GB2198123A - A method of and apparatus for treating waste water - Google Patents

Abstract

A system for treating waste water, comprising first and second reactor vessels selectively connected to feeding means for incoming liquor to be treated, and including aeration means and containing biomass support bodies. Further means are provided for periodically cleaning the support bodies to remove contained biomass therefrom, and control means to direct the flow of liquor sequentially to the two reactor vessels, and to drain effluent liquor sequentially therefrom so that whilst one vessel is aerating and mixing the liquor, clarification by sedimentation is taking place in the other, and the roles are periodically reversed.

Description

A METHOD OF AND APPARATUS FOR TREATING WASTE WATER THIS INVENTION concerns a system for treating waste water and is particularly, though not exclusively applicable to small or medium sized waste water treatment plants such as those used on industrial sites or in selfcontained municipal areas.

Such waste water treatment plants of a conventional design may comprise a contact stabilisation vessel in which incoming waste water is initially aerated, and in some instances, there might be a residence period therein for partial sedimentation or general conditioning of the sludge prior to an extending aeration stage when the liquor is fed into a vessel where vigorous aeration takes place and the sludge is activated in the presence of a micro-biological population which feeds on the waste water as nutrient.

To promote the growth of biological material (biomass) it is known to include within the vessel a plurality of freely movable biomass support bodies, for example, of reticulated foamed plastics. Small cubes (pads) of this material move freely within the vessel.

Their surfaces remain free of biomass by virtue of the repeated knocking or rubbing action between the pads, but the substantial voidage defined by the reticular material of each pad provides an environment in which the biomass may develop and be sustained.

An object of the present invention is to provide a method and apparatus for treating waste water, utilising the aerated sludge process, and particularly using biomass support pads of the kind referred to.

According to the present invention there is provided a method of treating waste water comprising the steps of feeding the liquor to be treated sequentially to at least two reactor vessels each containing biomass support bodies; aerating and mixing the liquor and the bodies in one of said reactor vessels whilst, in the other reactor vessel separating the solid and liquid fractions by sedimentation, and sequentially reversing the said roles of the two vessels; and periodically or continuously cleaning the biomass support bodies to remove contained biomass therefrom.

Preferably, the liquor is initially treated to separate suspended solids therefrom.

Further according to the invention there is provided a waste water treatment plant comprising at least first and second reactor vessels selectively connected to feeding means for the liquor to be treated, and including aeration means, and containing biomass support bodies; means for periodically or continuously cleaning the bodies to remove contained biomass therefrom: and control means to direct the flow of liquor through said feeding means sequentially to the two reactor vessels, and to drain effluent liquor sequentially therefrom.

Preferably said plant includes a primary clarifier to receive incoming waste water and to separate suspended solids therefrom by sedimentation.

Preferably, the plant includes a sludge conditioning vessel to which may be fed the biomass removed from said bodies, and/or settled sludge discharged from the primary clarifier.

Further according to the invention the biomass support particles present in said reactor vessels act as a solids - liquid filtration medium during the discharge of treated effluent liquor from said vessels such that said effluent liquor remains substantially free of suspended solids.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a block diagram to illustrate the steps comprised by the method; and Fig. 2 is a schematic plan view of a plant constructed to carry out the method.

Waste water to be treated and containing suspended solids, is initially fed to a primary clarifier 10. Two batch reactor vessels 11 and 12 are adapted to be fed sequentially with liquor from clarifier 10 via valved pipes lla and 12a.

Each vessel 11, 12 contains biomass support bodies which in a preferred form are comprised by reticulated plastics foam pads each shaped approximate to a cube of some 25mm side dimension. Each pad defines within its reticulated filamentary structure a substantial voidage representing some 97% of the bulk unit volume of the pad, in which biomass may develop and be sustained by a nutrient liquor within the vessel. The bodies are of such a shape that they can move freely within the liquor in the vessel with a rubbing or knocking action to prevent a build-up of biomass on the outer surfaces of the bodies.

A device 13 is provided to take pads perIodically or continuously from each reactor 11, 12 and to squeeze them in order to free contained biomass therefrom whereupon the bodies are returned substantially empty to the reactor vessel from which they were taken.

A sludge re-aeration or contact vessel 14 is provided to receive primary settled sludge from clarifier 1, together with the sludge removed by the pad cleaner 13.

Sludge re-aerated and conditioned in vessel 14 can be reintroduced into clarifier 10 along with raw incoming waste water. Sludge surplus to the process requirements can be wasted from either reaeration vessel 14 or from primary clarifier 10.

An automatic control system generally indicated at 15 is arranged, as will be described, to direct the flow of supernatant liquor from clarifier vessel 10 to one or other of the batch reactors 11, 12. Generally, therefore, the sequence of the process steps is as follows: Raw untreated waste water is initially fed continuously to primary clarifier vessel 10. In a first phase of the process supernatant liquor from vessel 10 is fed via pipe lla to reactor vessel 11. Whilst the latter is filling its contents are vigorously aerated and mixed using, for example, an air sparge system in the base of the vessel. During this filling, aeration and mixing process in vessel 11, vessel 12 is undergoing a quiescent phase in which the contents which had been aerated and mixed during a previous stage of the process, are allowed to separate by sedimentation.After a short period of rapid sedimentation the supernatant liquor in vessel 12 is decanted via line 16 as clarified final effluent.

In the period during which the contents of vessel 12 are settled and the final effluent is decanted, vessel 11 will have completed filling and will be thoroughly aerated and mixed.

Control device 15 then discontinues the supply of air to vessel 11 and liquid flow in pipe lla, and simultaneously commences flow in pipe 12a and the supply of air to vessel 12. Thus the roles of vessels 11 and 12 are reversed so that vessel 11 undergoes a sedimentation stage leading to decanting of final effluent at line 16 whilst the increasing contents of vessel 12 are aerated and mixed.

The cleaning of the foam pads in vessels 11 and 12 is carried out continuously or intermittently by removing pads from the respective vessels into cleaner 13 which squeezes the sludge from the pads and returns them, substantially empty, to their respective vessels. This removal of sludge from the pads is sufficient to maintain a constant volume of solids within the vessels and to prevent a build-up therein.

Referring now to Fig. 2, there is shown a plan view schematically of a composite structure incorporating the principal parts of the apparatus described in connection with Fig. 1.

The central area represents the primary clarifier vessel 10 to which the waste water to be treated is fed as indicated by line 17 and arrow 18. Reactor vessels 11 and 12 are illustrated as segments of the circular structure, and the flow of liquor to them is illustrated by lines llb and 12b. In practice, the sequential flow may be controlled by variable-height weirs between vessel 10 and vessels 11 and 12.

Air sparges 19 are provided in the base of the structure.

In this embodiment, in each vessel 11, 12, a mesh screen 20 separates a first zone 21 from a second zone 22, the latter containing the biomass support pads. High intensity aeration takes place in zone 21 and the air circulates between the latter and zone 22 through screen 20.

The pad cleaner 13 is positioned between vessels 11 and 12, and, once again, numeral 16 represents the final effluent passages.

Finally, vessel 14 is illustrated as occupying the remainder of the circular structure. Flow of liquor and solids between the various vessels of the structure can be by overflow weirs or valved pipes as required.

Several advantages accrue from a batch reactor system of the kind described. In a small or medium sized plant as may be found on industrial sites or in small municipal areas, the sequential operation of the two reactor vessels with the attendant benefits of the biomass support pads in enhancing the development of biomass for the rapid treatment of the waste water, enables an increased hydraulic loading of the system so that its capacity is increased by 100t or more when compared with a system incorporating a single reactor vessel with or without biomass support bodies.

The filtration effect of the pads during the settling and decanting stage avoids the need for a secondary clarifier or other residence tank.

Many existing plants operating in a so-called contact stabilisation mode or in an extending aeration mode may be readily converted to operate in accordance with the invention since much of the plant equipment is present and needs only to be converted to provide two or more batch reactor vessels of generally equal size and capacity.

Relatively insubstantial changes in pipework and valving may be carried out without undue expense to enable the plant to be converted rapidly into a form which can operate in accordance with this invention. Thus, the process may be applied to many existing plants in addition to those yet to be built.

Experimental work has shown that effluent discharged from the batch reactor process should exceed the traditional requirments for secondary treatment of 30 mg/l BOD and 30 mg/l suspended, solids. Indeed, expected levels of performance will be less than 10 mg/l for both BOD and suspended solids during normal routine operation.

It is not intended to limit the invention to the above example only, many variations, such as might readily occur to one skilled in the art, being possible without departing from the scope of the invention.

For example, the screens 20 and 23 in vessels 11 and 12 may be omitted in certain cases where, for example, aeration is provided by means other than as illustrated and described. In addition, where necessary a separate pad cleaner may be provided for each reactor vessel.

Furthermore, the sludge re-aeration vessel 14 can be replaced by an aerobic or anaerobic digestion tank from which only a portion of the sludge is returned and mixed with the incoming waste water.

Claims (12)

1. A method of treating waste water comprising the steps of feeding the liquor to be treated sequentially to at least two reactor vessels each containing biomass support bodies; aerating and mixing the liquor and the bodies in one of said reactor vessels whilst, in the other reactor vessel, separating the solid and liquid fractions by sedimentation, and sequentially reversing the said roles of the two vessels; and at least periodically cleaning the biomass support bodies to remove contained biomass therefrom.

2. A method according to Claim 1, wherein the liquor is initially treated prior to entry into either of the two reactor vessels, to separate out suspended solids.

3. A method according to Claim 1, wherein treated effluent liquor is discharged from the reactor vessel under sedimentation, and the biomass support bodies therein act as a solids-liquid filtration medium during the discharge of said treated effluent liquor such that the latter remains substantially free of suspended solids.

4. A method according to Claim 1, wherein the biomass support bodies are of such a shape and have such surface characteristics that they can move freely within the liquor in the vessel with a rubbing or knocking action to prevent a build-up of biomass on the outer surfaces of the bodies, and without interlocking and packing together.

5. A method according to Claim 1, wherein each said biomass support body comprises a reticulated plastics foam pad of generally cuboid shape, each pad defining within its reticulated filamentary structure a substantial voidage representing some 97% of the bulk unit volume of the pad whereby biomass may develop and be sustained by nutrient liquor within the vessel.

6. A method according to Claim 5, wherein said foam pads are periodically removed from the reactor vessel and squeezed to remove contained biomass, and thereafter returned to the reactor vessel.

7. A method according to Claim 1, wherein the liquor is aerated by an air sparge system introducing air at the base of either of the two reactor vessels whilst it is filling with liquor to be treated.

8. A waste water treatment plant comprising at least first and second reactor vessels selectively connected to feeding means for the liquor to be treated, and including aeration means, and containing biomass support bodies; means for periodically or continuously cleaning the bodies to remove contained biomass therefrom; and control means to direct the flow of liquor through said feeding means sequentially to the two reactor vessels, and to drain effluent liquor sequentially therefrom.

9. A waste water treatment plant according to Claim 8, including a primary clarifier to receive incoming waste water and to separate out suspended solids by sedimentation.

10. A waste water treatment plant according to Claim 9, including a sludge conditioning vessel to which may be fed the biomass removed from said bodies, and/or settled sludge discharged from the primary clarifier.

11. Apparatus according to Claim 8, comprising a structure having a central vessel serving as a primary clarifier, and segmental vessels around the latter divided radially to provide said first and second vessels a further segmental vessel serving as a sludge conditioning vessel.

12. Apparatus according to Claim 11, including an aerobic or anaerobic digestion tank from which sludge separated from said effluent liquor is returned to one or other of the reaction vessels to be mixed with incoming waste water.