Patent Description:
The present invention relates generally to a method and an apparatus for wastewater treatment and, more specifically, to a method and an apparatus for wastewater treatment with gravimetric selection.

Gravity separation is usually used to remove solids associated with the activated sludge process. A methodology has been developed to improve settling of solids by gravimetric selection. This methodology might also be applied to decrease membrane fouling in a membrane bioreactor (MBR) process or to decrease membrane diffuser fouling. There are currently three approaches to select for solids that settle well. The first is strategies within an activated sludge process to select for well settling solids such as by using aerobic and anoxic or anaerobic zones or selectors to improve settling. However, there is a mixed history with the use of these selectors and it does not always work.

The second method includes using shear/agitation in a reactor to select for granular solids that settle well. This selection is also accompanied with an increase in the overflow rate of sludge in the mainstream solid-liquid gravity separator. This selection process is often gradual and tedious, and, since the selector is associated with the mainstream process, it can result in problems associated with meeting permit requirements. In most cases, only a sequencing batch reactor process allows the flexibility to increase over time and modify the overflow rate.

The third method includes selecting and wasting the poor settling foam and entrapped solids, often by collecting and "surface wasting" the foam and solids at the surface of a reactor using "classifying selectors". While this approach was originally intended to reduce foam, it also selectively washes out the solids that do not settle well, as these slow settling solids tend to accumulate near the surface in reactors. Hence, this method retains only the solids that settle well, thereby providing a method that may be useful in deselecting poor settling solids, but which may have limited use in selecting settling solids. In implementing this method the settling characteristics improvements are often inconsistent, as sometimes poor settling solids, if they are produced at rates in excess of, e.g., a classifier surface removal rate, are retained and remain in the sludge.

An unfulfilled need exists for a method and an apparatus for wastewater treatment that does not have the drawbacks of the methods currently used to select and separate solids from wastewater.

<CIT> discloses wastewaters comprising BOD, nitrogen and phosphorus treated by an activated sludge process. The process utilizes an activated sludge tank, a solid-liquid separator, and a bioreactor to significantly reduce, or eliminate, waste activated sludge within a sludge stream. <CIT> discloses that removal of non-biodegradable organic material from an anaerobic treatment process that employs recycle or retention of anaerobic bacteria increases the efficiency of the anaerobic treatment process by removing the non-biodegradable organic material without removing substantial amounts of partially or wholly undigested biodegradable organic material or anaerobic bacteria. <CIT> discloses an apparatus for treating wastewater, with: a first sedimentation basin; a biological treatment tank; a final sedimentation basin; an acid production tank; a granulated sludge production tank; and a first granulated sludge supplying line. <CIT> discloses a waste water treatment method comprising: feeding activated sludge from a continuous activated sludge or sequential batch reactor plant into a settling tank; allowing the sludge to settle; removing an upper fraction which has a high sludge index as overflow; passing the lower fraction with a low sludge index to another tank; and determining the sludge index of this fraction. <CIT> discloses a method and apparatus for treating a continuous stream of organic wastewater using highly concentrated activated sludge, elevated atmospheric pressure and high levels of oxygen. The first vessel receives the mixed liquor consisting of macerated sewage and return activated sludge and thoroughly aerates it with diffused air bubbles. The liquor then flows by gravity into the second pressurised vessel where flocculation and further aeration occur. <CIT> discloses a process for relieving a secondary settling tank by separating off a part of return sludge from the activation tank. The activated sludge is passed through a hydrocyclone, the overflow of the hydrocyclone, whose activated sludge load is about <NUM> to <NUM> times the original load, is passed into a secondary settling tank, and the outlet of the hydrocyclone whose activated sludge load is about <NUM> to <NUM> times the original load, is subdivided into a first and second part-stream, the first part-stream being returned to the activation tank and the second part-stream being removed as excess sludge. Finally <CIT> discloses a simplified sludge recirculation system to be added to a system for potable, or industrial water or waste water treatment, which may include a combination of methods from the group comprising coagulation, sedimentation, flocculation and ballast flocculation, in order to improve its efficiency by reducing ballast and water loss. It also relates to a specific fluid flow behaviour rendered possible specifically due to the addition of the simplified sludge recirculation system, and which furthermore improves the efficiency of the process.

According to an aspect of the invention, a method is provided for selecting and retaining solids with superior settling characteristics according to the set of claims. The method comprises: feeding wastewater to an input of a processor that carries out a biological treatment process on the wastewater; outputting processed wastewater at an output of the processor; feeding the processed wastewater to an input of a gravimetric selector that selects solids with superior settling characteristics; and outputting a recycle stream at a first output of the gravimetric selector.

The method further comprises outputting a waste stream at a second output of the gravimetric selector to solids handling, where solids handling includes at least one thickening, stabilizing, conditioning, and dewatering. The waste stream is rejected and the recycle stream is returned to the processor. The waste stream comprises solids with poor settling and filtration characteristics or that have increased potential for membrane fouling.

The method further comprises supplying the recycle stream from the first output of the gravimetric selector to the processor. The recycle stream comprises solids with superior settling characteristics.

The treatment process may comprise: a suspended growth activated sludge process; a granular sludge process; an integrated fixed-film activated sludge process; a biological nutrient removal process; an aerobic digestion process; or an anaerobic digestion process.

The treatment process comprises a biological treatment process. The biological treatment process may comprise an in-line solid-liquid separation process.

The processor comprises a membrane separator.

The processor may comprise a cyclone that accelerates the wastewater and provides shear-force to the wastewater to separate solids with good settling characteristics from solids with poor settling and filtration characteristics.

The processor may comprise a centrifuge that provides centrifugal and shear force to separate solids with good settling characteristics from solids with poor settling and filtration characteristics in the wastewater.

The feed rate to and a geometry of the cyclone may be configured to adjust a velocity of the wastewater in the cyclone to select for larger or more dense solids or increase a time available for separation in the cyclone.

The process of feeding the processed wastewater to the input of the gravimetric selector may comprise: feeding the processed wastewater to an input of a separator that separates the wastewater into an underflow and effluent; receiving the underflow from the separator; and gravimetrically selecting solids with superior settling characteristics from the underflow and supplying the recycle stream to the first output.

The method may further comprise controlling a velocity of the wastewater in the cyclone so that solids of a predetermined size or density are retained.

The method may further comprise controlling a hydraulic loading rate to select settling solids of a predetermined size or density.

According to a further aspect of the invention, an apparatus is provided that selects and retains solids with superior settling characteristics according to the set of claims. The apparatus comprises: a processor that comprises an input and an output, the processor being configured to carry out a treatment process; and a gravimetric selector that comprises an input, a waste stream output and a recycle stream output, wherein the recycle stream output of the gravimetric selector is coupled to the input of the processor.

The input of the gravimetric selector is coupled to the output of the processor.

The input of the gravimetric selector is coupled to an underflow output of a separator.

The recycle stream output of the gravimetric selector supplies a recycle stream to the processor, the recycle stream comprises solids with superior settling characteristics.

The treatment process may comprise: a suspended growth activated sludge process; a granular process; an integrated fixed-film activated sludge process; a biological nutrient removal process; an aerobic digestion process; or an anaerobic digestion process.

The processor comprises a bioreactor process. The bioreactor process may comprise an in-line solid to liquid separation process.

The feed rate a geometry of the cyclone may be configured to adjust a velocity of the wastewater in the cyclone to: select for larger or more dense solids; or increase a time available for separation in the cyclone.

The apparatus may further comprise a separator that has an input coupled to the output of the processor.

The cyclone may control a velocity of the wastewater to adjust an overflow rate so that settling solids of a predetermined size or density are retained.

The cyclone may control a hydraulic loading rate to select settling solids of a predetermined size or density.

The accompanying drawings, which are included to provide a further understanding of the disclosure, are incorporated in and constitute a part of this specification, illustrate embodiments of the disclosure and together with the detailed description serve to explain the principles of the disclosure. No attempt is made to show structural details of the disclosure in more detail than may be necessary for a fundamental understanding of the disclosure and the various ways in which it may be practiced. In the drawings:.

The present invention relates to a method and an apparatus for treating wastewater by means of gravimetric selection as defined in independent claims <NUM> and <NUM>. Preferred embodiments of the method and apparatus are defined in the dependent claims.

The disclosure and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments and examples that are described and/or illustrated in the accompanying drawings and detailed in the following description. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale, and features of one embodiment may be employed with other embodiments as the skilled artisan would recognize, even if not explicitly stated herein. Descriptions of well-known components and processing techniques may be omitted so as to not unnecessarily obscure the embodiments of the disclosure. The examples used herein are intended merely to facilitate an understanding of ways in which the disclosure may be practiced and to further enable those of skill in the art to practice the embodiments of the disclosure. Accordingly, the examples and embodiments herein should not be construed as limiting the scope of the disclosure. Moreover, it is noted that like reference numerals represent similar parts throughout the several views of the drawings.

<FIG> shows an example of an activated sludge process and a system <NUM> for carrying out the activated sludge process. The system <NUM> may include pretreatment, which may include a bar screen <NUM>, a grit remover (not shown), a pre-treatment chamber <NUM>, and an influent pump (not shown). The system <NUM> may further include a primary separator <NUM>, a processor <NUM>, and a secondary separator <NUM>. The system <NUM> may receive wastewater <NUM> from an external source (not shown), such as, e.g., a sewage system, and process the wastewater <NUM> in a pretreatment stage which may include, e.g., a bar screen <NUM> to remove larger objects such as cans, rags, sticks, plastic packets, and the like, from the wastewater <NUM>. The pretreatment stage may also include a pre-treatment chamber <NUM>, which may contain, e.g., a sand or grit chamber, to adjust the velocity of the incoming wastewater <NUM> and thereby allow the settlement of, e.g., sand, grit, stones, broken glass, and the like. The pre-treatment chamber <NUM> may be replaced by, e.g., a sand or grit channel. The pretreatment stage may further include a small tank for removal of, e.g., fat, grease, and the like.

Following the pretreatment stage, the remaining solid-liquid mixture 4A, which includes excess wastewater containing accumulated solids, may be sent to a primary separator <NUM> for gravity settling. The primary separator <NUM> may include a tank (e.g., a clarifier tank, a sediment tank, etc.), which may have one of a variety of shapes, such as, e.g., rectangular, cone shape, circular, elliptical, and so on. The primary separator <NUM> may have a chemical or ballast material added to improve solids removal. The primary separator <NUM> settles the heavier solids from the solid-liquid mixture 4A. The resulting underflow 8A may be output from the primary separator <NUM> and sent to solids handling for further treatment, such as, e.g., thickening, stabilization, conditioning, dewatering, sludge processing, and so on, as is known by those having ordinary skill in the art.

The resulting solid-liquid mixture 4B containing soluble organic and inorganic contaminants and particulate materials is then be sent to the processor <NUM>. The processor <NUM> includes a bioreactor. The processor <NUM> may include an aeration tank (not shown) and live aerobic and facultative bacteria. Air may be added to the mixture 4B to feed a bioreaction process (where aerobic bacteria are grown) in the processor <NUM>. The aerobic bacteria will digest organic material in the presence of the dissolved oxygen.

The processor <NUM> includes a membrane module (not shown) for separating relatively pure water from the suspension of organic matter and bacteria. If the membrane module is included in the processor <NUM>, then the separator <NUM> may be omitted from the systems <NUM> (shown in <FIG>) and <NUM> (shown in <FIG>). The aerobic bacteria and the membrane module may be set up to run in succession in the membrane bioreactor (MBR). For example, the solid-liquid mixture may flow first through the bioreactor, where it may be held for as long as necessary for the reaction to be completed, and then through the membrane module.

The air may be added to the processor <NUM> via any known method that can supply air to the solid-liquid mixture 4B. A common method is through the addition of compressed air to fine bubble diffusers (not shown) constructed of perforated flexible membrane materials including EPDM and polyurethane. The processor <NUM> outputs an oxygenated solid-liquid mixture commonly known as mixed liquor 4C, which is then forwarded to the secondary separator <NUM>,.

The secondary separator <NUM> separates the oxygenated solid-liquid mixture 4C to produce an underflow 4F, which may then be recycled as part of a separated sludge <NUM> and sent back to the bioreactor <NUM>, and clarified wastewater as an effluent <NUM>. A portion of the underflow biomass 8B (or mixed liquor) may be wasted from the process and sent to solids handling for further treatment, such as, e.g., thickening, stabilization, conditioning, dewatering, sludge processing, and so on, as is known by those having ordinary skill in the art.

The processor <NUM> includes a membrane (not shown) that may be suspended in the slurry in the processor <NUM> (instead of the secondary separator <NUM>), which may be appropriately partitioned to achieve the correct airflow, with the surplus withdrawn from the base of the processor <NUM> at a rate to give the required sludge retention time (SRT).

It is noted that instead of, or in addition to the processor <NUM>, the system <NUM> may include, e.g., a granular sludge process, an integrated fixed-film activated sludge process, a biological nutrient removal process with various anaerobic, anoxic and aerobic zones with associated internal recycles, an aerobic digestion process, an anaerobic digestion process, and the like, as is known in the art.

<FIG> shows an example of a system <NUM> for carrying out the activated sludge process that is constructed according to the principles of this disclosure. The system <NUM> may include a similar set up as system <NUM>. The system <NUM> may include a cyclone (not shown), a hydrocyclone (not shown), a centrifuge (not shown), a sedimentation tank (not shown), a sedimentation column (not shown), a filter (not shown), and the like. Further to the components in the system <NUM>, the system <NUM> includes a gravimetric selector <NUM>. The system <NUM> has the ability to select for good settling solids by means of gravimetric selection in the gravimetric selector <NUM> through, e.g., direct wasting from the mixed liquor (or oxygenated solid-liquid mixture 4D). Good settling solids may include solids that exhibit a sludge volume index (SVI) of, e.g., less than <NUM>/gm, and preferably less than or equal to <NUM>/gm.

The gravimetric selector <NUM> may include, e.g., a clarifier, a settling tank, a cyclone, a hydrocyclone, a centrifuge, and the like. The gravimetric selector <NUM> of the invention comprises a filter, a cyclone, a hydrocyclone or a centrifuge. The gravimetric separator <NUM> includes an input and a plurality of outputs, including a waste stream output and a recycle stream output. The gravimetric separator <NUM> is positioned to receive the oxygenated solid-liquid mixture or mixed liquor 4D at its input from an output of the processor <NUM>. Alternatively (or additionally), the stream 4C may be input to the gravimetric selector <NUM>. During operation, the gravimetric selector <NUM> may classify, separate and/or sort particles in the mixture 4D, which may include a liquid or liquid-solid suspension, based on, e.g., the ratio of the centripetal force to fluid resistance of the particles. The gravimetric selector <NUM> may separate good settling solids from the mixture 4D and output the solids at its recycle stream output as an underflow 4E, which may be fed back to the processor <NUM> for further processing (e.g., bioreaction, digestion, etc.). The gravimetric selector <NUM> may output the remaining liquid/liquid-suspension at its waste stream output as a waste stream 8C, which may contain smaller particles and colloids that have the potential to cause MBR membrane fouling, cause turbidity in effluent <NUM>, and induce membrane air diffuser fouling, that may be output from the system for further treatment such as, e.g., sludge processing, dewatering, and so on.

<FIG> shows yet another example of a system <NUM> for carrying out the activated sludge process that is constructed according to the principles of this disclosure. The system <NUM> may include a similar set up as system <NUM>. Further to components in system <NUM>, the system <NUM> may include the gravimetric selector <NUM>, which may be positioned so as to receive an underflow 4F at its input from an output of the secondary separator <NUM>. The system <NUM> has the ability to select for good settling solids by means of gravimetric selection in the gravimetric selector <NUM> through, e.g., direct wasting from the more concentrated return <NUM>.

The gravimetric selector <NUM> may process the underflow 4F, separating heavier solids from the liquid-solid mixture and outputting the heavier solids as underflow 4E at the recycle stream output and the resulting overflow 8C at the waste stream output of the gravimetric selector <NUM>. The overflow 8C may be forwarded to solids handling for further treatment such as, e.g., stabilization, dewatering, and so on. The underflow 4E may be recycled together with the separated sludge <NUM> and returned to the processor <NUM> for further processing.

According to an alternative aspect of the disclosure, wasting of a portion (or all) of the sludge can occur directly from the underflow of the secondary separator <NUM>, which is not shown in the figures.

The gravimetric selector <NUM> may include any one or more gravity separation devices for selecting and separating solids from a liquid-solid mixture, including, for example, a settling tank, a settling column, a cyclone, a hydrocyclone, a centrifuge, and/or the like. The gravimetric selector <NUM> of the invention comprises a filter, a cyclone, a hydrocyclone or a centrifuge. In the gravimetric selector <NUM>, the overflow rate, which is also called the rise rate, can be used as a parameter in selecting good settling solids from the liquor (or sludge). This overflow rate can be adjusted to increase the wasting of poor settling solids, while only retaining good settling solids. An increase in the overflow rate can promote the selection for good settling solids until a certain point is reached, when the detention time is insufficient for proper classification of the solids. The target overflow rate of the gravity selection device should be based on the desired SRT of the process, and the associated need to remove a particular mass of biomass from the system. The specific overflow rate must be tuned to the particular device used, but would generally be expected to be <NUM> to <NUM> times the overflow rate of the secondary separation process <NUM>.

Hydrocyclone separation occurs under pressure, and a pressure drop may be used as the energy source for separation. Accordingly, if the gravimetric selector <NUM> includes a hydrocyclone, the hydrocyclone should be configured so that the input is positioned to feed the incoming liquid-solid mixture tangentially in the hydrocyclone to develop a high radial velocity. Further, the hydrocyclone may have a tapered shape. Hence, a spinning motion may be initiated and acceleration of the fluid may result from the tapered shape of the hydrocyclone. This creates a shear-force that improves settling characteristics of particles by actions such as, e. g, destruction of filaments or displacement of interstitial or bound water. A change in the initial velocity and/or the diameter (size) of the cyclone may result in the selection of different separation rates of desired solids fractions, or conversely results in overflow of non-desirables.

For example, a pair of hydrocyclones may be installed in the waste sludge line of the system <NUM> (or <NUM>) and configured for a wasting rate of, e.g., about <NUM><NUM>/hr each. The pressure may be set to, e.g., about <NUM> bar. An online pressure sensor (not shown) may be included in the system <NUM> (or <NUM>), which may provide a control signal for the frequency drive of, e.g., a pump (not shown), which may also be included in the system <NUM> (or <NUM>). The underflow nozzle(s) in the system <NUM> (or <NUM>) may have a diameter of, e.g., about <NUM>, thereby reducing any likelihood of vulnerability to clogging. <FIG> show SVI (mL/g) versus time charts for this example.

According to another example, a plurality of cyclones (e.g., a battery of seven cyclones) may be installed in the system <NUM> (or <NUM>). Each of the cyclones may be configured for a flow rate of <NUM><NUM>/hr. The pressure may be set to, e.g., about <NUM> bar and the diameter of the underflow-nozzle(s) may be set to, e.g., about <NUM>. The system <NUM> (or <NUM>) may include one or more inline sieves of, e.g., about <NUM> width to protect the cyclone(s) from clogging. <FIG> shows an SVI (mL/g) versus time charge for this example.

Centrifuge separation often occurs using a solid bowl centrifuge, where an increase in rpm of the centrifuge (e.g., in the range of <NUM> - <NUM> rpm) increases the gravitational force and thus the settling rate. Accordingly, if the gravimetric selector <NUM> includes a centrifuge that has a bowl, scroll and pond sections, the centrifuge may expose the liquid-solid mixture in the gravimetric selector <NUM> to many times the gravitational force that may occur, e.g., in a settling tank. A very small differential rpm (e.g., usually in the range of <NUM>-<NUM> rpm) between the bowl and the centrifuge scroll in the centrifuge can be used to separate the better settling solids from the poorer settling solids that are discharged in the overflow pond section of the centrifuge. Accordingly, by controlling hydraulic loading rate, centrifuge rotational speed, bowl/scroll differential rpm, and managing these rates between predetermined thresholds, the selection of larger and/or more dense solids may be controlled. For example, an increase in the hydraulic loading rate or bowl/scroll differential rpm may improve election of larger and/or more dense solids, while a decrease in these rates may help to increase retention time available for gravimetric separation, and a balanced rate may be used to manage the process. The solids in the pond section are wasted and the heavier scrolled solids can be retained and returned to the processor <NUM>.

An important characteristic of the gravimetric selector <NUM> is its capability of using an aggressive overflow rate to retain good settling solids in separate equipment associated with a solids waste stream. These good settling solids tend to be both more dense and larger, with the better settling being based on Stokian settling which allows for rapid removal of the material in the gravimetric selector <NUM>. Another important characteristic is the selective removal of smaller particles and colloids from the liquid/liquid-solid mixture that have the potential to cause MBR membrane fouling and/or turbidity in effluent <NUM>, and induce membrane air diffuser fouling in, e.g., the processor <NUM>.

<CIT> discloses a method for the biological purification of ammonium-containing wastewater. The disclosed method provides gravimetric separation (e.g., using a hydrocyclone, a centrifuge, or sedimentation) of heavy sludge phase containing slow-growing anaerobic ammonia oxidizing bacteria (ANAMMOX) from light sludge phase and returning the heavy sludge phase to the aeration reactor treating ammonia containing wastewater while feeding light phase sludge to a digester for gas production.

<FIG> illustrate improvements in the sludge settling properties resulting from implementation of the principles of the disclosure, including implementation of the system <NUM> (shown <FIG>) or <NUM> (shown in <FIG>). The sludge volume index (SVI) represents the volume of a sludge blanket settled for <NUM> minutes in a test cylinder normalized to one gram of solids and is a standard measure of settleability. Often a SVI greater than <NUM>/g is an indicator of poor settleability of sludge and an SVI less than <NUM>/gm, and preferably less than or equal to <NUM>/gm is an indicator of good settleability. Settleability of sludge determines the maximum mixed liquor solids operation that can be operated in an activated sludge plant. Even at many well operated treatment plants, the settling performance tends to deteriorate during certain periods of the year e.g., typically at the end of the winter season.

As seen in <FIG>, the use of the gravimetric selector <NUM> provides and maintains a good settleability, such as, e.g., less than <NUM>/gm, and preferably less than or equal to about <NUM>/gm.

<FIG> shows a graph comparing the deterioration of sludge settling properties in the process of system <NUM> with the improved settling performance of the activated sludge processes of systems <NUM> and <NUM>. This graph demonstrates the benefits of implementing the gravimetric selector <NUM> according to the principles of the disclosure. In particular, the graph illustrates a comparison of settling properties using the system <NUM> (or <NUM>) as compared to the settling properties using the system <NUM> (shown in <FIG>), which does not include the gravimetric selector <NUM>. In particular, this graph displays results where a pair of cyclones are installed in the waste sludge line of the system, and where the cyclones are designed for a wasting rate of <NUM><NUM>/hr each at a pressure of <NUM> bar with a <NUM> diameter underflow-nozzle, as noted earlier.

In <FIG>, the graph compares the deterioration of sludge settling properties in the system during the winter-spring season (e.g., December <NUM> to May <NUM>) for a three year period. As seen in the graph, although the SVI reached levels of up to about <NUM>/g at the end of the winter season, with the improved settling performance during the same period for the SVI remained below <NUM>/g using the system <NUM> (or <NUM>).

<FIG> and <FIG> show graphs comparing the deterioration of sludge settling properties at one process lane in a typical system with improved settling performance of a parallel lane in the system <NUM> (or <NUM>). In particular, the graphs display results from a full-scale pilot test at the WWTP Glarnerland plant where a battery of <NUM> cyclones were installed, each designed for a flow rate of <NUM><NUM>/hr. The design pressure was set to <NUM> bar and the diameter of the underflow-nozzle was set to <NUM>. An inline sieve of <NUM> width was installed to protect the cyclone from clogging. The results show comparison of the deterioration of sludge settling properties (SVI over <NUM>/g) at one liquid process lane with the improved settling performance of the parallel lane during an experimental period (SVI remains constant around <NUM>/g). At the WWTP Glarnerland, the performance comparison appears more direct where one treatment train was operated without the gravimetric selector and the other parallel one was operated with a gravimetric selector as seen in system <NUM> (or <NUM>) during the same period.

In <FIG>, the graph also displays the results from a test at the WWTP Strass plant where a pair of cyclones were installed in the waste sludge line designed for a wasting of <NUM><NUM>/hr each. The design pressure was set to <NUM> bar and an online pressure sensor was included to provide the control signal for the frequency drive of the pump used in the system. Due to the size of the underflow-nozzle, which had a diameter of <NUM>, no vulnerability to clogging was observed.

As evident from <FIG>, the application of the gravimetric selector <NUM> in system <NUM> (or <NUM>) may mitigate the deterioration of settling performance that would otherwise occur and which would otherwise lead to operational problems and to a bottleneck in design.

An activated sludge process may include a bioreactor that may be used for the treatment of wastewater. The activated sludge process may further include alternative processes for treatment of wastewater e.g., a granular process, an integrated fixed-film activated sludge process, an aerobic digestion process, an anaerobic digestion process, and so on. Any of these processes can be connected to a separation device utilizing gravimetric separation for the recycling or removal of biomass.

Claim 1:
A method for treating wastewater having improved settling of solids by means of gravimetric selection, the method comprising:
feeding a wastewater as a solid-liquid mixture (4B) that contains soluble organic and inorganic contaminants and particulate materials to an input of a processor (<NUM>) that includes a bioreactor;
processing, at the bioreactor, the wastewater as a solid-liquid mixture (4B) by a biological treatment process to obtain an oxygenated solid-liquid mixture as mixed liquor characterized in that the method further comprises:
the processor (<NUM>) further includes a membrane module that operates in succession of the bioreactor to separate the oxygenated solid-liquid mixture in the bioreactor by means of the membrane module into relatively pure water and a suspension of organic matter and bacteria and, then,
feeding the suspension of the organic matter and bacteria to a gravimetric selector (<NUM>) arranged at an output of the processor (<NUM>),
wherein the gravimetric selector (<NUM>) comprises a filter, a cyclone, a hydrocyclone or a centrifuge configured to select and separate by gravimetric selection into a waste stream and a recycle stream, the waste stream including small particles and colloids as poor solids that have potential for membrane fouling and are rejected for solids handling and the recycle stream including dense and large particles as solids with superior settling characteristics that are recycled to the processor (<NUM>) for further processing;
thereby the separation and the gravimetric selection allowing to separate and retain solids with superior settling characteristic for further processing, from poor solids that have potential for membrane fouling that are rejected,
thereby the method improving settling performance as result of increasing solids with superior settling characteristics that are retained and recycled to the processor (<NUM>) for further processing, and also as a result of decreasing solids with poor settling and filtration characteristics or that have increased potential for membrane fouling that are rejected.