Liquid purification or separation – Processes – Treatment by living organism
Patent
1991-11-19
1994-11-22
Wyse, Thomas
Liquid purification or separation
Processes
Treatment by living organism
210615, 210626, 210631, 210903, C02F 306
Patent
active
053666330
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention is in the field of waste water treatment. In particular, the invention relates to a process for the treatment of water containing sulphide or containing sulphur compounds having higher oxidation states such as sulphate, sulphite and thiosulphate which may also contain organic matter, wherein the sulphur compounds are oxidized in a reactor using sludge (biomass) containing aerobic bacteria. Furthermore, the invention relates to a process for the removal of heavy metals from aqueous effluents by precipitation of the heavy metals as sulphides.
BACKGROUND
The presence of sulphur compounds, such as sulphide, in waste water has many adverse consequences, such as: water after discharge of the waste water, involving environmental polution and/or high environmental levies,
While sulphide can be removed from waste water by chemical oxidation, stripping and precipitation, biological purification methods have become increasingly important. Biological removal of sulphide can be performed using phototrophic sulphur bacteria (also accompanied by sulphur production) as well as using denitrifying bacteria. Sulphide can also be converted to sulphate by oxygen consuming bacteria in activated sludge. Sulphur production using oxygen consuming bacteria has advantages over the use of phototrophic bacteria since aerobic conversion proceeds much faster than anaerobic (phototrophic) conversion and light supply in a turbid sulphur reactor is not easy, whereas oxygen can be supplied in an aerobic reactor in a simple way without problems. Nitrate is necessary in the case of denitrifying bacteria.
Advantages of conversion of sulphide into sulphur rather than sulphate include:
A process for the removal of sulphide from waste water by oxidation of the sulphide to elemental sulphur is known from Dutch patent application 8801009 according to which the production of sulphur can be promoted by using a lower oxygen supply than the stoichiometric amount that is needed for sulphate formation. Although a substantial amount of sulphur is produced using this known process, there is a need for improvement of this production, in order to minimize the discharge of soluble sulphur compounds such as sulphide and sulphate.
Another problem connected with biological waste water systems is that sulphide adversely effects the purification efficiency and the sludge retention during aerobic purification of waste water based on a process wherein activated sludge is used. One of the reasons is that sulphide oxidizing, filamentous bacteria such as those of the genera Thiothrix and Beggiatoa can develop in the treatment plants. These filamentous bacteria hamper an efficient settlement of sludge, causing sludge to wash out (bulking out). This has two undesired consequences: a: decrease of the activity of the waste treatment plant resulting in a lower purification performance; b: increase of levies as a result of the increase of the COD load by the washed-out sludge.
The presence of a high amounts of other sulphur compounds, for example a sulphur content of more than 350-500 mg S/l, or a sulphur related chemical oxygen demand (COD/S) of less than 10, in waste water also causes difficulties in the anaerobic treatment of the waste water, since the sulphide that is formed inhibits the methane producing bacteria. However, anaerobic purification of waste water generally has advantages over aerobic purification: low energy consumption, little sludge increase, methane production, etc. Thus, there is great need for a process allowing anaerobic purification of organic waste effluents, even when these contain high amounts of sulphur compounds.
A process for the anaerobic treatment of waste water containing sulphur compounds is known from European patent application 0.241.999, according to which waste water containing sulphate is purified anaerobically whereby sulphate is reduced to sulphide. The sulphide is then removed from the waste water in the form of hydrogen sulphide (H.sub.2 S). This process has drawbacks in that measures have
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Paques B.V.
Wyse Thomas
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