GB2336359A - Apparatus and method for wastewater treatment - Google Patents

Abstract

Apparatus (2) for the treatment of wastewater comprises a fixed structured first medium (12) and a granular second medium (14) in two distinct zones (6, 10) and an inlet (30) for supplying oxygen or a source of oxygen to the apparatus. The two media (12, 14) support, in use, microorganisms for treatment of the wastewater (e.g. carbonaceous treatment, denitrification and/or nitrification). The second zone (10) containing the granular second medium (14) has a depth of less than 2 metres, thereby improving the oxygen transfer efficiency of the apparatus. The apparatus is arranged such that, in use, the oxygen supplied flows through one medium and subsequently through the other medium so that oxygen not consumed by the microorganisms of one medium is available for consumption by the microorganisms of the other. The first medium (12) preferably has a specific surface area of less than 250m<SP>2</SP>/m<SP>3</SP>. The granular second medium (14) can act as a filtration barrier to filter off biomass which has become detached from the first medium (12) and/or to filter off solids present in the wastewater prior to treatment. An empty expansion zone (8) can be provided between the two media (12, 14) into which the granular medium (14) can move during backwashing/scouring of the apparatus.

Description

2336359

DESCRIPTION -APPARATUS FOR WASTEWATER TREATMENT AND METHOD

The present invention relates to an apparatus for the treatment of wastewater and a method of treating wastewater, for example a biological filter Z:1 (biofilter) apparatus, and a method of treatment of wastewater.

Biological filter systems are well known and much used for the treatment of wastewater (e.g. sewage). These filters comprise a medium or media which serve as a support system on which a slime or sludge of aerobic microorganisms grows. Generally, the wastewater is distributed on the top surface of the filter media and trickles down through void spaces in the media. In trickling or percolating filters, air flows through the medium (either upwards or downwards) by natural ventilation. In biological aerated filter (BAF) systems, air is injected at the base of the medium to rise upwards.

In "carbonaceous" water treatment, aerobic heterotrophic microorganisms, e.g. heterotrophic bacteria, use the oxygen provided to digest the dissolved organic matter (substrate carbonaceous material) in the wastewater, thereby decreasing the Biochemical Oxygen Demand (BOD) and/or Chemical Oxygen Demand (COD) of the wastewater. Typically, the BOD is reduced from 150 mg 1 -1 to 30 mg 1-1. In addition to dissolved organic matter the heterotrophic microorganisms also break down the organic fraction of the suspended solids content of the waste water (typically 70% of the solids of raw domestic sewage are organic solids and 30% are inorganic solids).

Normal domestic sewage also contains about 30 - 50 mg nitrogen 1-1, mainly in the form of ammonialammonium compounds deriving partially from hydrolysed urea. Carbonaceous treatment by heterotrophs does not substantially decrease the amount of ammonia present in the waste. Therefore a second "nitrification" stage is usually required to decrease ammonia levels to meet water quality standards. Nitrification uses a medium with an attached sludge of autotrophic nitrifying microorganisms (e.g. bacteria) or "nitrifiers" for oxidising ammonium, which use C02, rather than organic matter, as their carbon source. Bacteria such as Nitrosomonas use oxygen in the air provided to the biofilter to oxidise the ammonium (NH,+) to nitrite (N02-) and bacteria such as Nitrobacter further oxidise the nitrite produced to nitrate (N03_). In this way, a good quality nitrate-containing effluent is produced with ammonia levels reduced from typically 30 - 50 mg I to typically 5 - 15 mg 1-1.

A more recent development is to return part of the nitrate-containing effluent from the nitrification stage to the first carbonaceous stage. The nitrate provides an alternative oxygen source for the heterotrophic bacteria which allows the supply of oxygen/air provided in the first stage to be decreased. This process, involving a chemical reduction of nitrate to nitrogen gas, is 1 Z:1 called "denitrification" (DN) and performs the same function of reducing BOD/COD levels as carbonaceous treatment.

Biological filters can treat wastewater at different rates depending on the media used and the quantity of effluent desired. Relatively low loading -3rates (measured in Ka BOD/m/day) are possible using granular media like stones, gravel, ash, clay or plastic granules. A variety of granular BAF media have been tested (T.D. Kent et al., Water Sci. Tech., 1996. Vol. 34, p 363). Higher rates can be achieved using fixed structured plastic modules with open channels through which the wastewater passes.

Single-bed flooded filters (BAFs) containing a granular medium supporting both heterotrophs and autotrophs (nitrifiers) are widely used in sewage works for combined carbonaceous/DN and nitrification treatment. As the wastewater passes down the single bed, the carbonaceous load is progressively removed to allow nitrification to occur near the bottom of the bed. The disadvantage of these systems is that the loading rate must be carefully restricted to less than 2.5 kg BOD/m1May to ensure sufficient carbonaceous load is removed during passage through the bed to enable sufficient nitrifying bacteria to exist near the bottom of the bed to achieve nitrification. Even at this BOD load, only a modest decrease in ammonia is achieved. In order to achieve a 90% ammonia removal efficiency, which is cornmonly required in industry, a maximum loading rate of about 1.8 kg BOD/m'/day must be used. In addition, these single granular bed BAFs rapidly block up (blind) at the top at very high loading rates (eg. 5ka BOD/m'/day) due to rapid biomass buildup.

S.L. Harris et. al. of Cranfield University in conjunction with Peter Pearce of Thames Water Utilities have used off-gas analysis to study the oxygen transfer efficiency (OTE) of single-bed granular media BAR, -4comprising 0.49-0.5m diameter columns with a granular bed depth of 2 metres, under varying conditions (S.L. Harris et. al., Water Science & Technology, 1996, Vol 34, pp. 307-314; P.A. Pearce, paper presented at the 2nd Symposium on Biological Aerated Filters (BAF2) at Cranfield University on 12 June 1996). The first paper discloses that OTE increases with rising loading rate (between 1.0 and 2.3kg BOD/m'/day), but this improvement ceased at a very high load of 2.7kg BOD/M3/day. Both papers disclose that OTE increases markedly for air flow rates (through the medium) of below 5-10 m/h, and more specifically below 7 m/h. The second paper points out that small increases in OTE produce large energy savings: if OTE can be increased by 3-5 % then process air requirements can be reduced by 40 %. Therefore, in the BAF under test, power costs can be halved by operating at air flow rates in the 4-7 m/h range compared to operating in the typically 10-15 m/h range. However the second paper states that these benefits are not available at higher organic loadings (in the context of the paper, this appears to mean 2.3 kg BOD/m'/day or greater) or at media depths greater than 2 metres. The second paper also discloses that good air distribution is easy to attain in small (0.5m diameter) columns but may be less so in full scale units.

A common alternative to the single-bed biofilter is to have a first treatment vessel for carbonaceous/DN treatment in series with a second separate treatment vessel for nitrification. The first vessel reduces the BOD to levels that allow the nitrifiers (not the heterotrophs) to prevail in the separate second vessel. For example, a Swiss company (SuIzer) market a -5two-stage apparatus comprising a carbonaccous removal unit in series with a second unit for nitrification, each unit having its own oxygen supply and using a high-rate fixed structured medium, the latter allowing a high loading rate of up to 5 kg BOD/nil/day to be achieved. EP-A- 0524794 to Thames Water Utilities discloses an apparatus comprising separate primary and secondary treatment vessels in series, each vessel containing granular media and having a separate oxygen supply. This apparatus reduces BOD and ammonia levels at a low loading rate of 0.9 BOD kg/mIday.

There are several problems associated with a carbonaceous/DN filter in series with an entirely separate nitrification filter, however. Firstly, there are two separate oxygen/air supplies (one for each biofilter). This means that any oxygen or air supplied to but not consumed by the organisms in each filter is wasted. Where oxygen/air is injected into the filter by a blower, this represents a waste of some of the energy required to power the blower. Secondly, in all biofilters some of the biomass sloughs off and washes down through the filter during use and appears as "humus" in the effluent, increasing the suspended solids content. This is particularly the case with high-rate fixed structured plastic media with high voidage volume. Normally, a humus settlement tank is provided to separate the humus from the remaining effluent. Thirdly, large pairs of carbonaceous/DN and nitrification tanks (as well as single tanks combining both stages) are inflexible when faced with a variable wastewater load the plant is either underutilised or is unable to cope with heavy load periods.

Some workers have combined the two separate treatment vessels (e.g. as disclosed in EP-A-0524794) into a single-vessel BAF containing two superposed beds of various media for supporting microorganisms. For example, FR-A-2,624,847 to SOGEA discloses a method and apparatus for liquid treatment, wherein the liquid successively passes in a descending. flow through a first (upper) granular bed and then through a second (lower) granular bed disposed below the first bed. The first bed is aerated by means of air injectors positioned at the bottom of this bed (i. e. in the middle of the apparatus). The second bed (below these air injectors) is non-aerated. Preferably, the first bed contains larger granular particles than those present in the second bed, and there is a gap between the two beds.

Similarly, W095125695 to Thames Water Utilities discloses a process and apparatus for treating fluids comprising a plurality of filter beds (preferably two beds superposed) for supporting. attached microorganisms. Preferably, the upper bed comprises relatively high voidage granular or structured media (more preferably granular), the lower bed comprises relatively low voidage granular media, the two beds are separated by a gap, and the fluid flows downwards during use. The apparatus is different from that disclosed in FR-A-2,624,847 in that air is introduced at the base of the vessel and can rise upwards through all the beds - i.e. both beds are aerated. Preferably, however, an air removal system is provided between the two beds to capture air exiting the top of the lower bed - i. e. the lower bed is aerated, the upper bed non-aerated. The biofilter purports to achieve combined nitrification and denitrification in a single treatment vessel.

Finally, WO 91103429 to Helm and Vandervelde discloses an apparatus containing multiple juxtaposed beds of granular media, being progressively graded from a first coarsest layer at the bottom where the sewage is injected to a final, finest layer at the top where the treated sewage exits. This apparatus is fundamentally different to BAFs in that no air is injected (passive ambient air contact at the top only) and the sewage is injected at an extremely low rate and resides within the apparatus for several days or weeks.

Problems exist with two-bed BAFs such as those disclosed in FR-A2,624,847 and WO 95/25695, regarding optimising the oxygen transfer efficiency (OTE) and hence energy efficiency of the apparatus, reducing the rate at which either or both of the beds blocks up (blinds) during use due to biomass growth, and maximising the volumetric loading rate (in BOD/m'/day) of the apparatus while preferably maintaining a population of nitrifying microorganisms sufficient to perform effective nitrification of the wastewater.

An object of the present invention is to seek to provide an apparatus for or method of treatment of wastewater According to the first aspect of the present invention there is provided an apparatus for treatment of wastewater comprising:

(a) a first zone containing a fixed structured first medium for supporting, in use, microorganisms for treatment of the wastewater; (b) a second zone distinct from the first zone and containing a granular second medium for supporting, in use, microorganisms for treatment of the -8wastewater, wherein the second zone has a depth of less than 2 metres; and (c) an inlet for supplying oxygen or a source of oxygen to the apparatus, the apparatus being arranged such that the oxygen or source of oxygen flows through one medium and subsequently through the other medium during use of the apparatus.

By using a single oxygen supply, the oxygen or source of oxygen not consumed by the microorganisms of one medium is available for consumption by the microorganisms of the other medium thereby increasing the OTE of the system.

The depth of the second zone containing the granular second medium is limited to less than 2 metres for several reasons. Such a shallow bed has the advantage that the pressure differential across the top and bottom surfaces of the bed during use is minimised. This leads to a small headloss associated with the granular bed. The small pressure differential also effects the migration of the air bubbles through the granular medium. An air bubble passes through the granular bed along a decreasing hydrostatic pressure aradient and therefore expands during its passage. As the bubbles expand, n they collide forming even larger bubbles. Smaller bubbles are better distributed between the voids of the media and the oxygen within more efficiently absorbed by the microbial community.

Limiting the depth of the granular bed and the pressure differential across it therefore ensures the microbes thereon are serviced by frequent small -gbubbles evenly distributed across the bed, as opposed to infrequent large (long) bubbles unevenly distributed. The oxygen transfer efficiency (OTE) within the granular bed is thereby increased, and hence the energy efficiency of the apparatus overall.

The depth of the second zone is preferably 1-2 metres, more preferably 11.5 metres, even more preferably 1.2-1.5 metres. The depth of the second zone may be calculated from the mass in Kg of granular medium used, the packed density of the medium (Kg/1f), and the interior cross-sectional area of the apparatus. Alternatively, the apparatus can be provided with a mark or marks on a sight glass corresponding to the fill level(s) of the granular medium.

The fixed structured first medium typically comprises a "cross-flow" medium including a number of intersecting channels flowing in two different directions. A high loading and throughput rate is possible. In this "mixed system", downflowing, liquid is split up into two different paths at each intersection. Liquid can thus be distributed efficiently across the whole width of the medium as it passes downwards through it.

Preferably, the fixed structured first medium has a specific surface area not exceeding , 250m2per m' volume of the first zone (as determined by the manufacturer).

At these low specific surface areas, the fixed structured medium has an open structure which is less prone to"blcking with biomass buildup and which yields a very low headloss, in comparison to media with a higher surface area 10- of, say, 300-400 m'/m'. Importantly, this allows a volumetric loading rate of 3 3 ) - 7, preferably 4 - 6, more preferably about 5kg BOD/m media/day to be used without the media rapidly blocking via biomass formation. Where the medium is used to achieve carbonaceous treatment andlor denitrification, a reduction of BOD (e.g. to about 1.9 kg BOD/nil/day) can be achieved across the medium (which acts as a roughing stage), sufficient to allow sufficient nitrifiers to grow on the granular second medium to allow efficient nitrification to be effected thereon. This is a substantial advantage over, in particular, single granular bed BAFs, e.g. as disclosed in S.L. Harris et al., Water Science & Technolog, 1996, Vol 34, pp. 307-314, which cannot be loaded at y more that 2.5 (preferably 1.8) kg BOD/mIday, without blockage occurring and effective nitrification ceasing.

It is noted that all references herein to BOD refer to BOD,, i. e. the Biological Oxygen Demand measured over a 5-day period. This is how much C dissolved oxygen (DO) is absorbed by a liquid microorganism culture in a scaled tube containing no gaseous phase over 5 days, as calculated by measuring the DO levels at the beginning and end of that period.

Preferably, the specific surface area of the fixed structured first medium is 140-250, more preferably 200-250, even more preferably about 240m2 per m3 volume of the first zone.

Preferably, the apparatus is. substantially circular in cross-section. In comparison with square/rectangular BAFs commonly used, OTE is increased and the strength of the column is increased (i.e. less steel or other materials andlor fewer reinforcing rings or attachments are needed to accomplish the necessary strength, and construction costs are reduced).

Preferably, the first medium is for supporting, in use, heterotrophic microorganisms for carbonaceous treatment and/or denitrification of the wastewater. Preferably, the second medium is for supporting, in use, autotrophic (e.g. nitrifying) microorganisms for nitrification of the wastewater.

Preferably, the oxygen or source of oxygen is an oxygen-containing gas, more preferably air, usually supplied to the apparatus by a blower. Other chemicals which can act as a source of oxygen for the microorganisms (e.g. any gas, or any water-soluble chemical) could also be envisaged.

Preferably, the apparatus is arranged so that the first and second zones are superposed and the inlet for the oxygen or oxygen-containing gas is positioned relative to the lower medium such that bubbles of the oxygencontaining gas supplied can rise through the lower of the two media and subsequently through the upper of the two media. In this way, oxygen not consumed by the microorganisms living on the lower medium is available for use by those living on the upper medium. This avoids the need for two separate inlets for supplying oxygen,,one for each of the two media, and results in less of the oxygen supplied being wasted as compared to separate carbonaceous/denitrification and nitrifXadon systems. As the oxygen is usually supplied by a blower which requires power, this increased oxygen u'lization aives the present invention an improved energy efficiency.

ti c Preferably, the oxygen-containing gas flows through the second medium -12for nitrification and subsequently through the first medium for carbonaceous treatment/denitrification. This is preferable because the oxygen requirements of the nitrification stage are generally greater than those of the carbonaceous/DN stage.

Preferably, the apparatus is arranged such that in use the wastewater flows through the first medium for carbonaceous treatment/DN and subsequently flows through the second medium for nitrification. In this way, high levels of BOD/COD can be reduced to levels compatible with efficient functioning. of the second nitrification stage before the wastewater contacts the second stage.

This most preferable embodiment therefore is as follows: the fixed structured first medium for carbonaceous treatment/denitrification is positioned above the eranular second medium for nitrification; the apparatus is arranged such that the wastewater flows downwards through the first medium and subsequently through the second medium; and the inlet for the oxygen/source of oxygen is positioned so that the oxygen- containing gas can rise, in a direction counter to that of the wastewater flow, through the second (lower) medium and subsequently through the first (upper) medium.

Alternative embodiments could however be visualised, and ar considered within the scope of the invention, for example positioning the second medium above the first medium and/or arranging for the wastewater to flow upwards from the lower medium to the upper medium in the same direction as the flow of the oxygen-containing gas. A substantially horizontal -13configuration could also be envisaged.

Preferably, fixed structured media made of plastic, e.g. polyvinyl chloride (PVC), are used.

The granular second medium preferably comprises clay (e.g. expanded clay) or plastic such as polypropylene.

Especially but not exclusively when the fixed structured first medium is positioned above the granular second medium and the wastewater flows downwards from the first medium to the second medium, the granular second medium also acts as a filtration barrier to filter off any biomass (humus) which has become detached (sloughed off) from the first medium above, as well as any solids, especially inorganic solids, initially present in the wastewater. This solves the problem of the significant humus sloughing found especially when the first medium is a high-rate fixed structure medium. The suspended solids content of the effluent issuing from the biofilter is substantially reduced; in general, only sloughed off solids arising from the second nitrification stage are found in the biofilter effluent. In this way, the need for a separate humus settlement tank may perhaps be avoided, depending on the standards required.

The first medium and the second medium can be juxtaposed. Preferably, however, the first and sc=nd media are spatially separated by a third zone formed as a gap, which when the apparatus is filled with wastewater forms a liquid barrier zone containing firee-swimming microorganisms, and which provides an expansion zone between the two media into which the granular second medium can move during washing/scouring of the apparatus.

After build-up of excess biomass, the biofilter can be scoured with an upflow of air and/or washed with an upflow of water to remove any excess of biornass present on the first and/or second media to waste. Where the third zone formed as a gap is present, this scouring/washing allows the granular second medium to be disrupted and to expand into the third zone, facilitating the displacement of attached biomass.

Another preferable feature of the apparatus is a recycle inlet for recycling part or all of the effluent issued from the biofilter back into the biofilter so that any nitrate in the effluent forms a second source of oxygen for any heterotrophic microorganisms supported by the first medi In s way, urn. thi the first medium will operate in denitrification mode.

According to a second aspect of the invention there is provided a method of treatment of wastewater comprising the steps of:

(a) bringing the wastewater into contact with microorganisms supported by a fixed structured first medium contained in a first zone, so that treatment of the wastewater is effected; (b) bringing the wastewater into contact with microorganisms supported by a granular second medium so that treatment of the wastewater is effected, the granular second medium being contained in a second zone distinct from the first zone, wherein the second zone has a depth of less than 2 metres; and (c) supplying oxygen or a source of oxygen to the microorganisms so that the oxygen or source of oxygen flows through one medium and subsequently through the other medium in such a way that the oxygen or -is- source of oxygen not consumed by the microorganisms of one medium is available for consumption by the microorganisms of the other medium.

Preferably, the method is performed by the apparatus as described above. Preferable embodiments of the method are as described above for the corresponding apparatus.

Preferably, the method comprises removing at least 80%, more preferably 90%, of any ammonia in the wastewater.

Preferably, the method comprises bringing untreated wastewater into contact with the microorganisms supported by the first or second media at a volumetric loading rate of 37, more preferably 4-6, and even more preferably about Rg BOD/M1 media/day.

The method of treatment preferably uses the granular second medium as a filtration barrier to filter biomass which has become detached from the first medium and/or to filter off solids present in the wastewater prior to treatment.

Another preferable step in the method of treatment is recycling treated wastewater back into contact with any heterotrophic microorganisms so that any nitrate in the treated wastewater forms a second source of oxygen for the heterotrophic microorganisms..

It will be understood that an apparatus (per se, or during use) and/or a method according to the present invention may have any of the following features which are independent of the presence of any other features. Any one of the following features may form the basis of the invention with or without other features previously described as being essential (or preferred) -16features of the invention. For example, the second zone need not necessarily have a depth of less than 2m, and the oxygen or source of oxygen need not necessarily flow through one medium and subsequently through the other medium during use. The present invention thus includes an apparatus for or method of treatment of wastewater which includes one or more of the features listed below either with or without any other features (even the features in the statement of the invention) mentioned above. The features are:

4.

A first zone containing a first medium, a second zone distinct from the first zone and containing a second medium, wherein the media are for supporting, in use, microorganisms for treatment of wastewater.

A supply of (or an inlet for supplying) oxygen or a source of oxygen arranged such that the oxygen or source of oxygen flows through one medium and subsequently through the other medium.

The first medium in feature 1 being a fixed structured medium.

3a. A fixed structured medium comprising plastic, preferably polyvinyl chloride.

A fixed structured medium havin a specific surface area not exceeding g 250 m' per M3 volume of the zone which contains it, preferably 140 - 250 m'/m3, more preferably 200 - 250 MI/M3, and even more preferably about 240 M11M3.

5. The first medium in feature 1 being for supporting, in use, heterotrophic microorganisms for carbonaceous treatment andfor denitrification of the wastewater.

6. 7.

8.

9.

10. 11.

12.

13. 14.

15.

-17The second medium in feature 1 being a granular medium. A zone containing a granular medium with a depth of less than preferably 1-2m, more preferably 1-1.5m, even more preferably 1.21.5m. A granular medium comprising clay, e.g. expanded clay, or plastic, e. g. polypropylene. The second medium in feature 1 being for supporting, in use, autotrophic (e.g. ninifying) microorganisms for nitrification of the wastewater. An apparatus substantially circular in cross-section. A volumetric loading rate of 3-7 kg BOD/nil media/day, preferably 4-6 kg BOD/nil media/day, more preferably about 5kg BOD/m' media/day, being used. In combination with or independent of feature 11, effective nitrification being achieved, preferably at least 80% of any ammonia in the wastewater being removed, more preferably at least 90% of any ammonia being removed. The first and second zones in feature 1 being superposed. The source of oxygen in feature 2 being an oxygen-containing gas, preferably air. In combination with features 13 and 14, the oxygen or oxygencontaining gas rising through the lower of the two media and subsequently through the upper of the two media.

16. In combination with features 1 and 14, the oxygen or oxygencontaining gas flowing through the second medium and subsequently through the first medium.

17. In combination with feature 1, the wastewater flowing through the first medium and subsequently through the second medium.

18. The granular second medium in feature 6 acting or being used as a filtration barrier to filter off biomass which has become detached from the first medium and/or to filter off solids present in the wastewater prior to treatment.

19. A third zone formed as a gap between the first and second media of feature 1.

20. The third zone of feature 19 acting as an expansion zone into which the granular medium can move during washing andlor scourin of the 9 apparatus.

21. Recycling (or a recycle inlet for recycling) treated wastewater back into the apparatus or into contact with the heterotrophic microorganisms in feature 5 so that any nitrate in the treated wastewater forms a second source of oxygen for the heterotrophic microorganisms.

A specific embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawinas, in which:Fig. 1 is an elevation view of the exterior of a biological filter; Fig. 2 is a top plan view of the biological filter apparatus shown in Fig.

1; and Fig. 3 is a view of a vertical cross-section, passing through line AA' in Fig. 2, of the biological filter apparatus shown in Figs. 1 and 2 showing the media inside the apparatus. Fig. 4 is a perspective view, partially cut away, of the fixed structured

medium 12 used in the apparatus shown in Fig. 3.

Fig. 5 is a mass balance diagram for combined carbonaceous treatment and nitrification using the apparatus shown in Figs. 1-3.

Figs. 1 to 3 illustrate a vertically-orientated biological filter apparatus 2 for treatment of wastewater which takes the form of a single vessel. The apparatus has an outer cylindrical housing 4 containing three major distinct zones 6, 8, and 10 (see Fig. 3).

Referring to Fig. 3, the first (upper) zone 6 encloses a fixed structured first medium 12 which is designed for treatment, in particular carbonaceous treatment and/or denitrification, of the wastewater. During carbonaceous treatment/DN, this medium is to act as a support for a biomass of heterotrophic microorganisms, (heterotrophs), e.g. heterotrophic bacteria, which can be -rown on this medium ano which accomplish this treatment.

The second (lower) zone 10 encloses a granular second medium 14 designed for treatment, in particular nitrification, of the wastewater. During nitrification, this medium acts as a support for a biomass of nitrifying microorganisms (nitrifiers), e.g. nitrifying bacteria, which can be grown on this medium and which accomplish this treatment.

The third (middle) zone 8 is formed as a gap between the upper and lower zones 6, 10 and forms (when the biofilter is full of wastewater) a liquid barrier zone to segregate the heterotrophic and nitrifying microorganism communities in the media above and below. Free-swimming microorganisms also reside in liquid barrier zone 8. It also acts as an expansion zone into which the granular medium 14 below can move during backwashing of the apparatus 2.

The upper fixed structured medium 12 is supported in place by support bars 16 attached to the housing 4. The lower granular medium 14 is supported by a gravel layer 18 which in turn rests on the base of the apparatus 2.

The upper fixed structured medium 12 is a single coherent unit made of plastic (usually polyvinyl chloride) and has a high voidage volume comprising open channels through which the wastewater passes. The structure of this medium is shown in detail in Fig. 4.

Fig. 4 shows a fixed structured medium 54 comprising a plurality of fused corrugated PVC sheets (e.g. 56, 58). The corrugations of each sheet (c. 56) are disposed diagonally to the top surface of the medium 54 and form open channels (e.g. 60) through the medium 54. The corrugations of each ad acent sheet (e.g. 58) are disposed in an opposing diagonal direction and j form open channels (e.g. 62) in that opposing direction through the medium 1 54. The opposing diagonal channels 60,62 intersect. Therefore liquid flowing. down channel 60 is split between two channels 60,62 at the intersection of those channels 60,62 and is, continually re-split at every intersection -21thereafter. The liquid is thereby evenly distributed across the entire width of each sheet 56,58 as it passes down through the medium 54, representing a "cross-flow" or "mixed" system rather than the "plug flow" system prevalent with granular media.

Suitable commercially available fixed structured media 12,54 are marketed by Munters (UK, US, Germany) and Mass Transfer.

It has been found that the sl=ific surface area of the fixed structured medium 12 is important to obtain an optimal balance between BOD reduction and blocking (blinding) characteristics. Specific surface area is measured as the surface area (internal and external) of the medium 12,54 in M' per unit volume (in m') of the imaginary block which encloses the medium 12,54 (i.e. the first zone 6). Fixed structured media 12,54 with closely-spaced corrugations and narrow channels having a high specific surface area (e.g. 300-400 m'/m) are efficient at BOD removal but suffer from the problem of blocking. with biomass at high loading rates - Very Low surface area media cannot support a sufficient microbial community to achieve effective BOD removal. It has been found that media 12 with a specific surface area of 240m'/m' allows good BOD removal but represents a sufficiently open structure which minimises biomas&blockage problems even at high loading rates (e.g. 5kg BOD/rri'd) and yields a very low headloss across the media 12 during use. Munters medium FB10.12 (240 m2/m) is ideal. Lower specific surface area media are also possible e.g. 19Om'-lm (Munters FB12.19), 150m'lml (Munters FB10.19), 100m21m (Munters FB10.27), but as the -22surface area decreases the BOD removal efficiency decreases too. The surface area of the fixed structured media is determined by the manufacturer of the media.

deep.

The fixed structured medium 12 is 1.2-1.8 m deep, preferably 1.2m The granular medium 14 of choice is made of expanded clay, with a particle size of 3-6 mm, and a voidage of 0.3. A commercially available medium this of type is Arlita TA8 (see T.D. Kent, Water Science and Technology, 1996 Vol. 34, pp 363-370 for properties of this medium). Plastic (e.g. polypropylene) granular shapes, with or without internal or external projections to increase surface area, can also be used. The structure and particle size of the granular medium 14 is such that it acts as a filtration barrier as described below.

In order to maximise OTE, as described above, the granular bed 10 is limited to less than 2m deep, preferably 1-2m deep, more preferably 1-1.5 m deep, even more preferably 1.2-1.5m deep. Suitable bed depths are 1.0m, 1.2m and 1.5m. The depth of the granular bed 10 may be calculated from the mass in Kg of granular medium used, the packed density of the medium (Ka,/m'), and the interior cross-sectional area of the apparatus. Alternatively, the apparatus can be provided with a mark or marks on a sight glass 36 (described hereafter) corresponding, to the fill level(s) of the granular medium.

The third zone formed as a gap 8 is ideally 0.3m deep, and should be no more than 0.5m deep.

The apparatus is provided with a detachable joint 64 in the middle of the second zone 10. The top and bottom of the apparatus can be separated at this joint and an expansion cylindrical section (not shown) inserted. This will allow a deeper granular bed 10 to be used while keeping the depth of empty zone 8 constant.

The apparatus 2 has a wastewater inlet 20, which runs into a trough 22 (150mm wide) extending across the width of the apparatus 2 and above the fixed structured medium 12, for supplying wastewater to the apparatus 2. A lateral outlet system 24 is provided in the gravel layer 18 at the base of the apparatus 2. Lateral outlet system 24 has a wastewater outlet 26 at one end; and at the other end system 24 has a backwash inlet 34 to allow ingress of backwash water during backwashing/scouring of the apparatus 2, and a drain port 46. Wastewater outlet 26 is connected, during use, to a drain or to a further processing/sedimentation stage, and can also be connected by standard means to a recycle inlet 28 which forms one end of the lateral inlet system 22 at the top of the apparatus 2.

The apparatus 2 also has an air inlet 30 connected to a header 31 and perforated lateral air inlet system 32 in the gravel layer 18 at the base of the apparatus 2. The air inlet 30 is connected to a powered blower (not shown), which supplies air to the apparatus 2 when in use.

The air distribution system 30, 32 and effluent collection and backwash systems 24, 26, 34 can be header and perforated lateral systems as shown in -24Fig. 3, or of a nozzle design (not shown).

Finally, as shown in Figs. 1 and 2, the apparatus 2 is also provided with a sight glass and media inlet 36 at the level of the empty expansion zone 8, allowing visual inspection of the inside of the apparatus 2 and being an inlet through which the gravel 18 and granular medium 14 can be poured. Also provided in the expansion zone 8 are a dissolved oxygen (DO) measurement probe(s) 42 to help analyse and control the functioning of the apparatus during use, and a pressure measurement device 52 to measure the pressure differential across the granular bed 10. At the base, at the intersection between the gravel layer 18 and granular medium 14, is a handhole 44 for the removal of the granular medium.

There is also provided a V-shaped housing 48 (see Figs. 1 and 2) running down the side of the apparatus 2. This defines an outer sludge outlet reception vessel 48 outside the apparatus 2. A hole (not shown) is provided in the cylindrical housing 4, near the top of and enclosed by the Vshaped housing 48, so that backwash liquids and sludge can pass from the top of the apparatus 2 into the outer vessel 48. The hole (not shown) is below the level of the upper lip 50 of the apparatus. The bottom of the outer vessel 48 is provided with a sludge outlet 38 and drain 40.

Referrina now to Fias. 2 and 3 and the construction of the apparatus 2, the outer housing 4 can be fabricated in stainless steel, carbon steel, glass reinforced plastic or concrete. Fig. 2 shows a cylindrical housing 4 with a circular cross-section, but the housing can also be rectangular or square in -25cross-section. A circular section has certain advantages (see above). Cylinders 2 with a diameter of 1 metre or 3 metres are envisaged as ideal, depending upon the application, but cylinders with any diameter can be used. The bed depths (see above) remain substantially the same as the diameter varies. The apparatus 2 can be surface mounted or submerged.

The use of the apparatus 2, and the method of wastewater treatment using the apparatus 2, will now be described, again by way of example only.

The apparatus 2 can be used for:

(a) combined "carbonaceous" treatment and "nitrification" of wastewater; (b) combined "denitrification" and "nitrification" of wastewater; (c) "carbonaceous" treatment only; or (d) "nitrification" only.

During use of the apparatus 2 in the carbonaceous-nitrification mode, wastewater (e.g.. settled sewage) is distributed, via the wastewater inlet 20 into the trough. The wastewater weirs over the sides of trough 22 and falls onto the top surface of the upper fixed structured medium 12 or into a head of wastewater (not shown) above the me 12. The wastewater passes down through the fixed structured medium 12 where carbonaceous (organic) solutes and solids are broken down by the heterotrophs supported by the medium 12 ("carbonaceous" treatment). After passing through the intervening volume 8, the wastewater continues down through the lower granular medium 14 where nitrifiers supported thereon reduce the ammonialammonium content of the -26wastewater ("nitrification"). The granular medium 14 also acts as a filtration barrier for the purposes described below.

The treated wastewater leaves the apparatus 2 via the lateral outlet system 24 and via wastewater outlet 26 to discharge. Some effluent can be recycled back into the apparatus via the recycle inlet 28.

During use of the apparatus 2, air is blown by means of the powered blower into the apparatus through the air inlet 30 and the header and perforated lateral air inlet system 32. The air emerging through the perforations in the lateral system 32 is redistributed by the gravel layer 18 and then rises up through the lower granular medium 14. Any air not utilised by the nitrifiers on the Jgranular medium 14 continues upwards through the liquid barrier zone 8 and the upper medium 12 to provide oxygen to the heterotrophs therein. In this way, the utilization of oxygen is improved compared to separate carbonaceous and nitrification systems.

It will be seen that wastewater passes down the apparatus 2 and air rises up the apparatus 2 countercurrent to the wastewater flow.

The airloxygen is supplied to the apparatus 2 as follows. The nitrifying microorganisms have a higher oxygen requirement than the heterotrophs. Generally, therefore 4.3 -4.5 kg oxygen per kg of ammonia eliminated from the wastewater, and an additional 0.5 - 1.0 kg oxygen per kg of BOD eliminated from the wastewater, is provided. Air is therefore supplied at an appropriate rate, having regard to the influent flow, and the loadings of BOD, ammonia, and suspended solids. The air flow into the apparatus 2 is measured -27in N (normal) m'/day, i.e. the volume in m' of free air at 1 atmosphere supplied per day.

The oxygen requirement is controlled by monitoring the dissolved oxygen (DO) concentrations at the outlet 26 of the plant and at the liquid barrier zone 8 against predetermined setpoints. DO measurements are taken using a probe (not shown) near to outlet 26 and using probe(s) 42 in the barrier zone 8.

Fig. 5 (a) and (b) is a mass balance diagram which shows the ideal loading rates and air injection rates used, and the effluent quality achieved by a 1 metre diameter cylindrical apparatus 2 with a 1.2m deep fixed structured bed 6 and a lm deep granular bed 10, operating in combined carbonaceous nitrification mode. It is noted that a high organic loading of 4.71 kg BOD/day (corresponding to a 5 Kg BOD/m fLxed structured medium/day) is applied to the fixed structured medium 12 (as measured using the BOD5 five-day test described earlier). This medium acts a roughing stage, effecting reduction of 60% of the organic load to 1.88 kg BOD/day in the intermediate zone 8. This organic load is low enough to allow nitrifiers (as opposed to heterotrophs) to prevail on the lower granular medium 14, and to slow down the rate of blockage of that medium. The wastowater then passes through the granular medium 14, and nitrification is effecod (ammonia levels reduced from 1.3 kg/day to 0. 13 kg/day). A further reduction in BOD to 0. 56 kg/day is also achieved in passing through the gramdu medium 14. The process air is supplied to the apparatus at a flow of 168 N (normal) M3 /day. The ideal air -28flow velocity through the media is 8.9 m/h, to obtain optimum OTE. Any rate between 510 m/h, preferably 7-9 m/h can be used.

When the apparatus 2 is used in the denitrification-nitrification mode of operation, a proportion of the treated wastewater exiting the outlet 26 is recycled back to the top of the apparatus via recycle inlet 28. The recycled wastewater contains nitrate (from the nitrification stage) which forms an alternative source of oxygen for the heterotrophs in the upper medium 12. This reduces the quantity of oxygen which has to be made available from the nitrification medium 14 below (via up-flow of air). Thus the oxygen requirements of the whole apparatus 2 are reduced. In other respects, however, this mode of operation is identical to the carbonaceous-nitrification mode described above.

The apparatus 2 should be operated ideally at or above CC (e.l.. 10'C), to achieve both carbonaceous treatment/DN and nitrification. Below PC, growth of nitrifying microorganisms is seriously impeded, both media become dominated by heterotrophs, and the apparatus 2 will operate substantially in carbonaceous mode only.

The apparatus 2 will also work in carbonaceous mode only if it is provided only with an organic carbon source or if for some other reason heterotrophs dominate both media 12, 14 of the apparatus 2. In this mode, BODICOD is reduced, solids are filtered off using granular medium 14 (see below), but ammonia reduction is minimal.

In use, a biomass of microorganisms is capable of growing as a slime -29on the surfaces of the channel of the fixed structured medium 12 and the surfaces of the granules 14 and this biomass effects the treatment.

These microorganisms (heterotrophic/nitrifying bacteria) are present within most common wastewaters (e.g. sewage) and therefore are provided to the apparatus 2 merely by passing the wastewater into it.

The apparatus 2 should not be left starved for more than 1-2 days, or cannibal protozoa will take over and deplete the heterotrophs and nitrifiers.

The structure and particle size of the granular medium 14 used in the nitrification stage is such that it acts as a filtration barrier to filter off:

(a) any biomass (humus) which has become detached from the upper medium 12, and (b) any solids, in particular inorganic solids, present in the wastewater prior to treatment and not broken down by carbonaceous treatment/denitrification.

Having a dual-function granular medium 14, which both nitrifies and filters the water, has the advantage that the problem of significant slou. g hing off of biomass from the high-rate structured medium 12 is solved. The inorganic solids are removed too. There will still be some biomass solids appearing in the effluent from the apparatus 2, deriving from the nitrifying bacteria on the granular medium 14. However, the end result is that the suspended solids content of the treated wastewater is reduced in comparison with a module without a built-in filtering function. This may reduce the need for a separate sedimentation tank to remove biomass.

Biomass does eventually build up on both media 12, 14 to a thickness of 0. 2-0.5mm. This needs to be removed/controlled periodically to maintain an acceptable biomass thickness and the optimum efficiency of the module, to minimise the headloss across the granular bed 10 and to prevent blockage of the granular "filter" 14 in particular. This is achieved by scouring the media 12, 14 with an up-flow of air to detach the excess of biomass from the media 12, 14 followed by or concurrent with backwashing the media 12, 14 with a hydraulic upflow of water to displace the detached biomass. The backwash waters and biomass sludge pass up the apparatus 2 and through the hole (not shown) in the top of the housing 4 into the sludge outlet reception vessel 48 and via sludge outlet 38 to waste. The backwash water is supplied by the backwash inlet 34 (Figs. 2 and 3), attached to the lateral waste outlet system 24, and flows from bottom to top. During backwashing, the granular medium 14 is disrupted and mobilized, expanding into the empty expansion zone 8. This allows efficient displacement of the biomass from the granular medium 14.

It will be seen that an additional advantage of the two-media apparatus 2 is that only one backwash is. required for the whole apparatus 2, compared to two separate washes for separate carbonaceous/DN nitrification systems.

It will be appreciated that many variations of the device shown in the drawings may be made whilst remaining within the scope of the invention. For example, another embodiment of the invention comprises a two-media system as described above but without necessarily utilising a single air supply -31for both media or a second zone depth of less than 2 metres. A second air supply (not shown) may be provided, so that each of the two media 12, 14 have their own separate air supply.

Another embodiment of the invention is where both media 12, 14 operate in carbonaceous treatment mode and no separate nitrification is achieved. This arrangement would provide a benefit over conventional singlestage carbonaceous-only systems in that the filtering capability of the granular medium 14 would allow a high-rate fixed structured upper medium 12 to be used while controlling the level of suspended solids and biomass in the treated water. As described above, if the apparatus 2 is operated at below CC, growth of nitrifiers is impeded and the apparatus will operate substantially in carbonaceous mode anyway.

The biofilter 2 is designed on a modular basis with the option to arrange modules 2 in parallel to obtain the necessary treatment capacity, and/or in series to increase the height of each module 2. A change in wastewater volumes can easily be accommodated by bringing on-line extra modules 2 in parallel to increase capacity. The modular system therefore is inherently more flexible and adaptable d= a single large wastewater treatment tank.

A variety of industrial applications of the biofilter 2 are envisacred. Single biofilters of 1-6 m'/hr capacity are envisaged as ideal for smallscale treatment of sewage, wastewaters from chemical processes, or effluent from food processes, dairy works, abattoirs or beverage works, or indeed any -32wastewaters having a measurable BOD/COD or containing ammonia/ ammonium compounds. Larger volume wastewater streams could use a single larger biofilter 2 or a number of smaller modules 2 working in parallel. For large scale treatment of domestic sewage, many individual large modules 2 arranged in parallel are envisaged receiving effluent from primary settlement tanks, providing additional treatment at an overall rate of, say, 1300 m/hr.

Claims (48)

-33CLAIMS

1. An apparatus for treatment of wastewater comprising:

(a) a first zone containing a fixed structured first medium for supporting, in use, microorganisms for treatment of the wastewater; (b) a second zone distinct from the first zone and containing a granular second medium for supporting, in use, microorganisms for treatment of the wastewater, wherein the second zone has a depth of less than 2 metres; and (c) an inlet for supplying oxygen or a source of oxygen to the apparatus, the apparatus being arranged such that the oxygen or source of oxygen flows through one medium and subsequently through the other medium during. use of the apparatus.

2. An apparatus as claimed in claim 1, wherein the depth of the second zone is 1-2 metres.

3. An apparatus as claimed in claim 1, wherein the depth of the second zone is 1-1.5 metres.

4. An apparatus as claimed in claim 1, wherein the depth of the second zone is 1.2-1.5 metres.

5. An apparatus as claimed in any one of the preceding claims, wherein the fixed structured first mefflum has a specific surface area not exceeding, 250m 2 per m' volume of the first zone.

6. An apparatus as claimed in claim 5, wherein the first medium has a specific surface area of 140-25Om' per m volume of the first zone.

7. An apparatus as claimed in claim 5, wherein the first medium has a specific surface area of 200-250M2 per m' volume of the first zone.

8. An apparatus as claimed in claim 7, wherein the first medium has a specific surface area of about 240 m' per m' volume of the first zone.

9. An apparatus as claimed in any one of the preceding claims substantially circular in cross-section.

10. An apparatus as claimed in any one of the preceding claims, wherein the first medium is for supporting, in use, heterotrophic microorganisms for carbonaceous treatment and/or denitrification of the wastewater.

11. An apparatus as claimed in any one of the preceding claims, wherein the second medium is for supporting, in use, autotrophic microorganisms for nitrification of the wastewater.

12. An apparatus as claimed in claim 11, wherein the autotrophic microorganisms are nitrifying microorganisms.

13. An apparatus as claimed in any one of the preceding claims, wherein the source of oxygen is an oxygen-containing gas.

14. An apparatus as claimed in claim 13, wherein the oxygencontaining gas is air.

15. An apparatus as claimed in claim 13 or 14, arranged so that during use of the apparatus the first and second zones are superposed and the inlet for supplying oxygen or the oxygen-containing gas is positioned relative to the lower medium such that the oxygen or oxygen-containing gas supplied can rise -35through the lower of the two media and subsequently through the upper of the two media.

16. An apparatus as claimed in any one of claims 13 to 15, arranged such that during use of the apparatus the oxygen or oxygen-containing gas flows through the second medium and subsequently through the first medium.

17. An apparatus as claimed in any one of the preceding claims, arranged such that during use of the apparatus the wastewater flows through the first medium and subsequently through the second medium.

18. An apparatus as claimed in any one of the preceding claims, wherein the fixed structured first medium comprises plastic.

19. An apparatus as claimed in any one of the preceding claims, wherein the fixed structured first medium comprises polyvinyl chloride.

20. An apparatus as claimed in any one of the preceding claims, wherein the granular second medium comprises clay or plastic.

21. An apparatus as claimed in claim 20, wherein the granular second medium comprises expanded clay or polypropylene.

22. An apparatus as claimed in any one of the preceding claims, wherein the granular second medium acts as a filtration barrier durin. use of the apparatus to filter off biomass which has become detached from the first medium and/or to filter off solids present in the wastewater prior to treatment.

23. An apparatus as claimed in any one of the preceding claims, wherein there is also provided a third zone formed as a gap between the first and second media.

24. An apparatus as claimed in claim 23, wherein during the use of the apparatus the third zone acts as an expansion zone into which the granular second medium can move during washing andlor scouring of the apparatus.

25. An apparatus as claimed in any one of claims 10 to 24, as dependent on claim 10, additionally comprising a recycle inlet for recycling treated wastewater back into the apparatus so that any nitrate in the treated wastewater forms a second source of oxygen for the heterotrophic microorganisms supported by the first medium.

26. An apparatus substantially as hereinbefore described with reference to and/or as illustrated in any of the drawings herein.

27. A combination of a plurality of modules connected in parallel and/or in series, wherein some or all of the modules comprise apparatus as claimed in any one of the preceding claims.

28. A method of treatment of wastewater comprising the steps of:

(a) bringing the wastewater into contact with microorganisms supported by a fixed structured first medium contained in a first zone, so that treatment of the wastewater is effected; (b) bringing the wastewater into contact with microorganisms supported by a granular second medium so that treatment of the wastewater is effected, the granular second medium being contained in a second zone distinct from the first zone, wherein the second zone has a depth of less than 2 metres; and (c) supplying oxygen or a source of oxygen to the microorganisms so that the oxygen or source of oxygen flows through one medium and -37subsequently through the other medium in such a way that the oxygen or source of oxygen not consumed by the microorganisms of one medium is available for consumption by the microorganisms of the other medium.

29. A method of treatment as claimed in claim 28, wherein the depth of the second zone is as defined in any one of claims 2 to 4.

30. A method of treatment as claimed in claims 28 or 29, wherein the fixed structured first medium has a specific surface area as defined in any one of claims 5 to 8.

31. A method of treatment as claimed in any one of claims 28 to 30, wherein the first and second zones are substantially circular in crosssection.

32. A method of treatment as claimed in any one of claims 28 to 31, wherein the microorganisms supported by the first medium include heterotrophic microorganisms so that carbonaceous treatment and/or denitrification of the wastewater is effected.

33. A method of treatment as claimed in any one of claims 28 to 32, wherein the microorganisms supported by the first medium include autotrophic microoraanisms, so that nitrification of the wastewater is effected.

34. A method of treatment as claimed in claim 33, comprising removing at least 80% of any ammonia in the wastewater.

35. A method of treatment as claimed in claim 33, comprising removing at least 90% of any ammonia in the wastewater.

36. A method of treatment as claimed in any one of claims 28 to 35, comprising bringing untreated wastewater into contact with the -38microorg ganisms supported by the first or second media at a volumetric loading rate of 3-7 Kg BOD/m' medialday.

37. A method of treatment as claimed in claim 36, wherein the volumetric loading rate is 4-6 kg BOD/m? media/day.

38. A method of treatment as claimed in claim 37, wherein the volumetric loading rate is about 5 kg BOD/m' media/day.

39. A method of treatment as claimed in any one of claims 28 to 38, wherein the source of oxygen is as defined in claims 13 or 14.

40. A method of treatment as claimed in claim 39, wherein the first and second zones are superposed and the oxygen or oxygen-containing gas supplied rises through the lower of the two media and subsequently through the upper of the two media.

41. A method of treatment as claimed in claims 39 or 40, wherein the oxygen or oxygen-containing gas flows through the second medium and subsequently through the first medium.

42. A method of treatment as claimed in any one of claims 28 to 41, wherein the wastewater flows through the first medium and subsequently through the second medium.

43. A method of treatment as claimed in any one of claims 28 to 42, wherein the fixed structured first medium is as defined in claims 18 or 19.

44. A method of treatment as claimed in any one of claims 28 to 43, wherein the granular second medium is as defined as in claims 20 or 21.

45. A method of treatment as claimed in any one of claims 28 to 44, -39comprising the additional step of using the granular second medium as a filtration barrier to filter off biomass which has become detached from the first medium andlor to filter off solids present in the wastewater prior to treatment.

46. A method of treatment as claimed in any one of claims 32 to 45, as dependent on claim 32, comprising the additional step of recycling treated wastewater back into contact with the heterotrophic microorganisms so that any nitrate in the treated wastewater forms a second source of oxygen for the heterotrophic microorganisms.

47. A method of treatment substantially as hereinbefore described with reference to and/or as illustrated in any of the drawings herein.

48. A method of treatment or apparatus as claimed in any one of the preceding claims, wherein the wastewater comprises sewage, effluent from chemical processes, effluent from food processes, effluent from dairy works, effluent from abattoirs, effluent from beverage works, water having a measurable Biochemical Oxygen Demand andlor Chemical Oxygen Demand, or water containing ammonia or ammonium compounds.