FR2461684A1 - Purificn. of organically polluted water contg. sulphate(s) - using anaerobic bacteria with sulphide(s) removed by gas entrainment - Google Patents

Abstract

The process using anaerobic bacteria with stirring of the reaction mass, comprises eliminating the sulphides formed from the sulphates by entrainment using a gas introduced into the reaction medium. The process is esp. applied to water from starch mfr. canning, abattains, stock-forming, distilleries and from molasses and yeast mfr. It avoids inhibition of the methane forming bacteria by the sulphides.

Description

The present invention relates to a process for biological purification
than anaerobic wastewater containing sulphates, more particularly
waste water with a high organic pollution of starch, canning,
of rendering or raising and especially waste water containing from 2 to 5
g / l of sulphates, molasses distilleries and yeasts.

It is known that anaerobic digestion is an economically
interesting although still little used, for purifying industrial waste water with high organic pollution; it ensures the elimination of pollution
from 80% to 95%. This process consists in carrying out a fermentation in the absence
of oxygen, making it possible to transform as completely as possible the
organic matter in methane and carbon dioxide.
is carried out in two phases which take place simultaneously in the mass
a liquefaction phase during which producing bacteria
of acid transform complex organic compounds into simple compounds,
namely namely acetic acid, propionic, and butyric ...; and an
gaseous phase # p '# n or methanization-during which bacteria
These methanics transform these compounds into methane and carbon dioxide.

 At the same time, the reduction of SO4 = in S by sulfato-reducing bacteria, in case of the presence of SO - ions, also takes place.

4
Facilities have already been described to implement
anaerobic digestion dice; they generally comprise a closed tank
digester, maintained at 30-370C, into which the water is
treat, with high organic pollution; under the combined action of bacteria
acidifying and methane, organic pollution is transformed into methane
and carbon dioxide that emerge. This gas mixture is partly recycled
to carry out the mixing of the liquid mass present in the digester, the
remaining gas. This product can be used as a fuel. The sludge is separated
of the liquid treated in a clarifier, some of these sludge is recycled
key in the digester - and the other part is eliminated.

Methane bacteria are the key digestive organs
anaerobic; but they are very variable to the variations of the temperature, the pH and the toxic acids present, in particular to sulphides; If the waters to be treated
contain too much sulphates, sulphides formed during
of the anaerobic process, inhibit anaerobic digestion. For example in the presence
of 200.-mg / l of S # the efficiency of the elimination of the pollution falls of 85
at 65% for a water to be treated containing 13 g / l of BOD5, and a digestion time of 5 days. In view of the fact that one works continuously, one is forced to operate in volumes of digestion important for obtain sufficiently long residence times; on the other hand, the presence of sulphides in the outgoing gas poses problems of odor and toxicity.

 The invention relates to a process which makes it possible to remedy these drawbacks and to treat with a good yield, in structures of small dimensions and under advantageous economic conditions, waters with a high organic pollution containing sulphates.

 The present process essentially involves entraining sulfide compounds formed from the sulfates during anaerobic digestion by means of a gas ("stripping"), which gas is injected into the liquid mass during fermentation.

 The gas used must be nontoxic for anaerobic digestion and is preferably the gas produced by the digestion itself, previously desulphurized by absorption of the sulfur compounds it contains, and simultaneously serving to stir the liquid mass.

 The ratio by volume: gas formed during the anaerobic process / stirring gas and stirring, is maintained between 3/1 and 20/1, preferably between 3/1 and 7/1. Below the lower limit, sufficient mixing is not assured; the upper limit is not restrictive, but it ensures the desired elimination of sulphides.

 In fact, in the case where the water to be treated contains sulphates, the gas produced by the digestion contains, besides methane and carbon dioxide, a large quantity of hydrogen sulphide - several% by volume - and other products. sulfur content, the content of which must be considerably reduced in order to use this gas as a stripping gas. Since, in addition, a part of the gas is likely to be used as a fuel, it is advantageous to carry out the desulfurization of all the gas formed.

 To carry out the desulfurization, it is possible to use two known methods.

 According to a first method, the sulfur-containing compounds are selectively absorbed on a mass of metal oxides, for example of iron and zinc, capable of fixing the sulphides. After absorption, these sulphides are converted into sulfur by oxidation in air. For this purpose, air is injected continuously into the mass after the absorption phase. The solid sulfur is removed from the surface of the absorbent mass.

 According to a second method, the sulfur compounds are absorbed in a liquid medium that selectively accepts the reducing sulfur products by transforming them into a form that can be removed from sulfur, for example solid sulfur. This liquid absorbent must not act with respect to the carbon dioxide present in the gas produced by the digestion, because it makes it possible to maintain the pH in the digester between 6.5 and 7.5, a value necessary for good digestion.

After reaching the limit of exhaustion, the absorbing liquid is regenerated in air: the sulfur formed is removed by decantation, dehydrated and recovered.

 According to the invention, the gas is evacuated above the mass in constant flow digestion; this flow rate must always be greater than the formation rate of gas in the mass in anaerobic digestion. The complement is ensured on the one hand by the desulfurized entrainment and mixing gas, and on the other hand, in the case where the flow of digestion gas was to decrease for any reason, by an additional injection of gas of desulphurized entrapment in the atmosphere above the mass in digestion, so as to maintain an overpressure of between 0.005 and 0.02 bar; the upper limit is fixed for reasons of fitting (resistance to pressure), the lower limit ensures the evacuation of gases without pumping.

 Thus, the reaction conditions in the digester and the gas flow rate in the desulphurization apparatus are approximately constant.

 The single figure schematically shows an installation by means of which the invention can be implemented.

 This figure represents the case where the stirring and entrainment is carried out using the gas coming from the digester, after desulfurization.

 The waste water to be treated, with high organic pollution and containing sulphates, is fed through the feed pipe 1 into the digestion vessel 2 in which the water is treated continuously, the purified water exits through the pipe 3.

 In the digester 2 are carried out the acidification and methanization reactions previously described in the form of a liquid mass 5 covered with gas bubbles, originating on the one hand from the microbiotic activities in progress and on the other hand from the gas of entrafnement injected in 6 composed of CO2 and CH3 in the case shown and whose flow rate at 6 is Q2. The gases developed in the mass in digestion give rise to a flow q. The outlet 7 ensures the evacuation of the gaseous mixture which is formed above the mass in activity (flow Q1) and which is fed to the desulfurator 8. At the outlet of the latter (flow Q1 ') a part of the gas is recycled to the digester (flow Q2) by means of a pump 9. The remaining flow, Q1 '~ Q2 is stored or used (output U), after any sampling of a flow q' brought to the atmosphere 4 of the digester by the entrance 10.

 This sampling is performed using a valve 11 controlled by the pressure in the digester.

 The invention will be described in more detail in the nonlimiting example below.

Example
Anaerobic digestion of 20 ml of a waste water containing 15 g / l of COD, 10 g / l of BOD5 and 2.5 g / l of 50 is carried out, on the one hand according to a conventional method A with simple stirring, d secondly according to a method B, with stirring and entrainment of sulphides by injection of desulfurized gas from the digestion.

 The results obtained are shown in Table I. Table II contains the quantities and percentages of the gases formed during digestion, without desulfurization in process A and after desulfurization in process B.

 The desulfurization of the digestion gas is carried out by liquid absorption.

the digester
In the process according to the invention, there is obtained at exit / 477. 73 m of gas,
3 3 after desulfurization 470 m remain. 300 m were recycled in the digester, during the test, a total of 100 m3 of desulphurized gas was reintroduced into the gaseous atmosphere of the digester, 70 m3 of gas was stored.

Table I

<tb><SEP> Water <SEP> processed
<tb> Pollution <SEP> Raw <SEP> Water
<tb><SEP> by <SEP> process <SEP><SEP> classic <SEP> next <SEP> the inventio <SEP>
<tb><SEP> A <SEP> B
<tb> COD <SEP> 15 <SEP> g / l <SEP> 9 <SEP> g / l <SEP> 7 <SEP> g / l
<tb> BOD5 <SEP> io <SEP> 8/1 <SEP><SEP> 4, <SEP> 5 <SEP> g / l <SEP> 3 <SEP> g / l
<tb> SO4 = <SEP> 2.5 <SEP> g / l <SEP> 0, <SEP> 5 <SEP> g / l <SEP> 0, <SEP><SEP> 5 <SEP> g / l
<tb> S = <SEP> 0, <SEP> 490 <SEP><SEP> g / l <SEP> 0, <SEP> 100 <SEP> g / l
<Tb>
Table II

<tb><SEP> Composition <SEP> Gas <SEP> product <SEP> sent <SEP> to <SEP> storage <SEP> or <SEP> to <SEP> a <SEP> used
<tb><SEP> tion <SEP> Any
<tb><SEP> Process <SEP> A <SEP> Invention <SEP> B
<tb><SEP> Quantity <SEP> 55 <SEP> m <SEP> 70 <SEP> m
<tb> CH4 <SEP> 64 <SEP>% <SEP> 67, <SEP> 5 <SEP>% <SEP>
<tb> CO2 <SEP> 31.2 <SEP>% <SEP> 32, <SEP> 2 <SEP>%
<tb> H2S <SEP>% <SEP><SEP>%<SEP>
<Tb>

Claims (5)

 A process for the purification of waste water with a high organic pollution containing sulphates by means of anaerobic bacteria in known manner with stirring of the mass in reaction, characterized in that the sulphides formed from sulphates during the biological process by means of a driving gas which is introduced into the mass in reaction.  2. Method according to claim 1, characterized in that the dragging gas is the gas formed during the biological process, which is desulfurized before reintroducing it into the reaction mass.  3. Method according to claim 1 or 2, characterized in that the same gas is used for stirring of the mass in reaction and entrafnement sulfides.  4. Method according to any one of claims 1 to 3, characterized in that the gas volume ratio formed in the mass / gas introduced into the mass is is understood between 3/1 and 7/1.  5. Method according to any one of claims 1 to 4, characterized in that one maintains the overpressure above the mass in response to a constant value between 0, 005 and 0, 02 bar.