CA1145867A

CA1145867A - Purification process for waste and/or industrial water containing hydrocarbons

Info

Publication number: CA1145867A
Application number: CA000351959A
Authority: CA
Inventors: Willy Marti; Michel Charriere; Alain Gilli
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-05-15
Filing date: 1980-05-14
Publication date: 1983-05-03
Also published as: EP0019552B1; DE3063954D1; FR2456539A1; EP0019552A1; FR2456539B1

Abstract

Abstract A process for purifying waste or industrial water containing hydrocarbons, comprising the filtration of the water in at least one filter formed from a granular material, the process comprising alternately a fil-tration phase and a counter-washing phase of each of the filters with water, this second phase being intended to remove from the filter the impurities retained thereby during the filtration phase, wherein, during the filtration phase, the water charge to be purified has an anionic polyelectrolyte added thereto before passing over the filter.
The incorporation of such polyelectrolytes into the water improves the process efficiency compared with known filter systems of this type.

Description

ll~S8~7 The water used in different indu~stries isJ during its use, fre-quently polluted by different impurities such as, for example, hydrocarbons.
This is the case particularly in oil refineries or petrochemical factories.
It is of course necessary to treat this water, called waste water, before discharging it into a river or the sea, so as not to be detrimental to the environment. The laws in force require furthermore that this discharge water satisfy certain standards such as:
a maximum hydrocarbon concentration, a maximum concentration of materials in suspension (designated hereafter "M.E.S."), a maximum phenol concentration, a maximum chemical oxygen demand Ccalled hereafter DC0), a maximum biological oxygen demand (called hereafter DB0).
The primary treatment of waste water containing impurities required to be eliminated consists in separating at least a part of these impurities by decantation. This primary treatment is however insufficient ~o separate all the impurities and a considerable proportion thereof remains in the water, particularly in the form of an emulsion or solution.
It is known, after a decantation step, to complete the purifi-cation of partially purified water~ to subject it to a secondary treatment, consisting of flocculating the impurities which it still contains. This process requires adding to this water a flocculent, such an hydrolysable metal salt, for example an aluminium or iron salt. The sludge formed during this flocculation must generally be incinerated, which involves an expensive installation.
It is also known, in order to improve the quality of the water leaving decantation pans, to filter this water on a granular material, for S8ti'7 example sand. The water leaving these Filters does not however in general satisf~ the standards of quality required and the filtration is usually followed by a biological treatment, such as passing over a bacterial bed.
The filters must be subjected periodically to counter-washing with water to remove the impurities, particularly the hydrocarbons and the materials in suspension retained by the granular material. The water used for the counter-washing must then contain no such impurities or little thercoF. It may be water leaving another filter, or purified stored water, coming possibly more-over from outside the unit.
The process of filtration on granular material has however disadvantages.
The counter-washing of the filters is often imperfect, and hydrocarbons remain fixed on the filter, despite the counter-washing, which reduces its efficiency. Because of this reduction of efficiency, hydro-carbons may be contained in a considerable quantity in the effluent of the filters. These hydrocarbons may then:
a) if there is no further treatment, be discharged with the water leaving the unit, which i5 naturally prejudicial to the environment, b) if there exists a further treatment, such as passing over a bacterial bed, disturb this treatment and reduce the quality of the water obtained.
French patent application no 2 377 830 proposes, to get round this disadvantage, adding to the counter-washing water for granular material filters a polyelectrolyte, and treating this water by floatation, before recycling it to the beginning of the treatment. The hydrocarbons are then removed in the form of flocculated sludge.
A new means has now been found for increasing the efficiency of 11~58~;7 granular material filters used in the treatment of waste and/or industrial water containing hydrocarbons and possibly other materials in suspension.
The purpose of the present invention is to improve the processes used for treating waste and/or industrial water containing hydrocarbons using filtration on a granular material.
The applicants have noted that, unexpectedly, the use of anionic polyelectrolytes was increasing significantly the efficiency of filters constituted with granular materials in the treatment of waste and/or industrial water containing hydrocarbons and was permitting to recuperate. During the back-wash of the filters, an amount of hydrocarbons much higher than those which were recuperated without the use of anionic polyelectrolytes.
The use of anionic polyelectrolytes for the filtration treatment of water containing suspended materials is described in the French patent 1.443.626. However, in that patent the treated water is running (river) water and not industrial water and contains no (or practically no) hydro-carbons.
If the theory of the way of acting of the anionic polyelectrolytes on a water containing oils or solid particles is known from "chemical engineering process, 73, 57, (1977)", nevertheless it was recommended in that document to treat the refinery oily water first by a cationic polyelectrolyte and, after, by an anionic polyelectrolyte. The applicants have now found that the use of anionic polyelectrolytes, alone, gives better results for the treatment of that type of effluents.
The invention provides a process for the purification of water contaminated with hydrocarbon impurities, said process comprising introducing the contaminated water into a first decantation pan and separating a first portion of said hydrocarbon impurities from the water by gravity; adding an ll~S8~7 anionic polyelectrolyte to the water; passing said water through at least one filter formed from a granular material, retaining a second portion of the hydrocarbon impurities in said filter and collecting puriEied water therefrom.
The anionic polyelectrolyte may be, in particular, a product of the sulfonic type or an acrylamide derivative, which may be in solid form, in solution or in latex form, and obtained particularly by copolymerization or by hydrolysis of a non-ionic po]ymer.
A latex copolymer of acrylic acid and acrylamide, having a molecular weight of about 10,000,000 and a degree of anionicity close to 30% has successfully been used in this process.
In the process of the invention, the polyelectrolyte may be added to the water charge preferably in an aqueous solution.
The polyelectrolyte may be added at a concentration between O.OS
and 5 p.p.m., concentration being expressed by weight of the dry product in relation to the weight of the water charge.
The size of the granular material used in the process of the invention may, for example, be between 0.5 and Smm. Sand of such a size has been successfully used by the applicants.
It has been discovered that the addition of an anionic polyelectrolyte to the water charge to be treated provides, at the output of the filter during filtration phase, an effluent containing much less hydrocarbons and materials in suspension.
Further it has been discovered that, during the counter-washing phase of sand filters, the water from the counter-washing contained at the output of the filter agglomerates in suspension in the water. These ag-glomerates in suspension in the water have a density close to 1 and containhydrocarbons and materials in suspension.
It is assumed that, because of their density, these agglomerates contain a very high proportion of water. The separation of the water is however fairly difficult, because of their density.
However, it is possible to break up these agglomerates and indeed to separate the water and the impurities which they contain, by re-moving the water from the counter-washing by means of a centrifugal pump.
Thus is obtained a good separation of the water and the impurities. It should be understood that any other means adapted to cause a similar mech-anical shock to that obtained by passing through a centrifugal pump comes within the scope of the process of the invention.
The invention is illustrated by the two figures of the drawings in which:
Figure 1 shows an installation for treating waste water of a known type comprising three sand filters, two of which are in the filtration phase and one in the counter-washing phase, and a bacterial bed.
Figure 2 shows, in a non-limiting way, an installation of the same type as the preceding one, using the process of the invention.
In figures 1 and 2, the valves for placing the filters success-ively in the filtration phase and the counter-washing phase have not been shown, for simplicity's sake.
Reference will be made, first of all, to figure 1. The waste ~ater to be treated, for example that collected in an oil refinery, is brought by line 1 to a decantation pan 2, the decantation forming a primary treatment.
In this pan, the largest part of the impurities, and particularly the hydro-carbons, are separated by gravity and removed through line 3. The hydro-ll~S~

carbons may in particular be recovered and redistilled.
The water imperfectly purified is brought by line 4 to a buffer pan 5, then to a secondary treatment unit. It is taken up for this purpose by line 6 and pump 7 to be brought by line 8 to sand filters 9a, 9b and 9c.
In the figure, three filters have been shown, but there may be more or even only two of them. ~ne or more filters may be in the counter-washing phase, while the others are in the filtration phase.
It would moreover be possible to use only a single filter, the washing water then coming from outside the unit or from a sufficiently large buffer pan.
Two of the filters 9a, and 9b are shown in the filtration phase.
The water filtered is collected by line 10 and passes over a bacterial bed 11, while the purified water leaving by line 12 is discharged.
When one of the filters is in the counter-washing phase (the case for filter 9c in the drawing), a part of the water collected by line 10 is brought by line 13 to this filter, for counter-washing it and removing the impurities retained thereby. The counter-washing water is brought back by line 14 to line l for retreatment.
The applicants have discovered that, in the installation de-scribed above, filters 9a, 9b and 9c lose their efficiency, probably because of saturation of the filter with hydrocarbons, the counter-washing with water at least partially purified not being able to offset this saturation.
An installation using the process of the invention and which removes these disadvantages is shown in figure 2. In this figure, the parts identical to those shown in figure l have been shown with the same numbers, to which the index ' has been added.
Into pipe 6', through which the water from buffer pan 5' leaves, 11458~;'7 there is injected by line 20 an anionic polyelectrolyte. The rest of the treatment is identical to that described previously as far as the discharge of the counter-washing water from filter 9'c through 14'.
The water from counter-washing is brought to a pan 21. The water leaving this pan through line 22 is then brought to a separation pan 23, or 2'.
It has also been noted that the amount of oil which is recovered by decantation in these pans is greater than the amount recovered by decan-tation in the absence of the anionic polyelectrolyte.
The applicants have discovered that, by passing the water from line 22 through a centrifugal pump 24, the decantation in pan 23 was facili-tated.
Moreover, the applicants have observed that separation may take place in two ways:
1) If hydrocarbons through line 25 are added to the water fed into pump 24, the impurities are separated from the water by floatation in pan 23 and may be removed through line 26, the water being able to be re-cycled to buffer pan 5' through line 27.
The same result could be obtained as a variation, by leading the water from pump 24, through line 28, to line 1'. The hydrocarbons contained in the waste water of line 1' allow the impurities to be separated by float-ation in pan 2'. This variation eliminates pan 23.

2) If addition of hydrocarbons is not effected through line 25, the impurities are separated from the water by decantation at the bottom of pan 23.
The first method, in its two forms, is preferable, because the floating impurities are easier to recover.

S8tii7 The advantages of the process of the invention will be better understood from reading the following example, which has no limiting character.
EXAMPLE
Three tests Tl, T2 and A were carried out :for the treatment of waste water coming from an oil refinery.
Reference test Tl was carried out in a known installation, such as that shown in figure 1.
Reference test T2 and test A were carried out in an installation such as that shown in figure 2.
In the three tests, the filters 9a, 9b, 9c, 9'a, 9'b and 9'c are identical and contain sand having a granulometry between 1 and 3 mm.
Each filter contains 20 tons of sand and has a section of 10m2.
The bacterial beds 11 and 11' each contained a polypropylene ring filling having a specific area of 190 m2/m3.
The average flow rate of water fed into the installations was 460 m /hour.
An identical charge was treated in tests Tl, T2 and A, whose characteristics, measured at the output of pans 5 or 5', were the following:
- hydrocarbon concentration about 100 p.p.m. by ~measured in accordance with weight, AFNOR Standard T 90203) - M.E.S. concentration about 50 p.p.m. by Cmeasured in accordance with weight, AF~OR Standard T 90105) In the case of test Tl, there was no injection of polyelectrolyte into the charge to be treated.

1~4S86~7 In the case of test T2, there was injected into line 6', through line 20, an aqueous s-olution of a cationic polyelectrolyte formed by a polyquaternary amine chloride in an amount such that the concentration of the polyelectrolyte in the charge of the sand filters was 0.15 p.p.m., this concentration being expressed as dry product.
In the case of test A, there was injected into line 6', through line 20, an aqueous solution of an anionic polyelectrolyte sold by the Nalco company under the name NALC0 D 4047, CTrade Mark) and formed of a copolymer of acrylic acid and acrylamide, in an amount such that the concen-tration of the polyelectrolyte in the charge of the sand filters was 0.2 p.p.m., this concentration being expressed as dry product.
The analysis of the effluent of the sand filters was carriedout and the results appearing in the table below were obtained.

Output of filters Output of filters Output of 9a, 9b, 9c 9'a, 9'b, 9'c filters 9'a, e-t 7l e-t T2 Test A

Hydrocarbon concentration ~ 50 p.p.m.~ 50 p.p.m. ~ 10 p.p.m.

hydrocarbon _ elimination C 50% ~ 50% ~ 90%
rate _ _ m.e.s.concen-tration 20 p.p.m. 20 p.p.m. ~ S p.p.m.

m.e.s.elimi-nation rate ~ 50% < 50% ~ 90%

The table shows that the efficiency of the sand filters is greatly _ g _ 11458t~

improved by the addition of an anionic polyelectrolyte. The efficiency of the filters is`clearly lower, when there is no injection of polyelectrolyte or when there is injection of a cationic polyelectrolyte.
Similar elimination rates may be obtained with the anionic poly-electrol~te for water charges having higher concentrations of hydrocarbons and matter in suspension.
It has also been discovered that, in the case of test Tl and T2, the pressure drop in a filter 9a, 9b or 9c remains constant whereas, in the case of test A, the pressure drop in a filter 9'a, 9'b or 9'c increases regularly, which proves an improved efficiency of the filter, when there is addition of an anionic polyelectrolyte upstream of the sand filters.
Test Tl was continued for 6 months and, at the end of 6 months, a sharp increase of the DCO was fo~nd in the effluent of line 12, which went from 110 mg of oxygen/l to 250 mg of ox~gen/l (DCO measured in accordance with AFNOR Standard T 90101). This increase may be explained by the poison-ing of the bacterial bed by the excess of hydrocarbons leaving the sand filters.
In the case of test A, the DCO of the effluent of line 12' was 110 mg of oxygen/l. Given the low hydrocarbon concentration passing over the bacterial bed, saturation thereof would be much more difficult.
In the case of test A, the counter-washing removed practically all the impurities (hydrocarbons-materials in suspension) retained by the filter. In fact, the initial pressure loss of the filter is restored at the end of the counter-washing.
This example shows then well the advantages of the process of the invention.

Claims

THE EMBODIMENTS OF THIS INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the purification of water contaminated with hydrocarbon impurities, said process comprising introducing the contaminated water into a first decantation pan and separating a first portion of said hydrocarbon impuri-ties from the water by gravity; adding an anionic polyelectrolyte to the water, passing said water through at least one filter formed from a granular material, retaining a second portion of the hydrocarbon impurities in said filter and collecting purified water therefrom.

2. A process according to claim 1, wherein said anionic polyelectrolyte is a latex copolymer of acrylic acid and acrylamide.

3. A process according to claim 1, wherein said anionic polyelectrolyte is an acrylamide derivative.

4. A process according to claim 1, which further includes periodically counter-washing said filter with wash water to separate and recover the retained second portion of said hydrocarbon impurities from the filter in said wash water.

5. A process according to claim 4, which further includes subjecting the wash water to a mechanical shock means to facilitate the separation and recovery of said second portion from the wash water.

6. A process according to claim 5, wherein the mechanical shock means includes a centrifugal pump.

7. A process according to claim 6, wherein the purified water collected from said filter is further purified by biological treatment.