GB2272171A - Treatment of effluents - Google Patents

Abstract

In a non-biological treatment of effluent, such as raw sewage and liquid industrial waste, suitable for use in remote locations, solids are separated from the effluent by sedimentation followed by membrane filtration 14 to provide a filtrate which is acceptable for environmental discharge. The membranes may be formed of ceramics or polymers. The separation steps may be optimised to take place naturally, or may be assisted by the controlled addition of chemicals such as flocculating agents and the treatment may also include ultraviolet sterilisation to kill any bacteria in the final filtrate. The components of the apparatus are designed for use in locations where space is at a premium and may be demountable to enable assembly and disassembly in confined quarters, such as those on board a ship. A metal mesh filter 12, which may be a separate unit or located in sedimentation tank 3, is located upstream of membrane filter 14. A macerator may be included upstream of tank 3. <IMAGE>

Description

IMPROVEMENT IN OR RELATING TO THE TREATMENT OF EFFLUENTS This invention relates to the treatment of effluents, such as raw sewage and liquid industrial wastes, and more particularly, but not exclusively, is concerned with treating effluents arising in marine applications and remote locations where municipal sewerage systems do not exist.

Conventional biological treatment of sewage is well known. The process usually involves coarse separation of solids followed by aeration to reduce the Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) of the residual soluble and insoluble components of the liquid to within the legal limit before filtration and discharge to a watercourse.

It is an object of the present invention to provide a non-biological treatment of effluent in which an optimised settling and filtration system, including extremely fine filtration, is used to separate the solids content and leave an essentially clean liquid.

After sterilisation, to kill any bacteria present, the liquid may then be re-used as appropriate.

It has surprisingly been found that this object can be achieved by making use of membrane filtration at the filtration stage.

According to one aspect of the present invention there is provided a method of treating liquid effluent containing solids including the step of separating solids from the effluent wherein the separation step is carried out by membrane filtration.

According to another aspect of the present invention there is provided an apparatus for the treatment of liquid effluent containing solids which apparatus comprises; (i) clarifying means for the liquid effluent wherein solids settle out of the liquid (ii) means for separating the settled solids and liquid containing residual solids, and (iii) membrane filtration means for separating residual solids from the liquid.

If desired, means may be provided for disintegrating or macerating the liquid effluent prior to clarifying and/or means for sterilising the liquid from which the residual solids have been separated.

The non-biological separation steps may be optimised to take place naturally or may be assisted by the addition of chemicals, such as flocculating agents, or other additives specific to a given known contaminant, e.g. an industrial waste. In the case of specific contaminants, the additive may either neutralise the contaminant, or react with it to form an insoluble compound which can be removed with the other solids.

In the case where chemical processes are involved, the apparatus may additionally comprise: (a) means of measuring the volumetric flow rate or concentration of the supply of liquid effluent, or parts thereof; (b) means to add metered quantities of chemicals to the liquid effluent; and (c) means to mix said metered quantities of chemicals uniformly with the liquid effluent.

Optionally, the apparatus may include means automatically to control the operation of the processes taking place.

If desired, the apparatus may also include screening means after said clarifying means and before said membrane filtration means.

In a preferred form of the apparatus of the invention, the clarifying means, in which solids are caused to settle out of the supernatant liquid, is the only location where solid material, separated from the effluent, is discharged. This is in spite of there possibly being one, or more, filters in the clarified liquid treatment line.

Preferably, the sterilisation means includes one or more powerful ultraviolet light sources past which the filtered product liquid passes in thin films so that any bacteria, etc. present are killed very quickly.

In an embodiment, the apparatus also includes means to recirculate the liquid to be filtered to said membrane filtration means. The very high circulation of liquid at high pressure past the membrane filter is a key feature of a preferred aspect of the present invention and enables energy efficiency and a high filtrate throughput to be achieved. A bleed may be provided to the recirculation means so that a portion of the liquid being recirculated is returned either directly to said clarifying means or upstream of said clarifying means.

Preferably the apparatus includes means associated with said membrane filtration means to create a highly turbulent flow regime adjacent to the membrane of said membrane filtration means.

In one preferred embodiment, the membrane filtration means includes spaced membranes formed of ceramic material, preferably fine pore ceramic material, and the liquid to be filtered is squirted into the spaces between the membranes at high velocity.

The purpose of this is to create a highly turbulent condition, i.e. where Reynolds Numbers are well in excess of 3000. This allows the normal filtration process to occur but discourages the formation of filter cakes. The required degree of turbulence can be achieved by subjecting the liquid to a high degree of recirculation. A bleed back to the clarifying means or to upstream of it is provided to return a portion of the recycled liquid to minimise the build up of solids.

Suitable ceramic membranes for use in accordance with the present invention are composite membranes comprising a porous metal support (e.g. a woven metal mesh) carrying one or more porous inorganic films of sintered inorganic particles. Such membranes are described in European Patent Applications No.

89305214.2, No. 89305213.4, No. 90902811.0 and No.

90915166.4.

In a particular application of one embodiment of the invention, the liquid to be filtered is fed into one of a plurality of manifolds whence it is ejected at high velocity via a series of nozzles directed towards the filter membranes thus creating a first pattern of turbulence. After a period of time, the input liquid is re-directed to a second of said plurality of manifolds and thence ejected to create a second pattern of turbulence. As each pattern of turbulence has localised areas of slack flow where a filter cake may start to form, switching manifolds in this way results in varying patterns of turbulence so that any filter cake which starts to form in one pattern is washed away by a second, or subsequent, pattern of turbulence. In this way, the membranes are largely kept free of solids during normal operation.The solids thus washed away will be removed from the recirculating liquid via the bleed back to, or upstream of, the clarifying means.

To avoid risk of nozzle blockage, the velocities of flow through each plurality of nozzles may be varied between higher and lower values on an alternating basis. The, or each, plurality of nozzles may not be fixed, but be free to swing about an arc over the membrane surfaces.

In another embodiment, the membrane filtration means includes one or more membranes formed of polymeric material having ultra fine pores. In this case, the apparatus may include backflushing means since, unlike ceramic membranes, polymer membranes are flexible and conventional backflushing is acceptable.

Thus, in this case, the degree of turbulence created may be less and backflushing can be used periodically to remove the filter cake that has formed and to return it to the clarifying means.

In a preferred embodiment, the filtrate from the membrane filtration means is passed in thin films past one or more powerful sources of ultra violet light of a wavelength such that any bacteria, viruses, etc.

present are killed very rapidly.

Preferably, the individual components of the apparatus are located in close proximity to each other.

For example, a structural member(s) forming part of a first of said components may also form a part of a second of said components. In an embodiment suitable for use on board a ship, or other location where space is at a premium, the individual components of the apparatus are nested as closely together as possible to produce a module with minimum overall dimensions. In such an installation, for example, a plate may form a side of two adjacent tanks. In another example, one or more tanks may not necessarily be cuboid or cylindrical in form, but have angled or curved faces to give the maximum internal volume in an irregularly shaped space.

In a variation of the design, the components of the module may be demountable so that installation may be retrofitted in a limited space in an existing ship with all components being loaded through normal shipping routes i.e. without having to cut special access holes in the hull etc.

In an embodiment the method of the invention uses only the physical processes of sedimentation and filtration to separate the solid and liquid fractions into forms suitable for further use or disposal without contravening environmental legislation. If desired, however, the method may also employ chemical or physico-chemical processes.

In a preferred embodiment of the method, the sedimentation and filtration processes are optimised together so that a minimum of solid content leaves the sedimentation stage in the liquid phase and that which does is returned from the filtration stage(s) back to the sedimentation stage so that, essentially the solid fraction (albeit wet) is discharged only from the sedimentation stage and the liquid fraction is discharged only as filtrate from the filtration stage.

Preferably, the method includes the process of sterilisation of the liquid fraction. In this case, the sterilisation may be performed by exposing the filtrate, in thin films, to a powerful source of ultraviolet light. Alternatively, it may be performed by the addition of a suitable chemical e.g. sodium hypochlorite.

Advantageously, the sedimentation and filtration steps are operated together in such a way as to create synergy between the two processes.

Preferably, turbulent conditions are provided during the filtration so that solids in suspension are discouraged from settling on the filter surfaces.

Also, recirculation of the liquid to be filtered is advantageously effected, together with a bleed either directly to, or upstream of, said sedimentation stage.

These techniques are preferred where ceramic membranes, which are not amenable to regular pressure cycling or flexing, are used. Ideally, liquid to be filtered is squirted at high velocity over the membrane surfaces so that, while liquid passes through the membrane, the solids are discouraged from settling thereon. By varying the pattern of turbulence, e.g. velocity or position of entry, solids which have settled may be dislodged.

In another embodiment, the filtration conditions are such as to encourage the settling of solids onto the filter surfaces so that small particles may be caused to collect together and aggregate into larger ones which may subsequently be removed, e.g. by backflushing, and recycled to the sedimentation stage.

This procedure is preferred when the filtration means is a polymeric membrane. Such materials are flexible and amenable to backflushing so that a lower level of re-circulation, or none at all, may be acceptable.

Normal cross flow filtration is preferred in all aspects of the invention. In the event that re-circulation is used, a bleed back to the sedimentation stage is provided. In a preferred design, a positive displacement pump is used to pressurise the liquid to be filtered, so that the tendency to break up any partly aggregated materials is less than if a centrifugal pump were to be used.

In a preferred design a compact settling tank is used and the flow conditions are carefully controlled to minimise turbulence so that the effluent is of good quality prior to passing to the membrane filter. Here turbulent conditions are maintained by a high rate of re-circulation so that conditions do not favour the formation of a filter cake. A bleed from the re-cycle returns to the settling tank. Chemical flocculents may be added to the feed before entry to the settling tank, but these are not essential. Using this preferred design, the following results have been achievable in tests.

Nature of Feed Feed (mq/l) Permeate (mq/l) SS BOD SS BOD Domestic Sewage 232 80 3 2.5 (settled) Marine Sewage 103 122 1 12 (settled) Marine Sewage 175 84 3 5 (flocculated and settled) Marine Sewage 107 66 2 4 (flocculated and settled) Nature of Feed Feed (mcr/l) Permeate (mq/l) SS BOD SS BOD Undiluted, vacuum 1000 600 14 28 collected marine sewage (approx.

figures) (SS = Suspended Solids; BOD = Biological Oxygen Demand).

The reasons for the extremely good levels of purification above are not known. One explanation could be that BOD/COD is directly associated with the solid content and hence reducing one reduces the other.

Another explanation could be that charged ions collect on the membrane surfaces and act to prohibit the passage of similarly charged particles. It will be appreciated that some of the BOD/COD may be due to the oxidation requirements of dissolved substances, e.g.

the oxidation of alcohols to organic acids or of inorganic nitrites to nitrates These are the sort of polar ions which could be absorbed on to the membrane surfaces, or become caught in the tortuous pores in the membrane body and so block the passage of like-charged ions. This effect could be enhanced by an appropriate chemical pretreatment. Ions which are physically large, e.g. sulphates, or carrying multiple charges, e.g. heavy metals, are likely to be rejected rapidly by such a pre-treatment.

The present invention is particularly useful for treating vacuum collected sewage. In ships and aircraft, sewage is often transported to, and collected in, a holding tank under vacuum in the presence of small amounts of water only. Thus, the solids content is high and a high concentration of ammonia is produced which inhibits the conventional biological treatment of the sewage.

For a better understanding of the invention and to show how the same may be put into effect, reference will now be made, by way of example only, to the accompanying drawings, in which: Figure 1 diagrammatically illustrates various sewage treatment processes, Figure 2 is a block flow diagram of an effluent treatment process according to the invention, Figure 3 is a variation of the block flow diagram of the effluent treatment process shown in Figure 2, Figure 4 is a sectional plan view of the clarifying means shown in Figure 5 along the plane AA, Figure 5 is a developed sectional elevation of the clarifying means shown in Figure 4 along a vertical plane, Figure 6 is a sectional elevation of a filter including a filter element, Figure 7 is a diagram showing one method of cleaning the filter element of Figure 6, Figure 8 is a sectional elevation showing an ultrafine membrane filtration means, Figure 9 is a modification of a part of the block flow diagrams of Figures 2 and 3, Figure 10 is a diagrammatic representation of the flow processes between adjacent filter membranes in the ultrafine membrane filtration means of Figure 8, Figure 11 is a sectional plan view of the treatment module shown in Figure 12, and Figure 12 is a developed sectional elevation of the treatment module shown in Figure 11.

In order to put the current invention in context with other known sewage treatment processes and emphasise its novelty, reference is now made to Figure 1 which shows the development of sewage treatment processes. In A, a house 100 discharges raw sewage directly into a watercourse 200, eg. a river, lake or sea. This situation was the historical norm and is still prevalent in remote locations and in underdeveloped countries. The next development (B) was the use of a settling or septic tank 300, where the sewage was allowed to accumulate and the liquid allowed to discharge to the watercourse 200 or the whole contents of the tank were removed for discharge or treatment. In procedures A and B, the discharge water typically includes about 200 mg/l suspended solids (SS) and about 200 mg/l BOD.C shows the normal biological treatment with bacterial digestion and settling 400, followed by aeration of the separated liquid 500 and discharge to the watercourse 200. Here the discharge water typically includes s 50 mg/l SS and c 50 mg/l BOD (denoted herein as 50/50).

Subsequent attempts to further improve the purity of the water discharged from a sewage treatment plant involved the addition of a tertiary polishing stage 600, as shown in D. Here the water from the aeration process 500 is passed through a membrane but the output water quality barely improved from 50/50 and did not come near to reaching the target of < 20 mg/l SS and < 20 mg/l B.O.D. (denoted herein as 20/20). After separation 400 the solids 700 may be disposed of by burning, spreading on land or dumping at sea, where regulations allow.

In accordance with the present invention, the whole treatment uses chemical and physical processes only, i.e. it is non-biological. Consequently, it is particularly suited to marine applications including undiluted vacuum-collected sewage. In this case, as shown in E, the sewage from the passengers and crew of ship 900 is subjected to a solid separation first stage 800 optionally including dosing with appropriate chemical compounds, followed by membrane filtration 1000 to give a 20/20 standard of purity. The solid waste 700 may be stored temporarily in a holding tank, e.g. while in port and coastal waters and discharged into shore facilities (where available) or into the sea outside territorial waters where regulations permit.

In this description, the same reference numerals are used for the same component fulfilling the same role.

The basic principle of the effluent treatment system is shown in Figure 2. Raw sewage 1 enters the system, e.g. directly from a sewer, or from a holding tank etc. Though the term 'raw sewage' is used, this is intended to include, for example, domestic sewage, waste water, agricultural effluent, effluent from water borne vessels or offshore structures, rain water, rain water run off and liquid industrial effluents or any combination thereof. A coarse filter is not shown, but is normal in the inlet line to remove gross materials, e.g. plastic bags, pieces of woods, etc.

The sewage passes through a disintegrator (or macerator) 2 to a clarifier 3, where the solid content settles to the bottom as a dense sludge and the supernatant liquid 4 flows over a weir 80 (Fig.5) for further processing. The detailed features of clarifier 3 will be described later. Clarifier 3 is a key component of the process and is effectively where the whole of the solid content of the input sewage 1 is removed. Weir 80 controls the height of the liquid in clarifier 3 and also the flow rate through it; it is thus an important control feature of the process. A sludge pump 5 operates continuously, or intermittently, as required, to remove the sludge from clarifier 3 as it builds up and to transfer it to sludge tank 6.

It is well known in sedimentation technology that, the longer the time available for settling, the denser will be the sediment. A feature of the invention is a high throughput so that residence times must, of necessity, be short. Thus significant quantities of liquid will be present with the sludge removed from the clarifier 3 via pipe 7. As a longer residence time will be available in tank 6, further settling will occur and clear, supernatant liquid 8 may be removed via pump 9 and recycled 10 to clarifier 3 (Fig.2) or recycled 10A upstream of clarifier 3 (Fig.3). The use of a very fine mesh filter in sludge tank 6 over the outlet of pipe 8 improves the quality of the supernatant liquid recycled 10 to clarifier 3.A suitable filter material is a fine polyester mesh which can have pore sizes of as little as 10pom.

Supernatant liquid 4 from clarifier 3 is passed via pump 11 through a filter or screen 12 including a very fine metal mesh filter element 12A (Fig.6). A non-corrodible metal, such as stainless steel is preferred. A pore size in the mesh of 150-250 micrometers (pm) is preferred. This is small enough to collect most of the particles which fail to settle in clarifier 3, so that the liquid 13 passing the mesh filter element 12A is of high cleanliness.

The arrangement shown of a filter 12 downstream of the pump 11 is one possible arrangement (Fig. 2) and the reverse is equally possible, i.e. locating the filter 12 on the suction side of pump 11 (Fig. 3). The latter option has some advantages, e.g.

(a) Pressure differential across a clogged filter element will be less.

(b) Internal pressure inside the filter chamber will be less.

(c) Partly agglomerated particles will not be broken up by the pumping action before reaching the filter element.

The man skilled in the art will be familiar with the use of pairs of filter units in parallel, so that one may be cleaned while the other is on-stream, and including appropriate pipes and valving to alternate from one unit to the other, and back again, so as to allow each unit to be used and cleaned in rotation.

Though only a single mesh filter 12 is shown in the Figures, it is to be understood that pairs of filters 12 and all the accompanying pipes and valves may be used in the above way.

Filtrate 13 from mesh filter 12 then passes to a second, even finer, filter 14 (Figs.2 and 3). Filter 14 is a membrane filter and, in one preferred design, consists of a stack of ceramic membranes 15 (Figure 8).

The pore size here is so fine that almost all of the residual solid content will be rejected to leave a filtrate which will meet current and likely future BOD and solid content standards. The final stage in the treatment process is sterilisation 16 to remove bacteria, viruses, etc. which will pass membranes 15.

Sterilisation 16 may be by the addition of chemicals, e.g. sodium hypochlorite, but a preferred method is to pass the filtrate, in thin films, past a strong source of ultraviolet (W) light. From steriliser 16, the water, now of a re-usable standard, passes to a storage tank 17 for subsequent usage 18 as appropriate e.g.

washing, toilet flushing, boiler feed water, etc.

Sludge 19 from sludge tank 6 may be disposed of in one of a number of ways. One method is removal by a tanker and spreading on land to improve soil structure and fertility. If toxic industrial wastes are present, deep burial or incineration may be necessary. If the sewage treatment plant is on a ship, the sludge may be pumped overboard in areas where the regulations permit such dumping. If this option is not available to the captain, the sludge must be pumped ashore for subsequent spreading on land, or incineration.

Though not appropriate for shipboard use, where further pumping of the sludge will be required, there is an economic imperative for land-based plant to produce sludge with a minimum water content. One way in which this can be achieved is to place a filter bag inside tank 6 and discharge sludge 7 into the bag in the tank. The liquid content of the sludge will preferentially pass through the bag and can be discharged 8. When full, the bag may be removed and suspended over tank 6 to drain. A second bag can then be placed in tank 6 to allow the process to continue.

Bags may be allowed to drain for extended periods of time, e.g. days. Fine mesh polyester cloths are suitable materials for the bags.

Backflushing for cleaning the filters 12 and 14 can be effected, as shown in Fig.2, using product water drawn from tank 17 by pump 20 and circulated via pipe 21 and valves 22 or 23 to filters 12 or 14 respectively and thence, via outlets 24 or 25 and return pipe 26, to the clarifier 3. In order to improve the efficiency of the overall process, a high recycle is used in connection with ceramic filter 14. The recycle system is discussed below. In order to avoid contamination of membrane 15 of filter 14 during backflushing of filter 12, a non-return valve 27 is incorporated into return pipe 26.

Where the flow rate of sewage 1 admitted to the treatment plant can be controlled to a slow and steady rate, e.g. via a holding tank (not shown), the process as shown in Figure 2 is satisfactory. If however, the flow rate is high or irregular, the residence time in clarifier 3 may be inadequate to reach the standards of purity required. In such circumstances, settling times may be reduced by the addition of flocculants 28. In such cases, an additional flocculator tank 30 (Figure 3) may be added to provide extra time for the flocculation process to start prior to actual settling 3. In such cases, the re-cycled liquids in pipes 10 and 26 may re-enter the process upstream of flocculator tank 30, as shown by 10A and 26A.

A plurality of pipes 28 is shown to indicate that a range of chemical additives may be added. These additives may be specific to known impurities, e.g.

industrial wastes, and can be metered in pre-determined quantities when a monitoring means 31 detects the presence of one or more of a given list of impurities.

Monitoring means 31 measures the concentration of the impurity(ies) and signals 32 the rate of addition of specific additive(s) 28. The specific additive(s) can be chosen to interact with the impurity(ies) in such a way as to render them harmless and/or to convert them to a solid form so that they precipitate in clarifier 3.

An important feature of the process is the clarifier 3, which is designed to be a compact, high efficiency unit. Figures 4 and 5 show the clarifier 3 in a sectional plan and as a developed vertical section, respectively. Raw sewage 1 from disintegrator 2 enters mixer tank 29 via pipe 2A where it is mixed into a uniform slurry by an impeller 33 driven by a motor 34. Any additives 28 are also incorporated into the slurry. From mixer tank 29, the slurry flows 30A into flocculator tank 30, where small particles start to aggregate into larger ones. The slurry leaves flocculator tank 30 by downwards flow 3A into upper section 35 of the clarifier 3. Flows 30A and 3A are ideally controlled by weirs 36 and 37 which maintain minimum quantities of slurry in each of tanks 29 and 30 respectively; this is particularly useful if the rate of flow 1 is prone to variability.Weirs 36, 37 may have a straight edge or be notched.

Tanks 29 and 30 are merely indicative of one option available and may be replaced with a single tank. If desired, tank 29 may include a disintegrator and tank 30 may be the mixer tank.

The clarifier 3 consists of straight sided upper section 35 and a conical lower section 38. Conical section 38 is preferably of an inverted pyramidal form so that the four sides taper to a point. A panel 39 divides the majority of clarifier 3 into two separate zones. As shown in Figure 5, panel 39 provides complete separation in the upper section 35 and extends down part of the way into conical section 38. Clarifier 3 is thus divided up vertically into two zones, i.e. a settling zone 41 on the left and an elutriating zone 42 on the right.

The slurry flowing 3A over weir 37 enters settling zone 41 with a downward velocity and generally follows flow path 40. The vast majority of the solids settle quickly following path 43 to collect as a layer of sludge 44 in the lower end of conical section 38. The liquid flows round the end of panel 39 and upwards into elutriating zone 42. Here, as the cross section of zone 42 gradually increases, so the upward velocity of flow 40 gradually decreases and small 45 and smaller 46 particles settle out of the liquid flow. Some very small particles may temporarily become suspended in the upward liquid flow - too big to be carried upward and too small to settle.Such particles will remain in this state for a short period until they contact another similar particle when they are likely to aggregate either because of static electrical charges, or because of the 'sticky' nature of their surfaces, or because of mass-mass attraction; the aggregate may now be massive enough to settle normally.

In one embodiment, the filter 12 is located within the clarifier 3 with its fine mesh metal filter element 12A mounted horizontally in the upper part of elutriating zone 42. The preferred mesh size is 150 250um. The wires of such a mesh size filter will not immediately catch the very finest particles, but will cause some particles to adhere to the metal fibres.

Gradually, as more and more particles adhere, bridges will be formed across the wires and a filter cake will become established catching all but the very finest of particles. Clearly, as the thickness of the filter cake grows, the filtrate flowing through will reduce in volume and cleaning will become necessary to restore an economic throughput.

Figure 7 shows one method by which the filter element 12A may be cleaned. A pipe 47 with a slit or holes 48 facing the filtrate side of element 12A is moved slowly across element 12A in direction 49. Water from tank 17 is pumped by pump 20 (Fig.2 or 3) into the bore of pipe 47 and exits via slit or holes 48 with a downward velocity 51 greater than that of the upward water velocity 52 in elutriating zone 42. The effect of downward velocity 51 is gently to dislodge filter cake 53 so that it breaks away in large flakes 54 which sink quickly to the sludge layer 44. The volume of elutriating flow 52 disturbed by backflushing flow 51 is small and localised and backflushing may be performed continuously, or intermittently, during normal elutriation to optimise overall process efficiency.

In a second embodiment, shown in Figure 6, the filter element 12A is housed in a separate housing 55.

In this case, liquid 4 from elutriating zone 42 enters chamber 55A and filtrate passes through mesh 12A into chamber 55B before leaving via pipe 13. Though backflushing to clean wire mesh element 12A would be possible, it may be preferable to remove filter element 12A and clean it externally, eg. with a hose pipe. To remove filter element 12A, hand nuts 56 are unscrewed and lid 57 lifted off. Element 12A may now be removed, cleaned and replaced in groove 58, thus making a seal between the two chambers 55A and 55B when lid 57 is refitted.

In practice, filters 12 may be used in pairs with interconnecting valving to allow each to be isolated from the flow line in turn for cleaning. This second embodiment is appropriate on ships where seawater may be used for cleaning, thus eliminating the. need to backflush with potable water.

It will be noted that filter housing 55 has flat sides and is thus not intended for use with high internal pressures. Thus it is preferably located upstream of any high pressure pumps 11, ie. as shown in Figures 3 and 9. However, high pressure filter housings 55 could be provided if desired.

Filtrate 13 now passes either directly (Figure 2) or via a pump 11 (Figure 3) to the ceramic filter 14.

In this filter 14, (Figure 8) a series of ceramic membranes 15, eg. double sided discs, is mounted about a central pipe 59 in a housing 60.. The ceramic discs consist of a fine grained outer membrane 15 supported on a coarse grained core 61. Filtrate percolates through the fine pores in membrane 15 and then flows through the core 61 and via holes 62 to the bore 63 of pipe 59. It is then discharged 64 from the filter 14.

Filtrate 13 enters ceramic filter unit 14 via a manifold 65, from which a plurality of nozzles 66 directs filtrate 13 past the surfaces of membranes 15.

The purpose of this feature will be described later.

The discharged filtrate 64 will now be almost entirely free of any solid content, but will contain bacteria, viruses, etc. Sterilisation is used to kill these microorganisms. The metered addition of chlorine, usually in the form of sodium hypochlorite, is the traditional method of sterilisation, but a preferred alternative is to pass the filtrate 64, in thin films, past a source(s) of intense ultraviolet (W) light. The particular wavelength of UV light used should be such as to sterilise the liquid quickly so that an in-line unit 16 with a short residence time would be appropriate.

The water 67 leaving steriliser 16 is of potable quality and passes via storage tank 17 for further use 18. Although this use could be for drinking, etc. it would more likely be for other domestic purposes. e.g.

for washing, toilet flushing, etc. or as boiler feed water. For land based treatment plants, the water 67 may be discharged to a water course.

The philosophy of the operation of the process will now be described with reference to Figure 9. In a real installation, the filtrate 13 from filter 12 whether located within clarifier 3 or not, would pass to a holding tank 68. This would accommodate surges in input flow 1 and allow other parts of the process to continue to operate while, for example filters 12 or 14 were being cleaned.

There are two basic methods of operation. In both methods, unless an external filter 12 (55) is used, the sole route of solid removal is via clarifier 3.

In a first method of operation, pump 11, which may be a convention centrifugal unit, draws liquid from tank 68 and raises it to high pressure compatible with ceramic filters 14. In this invention, the term 'high pressure' is used to distinguish it from the other low pressure lines; it would probably not exceed 550 kPa (5.5 bar). The high pressure liquid 13 is passed to ceramic filter 14 and re-circulated 70 via a conventional circulating pump 71. A bleed 72, controlled by a valve 73 returns liquid and solids back to clarifier 3 via pipe 74. Bleed 72 may typically be 20% of the inlet flow 13.A pressure relief valve 75 communicates with clarifier 3 via pipes 76 and 74 to protect the system, Fresh filtrate 13 and recycle 70 are squirted via nozzles 66 into ceramic filter 14 to create turbulent flow conditions (Figure 10) over the surfaces of the ceramic filter discs 15. The purpose of this is to allow pure liquid to reach and pass through the membranes 15, but not to allow a solid filter cake to form. Figure 10 shows a vector diagram in which the velocity 77 of the liquid from nozzles 66 is shown as 5 units in a direction parallel to surfaces 15. The velocity 78 of the liquid towards surfaces 15 is shown as 1 unit and the resultant velocity 79 is shown dashed. As shown, the resultant velocity 79 'misses' surfaces 15.

Fluid flow dynamics complement this effect as the drag forces are proportional to the actual, or projected, area of the particle i.e. proportional to d2 where d is the particle linear dimension. However, the mass of the particle is proportional to the cube of the linear dimension, ie. d3, and this determines the inertia. Thus, as particles become smaller, the inertia decreases by a cubic relationship, whereas drag forces decrease only by the square, so that, the smaller particles become, the more they are affected by the highly turbulent flow axially between surfaces 15 compared to the steady flow towards these surfaces.

Consequently, little, or no filter cake forms. In practice, the recycle flow 70 is very many times more than inlet flow 13, eg. 20 - 60 times as much. Bleed 72 controls the build up of solids in recycle loop 70.

The filtrate 64 passing membrane 15 leaves via valve 69 for steriliser 16.

Figure 10 is illustrative of the principle involved. In practice, very high recirculation rates are used e.g. 50 or more times the flow 64 through membrane 15. Thus, in practice, vector 77 may be 50 units long. As the recirculation line 70 is at high pressure, very little energy is required to recirculate the liquid via pump 71. The very high recirculation rate enables permeate 64 of the required quality to be obtained especially when treating concentrated sewage e.g. vacuum collected marine sewage.

Though the operating parameters are such as not to facilitate the formation of a filter cake, there may eventually be some build up on the surfaces 15.

Backflushing is not preferred as a means of cleaning, as repeated pressure cycling, ie. connection and disconnection to high pressure pump 11, and altering the direction of flow through the ceramic discs 15,61 are both likely to damage the brittle ceramic material.

One preferred method of removing the filter cake is to close valve 69 while maintaining pumps 11 and 71 running. This will stop the flow 78 towards and through surfaces 15 and allow the turbulence due to flow 77 to scour away the filter cake. The effect may be enhanced by switching the flow from a first manifold 65 and nozzles 66 to a second manifold and nozzles (not shown); this will create a different turbulent flow regime and act to remove the cake from less turbulent parts of the first manifold's flow regime. In practice, if two, or more, manifolds are used and the flow is alternated between them during normal filtration operations, the effect will be to remove some/all of the filter cake as fast as it is formed.

Thus the need for cleaning may be almost eliminated.

If the recirculating flow to one manifold is stopped completely, there would be a risk of a nozzle becoming blocked. This risk will be lessened if the flows in the manifolds are alternated between higher and lower values. In this way, patterns of turbulence are changed, but flow continues through each manifold.

Another option is to move the nozzles on the manifold so that they swing to and fro across membrane surfaces 15; this could be done by rotating manifold pipe 65 backwards and forwards through a suitable arc.

During filter cleaning by the closure of valve 69, as described hereinbefore, pump 11 would remain running and valve 73 may be opened a little more so that the solids dislodged are recycled 70 and removed in bleed 72 to clarifier 3. Thus, after a few minutes operation, all the solid build up in recycle loop 70 and filter 14 would be removed.

Other methods of cleaning membrane filters 15 may be used such as chemical systems and/or mechanical sweeping of the surfaces 15. Chemical treatment may be applied as and when necessary.

In a second method of operation, the filter membranes 15 in unit 14, are in the form of flexible polymer films which can be backflushed. In this case, the aim is to build up a filter cake and so filter 12 is not essential; the following description assumes that there is no filter 12 present.

In this case, pump 11 is preferably a positive displacement pump because the action of such a pump is less violent than that of the high speed rotor of a centrifugal pump so that partly aggregated particles are less likely to be broken up and more likely to be caught on the filter membrane 15. The recycle 70 can be much smaller than that in the first method, though pump 71 may be centrifugal in nature, as large particles will usually be caught in the filter cakes in their first pass. Using the reverse of the logic stated previously, the less turbulent flow regime and fluid dynamics now act to increase the chances of solids being caught in the filter cake, ie. the ratios of vectors 77 to 78 are different. There would still be a bleed 72.

In this second method, filter cleaning would be by normal backflushing, eg. via pump 20, pipe 21, valve 23 and pipe 26 (or 26A). Filters 14 may be arranged in pairs with one on stream and the other being backflushed, or ready for duty.

In the description hereinabove, only the main and inventive features have been discussed. Though some of the valving and piping details have been described, these are only those directly relevant to the main or inventive features. The man skilled in the art will be familiar with all the normal piping and valving arrangements appropriate to process systems that are applicable to the present invention.

Similarly, the process is designed for automatic unmanned operation, except where filter units as shown in Figure 6 are concerned. Though the control system has not been described in detail, its presence is inferred by use of the symbol, for a remotely operated control valve e.g. 22,23. A controller/monitor 31,32 is shown and is indicative of the many other pressure, flow rate, temperature, etc. and control measuring devices which are not shown but which would be included in practice in order to provide the feedback and feed forward for the fully automatic control e.g. via microprocessors, etc of plants constructed in accordance with the present invention.

In any control system, it is normal to have one, or more, parameters which are maintained constant. In accordance with the present invention, these parameters relate to the operation of membrane filter 14, which is the key to the quality of the output 67. The parameters may involve either maintaining a constant flow in pipe 13 into the filter unit 14 or maintaining a constant recycle flow 70.

Figures 11 and 12 show how the components of the apparatus of the invention may be assembled to form a compact module, suitable for installation in a ship, or other confined space. In this case, two chambers A and B are provided on top of clarifier 3. These chambers comprise the disintegrator 2 and the mixer tank 29.

(If a flocculator tank 30 is required, then three chambers would be provided). From chamber B, the effluent flows 3A over a weir 37 down into clarifier 3, where sludge settles and is removed via pipe 7 to tank 6 (shown hatched for clarity).

Supernatant liquor passes up elutriating zone 42 via (optional) filter element 12A, over weir 80 and down into holding tank 68 (also shown hatched). It will be noted that plate 38 forms structural parts of both clarifier 3 and tank 68, thus contributing to the compactness and modularisation of the apparatus. From tank 68, liquid flows 4 through filter 12 (if no filter element 12A is included in zone 42). Filter 12 is shown flat under tank 68, but it could be standing upright (Figure 6), alongside tank 68, if required.

From filter 12, the liquid flows to pump 11 and thence via pipe 13 to membrane filter 14 from whence the filtrate 64 passes to W steriliser 16 and out of the apparatus for use 18 as desired. The recycle 70 and recycle pump 71 serve membrane filter 14. The bleed 72 is not shown but indicated as recycle lines 10, 26, 26A, 74 to chamber B. Items 6, 11, 12, 68 and 71 are not shown on Figure 11 for clarity.

Figures 11 and 12 show that the apparatus of the invention may be formed into a compact module. For a small ship with a crew of about 50, the dimensions of the module may be approximately 1.0 x 1.0 x 1.5m.

Dimensions may be scaled up, or multiple modules used, for greater duties, as required.

The individual component of the modules are demountable to give individual units which are small enough to be taken to the desired location via normal shipping routes, e.g. hatchways, watertight doors etc., so that existing ships may be retrofitted with sewage treatment modules of the invention without major structural modifications.

An apparatus as above described has low running (energy) costs per unit volume of sewage treated and has low maintenance costs. Moreover, the apparatus is designed to operate largely under automatic control.

List of Numbered Items 1 Raw sewage 2 Disintegrator (or Macerator) 2A Inlet to mixer tank 29 3 Clarifier 3A Inlet to Clarifier 4 Supernatant liquid from clarifier 3 4A Valve 5 Sludge pump 6 Sludge tank 7 Sludge pipe from clarifier 8 Supernatant liquid from sludge tank 6 9 Pump 10 Recycle line 10A Recycle line 11 Pump 12 Filter (screen) 12A Metal mesh filter element 13 Filtrate from filter 12 14 Ceramic filter unit 15 Ceramic membranes 16 Steriliser 17 Storage tank 18 Product water usage 19 Sludge 20 Backflushing/cleaning pipe 21 Pipe 22 Valve 23 Valve 24 Outlet 25 Outlet 26 Return Pipe 26A Return Pipe 27 Non-return valve 28 Chemical additives 29 Mixer tank 30 Flocculator tank 31 Monitoring means 32 Signal 33 Impeller 34 Motor 35 Clarifier upper section 36 Weir 37 Weir 38 Clarifier lower section 39 Panel 40 Flow path 41 Settling zone 42 Elutriating zone 43 Sludge flow path 44 Sludge collecting in 38 45 Small particle settling path 46 Smaller particle settling path 47 Backflushing pipe 48 Slit or holes in pipe 47 49 Motion of pipe 47 50 Bore of pipe 47 51 Backflushing water velocity 52 Elutriating flow velocity 53 Filter cake 54 Dislodged flakes of filter cake 55 Separate filter housing 55A Inlet chamber of filter 55 55B Exit chamber of filter 55 56 Hand nuts 57 Lid 58 Groove 59 Central pipe 60 Ceramic filter housing 61 Core of filter discs 62 Holes 63 Bore of pipe 50 64 Ceramic membrane filter outlet 65 Manifold 66 Nozzles 67 Sterilised potable water 68 Holding tank 69 Valve 70 Filter re-circulation 71 Circulating pump 72 Filter circulation bleed 73 Valve 74 Pipe 75 Pressure relief valve 76 Pipe 77 Velocity of liquid out of nozzles 66 78 Velocity of liquid towards surfaces 15 79 Resultant velocity 80 Weir 100 House 200 Water course 300 Septic tank 400 Bacterial Digestion and settling tank 500 Aeration tank 600 Membrane filtration (tertiary polishing stage) 700 Solid waste 800 Solid separation 900 Ship 1000 Membrane filtration (secondary stage)

Claims (37)

1. CLAIMS: 1. An apparatus for the treatment of liquid effluent containing solids, which apparatus comprises; (i) clarifying means for the liquid effluent wherein solids settle out of the liquid, (ii) means for separating the settled solids and liquid containing any residual solids, and (iii) membrane filtration means for separating any residual solids from the liquid.
2. 2. An apparatus as claimed in claim 1 wherein discharge of solid material which has been separated from the effluent occurs from the clarifying means only.
3. 3. An apparatus as claimed in claim 1 or 2, the apparatus further including a means for disintegrating or macerating the liquid effluent prior to clarifying.
4. 4. An apparatus as claimed in any of claims 1 to 3 further comprising a means for sterilising the liquid after membrane filtration.
5. 5. An apparatus as claimed in claim 4 wherein the sterilisation means includes one or more powerful ultraviolet light sources.
6. 6. An apparatus as claimed in any preceding claim, the apparatus further including means to recirculate clarified liquid.
7. 7. An apparatus as claimed in claim 6 and including means to return a portion of the recirculated clarified liquid back to, or upstream of, the clarifying means.
8. 8. An apparatus as claimed in any preceding claim wherein the membrane filtration means includes one or more spaced membranes formed of ceramic material.
9. 9. An apparatus as claimed in claim 8 wherein the membrane filtration means includes a means for creating a highly turbulent flow adjacent to the one or more membranes of said membrane filtration means.
10. 10. An apparatus as claimed in any of claims 1 to 7 wherein the membrane filtration means includes one or more membranes formed of polymeric material.
11. 11. An apparatus as claimed in claim 10, the apparatus including backflushing means to clean the one or more membranes.
12. 12. An apparatus as claimed in any preceding claim, the apparatus further including a screening means between the clarifying means and the membrane filtration means.
13. 13. An apparatus as claimed in any preceding claim wherein a pump is used to pressurise the liquid effluent after passage through the clarifying means.
14. 14. An apparatus as claimed in claim 13 wherein the pump is a positive displacement pump.
15. 15. An apparatus as claimed in claim 13 or 14 wherein the pump is located between the screening means and the membrane filtration means.
16. 16. An apparatus as claimed in any preceding claim, the apparatus further including a holding tank.
17. 17. An apparatus as claimed in any preceding claim, the apparatus further including a storage tank.
18. 18. Apparatus as claimed in any preceding claim, the apparatus additionally comprising: (a) means of measuring the volumetric flow rate or concentration of the supply of liquid effluent, or parts thereof; (b) means to add metered quantities of chemicals to the liquid effluent; and (c) means to mix said metered quantities of chemicals uniformly with the liquid effluent.
19. 19. Apparatus as claimed in any preceding claim, the apparatus including means for the automatic operation of the process taking place.
20. 20. An apparatus as claimed in any preceding claim wherein the individual components of the apparatus are located in close proximity to each other to produce a module with minimum overall dimensions.
21. 21. An apparatus as claimed in any preceding claim wherein the components are demountable.
22. 22. A method of treating liquid effluent containing solids including the step of separating solids from the effluent wherein the separation step is carried out by membrane filtration.
23. 23. A method as claimed in claim 22 wherein sedimentation occurs prior to membrane filtration.
24. 24. A method as claimed in claim 23 wherein sedimentation is promoted by the addition of one or more chemicals in controlled amounts.
25. 25. A method as claimed in claim 23 or 24 wherein the physical processes of sedimentation and filtration alone provide a filtrate with a BOD of not more than 20 mg/l and an SS of not more than 20 mg/l.
26. 26. A method as claimed in any one of claims 23 to 25 wherein the liquid effluent is subjected to the further step of screening between sedimentation and membrane filtration.
27. 27. A method as claimed in any of claims 22 to 26 wherein liquid is recirculated.
28. 28. A method as claimed in claim 27 wherein a portion of the recirculated liquid is returned to, or upstream of, the sedimentation step.
29. 29. A method as claimed in any of claims 22 to 27 further including the process of sterilisation of the filtrate.
30. 30. A method as claimed in claim 29 wherein the sterilisation is effected by exposing the filtrate to a powerful source of ultraviolet light.
31. 31. A method as claimed in any of claims 22 to 30 wherein turbulent flow is provided over the membrane surface during the separation step.
32. 32. A method as claimed in any of claims 22 to 30 wherein the filtration membrane is cleaned by backflushing.
33. 33. A method as claimed in any of claims 22 to 32 wherein the liquid effluent is vacuum collected sewage.
34. 34. A method as claimed in any one of claims 22 to 33 wherein the liquid effluent is any one, or any combination of, domestic sewage, industrial effluent, agricultural effluent, effluent from water borne vessels or offshore structures or rain water run off.
35. 35. A method as claimed in any of claims 22 to 34 wherein effluent treatment is effected automatically.
36. 36. An apparatus as claimed in claim 1 substantially as hereinbefore described with reference to and as illustrated in Figures 1 to 12 of the accompanying drawings.
37. 37. A method as claimed in claim 22 substantially as hereinbefore described.