GB2025920A - Process and apparatus for the biological treatment of waste water - Google Patents

Abstract

In a process for the biological treatment of waste water by causing the waste water for flow upwardly through, and fluidise, a bed of particulate material, the waste water is introduced into the particulate material through orifices arranged at a multiplicity of locations across the bed of particulate material, the waste water at at least some of said locations being constrained to flow downwardly from the orifices at those locations prior to rising upwardly in the particulate material.

Description

SPECIFICATION Process and apparatus for the biological treatment of waste water This invention relates to a process and an apparatus for the biological treatment of waste water using the fluidised bed concept. By the term "waste water" I mean water containing biologically decomposable pollutants which may be of organic and/or inorganic nature.

Fluidised beds have previously been employed on a limited scale for the treatment of waste water. The process was first described by N. J.

Pugh in articles entitled "The Treatment of Doubtful Waters for Public Water Supply" Parts II and Ill in the Journal of the Instutution of Water Engineers 1949, 3, page 123 and 1957, 11, page 29, respectively, this work being primarily aimed at the removal of ammonia from river water using a fluidised bed formed from the natural accumulation of river silt which serves as the nucleus for a biological sludge blanket. Later work by C. S.Short, referred to in articles entitled "Removal of Ammonia from River Water" in Publication TP 101 of the Water Research Association, dated July 1973, and in Technical Report TR3 of the Water Research Centre, dated March 1975, employed graded sand (50 400 microns) at upflow rates of between 10 and 25 m/hour, but further work by the Water Research Centre for the nitrification (i.e. the biological degradation of ammonia first to nitrite and then to nitrate) and de-nitrification (i.e. the biological conversion of nitrate to gaseous nitrogen) of river waters used coarser sand (200~600 microns) at upflow rates of between 20 and 35 m/hour.The ammonia concentration in the feeds being generally of low order ( < 2 mg/1), in all the nitrification experiments no supplementary oxygenation was added since the naturaldissolved oxygen concentration in the feed water was sufficient to satisfy the oxygen requirements of the nitrification reaction. However, in the second of the two articles by C. S. Short referred to above, the author did suggest possible benefits of pre-oxygenation using pure oxygen and reported on brief experiments he had conducted on this basis.

Work by J. S. Jeris on the treatment of waste water using biomass attached to a solid particulate carrier in fluidised form is described in British Patent Specifications Nos. 1,430,410 and 1,433,582 and in U.S. Patents Nos. 3,846,289, 3,956,129, 4,009,098,4,009,099 and 4,009,105, although the principles involved had previously been observed during waste water treatment by activated carbon (see the article by W. J. Weber, Jr. and J. C. Morris entitled "Kinetics of Adsorption in Columns of Fluidised Media" in the Journal of American Water Works Association Volume 443 (1965)). Furthermore, in an article by D. A.Okun entitled "System for Bio-precipitation of Organic Matterfrom Sewage" in Sewage Works Journal 21, 763 (1949), there is suggested a process which involved the introduction of pure oxygen oxygenated waste water through a fluidised activated sludge blanket, which process was later operated by W. E. Budd and G. F.

Lambeth (see their article entitled "High purity Oxygen in Biological Sewage Treatment" in Sewage and Industrial Water, 29, 237 (1957)). A modification of this process is described in U.S.

Patent No. 3804255.

The work by J. S. Jeris relates principally to the carbonaceous oxidation, nitrification and denitrification of waste water by sequential fluidised beds, although in a paper entitled "Biological Fluidised Bed Treatment~BOD and Nitrogen Removal in less then One Hour Biological Treatment Time" presented by J. S. Jeris, R. W.

Owens, R. Hickey and F. Flood at the Annual Conference of the Water Pollution Control Federation in October 1977 (see the Journal of the Water Pollution Control Federation 1977, No.

49, p. 816) there was proposed a three stage system for carbonaceous and nitrogen removal involving three beds-anaerobic, aerobic and anaerobic denitrification, with a recycle from the aerobic bed to the first anerobic bed, and in which process, it is assumed, carbonaceous oxidation and nitrification were expected to occur concurrently in the aerobic bed. However, due to the high removal of nitrate and carbonaceous matter in the first stage anoxic bed, it is questionable whether a secondary anaerobic stage is necessary to achieve adequate removal of nitrogen and Biochemical Oxygen Demand (BOD).

The effectiveness of removing pollutants from waste water in fluidised beds is primarily related to the concentration of viable biomass that can be retained within the bed and to the contact time that is provided between this biomass and the water to be treated. Thus, for a given biomass concentration, there is a minimum contact period relative to a particular pollutant concentration and to the amount of pollutant requiring removal.

Having determined the optimum contact time for the particular pollutant and removal efficiency required, there is then a choice of bed geometry which at the extremes could be either a shallow bed with a large water surface area or a deep bed of small surface area. The hydraulic head loss through a fluidised bed depends upon the particle density and size, the solid consistency, and the depth of the bed, and may typically vary between 0.25 to 0.60 m/m of bed depth. The selection of a shallow bed would obviously reduce the amount of pumping energy expended during purification in the fluidized bed and this arrangement would therefore normally be preferred.

However, difficulties have been experienced by most workers in providing a simple bed inlet arrangement enabling operation of large surface area units without consequent high inlet pressure losses, problems of inlet blockage and economic bed construction costs. High inlet head losses necessitate the use of pumps of higher head capabilities than required for the normal operation of the bed, this being necessary, due to blockage of the feed pipework, to re-fluidise the bed if for any reason the flow to the unit is discontinued.

Arrangements involving distribution through perforated plates and inlet ports or through a base support media, for example gravel, are generally unsuitable for distribution of flow in a bed of a large surface area due to blockages on cessation of flow. Media distribution systems are particularly unsuitable for waste water treatment due to growth of biomass and entrainment of solids in the media resulting in pressure build over the distribition arrangement and eventually blockage.

Another arrangement which has been used, consisting of a vertical open-ended pipe feeding into the apex of a cone, also suffers from high inlet head loss (in excess of 0.5 metre) and additionally reduces the effectiveness of the bed, since a high shear force is generated at the inlet causing severe attrition of the solid particulate carrier, which effectively eliminates any appreciable biomass growth in the lowermost 1 metre or so of the fluidised bed.

The present invention aims to provide a process and apparatus for the biological treatment of waste water, using the fluidised bed concept, in which the method employed for introducing the waste water into the fluidised bed does not involve the disadvantages mentioned above.

According to one aspect of the invention a process for the biological treatment of waste water by causing the waste water to flow upwardly through, and fluidise, a bed of particulate material, is characterised in that the waste water is introduced into the particulate material through orifices arranged at a multiplicity of locations across the bed of particulate material, the waste water at at least some, and preferably all, of said locations being constrained to flow downwardly from the orifices at those locations prior to rising upwardly in the particulate material.

According to another aspect of the invention, apparatus for the bioligical treatment of waste water by causing the waste water to flow upwardly through, and fluidise a bed of particulate material, comprising means for introducing the waste water into the particulate material through orifices arranged at a multiplicity of locations across the bed of particulate material, the orifices at at least some of said locations being arranged so that the waste water issuing therefrom is constrained to flow downwardly prior to rising upwardly in the particulate material.

In one embodiment of apparatus in accordance with the invention, waste water is introduced into the bed of particulate material from at least one pipe provided with a plurality of orifices facing in a downward direction. This pipe may be provided with at least one shrouding member which ensures that waste water issuing downwardly from the orifices passes into a zone which is comparatively free from the particulate material of the fluidisable bed.

One particularly advantageous embodiment of apparatus in accordance with the invention, comprises means for re-cycling water from a first region at or adjacent to the upper surface of the fluidised bed to a second region of the fluidised bed situated at a lower level than said first region.

The means for introducing waste water into the particulate material may then be arranged to introduce the waste water at a level intermediate said first and second regions. In such an apparatus it is possible to create a fluidised bed of particulate material having an aerobic nature in one part and an anaerobic nature in another part. Such an apparatus can be employed for contemporaneous nitrification and de-nitrification of waste water as well as removal of carbonaceous matter. The apparatus may also be provided with an oxygenating means to satisfy the oxygen requirement of the waste water.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 is a schematic sectional elevation, taken on the line I-I of Figure 2, of a first embodiment of apparatus for carrying out the process of the invention, Figure 2 is a partly sectioned plan of the apparatus of Figure 1, and Figures 3 and 4 are sectional views, on an enlarged scale, of alternative embodiments of part of the apparatus of Figures 1 and 2.

The apparatus shown in Figures 1 and 2 comprises a tank 1 with a closed base. The tank shown is of right-circular cylindrical shape, but it may, of course, be of other cylindrical or polygonal shape. The waste water to be treated is introduced through a pipe 2 into a vertical tube 3 of circular cross-section, from which the water flows into a plurality of radially-disposed, horizontal, perforated pipes 4. In the example shown there are six pipes 4 disposed approximately mid-way between the top and bottom of the tank 1 and arranged at equi-angular intervals around the vertical axis of the tank. Each of the pipes 4 has perforations 5 at intervals through its length, along its underside, and is provided with vertically disposed shrouding plates 6 so that water issuing from the perforations 5 first flows downwardly before rising inside the tank 1, as indicated by the arrows A in Figure 1. Two possible cross-sectional shapes of the pipes 4, with their perforations 5 and shrouding plates 6, are shown in Figures 3 and 4.

In the upper part of the tank 1 there are twelve vertical, radially-disposed partitions 7, each extending from the tube 3 to the wall of the tank 1. Each of these partitions has its lower edge 8 disposed below the level of the pipes 4 and it extends upwardly to the top of the tank 1. The partitions 7 define six sector-shaped spaces 20 and six sector-shaped spaces 21 in the upper part of the tank 1, the spacing 20 alternating with the spaces 21 and the spaces 20 being of larger volume than the spaces 21. Each of the pipes 4 is disposed in a different one of the spaces 20.

Water flowing from the pipes 4 rises in the spaces 20 and enters a submerged effluent launder consisting of six radially-disposed, horizontal, perforated pipes 9, which are connected at their radially-inner ends to a vertical tube 10 within the tube 3. Each pipe 9 is disposed directly above a respective pipe 4 in each of the spaces 20.

Within the tube 10 there is an axial flow pump 11, driven by a motor 12 (shown only in Figure 1) and a shaft 13, which pumps the water entering tube 10 in the downward direction through a constriction 14 into a downflow bubble contact aerator 15. Oxygen or air is fed into the aerator 15 via a pipe-line 16, and the aerator provides a sufficient contact time for the required oxygen input into the water. The water exits from the aerator 15 through a plurality of radially disposed, horizontal, perforated pipes 17.

In the example shown, there are twelve pipes 17 disposed near the bottom of the tank 1 and arranged at equi-angular intervals around the vertical axis of the tank, alternate pipes 1 7 being disposed directly below a respective one of the pipes 4. Each pipe 17 has perforations 18 at intervals throughout its length, along its underside, and is provided with vertically disposed shrouding plates 1 9 similar to the shrouding plates 6 of the pipes 4. As a result, water issuing from the perforations 1 8 first flows downwardly before rising in the tank 1, as indicated by the arrows B in Figure 1. The pipes 17 may have the same cross-sectional shape as the pipes 4.

Water flowing from the pipes 1 7 rises in the tank 1 and divides into a plurality of separate flows when it reaches the lower edges 8 of the partitions 7. Some of the rising water passes into the spaces 20 containing the pipes 4 and 9 so that this water mingles with the water issuing from the pipes 4 and is re-cycled through the pipes 9, the tube 10 and the aerator 1 5. The remainder of the water rising from the pipes 17 is received in the spaces 21. At its upper end, the tank 1 is surrounded by a launder 22 into which water can spill from the tops of the spaces 21 over weirs 23.

The rate of discharge into the launder 22 is substantially equal to the rate of inflow via pipe 2, and therefore the rate of flow of water through the spaces 21 will vary as the rate of inflow to the apparatus varies. Effluent entering the launder 22 is led from the apparatus via an outflow pipe 31.

In use of the apparatus shown in Figures 1 and 2, the tank 1 contains sand which is fluidised by the water flowing upwardly from the pipes 4 and 17. A sand particle size of from 50 to 600 microns is suitable for an upward velocity of the water of from 6 to 40 m/hour. The chain line 24 indicates the approximate level of the upper surface of the fluidised bed of sand in the spaces 20. The chain lines 24 and 25 indicate the range of levels of the surface of the sand in the spaces 21 when the sand in these spaces is fluidised, the degree of fluidisation depending upon the rate of inflow of waste water via the pipe 2.When there is no inflow via the pipe 2, the level of the sand surface in the spaces 21 drops to the lower edges of the partitions 7, The sand below the partitions 7 is the level of the upper surface of the sand when the pumped flow is stopped and there is no inflow of waste water via the pipe 2.

When a new apparatus is first put into operation for purifying waste water microorganisms gradually become established on the surface of the sand grains and are responsible for the removal of the pollutants in the water. This "seeding" operation can, however, be accelerated by the addition of sand from an established (mature) bed, or by addition of suitable biological sludges containing in particular populations of the nitrofying bacteria Nitrosomonas and Nitrobacter, viz. Activated/humus sludges from a waste water treatment plant.

In a typical application of the apparatus shown in Figure 1 and 2, namely removal of carbonaceous matter, BOD and nitrogen from waste water, the rate of injection of oxygen via the line 16 would be regulated to maintain a dissolved oxygen concentration at or near the lowe edges 8 of the partitions 7 of less than 0.5 mg/1. The primary biological fluidised bed created in the spaces 20 above the lower edges 8 of the partitions 7 by the combined flow of incoming waste water and re-cycled flow would therefore operate under anoxic conditions. The lower or secondary biological fluidised bed, occupying the full cross-section of the tank 1 below the partitions 7, operating under aerobic conditions, would remove carbonaceous BOD and ammonia from the re-cycled flow, converting the ammonia by nitrification to nitrite and nitrate.Heterotrophic microorganisms in the primary bed, in the absence of dissolved oxygen, utilise the oxygen available from the nitrate in the recycled flow to reduce the BOD concentration of the waste water and convert the nitrate to gaseous nitrogen which bubbles through the primary bed and evolves at the water surface 27. In addition, anaerobic degradation of carbonaceous matter takes place in the spaces 20, resulting in a further reduction in the waste water BOD prior to it being re-cycled through the lower bed.

The water leaving the apparatus as effluent is displaced from the tank 1 at the same rate of flow as the instantaneous rate of inflow of waste water into the apparatus via pipe 2, and passes upwards through an anoxic tertiary bed in the spaces 21 to discharge over the weirs 23 into the effluent launder 22. An additional amount of BOD and nitrate may also be removed in this tertiary bed, the degree of this latter activity depending upon the BOD content of the waste water at entry to the spaces 21. Such removal of BOD and nitratescan be enhanced, if required, by injection of a suitable carbon source, for example methanol, acetic acid or waste water, within the tertiary bed.

Alternatively, overall BOD and-or ammonia removal can be increased by oxygenation within or to the tertiary fluidised bed. Improved effluent dissolved oxygen concentration can also be accomplished by this means, mug/1) of low alkalinity ( < 200 mug/1 CaCO3) usually suffers inhibition due to acidification caused by the accumulation of carbon dioxide in the waste water which also reduces the transfer efficiency of oxygen into the water during oxygenation. Acidification is particularly critical to nitrification, and, if not corrected, can at pH 6.5 result in a failure to nitrify.For example, recirculation aquaculture systems are particularly susceptible to carbon dioxide accumulation, in cases where biological water treatment processes are employed, due mainly to the high level of reuse employed. In such cases pit correction to pH 7-9 is vital. Low submergence air aeration has previously been employed for removal of CO2 in pure oxygen activated sludge systems designed principally for BOD D removal only. However, for treatment of waste water by fluidised beds when nitrification is required, pH correction by chemical addition is usually necessary.Such pH correction may be effected in the apparatus of Figures 1 and 2 by using for the lowest portion of the secondary fluidized bed a fluidisable particulate material with alkaline properties, for example a reconstituted dolomitic material, such as that known under the Trade Mark "Akdolit", or a dolomitic sand. In this case it may be necessary to vary the geometry of the lower part of the tank 1 to provide the upward velocities necessary to fluidise this material. A possible alternative shape is shown by the broken lines 28 in Figure 1. The reconstituted dolomitic material would be present up to about the level indicated by the chain line 29.

Alternatively, pH adjustment may be accomplished in the apparatus by shallow submergence air aeration carbon dioxide stripping in an additional chamber indicated by the dotted line 30 in Figure 1.

In the apparatus described above with reference to the drawings, each of the perforations 18 of the pipes 1 7 serves as an inlet for waste water to the fluidised bed of sand in the tank 1, with the result that waste water is introduced to the fluidised bed fairly uniformly over the entire cross-sectional area of the lowermost part of the bed. In general I prefer to provide at least ten inlets for waste water per square metre of the cross-sectional area of the lowermost part of the fluidised bed, although satisfactoryfluidisation can be achieved with a smaller number, for example seven or eight inlets per square metre.

In a typical apparatus of the kind shown in the drawings, each of the pipes 17 may have a crosssectional area of about 175 cm2 and each of the perforations 18 may have a cross-sectional area of from 1.0 to 5.0 cm2, the perforations being spaced apart at a distance of from 10 to 20 cm along the pipes 1 7. The shrouding plates 1 9 of the pipes 1 7 preferably extend below the lowermost surface of the pipes a distance of from 0.5 to 1.0 times the maximum transverse dimension of the pipes, and the lower edges of the shrouding plates 19 are preferably disposed above the tank bottom at a distance of from 0.2 to 1.0 times the maximum transverse dimension of the pipes 1 7. The pipes 4 may be dimensioned similarly to the pipes 17.

Thus, the shrouding plates 6 of the pipes 4 (see Figures 3 and 4) may extend below the lower surfaces of the pipes 4 a distance x which is from 0.5 to 1.0 times the maximum transverse dimension dof the pipes.

Various modifications may be made to the apparatus described in detail with reference to the drawings. For example, in place of the pump 11 an external pump may be provided having an intake connected to the tube 10 just below the level of the water in the tube 10 and its output connected to the top of the aerator 1 5. Again, an aerator of a type other than the downflow bubble contact aerator may be provided, either inside or outside the tank 1. Furthermore, the use of the shrouded perforated pipes 4, 1 7 for introducing water into the fluidised bed in not essential. For example, the water may be introduced by a multiplicity of downwardly-directed nozzles distributed across the tank.

The same design of pumping, oxygenation and flow distribution arrangements to the fluidised bed can be provided without the submerged pipes 9 and re-cycle facility in cases when either only low pollutant removal efficiency is required or where the initial pollutant concentration is low. A typical example of this would be simultaneous carbonaceous oxidation and nitrification by fluidised bed treatment of aquaculture waste waters (BOD < 30 mug/1 and NH3 < 5 mug/1).

Similarly, it may or may not be necessary to introduce re-cycle into a multi-bed apparatus if the concentration and/or the required removal of the individual pollutants is low.

Also, where the strength of the waste water is particularly high normally demanding a high oxygen concentration and hence a high rate of recycle, the rate of re-cycle may be reduced by pressurising the whole of the apparatus. The normal maximum rate of oxygen input by the aerator 15 in a non-pressurised system is up to 40 mg/l per pass. For each atmosphere increase in pressure at the oxygen injection point into the aerator, the oxygen input capacity increases by a factor of 100 per cent. Therefore, by pressurising the apparatus say to 3 atmospheres, the input of oxygen to the waste water will be increased up to 120 mg/l per pass. Pressurisation can be achieved by covering the tank 1 with a pressure-tight cover.

It will then be necessary to pump the waste water into the apparatus (via pipe 2) at a pressure head equal to the required operating pressure of the apparatus. This pressure is retained by a nonreturn valve in the inlet pipe 2 and a pressure outlet valve at the effluent outlet. This method of operation will not increase the working pressure against the axial flow pump 11 and the whole apparatus is pressurised.

In operation of the process and apparatus of this invention it will be necessary to remove sludge from the apparatus either continuously or periodically. This may be achieved by removing, either continuously or periodically, a portion of the fluidised solids from the bed, introducing this into a shearing device to shear off the biomass from the sand, and then passing the sand/biomass mixture through a separating device, for example a vibrating screen, from which sludge at from 3 to 6 per cent dry solids content would be directed to waste and the sand returned to the fluidised bed.

The process and apparatus of the invention may be used for the treatment of most waste waters which are normally encountered by Water Authorities and Sewage Authorities and for the treatment of industrial waste waters. Normally, raw sewage would be subjected to a primary settlement process or fine screening before the waste water therefrom is treated by the process and apparatus of the invention. However, solids in the waste water, which are capable of passing through the water distribution system to the fluidised bed, can cause no harm. Such solids float up in the fluidised bed and are abraded by the particulate material of the bed.

Other aspects of the apparatus and process described above with reference to the drawings are claimed in my co-pending Application No. 79 filed on the 15th June 1979 and entitled "Process and apparatus for the biological treatment of waste water".

Claims (15)

1. A process for the biological treatment of waste water (as hereinbefore defined) by causing the waste water to flow upwardly through, and fluidise, a bed of particulate material, characterised in that the waste water is introduced into the particulate material through orifices arranged at a multiplicity of locations across the bed of particulate material, the waste water at at least some of said locations being constrained to flow downwardly from the orifices at those locations prior to rising upwardly in the particulate material.

2. A process according to claim 1, in which the particulate material has a particle size of from 50 to 600 microns.

3. A process according to claim 2, in which the waste water is caused to flow upwardly through the bed of particulate material at a velocity of from 6 to 40 m/hour.

4. Apparatus for the biological treatment of waste water (as hereinbefore defined) by causing the waste water to flow upwardly through, and fluidise, a bed of particulate material, the apparatus comprising means for introducing the waste water into the particulate material through orifices arranged at a multiplicity of locations across the bed of particulate material, the orifices at at least some of said locations being arranged so that the waste water issuing therefrom is constrained to flow downwardly prior to rising upwardly in the particulate material.

5. Apparatus according to claim 4, in which the means for introducing waste water into the bed of particulate material comprises at least one pipe provided with a plurality of orifices facing in a downward direction.

6. Apparatus according to claim 5, in which said pipe is provided with at least one shrouding member which ensures that waste water issuing downwardly from the orifices passes into a zone which is comparatively free from the particulate material of the fluidisable bed.

7. Apparatus according to claim 6, in which said shrouding member extends below the lower surface of the pipe a distance which is from 0.5 to

1.0 times the maximum transverse dimension of the pipe.

8. Apparatus according to claim 6 or 7 and comprising a tank with a substantially horizontal bottom, for the reception of said bed of particulate material, in which said pipe is disposed substantially horizontally above the tank bottom with the lowermost edge of said shrouding member disposed above the tank bottom at a distance of from 0.2 to 1.0 times the maximum transverse dimension of the pipe.

9. Apparatus according to claim 4, in which each of said orifices is provided by a downwardly directed nozzle.

10. Apparatus according to any of claims 4 to 9, in which there are at least seven of said orifices per square metre of the cross-sectional area of the bed of particulate material.

11. Apparatus according to any of claims 4 to 10, comprising means for recycling water from a first region at or adjacent to the upper surface of the fluidized bed to a second region of the fluidised bed situated at a lower level than said first region.

12. Apparatus according to claim 11, in which said means for introducing waste water into the particulate material is arranged to introduce the waste water at a level intermediate said first and second regions.

1 3. Apparatus according to any of claims 4 to 12, comprising an aerating or oxygenating means for supplying the oxygen requirement of the waste water being treated.

14. Apparatus according to claim 13, comprising means for establishing a superatmospheric pressure in the fluidised bed.

15. Apparatus according to claim 13 or 14 when dependent on claim 11 or 12, in which said aerating or oxygenating device is arranged tQ supply air or oxygen to the water recycled tosaid second region.