Colloids and Surfaces A: Physicochemical and Engineering Aspects
Removal of hydrogen sulphide from wastewater and waste gases by biological conversion to elemental sulphur: Colloidal and interfacial aspects of biologically produced sulphur particles
Introduction
Hydrogen sulphide is emitted into the environment as dissolved sulphide in wastewaters and as H2S in waste gases. Emission into the atmosphere is mainly a result of volcanic activities and evaporation from oceanic waters [1]. Also high pressure natural gas may contain up to 30% (v/v) of hydrogen sulphide. In the atmosphere hydrogen sulphide causes acid rain because of its reaction with ozone to form sulphuric acid. Sulphide-containing wastewaters are generated by a number of industries such as petrochemical plants, tanneries, viscose rayon manufactures and as a result of the anaerobic treatment of sulphate-containing wastewaters 1, 2, 3. The latter are mainly produced by the paper and pulp industry.
Hydrogen sulphide and sulphur dioxide can be converted into elemental sulphur via the biological sulphur cycle [4]. The insoluble sulphur can be removed from the treated water, leading to a reduction of the total sulphur content of the waste stream. The recovered sulphur is a potentially valuable compound which can be re-used, e.g. it can be purified by melting at high temperatures. It is suitable for the production of sulphuric acid or it can be applied in bioleaching processes [5]. Obviously, a highly effective sulphur removal step is essential for the successful application of this technology.
In this paper, the biological removal of hydrogen sulphide by oxidation into elemental sulphur for cleaning polluted water or gas streams is described and the colloidal properties of the sulphur particles are discussed. The results obtained were used to develop a bioreactor in which the formation of a sulphur particles with good settling properties was achieved.
Section snippets
Removal methods for hydrogen sulphide
The removal of hydrogen sulphide is required for reasons of health, odour problems, safety and corrosivity problems. It causes an irritating, rotten-egg smell above 1 ppm (1.4 mg m−3), and at concentrations above 10 ppm the toxicological exposure limits are exceeded [6].
In order to remove sulphide from wastewater streams, a number of physicochemical processes are in common use today, which involve direct air stripping, chemical precipitation and oxidation. The relatively high energy requirements or
Colloidal properties of biologically produced sulphur particles
In 1887 Winogradsky described the build-up and disappearance of sulphur inclusions by Beggiatoa, depending on whether or not the aqueous medium contained H2S [22]. Many authors have since described the formation and the properties of this “elemental” sulphur from both phototrophic bacteria as well as aerobic Thiobacilli. According to these reports, S0 forms transparent droplets (globules) which are deposited inside or outside the bacterial cells (Fig. 2). The droplets reach diameters of up to 1
Reactor technology
An important issue in the field application of sulphide-oxidising bioreactors is the recovery of the sulphur particles. Among a number of separation processes such as filtration, flotation, extraction, and membrane processes, plain sedimentation of the sulphur particles represents the technically and economically most attractive removal method. Therefore, the formation of sulphur particles with good settling properties is a prerequisite. It was shown that the size of the sulphur aggregates
Conclusions
The biological sulphur cycle offers substantial opportunities for the treatment of polluted gas and water streams, in environmental technology and in process industry. By combining anaerobic sulphate reducing, and aerobic sulphide oxidising processes, almost any inorganic sulphur compound can be converted into elemental sulphur. However, to make the newly-developed technologies successful in practice an adequate sulphur-removal step is essential. Therefore, during the last 4 years, research has
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