  10  which is disposed in a similar manner as other waste solids.  
         [0021]    In a preferred embodiment of the invention (see FIG. 2), no chemical coagulants  14  are introduced during the treatment process. In lieu of chemical coagulants  14 , a return activated sludge feeder  24  is introduced into the secondary effluent stream  7  or, alternatively, into the tertiary treatment basin  11 . It is preferred that this return activated sludge feeder  24  runs continuously through a closed conduit in fluid communication with the secondary effluent stream  7 . However, it is also recognized as within the scope of the invention to apply the return activated sludge feeder  24  in batches. The preferred final concentration of return activated sludge in the tertiary treatment basin  11  is 2-5 milligrams per liter.  
         [0022]    The return activated sludge feeder in the tertiary treatment basin  11  acts as a natural flocculent and adsorbs small solids on the biomass. The wastewater in the tertiary treatment basin  11 , thickened by the addition of return activated sludge, forms a beneficial cake  17  on the exterior of the filters  12  (see FIG. 3). Since the cake  17  does not have the deleterious characteristics of sludges bearing chemical coagulants, it also acts as a supplemental filter improving solids removal in the tertiary treatment basin  11 . To the extent that the return activated sludge feeder  24  replaces the addition of chemical coagulants  14 , the net amount of solids generated during tertiary treatment does not increase.  
         [0023]    In an alternative embodiment (see FIG. 4), the return activated sludge needed for tertiary treatment is stored in an aerated tank  16  adjacent to the tertiary treatment basin  11 . It is recognized that whether the return activated sludge is fed continuously or in batches, electronically controlled valves and other instrumentation as known in the art may be installed to automate the process.  
         [0024]    The inventions apply to treatment basins of any size or shape. It is also recognized that the invention is equally effective for activated sludge treatment systems where two or more of the treatment stages are combined, where one or more stages is omitted, or in sequencing batch reactors. The preferred return activated sludge concentration of 2-5 milligrams per liter does not disclaim higher or lower concentrations. The preferred concentration has been selected for typical domestic wastes. It is recognized that a typical domestic wastes, commercial wastes and industrial wastes will require limited experimentation to determine the optimal concentration for each application. While in many cases no chemical coagulants  14  are needed, it is also within the scope of the invention to introduce chemical coagulants  14  in quantities lower than conventional treatment practices in combination with the return activated sludge feeder  24  addition to obtain the claimed benefits.  
         [0025]    In an alternative application of the invention (see FIG. 5), the addition of activated sludge can be used for the preconditioning of algal solids prior to filtration. In a typical system, a non-potable water held in a lagoon  26  or retention pond may develop blooms of algae from the quiescent hydraulic condition and available nutrients. As a substitute for a chemical algaecide, an activated sludge feeder  25  is added to the lagoon effluent  21 . The effluent  21  flows into a mixing basin  18  where the return activated sludge interacts with the solids and nutrients to produce a mixed effluent  22  which passes through a clarifying tank  27  to regulate the solids concentration. Excess solids are directed through a return line  28  to the lagoon  26  for additional processing. It is preferred that the activated sludge concentration in the mixed effluent  22  be maintained in the range of 5-10 milligrams per liter. Although this concentration is preferred for typical municipal waste streams, it is recognized that both higher and lower concentrations are contemplated by the invention. The most effective activated sludge concentration for any specific application must be tailored to the specific characteristics of the waste stream being treated.  
         [0026]    The mixed effluent  22  flows into a pre-filtration basin  19  containing filter media such as rotating disk filters  12 . The return activated sludge in the mixed effluent  22  acts as a natural flocculent and adsorbs small algal solids on the biomass. The wastewater in the pre-filtration basin  19 , thickened by the addition of return activated sludge, forms a beneficial cake  17  on the exterior of the filters  12 . The pre-filtration process produces a pre-treated effluent  20  that can be directed to the plant&#39;s headworks for additional treatment or to a receiving water. Waste solids  10  are disposed in a conventional manner as is known in the art.  
         [0027]    The above description is not intended to limit the meaning of the words used in the following claims that define the invention. Rather, it is contemplated that future modifications in structure, function or result will exist that are not substantial changes and that all such insubstantial changes in what is claimed are intended to be covered by the claims.