Liquid purification or separation – Processes – Treatment by living organism
Patent
1995-11-29
1997-06-10
Upton, Christopher
Liquid purification or separation
Processes
Treatment by living organism
210610, 210616, 210626, C02F 334
Patent
active
056372205
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a process for purifying waste water containing sulphide, comprising oxidizing the sulphide to elemental sulphur in a reactor with sulphide-oxidizing bacteria in the presence of oxygen, and separating from the waste water at least a part of the sulphur formed during the oxidation.
BACKGROUND OF THE INVENTION
Such a process is disclosed for example in International patent application WO 91/16269. According to that process, a minimum ratio between sulphide and biomass is used.
International patent application WO 92/10270 discloses a cyclic process for the removal of sulphur compounds from a gaseous effluent wherein an aqueous solution is alternately contacted with the gaseous effluent and subjected to sulphur-oxidizing bacteria. Elemental sulphur formed by the bacterial oxidation is separated off from the aqueous solution, in such a way that 0.1 to 50 g of elemental sulphur per l is left in the recycled aqueous solution.
All known processes for bacterial waste water treatment are faced with the problem of keeping the bacteria inside the reactor. This problem is usually solved by using a carrier material for the bacteria. Two types of carriers are generally proposed: (1) mobile carriers such as pumice; however, a disadvantage of mobile carriers is that a vigorous turbulence or fluidization must be maintained in order to keep them mixed with the waste water to be treated, and, furthermore, a part of the mobile carrier will interfere with the sulphur formed, which is detrimental for the quality of the sulphur; (2) fixed carriers such as structures of synthetic material; they have a disadvantage that these fixed carriers get clogged up quickly. Moreover, both the conventional mobile carriers and the conventional fixed carriers considerably increase the cost for operating the treatment plant.
SUMMARY OF THE INVENTION
It has been found now that the problems associated with the use of a carrier material can be solved by providing a process wherein a part of the elemental sulphur separated form the treated waste water is recycled into the reactor, in such a way that a concentration of elemental sulphur of at least 1 g/l is maintained in the reactor.
Preferably, the amount of separated elemental sulphur is recycled to the aerobic reactor is such that a sulphur concentration of at least 2 g/l, in particular at least 3 g/l, and more in particular at least 4 g/l is provided. It was found that the sulphur produced by the microbial oxidation settles more quickly at these high sulphur concentrations, so that a more effective separation of sulphur and liquid effluent can be achieved by using the same type of settler.
It was shown furthermore that at the high sulphur concentration the sulphide-oxidizing bacteria can become attached to the sulphur formed in such a way that an effective biomass-carrier system is obtained which renders the use of a separate carrier material unnecessary.
In the process according to the invention, sulphur aggregates are therefore preferably used as a carrier material for sulphide-oxidising bacteria. Sulphur aggregates are understood to be sulphur particles having a diameter which is considerably larger than the size of about 1 .mu.m occurring in sulphur sols. The sulphur aggregates preferably have a diameter of at least 50 .mu.m. These sulphur aggregates are formed when the sulphur concentration is sufficiently high; alternatively, sulphur aggregates may be added as such at the start of the biological sulphide removal.
Advantageously, a reactor is used which is provided with an internal settler, so as to separate the biomass and at least a part of the sulphur from the liquid effluent in the reactor.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts a so-called airlift-loop reactor having an internal settler;
FIG. 2 shows the settling profile of a sample taken from the reactor as a function of the sulphur concentration;
FIG. 3 shows a comparison of the settling profile of a sample taken from an airlift-loop reactor and from a mixed reactor; a
REFERENCES:
patent: 4384956 (1983-05-01), Mulder
patent: 4584271 (1986-04-01), Stern et al.
patent: 4966704 (1990-10-01), Sarner
patent: 5366633 (1994-11-01), Buisman
Paques B.V.
Upton Christopher
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