WO1997035654A1

WO1997035654A1 - Wastewater treatment system

Info

Publication number: WO1997035654A1
Application number: PCT/CA1997/000193
Authority: WO
Inventors: Dietmar J. Berger; Robert Dash; Pei Kuang
Original assignee: Berger Dietmar J; Robert Dash; Pei Kuang
Priority date: 1996-03-25
Filing date: 1997-03-20
Publication date: 1997-10-02
Also published as: AU1919797A

Abstract

A sedimentation device for treating municipal and industrial wastewater, ore slurries, pulp and paper recausticizing slurries, flue gas scrubbing slurries, and coal refuse slurries having a mixing chamber, a down-flow zone and an up-flowing clarification zone. The input and output of the mixing chamber being controlled so as to require short residence time in the mixing chamber of the influent being treated.

Description

WASTEWATER TREATMENT SYSTEM

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

The present invention generally relates to gravitational separation by sedimentation and, more particularly, to systems for continuously separating suspended solid materials from a feed stream by gravity settling.

STATE OF THE ART

Sedimentation devices which incorporate settling tanks are well known to separate suspended solids from streams of liquid, such as water and wastewater, by gravity settling. To increase the effectiveness of the liquid-solid separation, it is well known to employ various chemical flocculating agents (e.g., polyelectrolyte polymers), coagulating agent (e.g., mineral salts). When mixed with the influent, the agents combine with suspended solids to form rapidly settlable aggregates, called floe. It is also known that settling of floe particles can be enhanced in certain circumstances by mixing the flocculating agents with inert particles such as sand. Typically, the mixing of flocculating agents and inert particles with the influent is accomplished outside the sedimentation (or settling) tank proper, say in a pipe or mixing chamber and, may be accompanied by mechanical stirring of the mixture to provide contact opportunity and time for the resulting floes to grow.

SUMMARY OF THE INVENTION

The present invention, in very general terms, provides a sedimentation device for treating water and wastewater having a mixing chamber, a down flow zone and an up-flowing clarification zone. The input and output of the mixing chamber being controlled so as to require short residence time in the mixing chamber of the influent being treated.

More particularly, the present invention provides an improved sedimentation device for treating mineral slurries, industrial wastes and sewage, and the like, where the sedimentation device requires very short residence times in the mixing chamber while still providing good clarity in the clarified effluent. Influents which the sedimentation machine of the invention is intended to treat include, for example, ore slurries, pulp and paper recausticizing slurries, flue gas scrubbing slurries, coal refuse slurries, and municipal and industrial wastewaters.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other aspects of the present invention can be readily ascertained from the following detailed description and appended drawings, which are offered by way of example only and not in limitation of the invention, the scope of which is defined by the appended claims and equivalents. In the drawing:

Figure 1 is a cross-sectional view of a sedimentation device in accordance with one embodiment of the present invention, parts of which are shown schematically;

Figure 2 is a cross-sectional view of a sedimentation device in accordance with another embodiment of the present invention, again with parts shown schematically;

Figure 3 is a graph showing TSS of effluent versus fiocculation time for sewage treatment using 4 mg/1 A13+;

Figure 4 is a graph showing TSS of treated waste water versus fiocculation time for a solids contact chamber residence time of 34 seconds;

Figure 5 is a table showing test results from the continuous operation of a three meter diameter unit; and

Figure 6 is a graph showing TSS of effluent versus fiocculation time for variation of TSS reading with mixing time at different ferric dosage rates. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The sedimentation devices of each of the embodiments shown in Figures 1 and 2 include a settling tank or main vessel 2 in which separation by sedimentation principally occurs utilizing ballasted fiocculation. Preferably, the settling tank 2 is cylindrical in configuration, but a rectangular or other shaped configuration can be used. In the embodiment illustrated in Figure 1 , vessel 2 is defined by a sidewall 1 1 and a bottom wall 13. Preferably, bottom wall 13 slopes downward at an angle of about 50° to about 70° toward a collection cone 15 formed centrally in the bottom wall 13 of the tank 2. As illustrated in Figure 2, for bottom walls that have a slope of about 0° to about 15°, a raking mechanism 37 is provided to prevent any sludge from building up on the bottom wall 13 as the floes settle in the settling tank 2, as will be described in more detail below.

As further shown in Figures 1 and 2, a launder 17 is mounted in the sidewall 11 of the vessel 2. The launder includes an outlet pipe 19 and an overflow weir 60, which defines the liquid level 29 in the tank 2. Centrally disposed in the tank is a generally cylindrical vessel 23 which for purposes of identification herein will be referred to as a down-flow vessel. The down-flow vessel 23 can be supported from the bridge 24 by hanger 26 as shown (or other trusswork which traverses the tank) or can be supported by extensions (not shown) extending inwardly from the inner walls of the settling tank 2. (Bridge 24 can be provided with handrail 20 and lifting lugs 23.) In operation of the sedimentation system, the down-flow vessel 23 serves to partition the settling floes and treated water from the clarified effluent rising along the sides of the settling tank. Attached to the lower portion 21 of the down-flow vessel 23 at flange connection 38 is truncated cone 40. Although cone 40 is not necessary in all circumstances, it assists in bringing the settled floes to the center of the settling tank 2 for more efficient collection by collection cone 15.

Also supported from bridge 24 is solids contact or mixing chamber 14. Preferably, solids contact chamber 14 is supported by adjustable hangers (not shown) and comprises an upwardly or downwardly telescoping section that allows the volume of the solids contact chamber 14 to be controllably varied depending on the operating needs. It can now be appreciated that the outer surface 30 of chamber 14 and the inner surface 32 of down-flow vessel 23 define a down-flow zone 34; the function and operation of the down-flow zone 34 will be described in more detail below.

In the solids contact chamber 14, baffle plates/vortex breakers 36 can optionally be mounted. The baffle plates 36 are vertically mounted at four spaced-apart positions within the solids contact chamber 14 and extend radially across the vessel for 1/12 to 1/10 the diameter of the solids contact chamber. In operation, the baffles prevent disruptive vortexes or swirls from being established in the solids contact chamber that might break up the floes or entrap air. Typically, four baffles are provided, but the number and configuration of the plates is a matter of design choice. As still further shown in Figures 1 and 2, at least one inlet feed 43 is provided in fluid communication with the interior of the solids contact chamber 14. By way of inlet feed 43, a stream of influent is fed in to the interior of chamber 14 through a distributor 44. In operation of the system, the distributor 44 is sized relative to the influent flow rate to distribute the influent evenly in the solids contact chamber. Adjacent the outlet of distributor 44 is mounted a mixing impeller 47. The impeller 47 is coupled to the drive unit 25 through shaft 7 or to an auxiliary rotary drive (not shown). Also located near the impeller 47 in the solids contact chamber 14 is an outlet 16 which, as will be explained in greater detail below, injects recycled inert particles into the chamber from a hydrocylcone underflow. Still further, a polymer or flocculating agent injection outlet 48 is located near the impeller 47 in the solids contact chamber 14. In practice, it is particularly advantageous to introduce the inert particles and flocculant as close together as possible so that the flocculant and inert particles come together quickly. The inert particles employed in the system are typically 150 micrometers in diameter or less, but should be sufficiently large to maintain a relatively high settling velocity in the down flow zone after floes have formed around them. The inert particles also aid in taking advantage of the differential sedimentation phenomena based on the heterogenous curvilinear fiocculation theory.

In practice, impeller 47 is rotated at a speed such to provide just enough agitation to produce sufficient mixing of the suspended solids with the influent, inert particles and flocculant to promote the formation of floes without inducing shear that would break the formed floes apart. Further in operation of the system, the impeller 47 is designed and operated to impart just enough turbulence to maintain the solids in suspension with as little shear as possible as floes are forming. The mixing in mixing chamber 14 advantageously utilizes the differential sedimentation phenomena for floe production with the addition of the inert particles. Thus, the floes and treated water that overflow the top edge 3 of the solids contact chamber 14 into the down flow zone are generally uniformly mixed.

At this juncture, it should be appreciated that the solids contact chamber 14 is relatively small; it is designed and sized to provide sufficiently short residence times and the hydraulic velocities aid in keeping the inert particles suspended. The impeller 47 is required for uniform mixing of the flocculant, influent and inert particles over the range of normal load operations.

As the floes and treated influent overflow the top edge 3 of the solids contact chamber 14, the fluid velocity accelerates from less than 70 meters/hour at arrow 49 to over 90 meters/hour at arrow 50. The velocities through the down flow zone 34 generally are dependent upon the relative diameters of the solids contact chamber 14 and the down-flow vessel 23, at least to a first-order approximation. In practice, the relative diameters of the solids contact chamber 14 and the down-flow vessel 23, are sized such that the down flow velocities are relatively high without creating shear in the liquid that would break up the floes. After the floes and treated influent pass below the bottom wall 27 of the solids contact chamber, the fluid flow velocity decreases at arrow 51 (to less than 50 meters/hour), accelerates in the space of truncated cone 40 at arrow 52 and then decelerates as the flow exits truncated cone 40 at arrow 53 (e.g., to less than 50 meters/hour). Outside of the down-flow vessel 23, the clarified effluent rises in clarification zone 5 and through the settling plates 4 (e.g., at a velocity about 20 meters/hour and higher) and, finally, overflows into the launder 17.

With regard to the embodiment shown in Figure 2, drive unit 25 of conventional construction is mounted on the bridge 24 to drive the impeller 47 and optional raking mechanism 37. (The rotation rates of the two devices can, of course, be different; typically, the impeller 47 rotates at a much faster rate than the raking mechanism.) In the illustrated embodiment, the raking mechanism 37 is of conventional construction and is mounted to rake settled solids across the bottom or floor 13 of the tank to the aforementioned collection cone 15 such that no sludge build up occurs on the bottom wall. For relatively small settling tanks (FIG. 1 ), a sloping bottom wall 13 of 60° or greater is used so that there is no sludge build up on the bottom wall and so that the sedimentation device has a smaller footprint. For larger settling tanks (FIG. 2), a sloping bottom wall 13 of any angle from 0° to 15° is used in combination with the raking mechanism. In the illustrated embodiment, an optional deflection cone 9 should be understood to be fixedly connected to the shaft 7 for rotation. Sludge scraper 3 attached to shaft 7 below bearing 4 rotates within collection cone 15 to prevent the build up of sludge in the cone.

From the collection cone 15, settled solids and inert particles are pumped to inlet 12 of separation device 6 (e.g., hydrocyclone) via conduit 39 by high shear recirculation pump 8. Pump 8 withdraws the settled solids and inert particles from the collection cone 15 at a sufficient rate to remove the inert particles and settled solids. In some operations, pump 8 prevents a build up of sludge in the bottom of the settling tank or in the collection cone. In operation, high shear pump 8 breaks the bonds between the sludge and inert particles to assist the separation device 6 in separating the inert particles, which are re-cycled into the solids contact chamber 14 through separating device underflow 16, and the sludge which is discharged through outlet 18 of separating device 6. A flush-out connection 10 can be provided in conduit 39.

Concomitant with the removal of thickened, settled solids via the collection cone 15, clarified effluent is removed from the liquid surface in the settling tank 2 via the launder 17 and launder outlet 19. Various suitable launder arrangements are well known in the sedimentation art.

In summary to this point, the interior of the settling tank 2 can be understood to comprise three zones: a zone for receiving and mixing the stream of influent, inert particles and flocculant; a down-flow zone with no mechanical agitation and/or no or very little shear for directing clarified liquid and floes to the bottom of the settling tank; and an up-flow zone in which clarified liquid is withdrawn. It should be understood that the received inert particles are recycled from the sediment (i.e., sludge) collected at the bottom of the clarification zone. And, as will be explained further below, the reintroduced (i.e., recycled) inert particles are normally provided from the separation device after being separated from the mixture of settled solids and inert particles that were previously removed from the settling vessel 2.

At this juncture, it can be appreciated that the above-described sedimentation device is a unique combination of a chemical reactor and clarifier within a single vessel. It is a highly compact unit that can be used in a liquid-solid separation process with excellent separation efficiency (i.e., high capacity) due to very short residence times in the solids contact chamber 14. The improved separation efficiency is due as much to the high settling rates in the clarification zone as it is to the short residence times in the solids contact chamber. These two principles work together. The inert particles increase the fiocculation rate resulting in a short residence time and a small mixing chamber. The high settling rate resulting from the use of the inert particles and polymer greatly reduces the necessary size of the clarification zone. One important feature of the process is the introduction of inert particles in the chemical treatment process which are recovered and recycled within the system. The inert particles, with high specific gravity, function as nuclei for binding with fine suspended solids by a long chain polyelectrolyte to produce large and dense floes. The floes formed in such a way settle rapidly and are easily separated from the liquid phase. The inert particles also work as a fiocculation aid and provide a large surface area that substantially increases the probability of particle collision, speeding the agglomeration of the floes and enhancing separation efficiency. As a direct consequence, the overall size of the clarifier is significantly reduced and the capital equipment costs are reduced. In the following, various process steps will be described employing the above-described sedimentation device.

PROCESS STEP 1

Pre-treatment of influent (e.g., to screen, adjust pH, add coagulant and to provide other chemical/physical conditioning) as required. In each wastewater/sewage treatment application the influent is analyzed and pretreatment is provided based on what is required to bring the influent into a normal processing range.

PROCESS STEP 2 The solids contact chamber 14 is used to bring the influent to be treated, the chemical flocculant, and the inert particles together in a controlled manner which results in a rapid, one step fiocculation of suspended solids which are then removed from the influent in the settling tank 2. The short residence time in the solids contact chamber, in the range of 10 to 240 seconds, results in much smaller equipment requirements to accomplish an equivalent level of wastewater purification. The reactions, both chemical and physical, which occur in the solids contact chamber 14 are controlled to provide adequate mixing and contact of the suspended solids with the flocculant to provide floes. The mixing is not so turbulent or long lasting so as to break up the delicate floes produced. Adequate but not excessive mixing and adequate but not excessive residence time are based on the influent characteristics and coagulation mechanisms.

Examples of the process variables are listed in the following table. All of the values are approximate and therefore may be greater or less for any particular operating process. The range provided for each of these variables allows for adjustment in the process.

Nominal Range

residence time in first mixing chamber 40 10 to 240

(seconds) inert particles added 10 5 to 100

( grams/liter of influent) flocculant added 2 0.1 to 10

(mg/liter of influent)

mixing characteristics

(DT, impeller 0.7 0.5 to 0.8 Nominal Range impeller tip speed (m sec) 2 0.5 to 6 velocity gradient, G (sec^*1) 120 100 to 1000

PROCESS STEP 3

The down-flow zone 34 in the process provides a transportation path free of mechanically induced turbulence and no or a minimum of shear for the floes and influent mixture into the settling tank. The operational parameters on this step are such that (a) no turbulence is induced and any shear which occurs does not destroy the floes and (b) that the path to the settling tank is short, downward and efficient. In the systems having a circular cross-section shown in Figures 1 and 2, this zone is around the outer surface 30 of the solids contact chamber 14 and is surrounded by the inner surface 32 of down-flow vessel 23. Down-flow vessel 23 prevents short circuiting of the fluids to the clarification zone 5. In a rectangular system (not shown) this zone is rectangular or circular. There is no particulate suspension or mixing required or induced in the down-flow zone. The treated influent and floes pass through the down-flow zone without further mechanical agitation.

PROCESS STEP 4

The configuration of the down-flow zone 34 insures that the mixed influent and particulate floes have been transported to the bottom of the settling tank 2 without substantial deleterious turbulence or shear. There is no turbulent flow in the region where the mixed influent is passed from the down-flow zone to the clarification zone 5. Typically, flow rates of less than 60 meters/hour are maintained in this transition region so that the particulate loaded floes are not destroyed. The clarification zone 5 can operate with or without separator or lamella plates 4 or tube settlers. In the system without lamella plates, a rise rate of 10 m/hr to 20 m hr can be used compared to rise rates of 1 to 2 rn/hr in conventional system clarifiers. The higher rise rates for this system compared to the conventional system result primarily from the use of the inert particles and polymer added to the influent. The use of lamella plates can be used to increase the capacity of the system. The upper part of the clarification zone 5 contains a collection system 17 for directing the clarified effluent to the system outlet 19. The sludge sedimented from the influent is collected at the bottom of the collection cone 15 and pumped to a separation device 6.

PROCESS STEP 5

The sludge/inert particle separation operation separates the inert particles from the sludge using a high shear recirculation pump 8 to transport the sludge mixture to the separating device 6. The inert particles are then recycled into the solids contact chamber 14 to be reused. The sludge is transported out of the system to be disposed of using known devices such as a belt press.

TESTS Tests of the above-described apparatus and process were carried out in the laboratory and the field.

FIGS. 3-6 illustrate the results of field tests conducted on sewage wastewater using various flocculant and types of agitation. In addition, FIG. 5 illustrates a long-term full-scale performance results of a 3 meter diameter sedimentation device. In FIGS. 3-6, TSS is the abbreviation for total suspended solids,

COD is the abbreviation for chemical oxygen demand, SCC is the abbreviation for solids contact chamber and RT is the abbreviation for residence time.

Modifications and variations of the present invention will be apparent to those having ordinary skill in the art having read the above teachings, and the present invention is thus limited only by the spirit and scope of the following claims.

Claims

What is Claimed is:

1. A process for influent liquid treatment by sedimentation, comprising: combining a flow of pretreated influent, flocculent and inert particles with agitation in a solids contact chamber to produce further-treated influent and floes; passing the further-treated influent and floes from the solids contact chamber into a down-flow zone; passing the further-treated influent and floes through the down-flow zone, without further mechanical agitation into a settling zone; and removing the further-treated influent from an upper region of the settling zone and settled floes and other settled materials from a lower region of the settling zone.

2. The process of Claim 1 further comprising: separating the settled floes into sludge and the inert particles; and recycling the inert particles into the solids contact chamber.

3. The process of Claim 1 wherein the pretreated influent, flocculant and inert particles are resident in the solids contact chamber for about 240 seconds or less.

4. The process of Claim 1 wherein the pretreated influent, flocculant and inert particles are resident in the solids contact chamber for a period of time in the range of about 10 to about 240 seconds.

5. The process of Claim 1 wherein the pretreated influent, flocculant and inert particles are subjected to just sufficient agitation in the solids contact chamber to produce sufficient mixing of the influent, flocculant and inert particles to form the flocculated mixture without inducing shear that breaks apart the flocculated mixture.

6. The process of Claim 1 wherein the pretreated influent, flocculant and inert particles are combined utilizing differential sedimentation for floe production.

7. Apparatus for influent treatment by sedimentation, comprising: a solids contact chamber having an inlet for pretreated influent, an inlet for flocculent and an inlet for inert particles to produce further-treated influent and floes; a settling zone below the solids contact chamber having an outlet for removing settled floes and other settled materials; a down-flow zone connecting the solids contact chamber to the settling zone such that the down-flow zone passes the further-treated influent and floes from the solids contact chamber into the settling zone without mechanical agitation; a clarification zone above the settling zone; and a separation device for separating the inert particles from the settled floes removed from the settling zone.

8. The apparatus of Claim 7 further comprising: an impeller located in the solids contact chamber for imparting just sufficient agitation in the solids contact chamber to produce sufficient mixing of the influent, flocculent and inert particles to form the floes without inducing shear that prevents the formation of the floes or breaks apart the floes.

9. The apparatus of Claim 7 wherein the inlets for the influent, inert particles and flocculent are in close proximity.

10. The apparatus of Claim 7 wherein the volume of the solids contact chamber can be varied by telescoping a portion of the solids contact chamber.

11. The apparatus of Claim 7 further comprising a recirculation system for recirculating the separated inert particles into the solids contact chamber.

12. The apparatus of Claim 7 wherein the separation device comprises a high shear pump.