CA2565052A1 - System for improved dissolved air floatation with a biofilter - Google Patents

Abstract

The system integrates dissolved air flotation (DAF) with a biologically active filter (or biofilter) to produce a treatment system. The DAF provides for effective removal of most particulates, whereas the biofilter enhances the removal of very fine particulate not efficiently removed by the DAF. The biofilter relies on the growth and proliferation of biofilm (attached colonies of micro-organisms) on a support media to effect particulate removal.

Description

System for Improved Dissolved Air Flotation with a Biofilter DESCRIPTION
[Para 1] Field of the Invention [Para 2] This invention relates to the treatment of wastewater using a biological process and more particularly to a system for improved dissolved air flotation with a biofilter.

[Para 3] Background of the Invention [Para 4] Dissolved Air Flotation (DAF) systems are commonly employed for the removal of suspended solids from water. However, the efficiency of removal generally declines with decreasing particle size. Ten (10) um is commonly regarded as a lower limit for efficient removal with this process. Water and wastewater can commonly contain many particles smaller than this threshold, for example, colloid particles. There is often a need to remove these fine particles. Chemicals called coagulants and flocculants are often used in conjunction with a solids removal process such as sedimentation or DAF for increased removal efficiency of these particles. These chemicals are costly and their use can be operationally complex. Fine filtration may also be used (with or without coagulants and flocculants) to remove the fine particles not removable by settling or flotation alone.

[Para 5] The two most common types of filters employed for fine filtration of wastewater are granular media filtration and membrane filtration. Both of these processes are primarily physical filters where the pore size of the filter determines the minimum size of particulate removed. Both types can have relatively high operating cost because the granular media Page 1 of 30 filters need vigorous backwashing for cleaning; the membrane filters need liquid turbulence and/or "scour" air to retard membrane fouling; they also require periodic chemical cleaning procedures.

[Para 6] Biofilms are known to be effective for the removal of fine particulate in water.
Mechanisms include passive interception of particles, adsorption of particles to biofilm and ingestion (grazing) activities of certain microorganisms such as sessile ciliates that utilize the particulate as a food source. The sessile ciliates are particularly important because of their ability to sweep particles from their vicinity and ingest them. It is known that the water vortices generated by ciliate feeding activity can extend up to 400 um from the organism itself. This has the effect of disturbing the liquid boundary layer around the organism and making particles outside the boundary layer available for capture. Ciliates often occupy biofilm protrusions that can extend 1 mm above the surface of the basal biofilm. Given that biofilms can often be 500 nm or more in thickness, it can be readily seen that a ciliate-rich biofilm can exert a cleaning effect 2mm or more from the biofilm substrate.

[Para 7] To date some commercial use of biofilm filtration exists using granulated activated carbon media. However these filters act principally as physical filters and require conventional backwashing. They typically receive a wide range of particles, rather than micron and sub-micron particles. No attempt is made to optimize conditions for the performance of sessile ciliates for removal of fine particles. Therefore there is a continued need to improve water and wastewater treatment processes and apparatus by a more effective means of using sessile ciliates in biofilm filtration.

[Para 8] Summary of the Invention Page 2 of 30 [Para 9] The invention provides advanced treatment of unsettled secondary effluent in a single unit operation. This includes gross solids removal, biological oxidation of residual organic matter and removal of fine (including submicron) particulates. The invention comprises a novel combination of dissolved air flotation (DAF) to remove fine particulates from wastewater and a biofilter specifically designed to maximize the particulate removal performance of naturally occurring microbiological flora particularly sessile ciliates. The result is a novel and inventive wastewater treatment system having cost and performance advantages over existing systems commonly used for such purposes. The DAF
provides for effective removal of most particulates. The biofilter enhances the removal of very fine particulate not efficiently removed by the DAF. The biofilter relies on the growth and proliferation of biofilm comprising colonies of micro-organisms attached to a support media or biomass carriers for particulate removal. The present invention provides optimized conditions for the proliferation and performance of sessile ciliates and similar organisms that have superior capabilities for the capture of fine particulate organic matter such as colloids. The invention requires periodic harvesting of biofilm. In the embodiments described herein, the invention provides a convenient method for removal of the biofilm flocs formed in the cleaning operation.

[Para 10] The invention comprises a vessel or tank capable of containing water. The tank receives feed flow in the form of unsettled secondary effluent. The feed flow enters the top of the tank by way of a feed conduit and discharges purified water from the bottom of the tank by way of a discharge conduit. The discharge conduit is in communication with a weir assembly. The tank is maintained full of water to a level determined by the weir so that it does not overflow.

Page 3 of 30 [Para 11] The tank is divided into two zones. The upper zone is called the DAF
zone, wherein recirculated water is aerated and mixed with new feed flow in a DAF
nozzle assembly. The DAF nozzle assembly generates small air bubbles that contact and adsorb particulate matter in the feed flow and DAF zone and float these solids to the surface where they collect as a "DAF float". The DAF float contains a significant amount of particulate matter by weight and is removed from the top of the tank by a skimming mechanism.

[Para 12] The bottom of the DAF zone is marked by a first media screen with a large percentage of open area (at least 60%) but with apertures small enough to prevent upward passage of any biocarriers from an adjacent and lower zone called the biofilter zone. The DAF may optionally include a particle interception layer consisting of a "plate pack" of inclined plates or other media (common to DAF and clarifier design in the industry). These are designed to stabilize and regularize velocities, encourage laminar flow and increase particulate removal efficiency.

[Para 13] The biofilter zone is divided by a watertight bulkhead(s) into two or more side by side compartments each containing plastic biofilm support media called "biomass carriers". Each compartment is further divided into upper (primary) and lower (secondary) layers. Under normal operating conditions the water leaving the DAF zone (including both feed water and all recycle flow) traverses the primary layers of both compartments on its way through the system.

[Para 14] A substantial potion of the water traversing the primary layers of the compartments is withdrawn and recycled back to the DAF zone through a process described Page 4 of 30 herein. The balance of the water traverses the secondary iayers of the compartments. The bottom of the compartments is delineated by a second media screen similar to the first media screen adapted to prevent the passage of biomass carriers. Water traversing the secondary layers of the compartments leaves the tank through a set of manifolds located below the second screen. Another set of manifolds is located at the bottom of the compartments and is employed in the cleaning cycle.

[Para 15] The biomass carriers are randomly packed discrete plastic elements having shapes, internal structures and large surface areas that promote biofilm growth and ensure intimate contact between the water and biofilm. These carriers preferably have a specific gravity greater than 1.0 so that they settle on the lower screen leaving a clear space of several cm between the top of the media layer and the upper screen. All water entering the DAF zone from the DAF nozzle assembly, feed flow and recycled water is compelled to traverse the biomedia where a variety of processes contribute to the treatment and clarification of the water as described herein.

[Para 161 The primary recycle flow is drawn from a set of manifolds into a continuously operating pump. The pump is a high-head, low flow type centrifugal pump. A
cavitating air injector or venturi designed for air suction and liquid motive flow injects air into the discharge of the pump. The cavitating nature of the venturi implies that within the normal operating range, back pressure on the outlet side of the venturi affects the degree of air suction but does not substantially affect the liquid flow rate. Conversely, imposed changes in air flow rate do not affect the liquid flow rate. The pump and venturi must be very carefully selected for compatible operation and to meet required targets for water and air Page 5 of 30 flow rate as well as pressures. A valve and flow meter are provided for monitoring and control of air flow.

[Para 17] Air is immediately mixed with water in the venturi, however dissolution of air within the venturi is incomplete. The venturi discharge mixture immediately enters a very precisely sized conduit designed to produce a specific two-phase flow regime known in the mass transfer engineering field as "bubble flow" or "froth flow" that most efficiently dissolves air in water. This approach requires a conduit having a smaller diameter than is typically used for DAF recycle lines. The conduit is typically small diameter plastic tubing.
Consequently, the length of the line must be minimized while still ensuring complete dissolution of air in the liquid flow. If not, the pressure drop in the conduit will severely restrict dissolved air capacity and cause a deterioration of the bubble flow regime within the conduit.

[Para 18] The bubble flow from the venturi enters the DAF nozzle assembly and is mixed with fresh feed flow on an impingement plate suspended above the nozzle assembly a predetermined and adjustable distance. Bubble flow is discharged into the impingement plate through a calibrated discharge orifice and experiences a rapid drop in pressure. The flow is redirected in a uniform radial pattern from the impingement point along the surface of the plate to its edge. The consequence is that very small bubbles (with rise times of 10 cm / min or less) are generated in immediate proximity to the fresh feed flow.
The hydrodynamics of the DAF nozzle assembly are such that the feed water is inducted into the area between the orifice and the impingement plate creating intimate contact between feed water containing particulates and recycle flow with dense bubble formation.
This condition Page6of30 maximizes the potential for particulate attachment to the bubbles and therefore enhances DAF particulate removal efficiency.

[Para 19] The bubbles generated in the DAF nozzle assembly pervade the DAF
zone and adsorb to or are physically entrapped by particles originating in the feed water (or particles already ambient in the DAF zone as in the aftermath of a cleaning cycle, as described below). This causes the captured solids to rise to the surface of the DAF zone where they gradually form a float layer. The float layer can grow to considerable thickness (up to several cm) and so must be harvested periodically and prior to every cleaning event. In operation with unsettled secondary effluent from a domestic source the float layer becomes more and more compact with time and rises above the water level. The result is that the upper portions of the float layer will have a solids content exceeding 15% dry solids by weight and the layer will have an average solids content exceeding 7%. The surface of the float layer gains a rubbery and elastic characteristic such that large cohesive chunks of the matter can be easily scooped off the surface of the DAF with minor loss of matter to the water phase. This progressive and natural dewatering of the float layer is an advantageous characteristic of the process. Removal of the float layer may be accomplished by a variety of established methods such as mechanical scraping or skimming. However, the preferred mechanism is a moving porous belt device that is immersed in the DAF zone at one end and rises above the DAF zone on the other side so that it will lift and pull the float layer out of the DAF and dewater it further while conveying it to a vessel located outside of the DAF for further dewatering and/or disposal. In the preferred configuration, the float would drop into a fabric dewatering bag where further removal of moisture occurs through gravity drainage as well as convective evaporation/drying possibly using forced air from sources of waste heat such as air blowers used in the secondary treatment process.
Alternatively, the Page 7 of 30 bags can be removed after partial drying to an indoor or outdoor storage facility where final dryness may be achieved. Final disposal of the solid byproduct can be to a land fill, land farming site or composting facility depending on local facilities and regulations.

[Para 20] Water traversing the secondary biofilter zones will equal (on average) the feed flow plus any water collected through the bottom manifolds and contributed to secondary recycle flow. This secondary recycle flow from the bottom manifolds can be adjusted in response to any need to optimize the variable of oxygen content, water residence time and channel velocities in the secondary biofilter layers. The secondary recycle flow traverses a supplementary aeration device before it is added to the feed flow so as to add oxygen to the system when required. In some situations there may be no need for a secondary recycle flow. In normal operation, water leaving the biofilter that is not drawn into the recycle flow will discharge through manifolds to the effluent weir and become final effluent discharge flow. The weir is an external horizontal weir that may be attached to a side of the tank. It is designed to minimize changes in the water level in the tank so that water does not overflow and the float layer is positioned vertically for efficient removal. Flow equalization must be accomplished prior to the DAF-Biofilter operation. An additional end filter may be added to further remove residual particulate matter from the effluent flow.

[Para 21 ] Biofilter Function and Cleaning Modes [Para 22] The biofilters employed in this invention differ from conventional biofilters in several respects:

Page 8 of 30 o They are physically integrated with the DAF unit and occupy the same tank (as described above). This stacked configuration is particularly advantageous in space-constrained sites;
o The media employed is random-packed media that is designed to be suitable both for "static bed" normal operation as well as intermittent "moving bed" action wherein the entire bed of media is circulated during cleaning cycles;

o There is no backwash flow for the biofilters - cleaning is accomplished with no change in fluid direction;

o The media is designed to achieve specific typical flow channel dimensions;
and, o The biofilters are specifically designed to treat water with low organic content, such as secondary effluent with soluble BOD values of 50 mg/L or less, preferably 25mg/L or less.
[Para 23] The biofilter relies on the growth and proliferation of biofilm (attached colonies of micro-organisms) on a support media to effect removal of particulates and residual soluble organic compounds. The present invention provides optimized conditions for proliferation and performance of particular micro-organisms known as sessile ciliates and similar organisms that have unusual capabilities to capture fine particulate organic matter such as colloids.

[Para 24] Key factors determining the micro-organism population include dissolved oxygen concentration, media design and the flow-channel geometry resulting from biofilm growth and accumulation of fine solids. The biofilm growth and accumulation of fine solids in the biofilters is progressive, resulting eventually in constricted flow channels and pressure loss through the biofilters. Consequently, periodic cleaning of the filter is required.
In the operation of the invention on unsettled secondary domestic effluent, the rate of accumulation of biomass in the filter is fairly slow so that a cleaning interval of one day or more should be sufficient in most applications.

Page 9 of 30 [Para 25] Unlike conventional filters thorough cleaning is not required. In fact it is beneficial to retain significant amounts of biofilm on the carriers to maintain their efficiency. The cleaning process needs to dislodge excess biomass and move it into the bulk fluid so that it can be transported out of the filter. No backwash is required. Instead, cleaning of a biofilter is accomplished by first removing the DAF float to avoid re-entraining any of the material in the water. Only one biofilter compartment will be cleaned at a time.
The system will have at least two and preferably three biofilter compartments.

[Para 26] During the cleaning procedure, 100% of the feed water will need to be processed through the other compartments. Consequently, this operation is best done at "off-peak" times or by using equalization capacity in the secondary treatment equipment to temporarily reduce the feed flow rate. However, we have observed that the biofilters are quite capable of tolerating flows up to double the design values for short periods and no interruptions in feed flow will be required if three biofilter compartments are employed.
[Para 27] The design of biomass carriers for the biofilters is critical. Of most importance are the effective sizes of flow channels resulting from random-packing of a particular media. The filter will be designed specifically for optimal particulate removal efficiency by micro-organisms. In this regard, the intent is to provide particular flow channel dimensions that maximize the opportunity for mature biofilms to capture fine particles.
Consistent with the preceding discussion, the ideal spacing between biofilm surfaces is 2 - 5 mm. The media must predominantly exhibit this spacing. In addition, the random packing must not result in inter-carrier flow channels that are too large in relation to the intra-carrier flow channels. Practically, inter-carrier channels will be larger and this will be tolerable as long as the ratio is not excessive. In this invention, the maximum ratio of inter-carrier channel Page 10 of 30 size to intra-carrier channel size should not exceed a value of approximately 3. Another consideration is that the media must enforce tortuous flow paths and not allow short-circuiting [Para 28] A final consideration for the carriers is that they should have a specific gravity greater than 1, ideally in the range 1.1 to 1.3, so that they will not float against top retaining screens and will be easily fluidized by air sparging, for efficient cleaning without excessive scour and turbulence.

[Para 29] The invention achieves high effluent quality prior to the final filter (consistently < i NTU operating on domestic secondary effluent) despite utilizing a filter pore size that is much larger than that required to produce similar effluent quality in conventional filters. In certain applications where further enhancement of effluent quality and a high degree of quality assurance is required, the invention may include a final filter that acts as a "physical barrier" against fine particles and makes further use of the microbiological filtration process described herein. A suitable and advantageous type of filter for this application is a filter constructed as a hollow, porous panel covered with industrial felt fabric of a specified micron grade (typically 0.5 to 5 micron). The effluent from the invention would flow to a tank containing one or more of these panels through the fabric into the core of panels and out of the panel through a bottom outlet nozzle connected by flexible conduit to the exterior of the tank. The tank will optionally have a low-capacity fine bubble aeration system as well as an air scour system. Because the DAF-filter has removed the vast majority of particulate and organic matter, these filters (if provided with sufficient surface area) will operate for long periods between cleanings. It has been observed that the final filter described herein operates for at least several weeks without requiring cleaning when Page 11 of 30 operated after the DAF-filter on typical secondary sewage effluent. Cleaning would be accomplished by air scour of the filter for a period of several minutes to one hour, followed by pumping the tank contents to the secondary treatment unit or the DAF-Filter. At infrequent intervals, panels will need to be removed for more thorough washing.

[Para 30] The DAF employed in this invention differs from the conventional DAF
design in the following respects:

o The DAF is physically integrated with a bio-filter, both units occupying the same tank;

o There is no bubble chamber (shallow or confined space at the vessel inlet where feed and aerated effluent is made to interact);

o There is no local confinement of flow, in fact, the invention requires a general ability to process particulate-containing water ambient in the DAF section as well as feed water - this is crucial to the cleaning process;

o Higher recycle to feed rations are required in order to supply sufficient oxygen to support biological oxidation in the biofilters;

o Due to the presence of the biofilters, the single pass removal efficiencies in the DAF do not need to be as high as in conventional DAFs since fine particles not captured in the DAF
will be trapped in the biofilters;

o The higher recycle rates increase the overall probability of capture.
Consequently, the present invention operates with higher recycle rates (typically 50% to 150%) and tolerates higher velocities in the DAF; and, o The float is removed intermittently (rather than continuously) after the float has had an opportunity to consolidate and partially de-water.

[Para 311 Advantages of the present invention.
Page 12 of 30 [Para 32] The present invention has the following advantages:

o Ability to remove fine particulates (including colloids) from the secondary effluent without the use of coagulants or flocculants;

o Space saving due to the fact that the DAF and biofilter are stacked;

o Enhanced removal of soluble organic compounds including some xenobiotic compounds, due to the microbiological diversity within the biofilter and high sludge age of the biofilm;

o Provides an already dewatered sludge residual eliminating the need for a separate dewatering device;

o Lower energy consumption than other tertiary filtration devices; and, o No backwash flow to be managed.

[Para 33] Description of the Drawings [Para 34] Figure 1 is a partial schematic drawing of one embodiment of the invention.
[Para 35] Figure 2 is a partial schematic drawing of the same embodiment of the invention as shown in Figure 1.

[Para 36] Figure 3 is a complete schematic drawing of the same embodiment of the invention as shown in Figure 1 and Figure 2.

[Para 37] Figure 4 is a schematic drawing of secondary aeration means in one embodiment of the invention.

[Para 38] Figure 5 is a schematic drawing of the nozzle assembly of one embodiment of the invention.

[Para 39] Figure 6 is a schematic drawing of a biomass carrier unit of one embodiment of the invention.

[Para 40] Detailed Description Page 13 of 30 [Para 41 ] Referring to Figure 1, my invention (10) is a system for improved treatment of wastewater using dissolved air floatation treatment and a biofilter. Figure 1 is a partial schematic and does not show all elements of the invention in order to avoid crowding in the diagram. The system comprises a source of continuous wastewater flow shown generally as (12), means for receiving and containing a predetermined volume of wastewater for treatment shown generally as (14), means for aerating the wastewater shown generally as (16) and means for purification of the wastewater shown generally as (18) located within containing means (14).

[Para 42] The source of continuous wastewater flow (12) is secondary effluent (19) as pressurized feed flow or gravity flow through conduit (20). The flow may be regulated by way of valve (22) to temporarily stop feed flow during cleaning cycles. The valve may be a remote control valve or a manually operated valve depending on the installation. The feed flow is generally unsettled secondary effluent containing particulate matter the type of which might be found downstream from a secondary biological waste water treatment faci l ity.

[Para 43] The means for containing a predetermined volume of wastewater for treatment (14) comprises a tank (24) having a predetermined volume. The tank has an open top (26) a closed bottom (28) and sides (30) and (32). The tank can be shaped as depicted in Figure 1 or it can be cylindrical with a similar frusta-conical bottom. The tank can be manufactured from concrete, steel, plastic or some other material suitable for the purpose of waste water treatment. The tank is adapted to receive secondary effluent from conduit (20) through aeration means (16) which is more fully described below. The secondary effluent is treated using a biofilter as more fully described beiow and clarified water is discharged in a post-treatment stream through manifolds (35) and (37) connected to conduits (33) and (34) Page 14 of 30 respectively. Secondary recycle flow is established through manifolds (39) and (41).
(Primary recycle flow is described below).

[Para 44] Referring now to Figures 1 and 2, a partial schematic of the complete invention is shown in both figures. The flow from the manifolds (35) and (37) is controlled by valves (36) and (38) respectively which are both open in normal operation. The post-treatment flow can be diverted into a secondary recycle stream to control the dissolved oxygen content of the effluent stream. This is accomplished by opening valves (45) and (47) to the suction of pump (200). The recycle flow is aerated by aerator (99a) the operation of which is explained below. The recycle flow modifies water residence time within the tank (24) and channel velocities in the secondary biofilter layers.

[Para 45] The tank volume and level is controlled by weir (50). Weir (50) can be an external horizontal weir attached to the side of the tank (24). Weir (50) is adapted to minimize fluctuations in the level of water within the tank to prevent overflow and to facilitate removal of float as more fully described below. Post-treatment flow enters weir (50) by way of conduit (52) and is ultimately discharged from the weir by way of conduit (54).

[Para 46] Still referring to Figures 1 and 2, the treatment tank (24) includes a number of internal structures that facilitate treatment of the feed flow and subsequent backwashing of the system as required. Within the tank is a horizontal first zone (60) comprising means for aerating the wastewater (16) and a horizontal second zone (64) comprising a biofilter (18) for purification of the wastewater. The horizontal first zone is disposed above the horizontal second zone.

[Para 47] The horizontal second zone (64) has an upper boundary (66) and a lower boundary (68) delineated by an upper (70) and lower (72) screen respectively.
The screens determine the area of biological treatment of the effluent. The screens have apertures that Page 15 of 30 are sized to permit fluid flow but prevent the movement of biocarriers from the horizontal second zone. Below the first horizontal zone (60), the tank is divided into vertical compartments, each consisting of the corresponding portion of the horizontal second zone (64) and a manifold area below the second screen. Figure 2 illustrates a vertical first compartment (74) adjacent to a vertical second compartment (76). These two compartments are non-communicative and operate independently. Since they are sealed from each other one may be cleaned while the other still operates. Two such vertical compartments are shown for the purpose of illustration. In the preferred embodiment three such vertical compartments are employed.

[Para.48] Referring now to Figures 1, 2, and 3, there is shown in Figure 3 a complete schematic of the invention. The vertical first (74) and vertical second (76) compartments comprise a primary upper layer (80 and 81), a secondary layer (82 and 83) and a third manifold layer 85 and 87. The top two layers are separated by the primary recycle withdrawal manifolds X and Y. The second and bottom layers are separated by a screen (72). Within a compartment, all three layers are in hydraulic communication.
Water for the primary wastewater recycling is drawn from the primary recycle manifolds X and Y through valves (92) and (94) into recirculation loop conduit (96) through the supplementary aeration device (99b) and then into the suction of pump (100). Secondary recycling is accomplished by opening valves (45) and (47) and drawing water as required from manifolds (39) and (41) into the suction of pump (200) and aeration device (99a) on conduit (202) [Para 49] Referring to Figure 4 supplemental aeration device (99) is depicted.
Influent (101) flows by gravity (or pumping as the case may be) from the DAF to the standpipe (103) and flows downward in the standpipe (103). Simultaneously, air introduced into the bottom of the standpipe from air pump (107). Air flow is measured by flow meter (109) and controlled by control valve (111) on conduit (113). The flow is aerated by fine bubble Page 16 of 30 diffuser (115) within the standpipe (103). Air rises as fine bubbles (105) in the standpipe creating counter current two-phase flow thereby aerating the influent. Water exits the standpipe through conduit (117). Supplemental aeration devices (99) are employed to add oxygen to the primary and secondary recycles respectively.

[Para 50] Referring back to Figure 3, pump (100) is a high head low flow type centrifugal pump. On the discharge conduit (102) is disposed air entrainment means being a venturi-type cavitating air injector (104) adapted to inject air into the recirculation flow. Air is forced into the recycling flow and mixed with it. Within the normal operating range of the system, the back pressure on the discharge end of the venturi will not substantially affect the amount of air entrained into the recycle water stream but it will not affect the flow of recycle water itself. Hence a change of air flow will not affect the recycle water flow rate.
The pump (100) and the air injector (104) must be compatibly chosen. Control valve (110) and flow meter monitor (112) exist to control and monitor the air flow into the recycle stream.

[Para 51] Discharge conduit (114) from the venturi is precisely sized so that the entrained air within the recycle flow produces a two-phase mixture called a "bubble flow" or a "froth flow" representing the most efficient dissolution of air into the water flow. The diameter of conduit (114) is typically smaller than the diameter of the recycle flow loop line (96). Plastic tubing is often used. The length of the tubing needs to be minimized to while ensuring complete dissolution of entrained air otherwise the pressure drop within the tubing will severely restrict air capacity and cause a deterioration of the flow regime.

[Para 52] Referring now to Figures 1 to 5, Figure 5 shows a schematic of the DAF aerating nozzle assembly (120) of the invention. The nozzle assembly communicates with discharge conduit (114) containing a supply of pressurized recycled water flow. Conduit (114) is provided access through the wall (32) of tank (24) by way of stand-pipe (124).
The aerating Page 17 of 30 nozzle assembly is also in communication with conduit (20) containing a supply of pressurized feed flow.

[Para 53] Conduit (20) penetrates the wall of the tank (24) by way of a stand-pipe (126) and is connected to the aerating nozzle assembly at connection (128). The nozzle assembly includes a horizontal section (130) and a vertical section (132). The vertical section has an inside wall (134), a closed first end (136) and an open second end (138).
There is also a third conduit (140) having a vertical orientation, an outside wall (142), a first end (144) in communication with the source of pressurized recycled flow conduit (114) and a second open end (145) having a discharge nozzle (146) so that the pressurized recycled flow is discharged there from. Conduit (140) is disposed centrally within the second conduit vertical section (132) and penetrates the closed first end (136) in a sealed relationship hence forming an annulus (150) between the inside wall of the second conduit vertical section and the outside wall of the third conduit. The discharge nozzle (146) has a pre-calibrated discharge aperture (152).

[Para 54] Still referring to Figure 5, the aerating nozzle assembly further comprises an impingement plate (154) disposed a predetermined and adjustable distance (156) from the discharge aperture (152) and forming a gap (158) above the discharge aperture and below the impingement plate. The impingement plate is circular in shape and suspended by suspension means (159). Pressurized recycled flow discharged from the discharge aperture impinges upon the centre of said impingement plate.

[Para 55] When the pressurized recycled flow is discharged from the discharge aperture it experiences a pressure drop between the discharge aperture and the impingement plate so that dissolved air entrained in the flow forms air bubbles. The pressurized recycled flow discharged from the discharge aperture mixes with the pressurized feed flow discharged from the annulus (150). The result is that particulate matter contained in the pressurized Page 18 of 30 feed flow is in close dynamic proximity to said air bubbles thereby causing the particulate matter to attach to the air bubbles and float. In this manner a dewatered float layer (160) is formed at the top of zone (60) having a high concentration of particulates.
This float layer is subsequently harvested by harvesting means depicted as (162) and comprises a moving belt mechanism that lifts the float layer from the DAF water surface over the edge of the tank and into a holding tank for later disposal. The float layer typically contains between 7%
and 15% dry solid particulates by weight.

[Para 56] Referring back to Figures 1 to 3, the biofilter comprises a plurality of dynamic biomass carriers adapted to promote the growth of micro-organisms. The biomass carriers are active in the second horizontal zone (64) and between screens (70) and (72). Since the biomass carriers are small, the screens have apertures sized to prevent leakage of the biomass carriers from zone (64). The biomass carriers are adapted to promote the growth of micro-organism on their surfaces. The predominant micro-organisms generally include sessile ciliates.

[Para 57] Referring now to Figure 6 there is shown a typical biomass carrier (160) of the invention. Each biomass carrier of the plurality of biomass carriers within zone (64) are separated by an intra-carrier flow channel (162) having a first size expressed as a median value of channel diameter. The biomass carriers comprise a plurality of surfaces (164) adapted for optimized growth of biomass. These surfaces are arranged to create a plurality of inter-carrier flow channels (166) having a second size expressed as a median value. The surfaces (164) are disposed apart a distance of between 2mm and 5mm. The ratio of the first size to the second size is less than 3. Each biomass carrier has a specific gravity greater than one. In the preferred embodiment of the invention the specific gravity of each biomass carrier is between 1.1 and 1.3. Typical dimensions for the biomass carriers are shown in Figure 6.

Page 19 of 30 [Para 58] Alternative biomass carrier types, including some commercially available types, may be employed in the device to varying levels of effectiveness. The design described herein is optimized for this application.

[Para 59] Referring to Figure 3, the system may include an optional secondary filter.
Effluent from the weir (50) is directed by conduit (54) to a secondary filter tank (170) in which there is disposed a non-woven industrial felt fabric (172) having a grade of 0.5 to 5.0 microns. The secondary filter further comprises a low-capacity fine bubble aeration system (175) to keep particulates suspended and an air scour system (174) to clean the filter periodically. Effluent from the secondary filter is discharged through conduit (177).

[Para 60] Referring to Figures 2 and 3, to commence cleaning of vertical zone (74), valve (36) is closed to isolate the zone from the effluent discharge conduit (52).
If not already open, valve (45) is opened to pump (200). Valve (53) (air supply valve) is opened for a pre-determined period of time and (if not already operating) pump (200) is activated at the same time. The air supplied to the zone out of the air pipe (57) is just sufficient to cause the bed of biomass carriers to move and circulate, sloughing off and dislodging biomass. Media in the areas immediately above the pipe (57) will be lifted upward, dispersed toward the opposite side of the zone and then carried downward, creating a rotational motion. Multiple revolutions of the media will be produced with the result that all carriers move relative to one another and carriers assume different positions in the filter. During this process, biomass is liberated and becomes fully entrained in the bulk fluid such that the fluid in the biofilter zone (74) becomes highly turbid. Most of this biomass will be carried into zone (60) where flotation processes will take it to the float layer (160). Other solids are carried out with the flow through pump (200) and conduit (202) through aeration means (99a) and into the feed flow conduit (20) for return to zone (60). After an interval of typically one to Page 20 of 30 several minutes the air supply is shut down, allowing the biomass carrier bed to re-consolidate, with each biomass carrier assuming a new location in the bed.
Pump (200) continues to operate for an extended period of time (typically at least 30 minutes) to ensure that unattached biomass is removed and effluent quality standards are regained. At that point, valve (36) is reopened. Flow will be re-established through the compartment to the effluent weir. To clean the opposite zone (76) valve (38) would be closed and air supply valve (55) opened to air pipe (59).

[Para 611 During the cieaning procedure, 100% of the feed flow will need to be processed through the other zone(s). Consequently, this operation is best done at "off-peak" times or by using equalization capacity in the secondary treatment equipment to temporarily reduce the flow rate. However, we have observed that the biofilters are quite capable of tolerating flows double the design values for short periods and a system with three vertical zones will not require a cessation in feed flow for satisfactory cleaning.

[Para 621 Although the description above contains much specificity, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.

Page 21 of30 What is claimed is:

[Claim 1] A system for improved dissolved air floatation (DAF) treatment of wastewater wherein said system comprises: a controlled source of continuous wastewater flow containing unsettled secondary effluent containing a first size and a second size of particulate matter wherein said first size is larger than said second size; a tank having a first predetermined volume for receiving and containing a second predetermined volume of said wastewater for treatment and discharging a post-treatment stream; means for capturing the first size of the particulate matter for disposal comprising a DAF nozzle;
means for capturing the second size of the particulate matter for disposal; and cleansing means.
[Claim 2] The system of claim 1 wherein said means for capturing the second size of particulate matter is a biofilter and wherein said cleansing means is adapted to periodically cleanse said biofilter.

[Claim 3] The system of claim 2 wherein the second predetermined volume of the wastewater in the tank is determined by a weir, wherein said weir receives said post-treatment stream and discharges the post-treatment stream into a system discharge line.
[Claim 41 The system of claim 3 wherein the tank further comprises a horizontal first zone comprising said means for aerating the wastewater, a horizontal second zone comprising the biofilter disposed below said horizontal first zone, and a horizontal third zone disposed below said horizontal second zone wherein said first, second and third zones are in hydraulic communication.

[Claim 5] The system of claim 4 wherein the horizontal first zone and the horizontal second zone are separated by a first screen and wherein the horizontal second zone and the horizontal third zone are separated by a second screen.

Page 22 of 30 [Claim 6] The system of claim 5 wherein the horizontal second and third zones are divided into at least first and second vertical and adjacent compartments so that said at least first and second vertical and adjacent compartments are non-communicative and operate independently.

[Claim 7] The system of claim 6 wherein the second horizontal zone of the at least first and second vertical compartments is divided into a primary upper layer and a secondary lower layer, said primary upper and secondary lower layers separated by a wastewater recycle withdrawal manifold disposed within the second horizontal zone and wherein the third zone of the at least first and second compartments comprises a post-treatment stream discharge manifold disposed above a secondary recycle flow manifold.
[Claim 8] The system of claim 7 wherein said DAF nozzle comprises an aerating nozzle assembly disposed centrally within the first horizontal zone and above the second horizontal zone.

[Claim 91 The system of claim 8 wherein said aerating nozzle assembly communicates with a first conduit containing a supply of pressurized recycled flow drawn from said wastewater recycle withdrawal manifold and wherein said supply of pressurized recycle flow is in communication with a cavitating air injector for entraining dissolved air into said supply of pressurized recycle flow.

[Claim 10] The system of claim 9 wherein said first conduit is sized to promote air dissolution within the supply of pressurized recycled flow and to achieve a "froth flow" mass transfer regime and wherein the first conduit terminates at the bottom end of a third vertical conduit and further wherein said third vertical conduit top end comprises a discharge nozzle having a discharge aperture.

[Claim 111 The system of claim 10 wherein the aerating nozzle assembly communicates with a second conduit in communication with a supply of pressurized feed Page 23 of 30 flow containing particulate matter of a first size and wherein said second conduit comprises a horizontal section terminating in a vertical section, wherein said vertical section has an inside wall, a closed first end and an open second end.

[Claim 1 2] The system of claim 11 wherein the third vertical conduit is disposed within the vertical section of the second conduit thereby forming an annulus.

[Claim 1 3] The system of claim 12 further comprising an impingement plate disposed an adjustable distance above said discharge aperture thereby forming an adjustable gap between the discharge aperture and said impingement plate so that the pressurized recycled flow discharged from the discharge aperture impinges upon the centre of the impingement plate with pressurized feed flow discharged from said open end of the second conduit.

[Claim 14] The system of claim 13 wherein the pressurized recycled flow discharged from the discharge aperture experiences a pressure drop within said adjustable gap so that air bubbles are formed therein.

[Claim 1 5] The system of claim 14 wherein the flow of pressurized feed containing first sized particular matter that is discharged out of the open end of the second conduit is in sufficiently close dynamic proximity to said air bubbles in the pressurized recycled flow discharged from the discharge aperture so that the first size of the particulate matter attaches to the air bubbles and floats to the top of said horizontal first zone the result being the formation of a float layer for later harvesting.

[Claim 16] The system of claim 15 wherein said biofilter comprises a plurality of dynamic biomass carriers disposed and circulating within the horizontal second zone and adapted to promote the growth of micro-organisms there upon.

[Claim 1 7] The system of claim 16 wherein each biomass carrier of said plurality of dynamic biomass carriers are separated by an intra-carrier flow channel having a first size.
Page 24 of 30 [Claim 181 The system of claim 17 wherein each biomass carrier of the plurality of biomass carriers has a specific gravity greater than one and comprises a plurality of surfaces disposed a predetermined distance apart for optimized growth of biomass and wherein said plurality of surfaces are arranged to create a plurality of inter-carrier flow channels.

[Claim 191 The system of claim 18 wherein cleansing means comprises means for isolating the biofilter, means for introducing air into the biofilter to agitate the biomass carriers so that excessive biomass is shed; and, means for transporting said shed biomass to the first horizontal zone.

[Claim 20] The system of claim 19 further comprising a final filter located on the system discharge line.

Page 25 of 30

Claims (20)

1. [Claim 1] A system for improved dissolved air floatation (DAF) treatment of wastewater wherein said system comprises: a controlled source of continuous wastewater flow containing unsettled secondary effluent containing a first size and a second size of particulate matter wherein said first size is larger than said second size; a tank having a first predetermined volume for receiving and containing a second predetermined volume of said wastewater for treatment and discharging a post-treatment stream; means for capturing the first size of the particulate matter for disposal comprising a DAF nozzle;
   means for capturing the second size of the particulate matter for disposal; and cleansing means.
2. [Claim 2] The system of claim 1 wherein said means for capturing the second size of particulate matter is a biofilter and wherein said cleansing means is adapted to periodically cleanse said biofilter.
3. [Claim 3] The system of claim 2 wherein the second predetermined volume of the wastewater in the tank is determined by a weir, wherein said weir receives said post-treatment stream and discharges the post-treatment stream into a system discharge line.
4. [Claim 4] The system of claim 3 wherein the tank further comprises a horizontal first zone comprising said means for aerating the wastewater, a horizontal second zone comprising the biofilter disposed below said horizontal first zone, and a horizontal third zone disposed below said horizontal second zone wherein said first, second and third zones are in hydraulic communication.
5. [Claim 5] The system of claim 4 wherein the horizontal first zone and the horizontal second zone are separated by a first screen and wherein the horizontal second zone and the horizontal third zone are separated by a second screen.
6. Claim 6] The system of claim 5 wherein the horizontal second and third zones are divided into at least first and second vertical and adjacent compartments so that said at least first and second vertical and adjacent compartments are non-communicative and operate independently.
7. [Claim 7] The system of claim 6 wherein the second horizontal zone of the at least first and second vertical compartments is divided into a primary upper layer and a secondary lower layer, said primary upper and secondary lower layers separated by a wastewater recycle withdrawal manifold disposed within the second horizontal zone and wherein the third zone of the at least first and second compartments comprises a post-treatment stream discharge manifold disposed above a secondary recycle flow manifold.
8. [Claim 8] The system of claim 7 wherein said DAF nozzle comprises an aerating nozzle assembly disposed centrally within the first horizontal zone and above the second horizontal zone.
9. [Claim 9] The system of claim 8 wherein said aerating nozzle assembly communicates with a first conduit containing a supply of pressurized recycled flow drawn from said wastewater recycle withdrawal manifold and wherein said supply of pressurized recycle flow is in communication with a cavitating air injector for entraining dissolved air into said supply of pressurized recycle flow.
10. [Claim 10] The system of claim 9 wherein said first conduit is sized to promote air dissolution within the supply of pressurized recycled flow and to achieve a "froth flow" mass transfer regime and wherein the first conduit terminates at the bottom end of a third vertical conduit and further wherein said third vertical conduit top end comprises a discharge nozzle having a discharge aperture.
11. [Claim 11] The system of claim 10 wherein the aerating nozzle assembly communicates with a second conduit in communication with a supply of pressurized feed flow containing particulate matter of a first size and wherein said second conduit comprises a horizontal section terminating in a vertical section, wherein said vertical section has an inside wall, a closed first end and an open second end.
12. [Claim 12] The system of claim 11 wherein the third vertical conduit is disposed within the vertical section of the second conduit thereby forming an annulus.
13. [Claim 13] The system of claim 12 further comprising an impingement plate disposed an adjustable distance above said discharge aperture thereby forming an adjustable gap between the discharge aperture and said impingement plate so that the pressurized recycled flow discharged from the discharge aperture impinges upon the centre of the impingement plate with pressurized feed flow discharged from said open end of the second conduit.
14. [Claim 14] The system of claim 13 wherein the pressurized recycled flow discharged from the discharge aperture experiences a pressure drop within said adjustable gap so that air bubbles are formed therein.
15. [Claim 15] The system of claim 14 wherein the flow of pressurized feed containing first sized particular matter that is discharged out of the open end of the second conduit is in sufficiently close dynamic proximity to said air bubbles in the pressurized recycled flow discharged from the discharge aperture so that the first size of the particulate matter attaches to the air bubbles and floats to the top of said horizontal first zone the result being the formation of a float layer for later harvesting.
16. [Claim 16] The system of claim 15 wherein said biofilter comprises a plurality of dynamic biomass carriers disposed and circulating within the horizontal second zone and adapted to promote the growth of micro-organisms there upon.
17. [Claim 17] The system of claim 16 wherein each biomass carrier of said plurality of dynamic biomass carriers are separated by an intra-carrier flow channel having a first size.
18. [Claim 18] The system of claim 17 wherein each biomass carrier of the plurality of biomass carriers has a specific gravity greater than one and comprises a plurality of surfaces disposed a predetermined distance apart for optimized growth of biomass and wherein said plurality of surfaces are arranged to create a plurality of inter-carrier flow channels.
19. [Claim 19] The system of claim 18 wherein cleansing means comprises means for isolating the biofilter, means for introducing air into the biofilter to agitate the biomass carriers so that excessive biomass is shed; and, means for transporting said shed biomass to the first horizontal zone.
20. [Claim 20] The system of claim 19 further comprising a final filter located on the system discharge line.