GB2024795A - Purification of effluent - Google Patents

Abstract

Liquid effluents are treated by subjecting them initially to biological treatment on an aerobic bacterial bed at a high load then physico-chemical treatment such as flocculation without the need for intermediate decantation.

Description

SPECIFICATION Purification of effluent The present invention relates to treatment of liquid effluents which may be of urban and industrial origin.

For treatment of effluent there are normally used biological treatments which allow a considerable cleaning and in particular a very high yield with res peck to reduction in biological oxygen demand (DBO 5) and chemical oxygen demand (DCO), but they have however a number of disadvantages which create difficulties which may be serious, in particular: -they require expensive investment, -they are heavy and cumbersome, -theyaresubjectto poisoning, -they are badly adapted to rapid variations of input of effluent to which many industrial effluents are subject, as discharges from bathing stations and winter sports stations.

For this reason there has appeared in the past few years a certain interest in physico-chemical treatments. These treatments do not have the disadvantages of biological treatments but on the other hand their performances, even though they have improved, do not allow a sufficient cleaning to allow in all cases discharge of the treated water directly into the natural surroundings.

Thus, with respect to the DBO 5 of urban effluents physico-chemical treatments are not generally capable of a yield of greater than 70 to 75% whereas more than 90% is required to conform to legislation and is easily obtained only by biological systems. To achieve these yields it is necessary to complete the physico-chemical treatment by a finishing treatment which allows standards for discharged material to be met but then all or part of the advantages of physico-chemical systems are lost.

Physico-chemical treatment may be followed by a biological treatment intended to achieve purification which would allow standards to be satisfied but on the other hand this would, - increase the cost of investment to a large extent, as it is necessary to add to the physico-chemical system a vessel for biological treatment and an additional decanter for separation of biological sludge, - obstruct the functioning of the biological system as flocculants from the physico-chemical system would reduce biological activity, - produce an effluent which is less clear than that produced by physico-chemical treatment because of the presence of colloidal sludge resulting from the biological purification, - require a site area which is considerably greater.

The present invention relates to a method and apparatus intended to allow reduction to a great extent of these disadvantages while allowing a yield on purification which is high, compatible with discharge of the product into the natural surroundings.

According to one aspect of the invention, there is provided a method of purifying liquid effluents comprising aerobic biological treatment at high load by a fixed biomass followed by physical and/or chemical purification treatment.

Preferably the reduction in DBO Sin biological treatment is from 15 to 40%, most preferably between 20 and 30%. The physico-chemical treatment may have only external evacuation whereas the biological treatment may use an internal recycling.

According to another aspect, there is provided apparatus in which the quantity of fixed biomass is that which ensures, forthe biological system operat ing at high load, and elimination of 15 to 40% and preferably 20 to 30% or original DBO 5.

Embodiments of the invention will now be described by way of example with reference to the drawings, which show: Figure 1: a diagram of apparatus according to the invention, Figure 2: a variant of part of the apparatus of Figure 1, and Figure 3: a curve showing the response of a biologi cal bed having a fixed biomass of the "Flocor" type.

Figure 1 shows a diagram of a purification apparatus for liquid effluent combining a primary biological system working at a high mass load and a physico-chemical system.

The apparatus comprises conventional initial means A for pretreatment capable of stopping the largest solid material and eliminate certain phases in suspension. These means A are for example a grid 1 formed by a grill of bars which carries out coarse filtering, a sand remover 2, an oil remover 3 and optionally a primary decanter 4. Following this means A, there are arranged the biological system B and physico-chemical system C.The biological system B is a system using aerated flow having a biomass fixed on a support comprising a conventional bacterial bed or on plastics material shown schematically at 5, or biological discs which are irrigated by the effluent to be treated; the effluent is distributed by means of a device 6 ensuring its distribution, for example a rotary distributor or a system of channels feeding onto indented plates having numerous jets which discharge multiple droplets.

Advantageously there will be used as a bacterial bed Flocor modules of Imperial Chemical Industries Ltd., stacked inside a tower 7. The modules are constructed by assembly of flat plates and moulded plates having a double undulation, that is to say undulations of which the crest has a generally vertical direction and is itself undulating. These plates, intended to be covered with a film of bacteria, are further crimped in order to provide a good anchorage for bacterial sludges forming the purification medium. The number of modules used is proportional to the quantity of effluent to be treated and chosen such that the yield of purification in the biological system is from 15 to 40%. Below the tower 7 of Flocur modules a pan 8 recovers the effluent in a tank 9.From this tank 9 a part of the effluent is returned to the head of the bacterial bed by pumps 10 and another part is sent by overflow into a vessel 11; from there the effluent flows to the physicochemical system C.

The physico-chemical system C comprises first of all a flocculator 12 essentially formed by a vessel receiving the effluent to be treated and in which a mineral flocculent such as iron or aluminium salt and/or a polyelectrolyte prepared in a reservoir 13 is introduced by dosing pumps 14. This system then comprises a separator 15 having principally the pur pose of separating the flocs resulting from the preceding flocculation. It may be a decanter where the decanting is natural, lamellar or centrifugal, but it may further be a filter.

Alternatively, as shown in Figure 2 a floatation cell 16 using air fed from below or an electrofloatation cell may replace the decanter or filter 15.

Advantageously to allow regular feed of the systems B and C with effluent to be treated there will be arranged upstream of the biological system B a storage vessel 17 where the effluent is taken up by a constantflowpump 18 to deliver it to a distributor device 6 to traverse the bacterial beds.

To use the device according to the invention, first of all the DBO 5 of the effluent to be treated is determined, and knowing the response curve of a biological system having a fixed biomass for example of the Flocor type, as a function of the load applied, there is determined the volume of the bacterial bed to be used so that a yield of the order of 15 to 40% and preferably 20 to 30% is obtained. Figure 3 gives for a Flocor module such a curve in which the load applied at the purification station expressed as DBO 5 in kg per day and per cubic metre of bacterial bed is abscissa and the percentage reduction of DBO 5 is ordinate.

Thus in the case of Flocor modules a purification yield less than 40% will be obtained in a device dimensioned so thatthe volume load applied is greater than 6 kg DBO 5 per m3 and per day. By way of further example it may be indicated that a yield on purificatin less than 35% will be obtained for an applied volume load greater than 7 kg of DBO 5 per m3 and per day. It will be apparent that compared to biological systems of the prior art, that of the present invention is under-dimensioned.

The effluent to be treated, initially freed of oils by de-oiling, solid matter of large dimensions by the grid and then possibly having been briefly decanted, is introduced into the storing vessel 17. From there the pump 18 sends it a uniform rate to a distributor device 6 which feeds the bacterial beds 5 or the biological system B. The liquid flows through the active bacterial mass. It causes a regular erosion of the biological film and generates a permanent renewal of the living purifying mass which thus retains a rate of development and activity which is high.The volume of the bacterial bed having been chosen as a function of the rate of feed of effluent and its content of polluting substances so that the yield of the system will be small and of the order of 15 to 40%, the residence time of the liquid in this system is very short and of the order of 5 minutes.

It is known that effluents of the urban type generally contain organic pollution comprising about 1/3 of insoluble material, 1/3 of colloids and 1/3 of dissolved matter. Biological treatment has a large effect on the dissolved material which is attacked first, such that during the small time of contact with the bacteria practically only the dissolved materials are attacked.

The effluent after traversing the bacterial bed is recovered at the base of the biological system in the pane 8 from which it flows into the tank 9 and then to the vessel 11 and is introduced into the flocculator 12 where under the effect of aluminium sulphate flocculation of colloidal materials is produced, then into the decanter or filter 15 orfloatation cells 16. Part of the liquid recycled by the pump 10 from tank 9 ensures permanent wetting of the baterial bed even in the case of breakdown of the devices situated upstream. The effluent treated by the two systems, initially biological and then physico-chemical, is purified substantially completely and may easily achieve the degree of purification of 90% required for discharge into the natural environment.It is clear, contrary to liquid which even with an identical degree of purification has been treated entirely by biological means. On the other hand the biological sludges which result from the erosion of the biological film which in any other configuration of purification installation (whether an installation which is purely biological or a physico-chemical installation followed by a biological installation) would have to be recovered by decantation in a decanter reserved entirely for this purpose, are here eliminated in the physico-chemical system at the same time as the colloids and the insoluble matter of the initial liquid, thus reducing the cost of investment.

The absence of a decanter between the two systems A and B, which allows important economy in investment, requires necessarily the use of biological systems having a fixed biomass as all other systems need considerable recycling of biological sludge and this recycling is incompatible with the presence of chemical flocculants which reduce the biological activity.

Further such a purification assembly has a high speed of start-up and a great flexibility in use. The starting up of a physico-chemical system C is practically instantaneous and the biological system B requires only a few days for initial starting up, biological systems using bacterial beds generally having a small inertia. In any case during the first two or three days of use of the bacterial system the purification is not zero as would be the case with s system which is entirely biological, not yet operating, as some purification is carried out by the physical and physico-chemical systems A and C having an instantaneous response.

Further the cost of use is reduced because of the contribution to flocculation of the flocculants of biological origin which appear in the bacterial bed and which are introduced into the flocculator with the effluent. Additionally if the feed of polluted effluent is reduced for example outside seasons of popularity in bathing or winter sports stations, the flocculation which, during use, represents a certain cost may be discontinued. The biological system B and separation of sludge in system C may then remain in action on their own with this time an effi ciencywhich is much greaterthan that demanded of it when it operates at full load associated with a complete physico-chemical system so as to give a purification in accordance with legislation.

Claims (12)

1. A method of purifying liquid effluents comprising aerobic biological treatment at high load by a fixed biomass followed by physical and/or chemical purification treatment.

2. A method according to Claim 1, in which the load of the biological treatment is such that from 15 to 40% of the original DBO 5 is eliminated by the biological treatment.

3. A method according to Claim 1 or 2, in which the load of the biological treatment is such that from 20 to 30% of the original DBO 5 is eliminated.

4. A method according to any one of the preceding claims, in which said physical and/or chemical treatment comprises chemical flocculation followed by separation of floc.

5. A method according to Claim 4, in which the flocculation is carried out by a salt of iron and/or aluminium and/or a polyelectrolyte.

6. A method of purifying liquid effluents, substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.

7. Apparatus for purifying liquid effluents comprising a biological purification system comprising a system of bacterial beds having a fixed biomass and aerated flow followed by a system for physical and/or chemical purification of the effluent.

8. Apparatus according to Claim 7, in which the quantity of fixed biomass is that which ensures, for the biological system operating at high load, an elimination of 15 to 40 /O and preferably 20 to 30% of original DBO 5.

9. Apparatus according to Claim 8, in which the biological system is provided with an internal recycling circuit for effluent, whereas the physico-chemical system is not.

10. Apparatus according to Claim 8 or 9, in which the biological system is preceded by a regulator for regulating the rate of feed of effluent thereto, the regulator comprising a storage vessel for effluent and a pump operating at a constant rate to feed effluent from the vessel.

11. Apparatus according to any one of Claims 8 to 10, in which said physical and/or chemical system comprises a flocculator and a floc separator.

12. Apparatus for purifying liquid effluent, substantially as herein before described with reference to Figure 1 or 2 of the accompanying drawings.