GB2130571A

GB2130571A - A method for treating polluted liquid effluents

Info

Publication number: GB2130571A
Application number: GB08327873A
Authority: GB
Inventors: Jacques Simond
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-11-19
Filing date: 1983-10-18
Publication date: 1984-06-06
Also published as: GB2130571B; FR2536381B1; GB8327873D0; DE3337212A1; FR2536381A1; NL8303822A

Abstract

A method for treating polluted liquid effluents includes spreading the effluent to be treated through a heap of slag composed of the solid residues from the incineration of household refuse.

Description

SPECIFICATION A method for treating polluted liquid effluents The subject of the present invention is a method for treating polluted liquid effluents.

Before they are returned to the natural environment treatment must be carried out on polluted liquid effluents, such as the waste water left after the lixiviation (leaching) of household refuse and industrial waste, the polluted water from towns or the waste coming out from food industries such like abbatoirs.

Generally this treatment is carried out in biological and physicochemical purification plants which have a very bad economic return, to the extent that they are merely added on to an outlet for industrial waste or require burdensome conveyances.

The operation of a purification plant and the quality of the result obtained are all the more uncertain because the composition of the effluent liquids to be treated is very complex, due to the great variety of substances stored in the discharges, as the load of the pollutant varies in the course of time depending on the products discharged and due to the great variation in the flow of effluent emitted depending on meteorological conditions.

The present invention aims to obviate or mitigate these drawbacks.

According to the present invention there is provided a method of treating liquid effluents including spreading the effluent to be treated through a heap of slag composed of the solid residue from the incineration of household refuse.

According to another aspect of the invention there is provided means for treating liquid effluent comprising a heap of slag composed of solid residues from the incineration of household refuse.

The thermal effect resulting from the slow combustion of incombustible carbon found in the slag causes at least some part of the water to be eliminated by evaporation. This thermal effect is promoted by the black colour of the slag which enables maximum absorption of the incidental solar energy.

If the diffusion of polluted water through the heap of slag is greater than the rate of evaporation, part of the effluent passes through the heap without being evaporated and is recovered at the bottom of the heap.

Nevertheless it should be noted that the liquid effluent coming out from the heap of slag has undergone a great reduction in the load of pollutant it contains.

First of all this reduction in the load of pollutant is effected by filtration. In fact the slag appears as a mass of solid particles of very variable size. These particles of varying size act as a filter by retaining some materials in suspension and they effect clarification of the effluent. Now the materials in suspension form one component for assessing the degree of pollution of the water.

The passage of the effluent through the heap of slag is also indicated by the fixation of a part of the load of pollutant by physicochemical reactions of several types: - reactions giving rise to oxidation and reduction; for example metallic iron is oxidised to form Fe203,xH20 which is insoluble in water; - acid-base reactions of the calco-carbonic type; the lime present in the slag is thus changed into calcium carbonate; the traces of heavy metals present are precipitated as insoluble carbonates or hydroxycarbonates; - reactions giving rise to oxidation and decomposition with regard to compounds which can be oxidised such as cyanides, phenols or sulphides; - the reaction of active carbon with the absorption of organic molecules causing odours and colours.

Iron and its oxides as well as the oxides of various other metals act as catalysts for the reaction.

It is interesting to note that they are present in the slag as the removal of iron from the slag before it is made into a heap does not affect them.

Preferably the slag used in the heap has an organic carbon content of at least 6%. This minimum percentage is important to enable the thermal effect to be set in motion.

Preferably the slag consists of: - 4to 14% incombustible organic elements - 4 to 14% of lime (CaO), - 12 to 13% of scrap-iron, - 40 to 50% of silica (SiO2) made up of glass or other rubbish, - up to 30% water, - metal oxides such as Fe203, Al203, Na2O, MgO and Mn2O3, ensuring a complement of 100%.

Advantageously this process consists of bringing into being a heap of slag at least four metres thick, which appears as a truncated pyramid; of covering over the lateral surfaces of the heap with a coating of water-tight material such as a sheet of synthetic material; of fitting substantially horizontal ducts to form drainage tubes emerging on to the lateral surfaces of the heap and pointing for preference in the direction of the prevailing winds; and of fitting at least one vertical chimney pierced by radial holes and extending the full height of the heap, the bringing of the effluent into contact with the heap being effected by diffusion to the upper part of the heap.

Covering the lateral surfaces of the heap restricts the amount of rainwater brought in as this, in too large amounts, could swamp the slag and prevent the combustion of the carbon.

The drainage tubes at the sides ensure that the heap of slag is aerated and, as a consequence of the oxygenation of the heap, they promote the thermal effect throughout the entire mass of the pile. The chimneys, for their part, ensure that the evaporated part of the effluent passes out into the air.

Depending on the properties of the slag which are utilised and the type of effluents to be treated, it is possible to improve the purifying operations by adding to the slag a certain number of products, especially oxidising constituents, lime, active carbon, charcoal and zeolites.

Oxidants may, for instance, be incorporated as sodium hypochlorite when the DCO (chemical requirement for oxygen) is too considerable.

When the effluent has too high a metal content the incorporation of lime enables complete precipitation of the metal ions as insoluble hydroxides which remain caught up in the slag due to its filtering properties. It is also possible to use slag or cinders from blast furnaces for the same thing.

Active carbon may be added when the slag contains too little organic carbon and does not perform its function of purification efficiently with regard to organic pollutants and dyes.

The incorporation of charcoal enables the duration of the thermal effect to be lengthened, of which it should be observed that this relates to a phenomenon which has the shortest life-span.

Finally zeolites, which act by iron absorption and exchange, may usefully be incorporated in order to complete the purifying action of the slag.

Preferably the additives are retained in bags with permeable linings which are distributed throughout the interior of the heap of slag.

According to an alternative method the additives are put into a device through which the effluent passes after having been treated in the heap of slag.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 shows an effluent treatment installation in median longitudinal cross-section, and Figure 2 shows a view of the installation from above.

The installation shown on the drawing consists of a heap of slag 2, four metres thick formed of solid residues obtained by the incineration of household refuse and shaped like a truncated pyramid and resting on a V-shaped area of ground 3 at the bottom of which a collector 4 has been placed to collectthe discharged liquid.

The lateral surfaces of the heap are covered by a sheet of water-tight synthetic material 5.

Substantially horizontal ducts 6 are arranged at several levels inside the heap, opening out on to the lateral surfaces to form drain pipes which enable ventilation of the inside ofthe heap. The ducts are arranged in the direction of the prevailing wind.

Two chimneys 7 having radial perforations 8 extend the full height of the heap of slag and are located in the central area of the heap. The introduction of effluent is in part effected by a duct 9 above the heap which is equipped with sprays 10.

The slag consists of: - 4to 14% incombustible organic elements, - 4to 14% of lime (CaO), - 12to 13% of scrap-iron, - 40 to 50% of silica (six2) made up of glass or other rubbish, - up to 30% water, - metal oxides such as Fe203, Awl203, Na2O, MgO and Mn203, ensuring a complement of 100%.

From a practical point of view the liquid effluent distributed by the sprays 10 will go right through to the inside of the heap of slag, part of the effluent evaporating, especially via the funnels 7, while the remainder undergoes a detoxification process before being discharged by way of the collector 4.

A description is given below of one example in which the treatment of a polluted effluent is carried out.

Before processing this effluent is black in colour and has a fecal odour.

Its composition is as follows: DBO5 (biological requirement for oxygen) 4680 mg/litre, DCO (chemical requirement for oxygen) : 6422 mg/litre, Organic Nitrogen : 67,5 mg/litre, Total hardness : 122 F, Ammoniacal Nitrogen : 211,5 mg/litre, Iron : 13,5 mg/litre, Zinc : 0,25 mg/litre, Phenol : 14,5 mg/litre, Phosphorus (as PO4S) : 0,9 mg/litre.

The slag is in a heap weighing 4300 tonnes which has enabled 150 m3 of polluted effluent to pass through it for a time of 500 hours. Out of the 150 m3 effluent, 75 m3 have been recovered from the lower part of the heap, which means that half the volume of the effluent has disappeared by evaporation of the water by reason of the thermal effect.

The effluent leaving the heap of slag is colourless and odourless and the aggregate reduction in pollution has been assessed in detail as follows: DBO5 (biological requirement for oxygen) : 89%, DCO (chemical requirement for oxygen) : 85%, Total organic carbon ~ 85%, Organic Nitrogen : 71%, Total salinity : 39%, Ammoniacal Nitrogen : 63%, Phenol index : 99.6%, Iron (total) : 99%, Zn (Zinc) : 80%, Phosphorus : 75%, Hardness (water) : 91%.

It should be noted that at the end of 500 hours the processing capacity of the slag had not been exhausted.

Since it arises from what comes before, the invention brings about a great improvement in the existing technique by providing a process which especially possesses the following advantages in particular: - the raw material and its possible additives are waste products of no great economic value; - the method works without difficulty when the flow varies in quantity and quality; - placed up-stream from a biological purification unit the system enables the correct operation of the latter in eliminating toxic substances harmful to the bacterial growth; - there are no by-products of the method for which it is necessary to ensure further treatment as is the case with standard mills for treating waste water.Once the efficiency of the slag has been exhausted, this becomes non-polluting waste which can remain in a rubbish heap; - the operation costs of the system are minimal, since they are confined to the spraying of the heap of slag with the effluent to be treated; - if the effluent does not contain a heavy load of toxic products, this method can be sufficient to produce a non-polluting discarded material which can be cast back into the natural surroundings.

In a modified method additives are included in the slag, for example oxidising constituents, lime, active carbon, charcoal, zeolites. These are included in the heap in bags with permeable linings or may be contained in a device through which the effluent passes after having been treated in the heap of slag.

As it is understood, the invention is not restricted to the single way described above for putting this method into effect; on the contrary it includes all the variations embraced by the claims.

Claims

1. A method fortreating liquid effluents including spreading the effluent to be treated through a heap of slag composed of the solid residues from the incineration of household refuse.

2. A method according to claim 1, in which the slag has a minimum organic carbon content of 6%.

3. A method according to one or other of claims 1 and 2, in which the slag used includes: - 4 to 14% incombustible organic elements, - 4to 14% lime (CaO), - 12 to 13% scrap iron, - 40 to 50% silica (SiO2) made up of glass or other rubbish, - up to 30% water, - metal oxides such as Fe2O3, Awl203, Na2O, MgO and Mn2O3, ensuring a complement of 100%.

4. A method according to any one of claims 1 to 3, including forming a heap of slag at least two metres thick and in the form of a truncated pyramid; covering the lateral surfaces of the heap with water-tight material; such fitting substantially horizontal ducts are to form drainage tubes emerging from the lateral surfaces of the heap; and fitting at least one vertical perforated chimney which extends the full height of the heap, the effluent being brought into contact with the heap by diffusion to the upper part of the pile.

5. A method as claimed in claim 4, in which the horizontal ducts are arranged in the direction of the prevailing winds.

6. A method according to any one of claims 1 to 5 including adding to the slag any one or more of the group comprising oxidising constituents, lime, active carbon, charcoal and zeolites.

7. A method according to claim 6, in which the additive(s) are conditioned in bags with permeable linings which are distributed throughout the interior of the heap of slag.

8. A method for treating liquid effluents substantially as hereinbefore described with reference to the accompanying drawings.

9. Means for treating liquid effluent comprising a heap of slag composed of solid residues from the incineration of household refuse.

10. Means as claimed in claim 9, in which the slag has a minimum organic carbon content of 6%.

11. Means as claimed in claim 9 or claim 10, in which the slag used includes: - 4to 14% incombustible organic elements, - 4to 14% lime (CaO), -12to13%scrapiron, - 40 to 50% silica (SiO2) made up of glass or other rubbish, - upto 30% water, - metal oxides such as Fe2O3, Awl203, Na2O, MgO and Mn2O3, ensuring a complement of 100%.

12. Means as claimed in claim 9, 10 or 11, in which the heap of slag is at least two metres thick and in the form of a truncated pyramid; the lateral surfaces of the heap are covered with water-tight material; such substantially horizontal ducts are fitted to form drainage tubes emerging from the lateral surfaces of the heap; and at least one vertical perforated chimney is fitted which extends the full height of the heap, the effluent being brought into contact with the heap by diffusion to the upper part of the pile.

13. Means as claimed in any one of claims 9 to 12, in which the heap includes one or more of the group comprising oxidising constituents, lime, active carbon, charcoal and zeolites.

14. Means as claimed in claim 13, in which the additive(s) are contained in permeable bags.

15. Means for treating liquid effluents substantially as hereinbefore described with reference to the accompanying drawings.

16. Any novel subject matter or combination including novel subject matter herein disclosed, whether or not within the scope of or relating to the same invention as any of the preceding claims.