GB2306462A - Clarifying tank with buoyant filter - Google Patents

Abstract

A clarifying tank, eg a settlement tank, comprises a peripheral weir 2, with water flowing upwardly towards the weir and a buoyant medium 9 retained in the upward flow path. Preferably the buoyant medium consists of polythene tube-like members adapted to support microbiological growth, the members being held in a layer or basket-type mesh 8 and retained adjacent the weir. The buoyant medium may be retained at or below the surface 3 of the water. Preferably the medium is at least 100mm in depth. The clarification tank may be part of a sewage treatment system where the water is subject to biological oxidation prior to entering the clarifying tank.

Description

SEWAGE TREATMENT APPARATUS Field of the Invention This invention relates to sewage treatment apparatus, and in particular to tanks adapted for clarification and settlement, following biological treatment.

Backaround of the Invention The use of a buoyant material as a filter medium is known, especially in closed vessels in which the water to be treated is forced upwardly through a bed of the medium.

Buoyant materials have also been used for biological treatment, and materials have been specifically designed for this purpose, usually including recessed surfaces on which microorganisms can be retained. A particularly successful material of this type is the so-called "Kaldnes" medium, which is of the type described in EP-A-0575314.

Following the biological treatment of sewage, it may be passed to a clarification or settlement tank, of the type in which the water flows towards a peripheral weir.

In the region of the weir, there is net upwards hydraulic flow, whether the tank is relatively deep, e.g. of the Dortmund type, or shallow, as in the case of a horizontal flow tank.

Even the best of sedimentation plants may suffer from solids carry-over problems. Sludge can rise up, due to thermal effects, wind action, denitrification or flow rate variation. As standards become more demanding and costs have to be minimised, it is vital that intermittent solids carry-over is prevented without the need for significant capital expenditure.

Summarv of the Invention The present invention is based on the surprising discovery that buoyant material, e.g. Kaldnes medium, can be used as a filtration medium at the top of a settlement tank, to reduce suspended solids. As indicated above, this may follow biological treatment. Thus, both treatments may use the same material, but for different purposes.

For use in the invention, a buoyant medium is retained in the upward flow path towards the peripheral weir of a clarifying tank. By retaining the medium, e.g. in a suitable mesh, the invention is readily adapted for addition/retrofitting to existing final settlement tanks at a comparatively low cost. In a specific embodiment, the invention uses a unique system of Kaldnes micro-tube settlers, randomly packed into cages or restrained by mesh.

The cages are suitably placed around the periphery of final settlement clarifiers, just below the top water level.

Occasional back-washing quickly removes the suspended solids retained by the micro-tube settlers; no other maintenance is required.

This invention provides several advantages, at low cost, including removal of up to 30% of residual suspended solids from final effluent, dampening of wave action in clarifiers, formation of a barrier to prevent clumps of sludge floating to the surface following trickling formation, and the avoidance of added chemicals; the invention is environmentally-friendly.

Description of the Invention The tank may be of any suitable design, e.g. of the Dortmund or horizontal flow type, or the buoyant medium can be used to replace gravel in a Banks' clarifier. In perhaps the simplest embodiment of the invention, the material may simply be introduced into the tank, to provide a surface layer; in this case, the medium needs to be retained only by the walls of the tank or, preferably, by a peripheral mesh material that prevents any of the medium from passing over the weir. Depending on the size of the tank, however, it may be more economical to retain the medium as a layer adjacent to the weir, e.g. in a mesh or basket extending around at least the sides and base of the layer.

As indicated above, the medium may be used in place of gravel in a Banks' clarifier, in which case the medium will typically be retained below the surface of the water in the tank, i.e. involving the use of mesh material above the medium. Perhaps in order to reduce flow restriction, however, it is generally preferred not to retain the medium below the water surface.

Clearly, for efficient operation, the water flow path through the medium should be sufficiently long to have an effect, e.g. at least 200 mm, and preferably about 300 mm.

However, such quantitative restrictions will be determined, not in the abstract but with relation to the particular circumstances, including the local water characteristics, the nature and size of the tank, the nature of the medium, and other factors. Suitable mesh materials are also known, and will be chosen accordingly.

The invention will be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a sectional view of part of a clarifying tank of known type; Figures 2 to 4 are sectional views of clarifying tanks of the type shown in Fig. 1, but each modified by the inclusion of retained material (shown schematically, not to scale) so that it embodies the present invention; and Figures 5 and 6 are each cross-sectional elevations of settlement tanks embodying the present invention, respectively analogous to Figs. 2 and 3.

The following Examples illustrate the invention, with reference to the accompanying drawings.

Examples Tests were conducted on a conventional biological filtration works treating domestic sewage from a population equivalent of 12,300. Consented dry weather flow was 1,700 m3/d.

After coarse screening and primary treatment (three Dortmund tanks), the flow was split into two halves each with two circular biological filters, using motorised distributors and four Dortmund humus tanks. The combined humus tank effluent was "polished" by passing through a lagoon.

As shown specifically in Fig. 1 (without repetition of the same numerals for identical parts in Figs. 2-4), each tank 1 has an overflow weir 2 defining a top water level 3.

Each tank has also a sub-surface peripheral ledge 4 and a skeleton steel framework 5. This facilitates the retrofitting of aluminium mesh sheeting to retain Kaldnes media in the three different configurations of Figs. 2 to 4. The first tank (Fig. 1) has a scum board 6 in the stilling chamber'7, but this was removed from the other tanks.

Fig. 2 shows a layer of mesh 8 across the whole plan area of the tank, supported by existing steel girders (this is to prevent medium entering the body of the tank if the water level is lowered for any reason), and a "collar" of mesh 8a around the inside edge of the weir to prevent medium leaving the tank with effluent. Medium 9 covers the tank surface, to a depth of about 200 mm.

Fig. 3 shows a basket-type structure of mesh 8 around the periphery, to retain medium 9, to a depth of around 200 mm. Fig. 4 is designed on the Banks' clarifier principle, with a 200 mm deep layer of medium 9 sandwiched between two layers of mesh 8 around 400 mm beneath the water level.

Before carrying out these modifications, a preliminary survey was conducted to check the uniformity of the four humus tanks in terms of their individual performance. No significant differences were observed, when comparing the effluents from all four tanks.

The tanks shown in Figs. 2-4 all contained Kaldnes medium, i.e. randomly-packed polythene tube-like members, including an internal surface. Care was taken to minimise disruption to treatment. Snap samples of feed to the tanks plus individual effluents were taken 3-4 times per week from then on, and the results of chemical analysis compared. Flows and temperatures were noted on each visit.

Back-washing was done when deemed necessary by (a) turning off the feed to tank, (b) cracking open the sludge outlet valve (not shown), (c) pumping final effluent at high pressure onto the medium whilst allowing the water level to slowly fall, (d) leaving the tank quiescent for an hour, and (e) restoring normal operation. The frequency of backwashing was every 2-3 weeks.

The use of the Kaldnes medium lead to an overall improvement in solids removal in all configurations, but especially so in the tank of Fig. 2, the simplest arrangement. Here the average improvement in suspended solids removal was 29%, the others being 15 and 6% respectively.

Day-to-day performance was variable both for the control tank and the modified tanks. There was no evidence of any correlation with daily sewage volume. Compared with Banks' clarifiers, Kaldnes medium requires less frequent back-washing, and the procedure may take rather less time.

Using a high pressure pump, the time taken in this exercise varied from 15 mins for the tank of Fig. 3 to 30 mins for the tank of Fig. 2. Considerable quantities of black humus were thus released into the body of the tank and removed along with the settled humus sludge during the autodesludging cycle. The medium was visibly cleaner although some algal discolouration was retained.

In summary, this invention provides, at least in one embodiment, a modification to an existing final settlement tank following biological oxidation on a water-water treatment works. In passing through the medium, a proportion of the suspended matter in the supernatant liquor is entrained within the interstices and hollow structure of the medium, and the quality of the effluent is thereby improved. Cleaning is necessary from time to time, and is effected by manual back-washing using water or final effluent, the released solids being allowed to settle out in the tank.

Claims (10)

1. A clarifying tank of the type in which the water flows upwardly towards a peripheral weir, wherein a buoyant medium is retained in the upward flow path.

2. A tank according to claim 1, wherein the medium is of the type adapted to support microbiological growth.

3. A tank according to claim 2, wherein the medium is Kaldnes medium as defined in EP-A-0575314.

4. A tank according to any preceding claim, which is a settlement tank, and includes means for removing settled solids.

5. A tank according to claim 4, which is a Dortmund tank.

6. A tank according to any preceding claim, wherein the medium is retained by a mesh, as a layer adjacent to the weir.

7. A tank according to any preceding claim, wherein the medium is not retained below the water surface.

8. A tank according to any preceding claim, wherein the minimum flow path through the medium is 100 mm.

9. A tank according to claim 1, substantially as herein described with reference to any of Figures 2 to 6 of the accompanying drawings.

10. A sewage treatment system comprising apparatus in which the water is subjected to biological oxidation and then to clarification in a tank according to any preceding claim.