PROCESS AND INSTALLATION FOR TREATING WASTE WATER CONTAINING SULPHIDES AND AMMONIUM.
The invention relates to a process for treating waste water loaded with sulphides and with ammonium, in particular waste water of urban or industrial origin or materials fed back from digestion, condensates or leachates.
Waste water contains in particular carbon, nitrogen and sulphur in various forms of compounds which can be treated chemically by various processes. All the physicochemical treatments consist in separating the compounds of these elements from a liquid phase in order to concentrate them in another phase. They are in particular stripping (removal of gas from water by means of an entrainment gas), reverse osmosis, distillation, chemical precipitation or catalytic oxidation processes, the implementation and running costs of which are high. Removal of these compounds via the biological process presents itself as a treatment alternative.
Biological treatments for nitrogen and for carbon
Carbonaceous pollution and nitrogenous pollution of waste water are principally eliminated by the biological process. This conventional process is based on the ability of microorganisms to eliminate the pollution by assimilation or by biodegradation.
The carbonaceous organic matter is oxidized in an aerated medium by microorganisms, principally heterotrophic bacteria. These microorganisms use the colloidal and dissolved carbonaceous organic matter and convert it either to gas or to biomass.
As regards the treatment of nitrogen, the treatments mainly distinguished are treatments by nitrification and denitrification, by which the ammonium is oxidized in two steps under aerated conditions by autotrophic bacteria (first to nitrites then to nitrates) and, finally, reduced to nitrogen gas under anoxic conditions by heterotrophic bacteria.
The biological reactions are presented in the schemes below.
(Scheme Removed)
Limits of the nitrogen treatment:
- The nitrification/denitrification by the biological process is an effective process but it demands up to 1/3 of the total oxygen consumed by the purification station.
- The treatment is difficult to operate compared with the treatment of carbon, with resulting frequent additions of carbonaceous reactants (methanol, ethanol, etc.) in order to complete the denitrification.
- This treatment requires considerable holding times and tank volumes, since the nitrification kinetics are very slow.
Drawbacks of the presence of sulphur:
Many problems linked to the presence of sulphur in waste water flowing into purification stations have been clearly identified, namely:
- The emission of nauseating odours (rotten egg type). These unpleasant odours are detected well before they represent a danger to human beings since the olfactory sensors have a detection threshold at 0.15 ppm.
- The health risks related to H2S (hydrogen sulphide). In fact, this molecule is toxic starting from 10 ppm, with worsening of the effects according to the duration of exposure.
- Corrosion of concrete and metals due to oxidation of sulphides to sulphuric acid by certain bacteria of Thiobacillus type.
- The development of filamentous bacteria. Since the water is septic, filamentous bacteria are more competitive than the conventional bacteria of the flock with respect to oxygen. Some are capable of accumulating sulphur in the form of granules in their cells.
- The need to cover the works in order to prevent dispersion of H2S.
Sulphur treatments
Sulphates are naturally present in waste water and sulphur enters into the composition of intracellular proteins of microorganisms. The sulphur can be oxidized or reduced according to the conditions of the medium and the populations of bacteria present. In an aerobic medium, sulphides are converted to sulphates by sulphur-oxidizing bacteria, and in an anaerobic medium, sulphates are converted to sulphides by sulphate-reducing bacteria.

(Scheme Removed)
The treatment of waste water by autotrophic denitrification with oxidation of sulphides has been described in FR2841548-A1. This process uses three biological reactors in series with biomass fixed on a mobile support: anaerobic, anoxic and aerobic.
The first reactor makes it possible to reduce the sulphates to sulphides and to treat the carbonaceous load. In the second reactor, the sulphides are oxidized to sulphates by reduction of the nitrates originating from the recirculation from the aerobic tank to the anoxic tank. Finally, in the aerobic reactor, ammonium is converted to nitrates.
This process puts a constraint on expenditure since it requires a large volume of expensive support materials.
The limit of this treatment scheme is based on the amount of sulphides to be oxidized by autotrophic denitrification which consumes alkalinity. Since the proposed operating method consumes alkalinity (autotrophic nitrification in the aerated tank + autotrophic denitrification in the anoxic tank), in the event of an excess of sulphides to be treated, the anoxic medium would be limiting in terms of alkalinity, or even in terms of nitrates. The oxidation of the sulphides would not therefore be complete.
Moreover, the production of volatile fatty acids in the first anaerobic reactor also consumes alkalinity, which is all the more unfavourable to the sulphide-oxidation reactions in the anoxic second reactor.
The objective of the invention is especially to provide a process which makes it possible to remove the sulphides and the various forms of nitrogen economically and effectively.
The process according to the invention for the treatment of waste water loaded with sulphides and with ammonium, in particular of waste water of urban or industrial origin or materials fed back from digestion, condensates or leachates, is characterized in that:
- the waste water first undergoes a free-culture anoxic treatment via the biological process, according to which, in a first step, the organic carbon is essentially eliminated by heterotrophic bacteria, and in a second step, separate from the first, the sulphides are oxidized via the biological process by autotrophic bacteria with reduction of nitrates and/or of nitrites,
- the effluent leaving the second step is subjected to a free-culture aerobic biological treatment for conversion of the ammonium to nitrates.

Preferably, a fraction of the effluent which has undergone the free-culture aerobic biological treatment, and which contains nitrates, is recirculated to the second step and the first step of the anoxic treatment. The fraction of the effluent recirculated to the anoxic treatment is advantageously adjusted according to the amount of sulphides to be treated.
In the second step of the anoxic treatment, the S/N (sulphur/nitrogen) ratio by mass is preferably maintained between 0.5 and 3.
The recirculated fraction may be slaved to the amount of nitrates/nitrites required to oxidize all the sulphides present in the water in the second step of the anoxic treatment. Advantageously, the recirculated fraction is slaved so as to ensure, in the second step of the anoxic treatment, the S/N (sulphur/nitrogen) ratio by mass of between 0.5 and 3.
The oxidation of the sulphides via the biological process can be coupled to nitrite formation by means of an aerobic sequential biological reactor (SBR), with the second step of the anoxic treatment being fed with nitrites originating from the nitrite formation.
The feeding of the second step of the anoxic treatment with nitrites originating from the nitrite formation can be adjusted according to the amount of sulphides to be oxidized with the nitrites.
The invention also relates to an installation for implementing the process defined above, characterized in that it comprises two free-culture biological reactors in series, the first reactor being a two-stage anoxic biological reactor comprising a heterotrophic tank and an autotrophic tank which are separate, while the second reactor is an aerated free-culture tank.
Preferably, recirculation of the nitrates is carried out from the aerated tank to the anoxic tanks according to the concentration of sulphides to be oxidized. The nitrate recirculation can be carried out by step-feed from the aerated tank to the anoxic tanks according to the concentration of sulphides to be oxidized.
The installation comprises measuring probes in the anoxic tanks and in the aerated tank, for several parameters including the carbon content of the effluent flowing in, and the sulphide content in the autotrophic anoxic tank, these measuring probes being connected to a controlling device which controls the recirculation flow rates according to the parameters measured.
In the case of an installation for a purification station comprising a sludge system with an anaerobic digester and an aerobic sequential biological reactor providing partial nitrification and partial denitrification of effluents with a high concentration of ammonium, the aerobic sequential biological reactor can

be coupled to the autotrophic anoxic tank so as to feed this tank with nitrites.
Apart from the arrangements disclosed above, the invention consists of a certain number of other arrangements which will be described more explicitly hereinafter in terms of exemplary embodiments described with reference to the attached drawings, but which are in no way limiting. On these drawings:
Fig. 1 is a scheme of an installation according to the invention, and
Fig. 2 is a scheme of a variant of embodiment of the installation.
Referring to Fig. 1 of the drawings, an installation according to the invention for the treatment, according to a continuous flow, of waste water loaded with sulphides and with ammonium can be seen. This installation comprises an anoxic reactor 1 compartmentalized into a heterotrophic tank 1a followed by an autotrophic tank 1b. The two tanks 1a and 1b are separate and the effluent leaving the tank 1a is taken up by pumping means and sent to the tank 1b.
The tank 1a receives the flow Q of waste water containing carbon, sulphides and ammonium. Stirring means Aa, Ab are provided in each of the tanks 1a, 1 b. There is no blowing of air or oxygen into these tanks.
Heterotrophic and autotrophic bacteria are naturally present in the waste water and in the tanks 1a, 1b, in the form of free cultures.
Due to the fact that the anoxic tank 1a receives a flow Q loaded with organic carbon, the heterotrophic bacteria develop rapidly in this tank 1a, while the autotrophic bacteria develop much less rapidly under such conditions. The anoxic tank 1a is thus heterotrophic and makes it possible to treat the entering carbon, and to denitrify a surplus of nitrates originating from an aerated tank 2 and which are not used in the oxidation of the sulphides in the autotrophic anoxic tank 1 b. The aerated tank 2 is downstream of the tank 1 b.
The tank 1b receives the effluent leaving the tank 1a, the organic carbon having been for the most part removed from said effluent. The action of the autotrophic bacteria in the tank 1b is promoted by the low organic carbon load of the water originating from the compartment 1a. Thus, by separating the anoxic tanks 1a, 1b, conditions have been created which promote, in the tank 1a, the action of the heterotrophic bacteria and, in the tank 1b, the action of the autotrophic bacteria.
The autotrophic anoxic tank 1b makes it possible to treat the sulphides present in the raw water by virtue of the nitrites derived from the aerated tank 2 and/or of the nitrites derived from the nitrite formation step in the aerobic sequential biological reactor (SBR). The sulphides are oxidized by

autotrophic denitrification which consumes alkalinity. The heterotrophic denitrification which takes place in the tank 1a makes it possible to produce alkalinity. By virtue of this production of alkalinity in the tank 1a, the process is not slowed down or stopped by a decrease in alkalinity due to the consumption in the tank 1b.
The effluent originating from the tank 1b is sent, for aerobic free-culture biological treatment, to the aerated tank 2 which comprises stirring means Ac and aeration means B located at the bottom of the tank 2 for injecting bubbles of air or oxygen into the liquid of the tank. The aeration means B may be formed by perforated tubes for blowing air or by nozzles installed in the floor or by any other conventional means.
The biological treatment of the water in the reactor 2 leads to nitrification and the conversion of ammonium NH4+ to nitrate NO3". The effluent leaving the tank 2 contains sulphates SO42" originating from the oxidation of the sulphides using the nitrates and/or nitrites.
A recirculation pipe 3 is provided between the aerated tank 2 and the anoxic tank 1a, a pump 3p being mounted on the pipe 3. Similarly, a pipe 4 is provided for recirculation from the aerated tank 2 to the anoxic tank 1 b, a pump 4p being inserted in this pipe. The recirculation from the aerated tank 2 to the anoxic zones 1a, 1b in order to provide them with nitrates is therefore carried out by step-feed. The recirculation flow rate is slaved to the amount of nitrates required to oxidize all the sulphides present in the water according to an S/N (sulphur/nitrogen) ratio by mass of between 0.5 and 3.
To adjust the recirculation flow rate, an automated controlling device 5 controls the operating speed, and therefore the flow rate, of the pumps 3p and 4p. In order to make the proposed treatment reliable, the installation may comprise a series of flowmeters installed on the various pipes, and various sensors or measuring probes. The control instructions are established, in particular, according to parameters such as the organic carbon content in the inflow Q and in the heterotrophic anoxic tank 1a, the sulphide content in the inflow Q and in the autotrophic anoxic tank 1b and the nitrate content in the aerated tank 2. These parameters are obtained by measuring probes such as 6, 7 and 8 provided in the tanks 1a, 1b, 2 and connected to the controlling device 5. Other parameters such as pH, oxidoreduction potential, temperature or dissolved oxygen can be measured by appropriate probes. This information serves to optimize the operating parameters of the tanks 1a, 1b and 2. The probes and sensors connected to the controlling device 5 allow the evolution of the treatment to be continuously monitored and corrective actions to be

controlled.
Fig. 2 is a scheme of a variant of an installation according to the invention in the case of a purification station where there is a sludge system comprising an anaerobic digester (not represented) which provides dewatering supernatants loaded with nitrogen. These supernatants can be treated separately by the biological process in an aerobic sequential biological reactor SBR 9. The flow U of effluents with a very high concentration of N-NhV, of the centrate, filtrate, condensate or leachate type, is transferred into the reactor 9 which is a free-culture aerated tank reactor with stirring means and means for blowing in air or oxygen at the bottom, as for the aerated tank 2.
In the reactor 9, the treatment of effluents with a high concentration of ammonium is carried out by partial nitrification and partial denitrification. The ammonium is oxidized to nitrites which are reduced to nitrogen gas, without it being necessary to go through nitrite conversion to nitrates. This process, also called "nitrate shunt", described for example in EP-A-0826639, is theoretically capable of reducing the supply of oxygen for nitrification by 25% and the supply of biodegradable carbonaceous reactants for denitrification by 40%, and also the production of associated heterotrophic sludge.
The elimination of effluents with a high concentration of nitrogen in the reactor 9 comprises several fractionated feed, aeration and anoxia phases, respectively, the number and the duration of these phases and also the addition of carbonaceous reactants being adjusted by virtue of a series of real-time measurements in the effluent to be treated, in the waste, and in the biological reactor 9.
Nitrite feed for the autotrophic anoxic reactor 1b is provided by a pipe 10 which leaves the reactor 9 and opens out into the reactor 1b. A pump 11, controlled by the controlling device 5, is placed on the pipe 10. The feeding of the reactor 1b is carried out using a fraction of the volume of water treated by the reactor 9 during the aerated phase, during the course of which there is production of nitrites. A part of these nitrites is therefore sent to the tank 1 b. The flow rate in the pipe 10 is adjusted according to the nitrite needs for oxidizing the sulphides in the autotrophic anoxic reactor 1 b.
Treatment examples are given hereinafter.
In general, the concentration ranges for the various physicochemical parameters during the treatment described in the scheme of Fig. 1 are:

(Table Removed)
The above table reveals the drop in COD (chemical oxygen demand) produced by the treatment in the tank 1a, which corresponds to the elimination of most of the organic carbon, which elimination is completed in the aerated tank 2.
The conversion of the ammonium nitrogen N-NH4+ to nitrate nitrogen N-NOx takes place in the aerated tank 2.
The oxidation of the sulphides S2" to sulphates takes place in the tank 1b.
In the following two examples, the following operating conditions are, in general, found:
The anoxic reactors 1a and 1b have a mechanical stirring system and the aerated tank 2 has a system for blowing in air in addition to a mechanical stirrer.
The concentration of material in suspension in the tanks 1a, 1b and 2 is maintained between 1 and 5 g/l.
The degree of step-feed recirculation depends on the concentration of nitrates and of sulphides and oscillates between 50 and 400%.
In all the tanks, the pH is between 6.5 and 8.5.
The age of sludge in the tanks is between 6 and 20 days, depending on the temperature.
The hydraulic residence time or HRT in each of the reactors 1a and 1 b is from 2 to 3 h, and in the reactor 2 is from 4 to 6 h.
1st Example:
Two tanks 1a and 1b each with a HRT of 2 h, and a tank 2 with a HRT of 4 h.

Characterization of the raw and treated water:
(Table Removed)
It is noted that the chosen ratio of sulphides to be eliminated/nitrates consumed is 1.5. Under these conditions, to oxidize 30 mg/l of sulphides, 20 mg/l of N-NO3" are necessary. The recirculation of the nitrates from the tank 2 to the tank 1b should make it possible to provide this concentration. The excess nitrates, i.e. 25 mg/l, are denitrified by recirculation to the tank 1a.
The proposed system is capable of eliminating 100% of the sulphides present in the raw water, and of completely nitrifying and denitrifying.
2na Example:
According to the same initial operating conditions (pH, T°, O2, age of sludge,
etc.), but with raw water having a different typology:

(Table Removed)
The S/N ratio of 1.5 is no longer adhered to since there is a nitrate deficiency due to a low concentration of ammonium entering the station. It is still necessary to have 20 mg/l of N-NOx in order to oxidize the 30 mg/l of sulphides. In the present case, it was chosen to recirculate 50% of the flow leaving the aerated tank 2 to the heterotrophic anoxic tank 1a, i.e. 5 mg/l of N-NOx, since it
is essential to recover the TA (total alkalinity) for the autotrophic denitrification. 15 mg/l of N-NOx are lacking, but can be compensated for by the fraction of the volume of treated water provided by the reactor 9.
The invention allows treatment of waste water virtually without consuming expensive chemical reactants, and with reduced sludge production due to the low growth of autotrophic bacteria in the autotrophic anoxic tank.

CLAIMS
1. Process for the treatment of waste water loaded with sulphides and with
ammonium, in particular of waste water of urban or industrial origin or materials
fed back from digestion, condensates or leachates, characterized in that:
-the waste water first undergoes a free-culture anoxic treatment (1) via the biological process, according to which, in a first step (1a), the organic carbon is essentially eliminated by heterotrophic bacteria, and in a second step (1b), separate from the first, the sulphides are oxidized via the biological process by autotrophic bacteria with reduction of nitrates and/or nitrites, -the effluent leaving the second step (1b) is subjected to a free-culture aerobic biological treatment (2) for conversion of the ammonium to nitrates.
2. Process according to Claim 1, characterized in that a fraction of the effluent which has undergone the free-culture aerobic biological treatment (2), and which contains nitrates, is recirculated to the second step (1b) and the first step (1a) of the anoxic treatment.
3. Process according to Claim 2, characterized in that the fraction of the effluent recirculated to the anoxic treatment (1a, 1b) is adjusted according to the amount of sulphides to be treated.
4. Process according to one of Claims 1 to 3, characterized in that, in the second step (1b) of the anoxic treatment, the S/N (sulphur/nitrogen) ratio by mass is maintained between 0.5 and 3.
5. Process according to Claim 2 or 3, characterized in that the recirculated fraction is slaved to the amount of nitrates required to oxidize all the sulphides present in the water in the second step (1b) of the anoxic treatment.
6. Process according to Claim 2 or 3, characterized in that the recirculated fraction is slaved so as to ensure, in the second step (1b) of the anoxic treatment, the S/N (sulphur/nitrogen) ratio by mass of between 0.5 and 3.
7. Process according to any one of the preceding claims, characterized in that the oxidation of the sulphides via the biological process is coupled to nitrite formation by means of an aerobic sequential biological reactor (SBR) (9), with the second step (1b) of the anoxic treatment being fed with nitrites originating

from the nitrite formation.
8. Process according to Claim 7, characterized in that the feeding of the second step (1b) of the anoxic treatment with nitrites originating from the nitrite formation (9) is adjusted according to the amount of sulphides to be oxidized with the nitrites.
9. Installation for implementing a process according to any one of the preceding claims, characterized in that it comprises two free-culture biological reactors (1, 2) in series, the first reactor (1) being a two-stage anoxic biological reactor comprising a heterotrophic tank (1a) and an autotrophic tank (1b) which are separate, while the second reactor (2) is an aerated free-culture tank.

10. Installation according to Claim 9, characterized in that recirculation of the nitrates is carried out from the aerated tank (2) to the anoxic tanks (1a, 1b) according to the concentration of sulphides to be oxidized.
11. Installation according to Claim 9, characterized in that it comprises measuring probes (6, 7, 8) in the anoxic tanks (1a, 1b) and in the aerated tank (2), for several parameters including the carbon content of the effluent flowing in, the sulphide content in the autotrophic anoxic tank (1b) and the nitrate content in the aerated tank, these measuring probes being connected to a controlling device (5) which controls the recirculation flow rates according to the parameters measured.
12. Installation according to one of Claims 9 to 11, for a purification station comprising a sludge system with an anaerobic digester and an aerobic sequential biological reactor (9) providing partial nitrification and partial denitrification of effluents with a high concentration of ammonium, characterized in that the aerobic sequential biological' reactor (9) is coupled (10, 11) to the autotrophic anoxic tank (1 b) so as to feed this tank (1b) with nitrites.