Process for the decoloration and detoxification of aqueous effluents

Process for the decoloration and the toxification of highly polluted (COD.gtoreq.1000 mg/l) resistant aqueous effluents by an oxidation treatment of the effluents by hydrogen peroxide, continuously and in a homogeneous phase, at a pH of 2 to 5 in the presence of Fe ions and under irradiation by means of UV radiation. The AvOx/COD ratio by weight is not less than 0.5:1 and the AvOx/concentration of Fe ions ratio by weight is not less than 50:1.

The invention relates to a process for the decoloration and the 
detoxification of aqueous liquid waste and in particular of aqueous 
industrial discharges by an oxidizing chemical treatment. 
More particularly, it relates to a process for the decoloration and the 
detoxification of industrial aqueous effluents containing dyes and toxic 
organic compounds, which are resistant to conventional pollution-abatement 
chemical treatments, by treatment by means of peroxygenated compounds in 
the presence of ultraviolet radiation. 
Many industrial operations, such as the textile industry, the paper, 
cardboard and plastic packaging industry, the leather industry, and the 
like, produce highly polluted aqueous discharges containing a great 
variety of dyes and of organic products which are toxic to living 
creatures, among which are found intensely coloured compounds and/or 
compounds which are resistant to the majority of known chemical 
pollution-abatement treatments. The colouring of the effluents cannot be 
removed by a biological treatment. Moreover, the high toxicity of these 
discharges generally does not allow them to be treated in a biological 
operation by means of activated sludges, or even to be mixed, even in 
small proportions, with other effluents which would only contain organic 
substrates which are nontoxic for the biomass. 
A process for treating groundwaters and liquid effluents containing organic 
contaminants is known from U.S. Pat. No. 5,043,080, according to which the 
water is treated at a pH of approximately 2 to 4 with hydrogen peroxide in 
the presence of an ion of a transition metal, such as Fe or Cu, and 
irradiation is carried out with polychromatic ultraviolet light with 
wavelengths of 200 to 400 nm. In this process, H.sub.2 O.sub.2 
concentrations of 100 ppm, with [H.sub.2 O.sub.2 ]/metal ions ratios by 
weight of 10:1 to 1:1, and concentrations of polluting organic materials 
of 100 ppm of dioxane, of 100 ppm of trinitrotoluene and of a mixture of 8 
ppm of benzene, 7 ppm of toluene and 4 ppm of xylene are disclosed. Patent 
U.S. Pat. No. 5,043,080 does not, however, deal with the problem of the 
decoloration and of the detoxification of resistant effluents containing a 
high load of pollutants, such as the effluents encountered in the 
industries mentioned above. 
This known process, moreover, exhibits the disadvantage of bringing about 
precipitation of insoluble metal compounds on the transparent walls of the 
ultra-violet lamps and, for this reason, of rapidly decreasing the 
efficiency thereof. 
The invention overcomes the disadvantages of the known processes by 
providing an efficient process for the treatment of highly polluted 
effluents which does not affect the transparency of the walls of UV lamps 
and which makes it possible to continue the purification treatment by 
means of a biochemical stage, without endangering the viability of the 
biomass. 
To this end, the invention relates to a process for the decoloration and 
the detoxification of aqueous effluents by an oxidation treatment of the 
effluents by hydrogen peroxide, continuously and in a homogeneous phase, 
at a pH of 2 to 5 in the presence of Fe ions and under irradiation by 
means of UV radiation, characterized in that: 
a) the effluent to be treated is coloured and resistant and highly polluted 
and exhibits a chemical oxygen demand (COD) of not less than 1000 mg/l, 
b) the ratio by weight of the concentration of active oxygen (AvOx) 
(expressed as mg oxygen/l) of the continuous homogeneous phase to the COD 
of the effluent (expressed as mg/l) is not less than 0.5:1, 
c) the ratio by weight of the AvOx of the continuous homogeneous phase 
(expressed as mg oxygen/l) to the concentration of Fe ions (expressed as 
mg Fe/l) is not less than 50:1. 
Decoloration and detoxification of an effluent is understood to denote a 
purification treatment of this effluent which lowers its toxicity to a 
sufficient extent with respect to the biomass of the activated sludges 
which are used in biological purification processes for the viability of 
this biomass not to be compromised after several hours of contact with the 
detoxified effluent. 
The treatment according to the invention is carried out in a homogeneous 
aqueous phase in the presence of Fe ions. Any water-soluble source of Fe 
ions is generally suitable. For example, it will be possible to employ 
water-soluble Fe salts of organic or inorganic acids. The oxidation state 
of the Fe ions employed is preferably the II state, although Fe(III) ions 
can also be used, in particular when their concentration does not exceed 
20 mg/l. 
Fe(II) oxalate can be employed but, however, is not recommended because of 
its not insignificant toxicity with respect to the biomass of the 
biological treatment which sometimes follows the detoxification treatment. 
In practice, Fe salts of a strong inorganic acid, in particular Fe(II) 
salts, such as FeCl.sub.2 and FeSO.sub.4, are preferred. The sulphate 
FeSO.sub.4 has given good results. 
The concentration of Fe ions which is employed is generally not less than 5 
mg/l and preferably not less than 10 mg/l. It often does not exceed 50 
mg/l and, most often, does not exceed 40 mg/l. Concentrations of 10 and 20 
mg/l have given excellent results. 
The AvOx:concentration of Fe ions ratio by weight in the process according 
to the invention is preferably not greater than 1200:1. In a particularly 
preferred way, it does not exceed 1100:1. 
It is also preferable to set the AvOx:COD ratio by weight at a value of not 
less than 1:1. 
Likewise, it is preferable for the COD:concentration of Fe ions ratio by 
weight not to be greater than 1200:1. 
In a particularly preferred way, this COD:concentration of Fe ions ratio is 
set at a value of not less than 50:1. 
According to the invention, the UV radiation can be produced by any type of 
device available for operating in an industrial environment. 
For example, the UV radiation can be produced by one or several mercury 
vapour lamps arranged so as to illuminate the entire volume of the 
homogeneous liquid phase of the reactor. It is possible, to this end, to 
use a lamp of elongate shape which is introduced into a quartz tube placed 
in the axis of a reactor of annular shape and to supply the mixture of 
effluent to be treated, of hydrogen peroxide and of Fe ions via one of the 
bases of the annular volume delimited by the quartz tube of the axis of 
the reactor and the outer cylindrical walls of the latter. 
Mercury vapour UV lamps of the medium- and high-pressure type are 
preferably used. The spectrum of these lamps essentially lies in the UV at 
wavelengths ranging from approximately 210 to approximately 470 nm, in 
particular 254, 313 and 366 nm. These lamps also emit, at low intensity, 
radiation which lies in the visible region, that is to say in a wavelength 
range from approximately 470 to approximately 750 nm. 
The power delivered by the UV radiation is generally greater than or equal 
to 200 W/l of illuminated homogeneous liquid phase to be treated. The 
power is in particular greater than or equal to 250 W/l of liquid phase, 
powers of not less than 260 W/l being the most common. The power is 
usually less than or equal to 350 W/l of liquid phase, in particular less 
than or equal to 300 W/l of liquid phase. The power generally lies in a 
range from 200 to 350 W/l of liquid phase. A power of 260 to 300 W/l of 
liquid phase has given excellent results. 
The decoloration and detoxification treatment according to the invention is 
generally carried out at atmospheric pressure for reasons of ease and of 
reduced costs. However, there is no reason why the treatment according to 
the invention should not be carried out at pressures other than 
atmospheric pressure. It is possible, for example, to operate in a reactor 
which is overpressurized with respect to atmospheric pressure. 
The temperature of the decoloration and detoxification treatment according 
to the invention is generally equal to or greater than the ambient 
temperature. It is preferable to use a temperature greater than the 
ambient temperature. Temperatures ranging from the ambient temperature to 
80.degree. C. can be used. Temperatures of not less than 50.degree. C. and 
more particularly of not less than 60.degree. C. are preferred. The 
temperature of 80.degree. C. has given excellent results. 
The duration of the decoloration and detoxification treatment according to 
the invention is generally greater than or equal to 5 min, in particular 
greater than or equal to 10 min. The duration is usually less than 120 
min. Durations of less than or equal to 100 min are preferred. Durations 
greater than or equal to 5 min and less than 120 min are highly suitable. 
The process according to the invention is advantageously carried out in the 
absence of enzyme. 
In a specific embodiment of the process according to the invention, the 
oxidation treatment is carried out in the presence of ozone. The amount of 
ozone employed can vary from 5 g/l.h to 15 g/l.h. This embodiment leads to 
excellent results with respect to decoloration. 
According to an alternative form of the process which is the subject of the 
invention, the detoxification treatment is followed by a biological 
purification treatment by means of an activated sludge. It is generally 
advisable to allow the effluents to cool to a temperature of less than 
40.degree. C. and preferably of less than 35.degree. C., before bringing 
them into contact with the activated sludge. 
According to another alternative form of the process which is the subject 
of the invention, the detoxification treatment is preceded by a biological 
purification treatment by means of an activated sludge. 
These two alternative forms can optionally be combined. 
The invention also relates to the application of the decoloration and 
detoxification process described above to the decoloration and the 
detoxification of industrial effluents. It relates in particular to the 
application of this process to the detoxification and to the decoloration 
of coloured effluents and of dye liquors from the textile industry.

The following examples are given with the aim of illustrating the 
invention, without, for all that, limiting its scope thereof [sic]. 
EXAMPLE 1R (reference example not in accordance with the invention) 
Ten liters of a dye liquor containing: 
83.2 mg/l of Foron.RTM. yellow SEFL dye 
208 mg/l of Foron.RTM. red dye 
24.96 mg/l of Foron.RTM. blue dye 
0.46 ml/l of Sandazol.RTM. KBN dispersant 
0.46 ml/l of Lyogene.RTM. DFT dispersant 
in solution in demineralized water was [sic] heated in a beaker for 5 
minutes at 100.degree. C. 
The aqueous solution obtained is characteristic of effluents originating 
from dye liquors from the textile industry and is described by the 
following overall parameters: 
COD: 1800 to 2000 mg/l 
Extinction coefficient: 1.3 to 1.6 
pH 3.5 (adjusted with sulphuric acid). 
The effluent solution was introduced via the pipe 1 into the storage tank 2 
of a laboratory-scale plant described in FIG. 1. The solution was then 
circulated for 10 minutes through the plant, the flow rate of the pump 
being adjusted to 6 l/min. 
An amount of 15 g of FeSO.sub.4.7H.sub.2 O was then dissolved in the tank 
2, so as to produce a COD:concentration of Fe(II) ratio of 1:0.2. 
The solution is then recirculated through the plant for a period of 5 
minutes, the flow rate of the pump being adjusted to 6 l/min, so as to 
distribute the Fe(II) content homogeneously throughout the liquid phase. 
A 35% by weight concentrated hydrogen peroxide solution was then injected 
via the pump 4 with a flow rate of 0.5 ml/min at the inlet of the static 
mixer 5, after which the UV lamp 7 (HP lamp of TQ 2022 type manufactured 
by the firm W. C. Heraeus GmbH) equipping the reactor 6 was switched on. 
The electrical power supplied to the lamp was 1600 W and the volume of the 
reactor 3.5 l with an illuminated volume of 1.57 l. A total amount of 
1.5-1.8 g of 100% H.sub.2 O.sub.2 per liter of solution to be treated was 
thus added over a period of 78 minutes. The test was halted after running 
for 90 minutes. 
Samples were withdrawn via the pipe 9, for the purposes of analysis, at the 
beginning of the test and after running for 15, 30, 45, 60 and 90 minutes. 
The temperature of the effluent rose during the test from 20 to 30.degree. 
C., as a result of being heated during passage around the UV lamp. 
______________________________________ 
Duration of Reduction in 
treatment, min 
the COD, % 
Decoloration, % 
______________________________________ 
0 0 0 
15 37 28 
30 70 32 
45 71 57 
60 72 16 
90 70 8 
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The analyses were carried out according to the following standards: 
COD: Hach method of the USEPA (US Environmental Protection Agency), Federal 
Registration, Vol. 45, 1980 
Decoloration: measurement of extinction at 523 nm in a spectrophotometer 
equipped with a measuring cell with an optical path width of 1 cm. 
EXAMPLE 2 (in accordance with the invention) 
Example 1R was repeated in the same plant and with the same effluent, the 
temperature being modified to 80.degree. C. and the amounts of Fe ions 
being modified so as to achieve the following ratios: 
COD:AvOx of 1:0.5 
AvOx:concentration of Fe ions of 100:1 
______________________________________ 
Duration of Reduction in 
treatment, min 
the COD, % 
Decoloration, % 
______________________________________ 
0 0 0 
15 38 54 
30 82 84 
45 94 93 
60 96 99 
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