Removal of sulfur compounds from wastewater

Details Removal of sulfur compounds from wastewater Removal of sulfur compounds from wastewater

2002-11-25

2004-03-23

Prince, Fred G. (Department: 1724)

Liquid purification or separation

Processes

Treatment by living organism

C210S631000, C095S235000, C423S243010

Reexamination Certificate

active

06709592

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a process for anaerobic removal of sulfur compounds from wastewater.
BACKGROUND OF THE INVENTION
Anaerobic biological processes for the treatment of wastewater having a high COD (chemical oxygen demand) value are known in the art. In such known processes, organic compounds are converted to biogas (a gas mixture comprising CO
2
and CH
4
) at temperatures above about 25° C. When wastewater contains high amounts of sulfur compounds (e.g., more than ca. 100 mg S/dm
3
), such as sulfates, problems may occur in operating such anaerobic reactors due to the presence of sulfide, which is formed under anaerobic conditions from sulfate. Sulfide may inhibit the methane formation. Also sulfide will give rise to H
2
S formation at pH<9. H
2
S is a toxic and corrosive gas and requires measures to control odour. The sulfide may be reoxidized into sulfate, but this requires an additional aerobic step. Furthermore, regulations often impose a strong limitation on the amount of sulfur compounds that can be discarded into the environment.
As a consequence, there is a need for processes to remove sulfide from wastewater. In the art different approaches have been suggested for sulfide removal processes. For example, EP-A-0 766 650 describes an anaerobic process in which H
2
S is stripped from a methanogenic reactor using a stripping gas. The H
2
S rich stripping gas is fed to a scrubber where the H
2
S is converted into elemental sulfur. The scrubber is operated with a regenerable redox liquid that contains an iron(III) chelate or an iron(III) complex. During this absorption process sulfide is converted to elemental sulfur, while Fe(III) is reduced to Fe(II). Fe(II) is reoxidized to Fe(III) by air in a separate aerator. This known process is particularly suited for the treatment of alkaline wastewater, such as tannery wastewater.
However, the known processes have a number of drawbacks.
The anaerobic reactor in known processes is operated at a relatively high pH (8-8.5), in particular when alkaline wastewater is treated having a pH of 9-12. As a result, the stripping of H
2
S is slow, since H
2
S dissolves more easily in water at higher pH. To compensate for this, equipment of a large volume has to be employed, in particular the stripper column, which has to be operated with large volumetric flows. An example of such a process employing an external stripper is found in U.S. Pat. No. 5,500,123. Alternatively, acid, such as formic acid, can be added. Both alternatives bring about high equipment and/or operating costs. Another result of operating the anaerobic reactor at a relatively high pH is that carbonate may precipitate in the stripper, which leads to fouling and clogging of the equipment.
When the H
2
S loaded gas is scrubbed in the known processes, inevitably an amount of CO
2
, which is present in the stripped gas as well, is absorbed in the scrubber liquid. This CO
2
is eventually vented when the scrubber liquid is regenerated. This net removal of CO
2
from the system gives rise to a further increase of the pH. Moreover, the formation of carbonate salts due to the presence of CO
2
in the stripper equipment may give rise to fouling and clogging. The precipitation of carbonate salts may be prevented by lowering the pH, however, this lowers the rate of absorption and reaction of H
2
S, as a result of which larger absorbers and volumetric flowrates are required.
These drawbacks become apparent for example in the process of EP-A-0 766 650, where the aqueous effluent needs to be recirculated over the stripper a number of times in order to lower the sulfide concentration, since per pass only a small amount of sulfide is stripped. This is the result of the unfavourable equilibrium at higher pH values. This recirculation results in further disadvantages, since the anaerobic reactor is operated at a higher hydraulic loading rate, which may lead to rinsing out of methanogenic sludge from the reactor.
Yet another disadvantage of known processes is that the elemental sulfur that is formed does not accumulate exclusively in the sulfur settling tank, but also forms deposits on walls of the tubing, vessels, sprayers, blowers, pumps, packing material, etc., which causes the need for regular cleaning of the equipment.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a process which obviates, at least partly, some or all of the above mentioned problems. The process of the invention thus aims to provide an improvement to anaerobic wastewater treatment with elemental sulfur recovery.
In accordance with the present invention, there is provided a process for removal of sulfur compounds from wastewater, comprising the steps of: a) converting in a first anaerobic suspended sludge reactor organic compounds present in the wastewater to acid compounds, forming an effluent comprising the acid compounds; b) converting the sulfur compounds to sulfide compounds in the first reactor, the remainder of the sulfur compounds being also comprised by said effluent; c) removing sulfide compounds in said first reactor with a stripping gas, forming a gaseous effluent; d) converting the acid compounds in said effluent to biogas in a second anaerobic reactor; and e) converting sulfur compounds in said effluent to sulfide compounds in the second anaerobic reactor, forming a second liquid effluent.
It will be understood that by converting the compounds in steps a), b), d) and e), and by stripping in step c) is meant that at least an essential part of the respective compounds present are converted, stripped or removed, the non-converted, non-stripped and non-removed compounds leaving the reactor in the effluent.
The organic compounds present in the wastewater are converted to acid compounds, in particular organic (carboxylic) acids, such as (lower) fatty acids. The conversion of organic compounds to acid compounds, as well as the conversion of sulfur compounds to sulfide is established using microorganisms known to the skilled person. For acidification of organic compounds bacterial species from the genera Clostridium, Ruminococcus, Propionibacterium, Selenomonas, Micromonospora, the family of the Lactobacteriaceae, and thermophilic clostridia can be used, as well as anaerobic sludge in which they are present. For the production of sulfide from sulfur compounds bacterial species from the genera Desulfovibrio, Desulfotomaculum, Desulfobacter, Desulfococcus, Desulfuromonas, Desulfonema, Desulfobulbus and for thermophilic applications
Sulfolobus
can be used, as well as sulfate reducing sludge or anaerobic sludge in which they are present.
The first (acidification) reactor typically works at 10-40° C. (mesophilic range) or 40-80° C. (thermophilic range), preferably 30-35° C., pressures between 0.3 and 4 bara, mostly 1 bara and biomass concentrations between 0.1 and 10 g dry weight/dm
3
, mostly 3 g/dm
3
. The ratio between stripping gas flow rate and wastewater flow rate can be 3 to 100, but is typically 15.
The term ‘suspended sludge reactor’ is used in the present description and claims in its ordinary meaning and encompasses, as the skilled person knows, reactors in which the sludge is essentially free to move through the reactor, viz. reactors in which the sludge is essentially not bound to a static surface in the reactor. Such reactors do therefore not rely on measures to increase the surface area which may serve as a substrate for the microorganisms, such as packings, etc. The sludge in these type of reactors is kept in suspension by means of agitation, e.g. by mixing using an agitator, by recirculating the liquids or by bubbling gas through the liquids.
As a result of the production of acid compounds, the pH in the first reactor will be relatively low. The pH in the first reactor will depend on the type of wastewater to be treated, but is generally maintained at a value lower than 9, preferably lower than 8, most preferably between 6 and 7.5. Because of the low pH, the sulfide formed in the first reactor is relatively easily stripped. Moreover, only a r

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