Abstract:
In a sewage treatment system, microconstituents, including personal care products and pharmaceutical materials, often difficult to degrade biologically, are removed by supersaturating the untreated wastewater feed with ozone. This breaks down refractory microconstituents into more readily biodegradable materials, subsequently treated preferably in an activated sludge membrane bioreactor process. The oxygen biproduct of ozonation is utilized by feeding the oxygen into an aerobic part of the plant to meet a portion of the biological demand, thereby increasing efficiency of ozone use in the process.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention is concerned with wastewater treatment and especially efficient removal of refractory biodegradable compounds including so called microconstituents from wastewater in a membrane bioreactor (MBR) process. 
         [0002]    A great deal of research has been undertaken to characterize the ability of conventional activated sludge and membrane bioreactor (MBR) technologies to remove microconstituents. Microconstituents are dissolved pollutants that are usually measured on the parts per billion (ppb) or parts per trillion (ppt) level. They include personal care products, pharmaceutical materials and hydrocarbons. Microconstituents are often refractory, long-chain organic compounds that are difficult to degrade biologically given typical solids residence times (less than 30 days). 
         [0003]    In conventional activated sludge processes using sedimentation or membrane filtration for removal of suspended solids, post-disinfection is used to inactivate or kill pathogenic organisms. In addition to disinfection, post treatment including high pressure filtration (e.g. reverse osmosis) is sometimes employed to remove microconstituents. 
         [0004]    Submerged MBR (sMBR) technology has a unique advantage over CAS systems using sedimentation for the separation of solids for biologically treated wastewater in that activated sludge concentrations can be more than three times higher allowing for a longer SRT given the same volume. Research suggests that running at longer SRT can lead to better removal of some refractory compounds and specifically some microconstituents. However, results are mixed and studies have not shown a sufficient correlation between treatment efficiency and SRT; therefore, the efficacy of MBR as compared to CAS Systems remains unquantifiable. 
         [0005]    Prior art, whether CAS or MBR, often involves the use of high-pressure filtration such as reverse osmosis (RO) followed by post-oxidation (or post disinfection) of permeate using one or more oxidative compounds. The list includes ozone, chlorine and ultraviolet (UV) radiation. High-pressure permeate filtration, in some cases followed by oxidative post-disinfection, has been successful in destroying some microconstituents but is expensive and in many cases impractical. Moreover, the use of chlorine can lead to the formation of undesirable disinfection byproducts, some of them known carcinogens. 
       SUMMARY OF THE INVENTION 
       [0006]    In a system and process of the invention, pretreated (screened, degritted) wastewater is saturated with ozone or contacted with a second stream of ozonated permeate for partial treatment or conditioning of refractory biodegradable compounds including microconstituents. The degraded ozone forms oxygen which is then used to offset biological process requirements. 
         [0007]    The byproducts of wastewater ozonation are smaller, more readily biodegradable compounds and oxygen. The oxygen produced during ozonation is used to meet a portion of the total biological demand for aerobic processes in the system and in some cases may evolve as bubbles, partially offsetting the need for air scouring of submerged membrane separators. 
         [0008]    Ozonating wastewater breaks down non- or less-biodegradable compounds including microconstituents into more readily biodegradable compounds that can be subsequently treated in an activated sludge MBR process at a shorter SRT. Given a target mixed liquor suspended solids (MLSS) concentration a shorter SRT translates into a reduced tank volume, allowing for what is called process intensification (reducing plan area and tank volume requirements to achieve a given treatment objective). For example, ozone can break down benzene rings into smaller carbon molecules readily consumed by microorganisms reducing the SRT required for treatment from 30 days to less than 5 days. 
         [0009]    In one embodiment of the invention, pretreated (fine screened, degritted, etc.) wastewater (influent) is essentially saturated with ozone to concentrations ranging between 25 mg/l and 100 mg/l before being fed directly into an MBR. Given the dilute concentrations of carbon substrate in municipal wastewater, it is possible to safely aerosolize influent and contact with ozone for saturation but the range is limited to protect against auto-ignition (typically less than 80 psig depending on loading). In another embodiment, treated permeate with virtually non-detectable amount of carbon materials is essentially saturated and the ozonated stream contacted with pressurized influent. This method of contacting influent with ozone requires water to be filtered twice but allows for higher concentrations of ozone to be safely achieved, increasing viability of the invention. 
         [0010]    As explained above, the ozonated wastewater will contain a greater portion of more readily biodegradable oxygen demand (rBOD) and be oxygen-rich, making it more treatable and improving process efficiency in three ways by reducing: (1) the volume of the tank by more than 20%; (2) the supplemental oxygen requirement by 20%-40% and; (3) the amount of air required for scouring membrane separators by 5%-10% depending on the residual oxygen concentration (above 10 mg/l gas evolution will occur). The total amount of oxygen required for any biological process is a function of the pollutant loading and other site conditions. The invention uses the oxygen byproduct of ozonation to meet some or all of the demand set by the process. Additional oxygen will ordinarily be necessary to meet the total biological demand, but not in all cases. 
         [0011]    It is among the objects of the invention to improve the space efficiency and process efficiency of MBRs in removing microconstituents by contacting wastewater with ozone as a form of partial treatment or conditioning, thus breaking down refractory compounds into readily biodegradable materials, utilizing the oxygen byproduct of ozonation to supplement process oxygen, and in many cases offsetting membrane air scouring through gas evolution. These and other objects, advantages and features of the invention will be apparent from the following description of a preferred embodiment, considered along with the accompanying drawings. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0012]      FIGS. 1 and 2  are flow charts showing prior art liquid side wastewater treatment, with sedimentation or membrane filtration and with conventional post-treatment oxidation. 
           [0013]      FIG. 3  is a flow chart indicating one embodiment of the invention where ozonated influent is fed directly into an MBR for full biological treatment and solids separation (filtration). 
           [0014]      FIG. 4  is a flow chart indicating a second embodiment of the invention wherein ozonated influent is allowed to fully react before being fed into an MBR for full biological treatment and solids separation (filtration). 
           [0015]      FIG. 5  is a flow chart indicating a third embodiment of the invention wherein a ozonated, reacted influent is saturated with oxygen to meet all or most of the process oxygen requirements. 
           [0016]      FIG. 6  is a flow chart indicating a fourth embodiment of the invention wherein a slip stream of permeate is ozonated and then contacted with influent. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0017]      FIGS. 1 and 2  show systems used in the prior art. In the system of  FIG. 1  influent  10  enters the denitrification zone  12 , from which it passes to an aerobic zone  14 . Recycle back to the denitrification zone is shown at  15 , via a pump  16 . Process air is shown introduced to the aerobic zone by a blower  18 . As indicated, mixed liquor exiting the aerobic zone at  20  is introduced to a sedimentation tank  22 , from which settled sludge is withdrawn at  24  and delivered via a pump  26  to be recycled into the aerobic zone  14 , as shown at  28 . 
         [0018]    In the conventional system of  FIG. 1 , supernatant  30  from the sedimentation tank  22  goes to a membrane filtration zone  32 , where further solids are removed, and the permeate liquid may then be put through high pressure filtration in a reverse osmosis (RO) treatment  34 . The resulting liquid may be put through an oxidative post-disinfection treatment, indicated at  36  (this treatment could involve chlorination or other disinfectant or oxidative treatments). The membrane separation  32  can be for removing total suspended solids (TSS), turbidity, and pathogens. The RO treatment  34  can be for further removal of solids measured as silt density index (SDI), microconstituents, ionic species and pathogens. Post disinfection at  36  is generally for sterilizing or killing remaining pathogens (e.g. viruses) but can also be used to destroy microconstituents. These processes are expensive. 
         [0019]      FIG. 2  shows a prior art system very similar to that of  FIG. 1 , with the exception that a membrane filtration zone  38  replaces the sedimentation tank  22 . Air scour is shown introduced to the membrane zone  38  by a blower  40 , for cleaning the membranes and also to supply process air in the zone  38 . This zone  38  also replaces the membrane filtration shown at  32  in  FIG. 1 , and again, the permeate from membrane filtration can be put through reverse osmosis or high pressure filtration at  34 , and a post-disinfection treatment at  36 , the zones  34  and  36  being for the purpose of removal or destruction of pathogens, microconstituents and other components as noted above that have been present in the influent. 
         [0020]      FIG. 3  shows one embodiment of the invention. Here, influent  10 , which has been fine-screened or degritted in a pretreatment step, is shown as put under pressure by a pump  42 , so that it is delivered under pressure into the ozone saturation zone  44 . Pressure can be, for example, about 25 to 80 psig, or somewhat higher. The influent is aerosolized in the presence of gaseous ozone and saturated according to Henry&#39;s Law, to achieve saturation concentrations greater than 25 mg/l. However, the solids in the wastewater are combustible, so that aerosol pressure must be controlled to keep the aerosol at a safe level to prevent auto-ignition. 
         [0021]    In the system of  FIG. 3  the ozonated, saturated or essentially saturated influent passes into a membrane biological reactor or MBR, shown at  46 . From the point of saturation  44 , dissolved ozone rapidly reacts with carbonaceous materials including refractory organic compounds such as microconstituents. Depending on the hydraulic residence time (HRT) between saturation and discharge into the membrane biological reactor  46 , some or all of the ozone may be converted into oxygen. A blower  40  is shown introducing air for air scour in the MBR  46 . The ozonated, ozone rich influent entering the MBR is depressurized; the MBR zone is not under pressure. As a result, a small envelope or area around the point of discharge is temporarily supersaturated and conversion of remaining ozone to oxygen is rapid in the presence of mixed liquor (ML). Some of the oxygen byproduct may evolve as bubbles in the MBR or all of the oxygen can diffuse into the bulk solution. This helps supply process oxygen to the ML in the MBR. In addition, the ozone introduced in the zone  44  breaks down microconstituents or refractory materials as explained above, resulting in smaller, more readily biodegradable compounds as well as the released oxygen. The oxygen bubbles evolving in the MBR  46  can contribute to air scouring of the membrane separators, and the point of introduction of the ozonated influent in the MBR tank  46  can be arranged so that the evolving bubbles add to the air scour bubbles from the blower  40  for scouring membranes. 
         [0022]    As indicated in  FIG. 3 , permeate at  48  can be directed to an optional denitrification zone  50 , where a final denitrification step includes running the permeate through filters or other equipment for removal of nitrates. Denitrified permeate effluent from the zone  50  is shown at  52 . 
         [0023]    By use of the ozonation, particularly at saturation or near-saturation and under pressure, this produces a greater proportion of more readily biodegradable oxygen demand and results in reduction of tank volume, reduction of supplemental oxygen requirements and usually reduction in scour air requirements, depending on the residual oxygen concentration and the evolution of bubbles in the zone  46 . With enough process oxygen supplied to the MBR and with sufficient inventory of biological solids in the MBR, a separate aerobic zone is not necessary. By not putting the influent directly into an anoxic zone, as conventionally done, the system of the invention can take advantage of the oxygen created by ozonation. Although the system theoretically gives up the efficiency of using nitrates of a recycle stream (e.g. the stream  15  in  FIG. 1 ) to meet oxygen demand in an initial anoxic zone, the ozonation makes up for this through breakdown of refractories and reduction of supplemental oxygen requirements. 
         [0024]      FIG. 4  shows a variation of the system of  FIG. 3 . In this modified embodiment the ozonated influent is allowed to fully react before being fed into an MBR  46  for full biological treatment and solids separation by filtration. The treatment of influent  10  is similar to the system of  FIG. 3 , except that on leaving the ozonation zone  44  the ozonated influent is allowed to more fully react in a pressurized ozone reaction zone  54 . This permits the ozone to be kept in solution, preferably at near saturation under the pressurized conditions, for a longer period of time so that breakdown of microconstituents can be more thorough. The remaining steps and zones are similar to those of the  FIG. 3  system. 
         [0025]      FIG. 5  shows a third embodiment of the invention, a variation of the  FIG. 4  system. This system adds to  FIG. 4  an oxygen post-saturation zone  56  downstream of the ozone reaction zone  54 . The ozonated, reactive influent from the zones  44  and  54  is then saturated or essentially saturated with oxygen, preferably still under pressure, to provide sufficient oxygen to meet essentially all of the process oxygen requirements. Pressure may be in a range of about 80 to 110 psig, to produce an oxygen concentration of at least about 250 mg/l, and preferably about 300 mg/l. Again, the biological reactions occur in the MBR  46 , in which the influent is depressurized. Oxygen may evolve as useful scouring bubbles, reducing blower  40  requirements and providing process oxygen; soluble oxygen may also diffuse from a zone of supersaturation around the point of discharge and into the bulk solution at less than or equal to standard conditions from both the reacted ozone remaining in solution and the oxygen introduced in the zone  56 . 
         [0026]      FIG. 6  is another flow chart showing a further modification of the systems described above. In this form of system, influent at  10  is again pressurized via a pump shown at  42  and enters an ozone reaction zone  58  followed by an oxygen saturation zone  60 , both under pressure. The ozone in the stream comes from a pressurized permeate stream, a recycle stream  61  with ozonation, as described below. The influent stream, blended with permeate, is saturated or essentially saturated with oxygen at the pressure in the zone, which may be in the range of about 80 to 200 psig, and the influent at this point has ozone. The influent with dissolved oxygen is then delivered to an MBR  62 , where the liquid is depressurized and oxygen diffuses into the bulk solution, offgases forming scouring bubbles, or both, depending on conditions. Again, any bubbles formed can be used to supplement air scour via a blower  40 , lowering air scour requirements in most cases. Oxygen concentration can be maintained greater than about 8 mg/l to increase aerobic respiration rates and reduce necessary residence times. The permeate  64  from the MBR is then primarily directed to an optional denitrification zone  50 , if included in the system, and denitrified permeate effluent exits the system at  52  (or discharged as effluent without denitrification). Alternatively, the MBR zone  62  (as with the MBR zones in  FIGS. 3-5 ) can be maintained at a low residual oxygen concentration, less than about 2 mg/l, to induce simultaneous nitrification and denitrification, removing nitrates and ordinarily avoiding the need for zone  50 . 
         [0027]    However, a portion of the permeate  64  from the MBR is directed, as shown at  66 , through a pressurizing pump  68  and to an ozone saturation zone  69 , under pressure, as the permeate stream  61  described above. Thus the permeate at  61 , which may comprise about 25 percent or more of the permeate at  64 , is saturated or essentially saturated with ozone at the pressure under which it is maintained (range of approximately 20 psi to 100 psi). This pressurized, ozonated stream enters the influent stream downstream of the influent pump  42 , producing at  72  an ozone-laden influent mix which has combined the essentially ozone-saturated permeate at  61  with the raw and pressurized influent. This is the pressurized influent to the ozone reaction zone  58  and then to the oxygen saturation zone  60 , producing a heavily oxygen and ozone-laden influent on depressurized entry into the MBR  62 . Oxygen bubbles evolve in the MBR from both the oxygen and ozone contained in the influent, providing for essentially all process oxygen requirements in the zone  62  and potentially reducing air scour requirements by the blower  40 . As discussed above, this system safely ozonates the permeate water, which can be by high-pressure aerosol methods, rather than directly ozonating the influent as in the systems of  FIGS. 3 and 4 . 
         [0028]    In a variation of the system as described, oxygen alone can be saturated into the incoming wastewater. Such a system could be as in  FIG. 3  or  4 , but with oxygen saturation and reaction zones rather than ozone saturation and reaction zones. Oxygen pressures can be higher, such as a range of about 110 to 300 psig, and can produce an oxygen concentration greater than about 300 mg/l. 
         [0029]    All references to pressure in p.s.i. refer to gauge pressure (psig, above atmospheric). References to supersaturation are relative to saturation levels at standard temperature and pressure. 
         [0030]    The above described preferred embodiments are intended to illustrate the principles of the invention, but not to limit its scope. Other embodiments and variations to these preferred embodiments will be apparent to those skilled in the art and may be made without departing from the spirit and scope of the invention as defined in the following claims.