GB2143810A

GB2143810A - Process for biological reduction of sulphur oxides

Info

Publication number: GB2143810A
Application number: GB08415444A
Authority: GB
Inventors: Meint Olthof; Jan Alojzy Oleszkiewicz; Otto Milgram; Harold Garfinkle Weinreb
Original assignee: Duncan Lagnese & Ass Inc
Current assignee: Duncan Lagnese & Ass Inc
Priority date: 1983-06-20
Filing date: 1984-06-18
Publication date: 1985-02-20
Also published as: NL8401767A; GB8415444D0; DE3422959A1; FR2548213A1

Abstract

Sulphur oxide-containing material and organic material are fed into an anaerobic expanded bed or fixed film reactor that has been inoculated with a mixed culture of microorganisms. Hydrogen sulphide is removed from the fermented mixture by gas stripping inside and/or outside the reactor, and preferably partly within and partly outside the reactor. The hydrogen sulphide can be processed further to elemental sulphur, sulphuric acid or sulphide salts. The sulphur oxides reduction process may also generate soluble or insoluble carbonates which may be used as the microorganism growth medium or which can be recovered for use in industrial processes.

Description

SPECIFICATION Process for biological reduction of sulphur oxides This invention relates to the anaerobic biological conversion of a mixture of sulphur oxides and organic matter to hydrogen sulphide, carbon dioxide and water. The term "sulphur oxide" as used herein means an oxide of sulphur or a salt derived therefrom such as a sulphite or sulphate.

Significant quantities of sulphur oxides, such as gypsum or calcium sulphate, are generated as a byproduct of industrial processes. For example, the phosphate fertilizer industry generates about 5 tons of gypsum per ton of fertilizer. In addition, calcium sulphate waste sludges are formed in many air and wastewater treatment processes. Large volumes of gypsum are produced by power plants in scrubbing out sulphur dioxide from the off gases produced by combustion of sulphur containing fuels and are disposed of as landfill.

On the other hand, hydrogen sulphide, elemental sulphur and sulphuric acid are required by industry.

This invention provides a process whereby industrial wastes containing sulphur oxides and organic material are biologically converted to hydrogen sulphide, carbon dioxide and water. The hydrogen sulphide then may be converted by conventional methods to elemental sulphur, sulphuric acid or sulphide salts.

Other useful by-products, such as sodium, calcium or magnesium carbonate, may be recovered, depending upon the chemical structure of the molecule containing the sulphur oxides to be reduced. Thus, the invention both reduces the volume of industrial waste which must be disposed of and provides useful products for further industrial applications.

It is known that certain anaerobic bacteria can reduce sulphur oxides to sulphide. The genera of known sulphate-reducing bacteria are Desulfovibrio, Desulfotomaculum and Desulfobacter. Some of these, for example, Desulfotomaculum, are versatile and capable of methane fermentation as well. Sulphur oxide reduction has, in fact, been an undesirable side reaction in methane fermentation in that an accumulation of hydrogen sulphide in the reactor is toxic to the methane-forming bacteria. Some previous work has been done in attempting to understand and harness the sulphur oxides reducing capabilities of microorganisms. For example, S.G. Burgess et al., "Pilot-Plant Studies in Production of Sulphur from Sulphate Enriched Sewage Sludge," J. Sci. Food Agric., Vol. 12, pp. 326-35 (April 1961) and J.C. Sadana et al., "Microbiological Production of Sulphide from Gypsum," J. Sci. Indus.Res., Vol. 21C, pp. 124-27 (May 1962), disclose both batch processes and semi-continuous processes in which the fermentation reactors were fed with daily charges of gypsum and either sewage, sludge or acetic acid. The reactors were swept with nitrogen or a combination of nitrogen and carbon dioxide to remove the hydrogen sulphide formed by the fermentation. The Sadana reactor was mechanically stirred continuously and the Burgess reactor was stirred before and after charging. In addition, A.C. Middleton et al., "Kinetics of Microbial Sulphide Reduction," J. Water Pollution Control Fed., Vol. 49, pp. 1659-60 (July 1977), discloses a completely mixed suspended growth continuous process in which reactor gases were recycled to mix the biological reactor.

None of the processes of these references would be economical to run at the scale required for industrial application. Internal gas sweeping would require a great deal of energy and would interfere with the proper fluidization and stratification of the biological reactor. In addition, the semi-continuous process cannot accommodate the volumes and flow rates required by commercial facilities.

The present invention provides a method for the biological reduction of sulphur oxides with simultaneous biodegradation of organic materials to give hydrogen sulphide, carbon dioxide and water. A mixed culture, containing microorganisms capable of degrading organic molecules, oxidizing organic molecules and/or reducing sulphur oxides, is introduced into an anaerobic reactor, together with sulphur oxides and organic material. Fermentation occurs, the sulphur oxides are reduced and the organic material is degraded and oxidised.

The method of this invention makes it possible to recover commercially useful resources by biological conversion of sulphur oxide containing material in the presence of industrial or other organic wastestreams. This can significantly reduce the volume of solid waste material which must be disposed of at sea, in landfills or other approved depositories. The new method can accommodate high volumes of industrial wastestreams without sacrificing conversion efficiency, and requires only a low carbon-to-sulphur input ratio, which greatly reduces the organic material required for the reaction.

The new method may be operated to provide, as a by-product of the reaction, the self-generation of the reactor medium, and is stable and resistant to upset.

According to the present invention a method for reduction of sulphur oxides comprises fermenting a mixture of sulphur oxide-containing material, and organic material with a mixed culture of microorganisms, in an anaerobic reactor, and in particular an expanded bed or fixed film reactor, and removing accumulated hydrogen sulphide from the fermented mixture by gas stripping. Such gas stripping may be effected inside the reactor, or in a zone external to the said reactor or both internally or externally.

The reaction is preferably effected in a continuous upflow expanded bed reactor. The hydrogen sulphide concentration is kept just below the toxic limit by sparging the reactor with small amounts of gas.

The reactor effluent is conducted into a gas sweeping chamber or compartment, where a gas stream is used to remove at least a major part of the remaining hydrogen sulphide. At least a part of the effluent, after being rendered hydrogen sulphide-poor, is preferably recirculated back to the reactor. This staged gas sweeping provides a preferred and economical method of removing the toxic hydrogen sulphide from the reactor.

The materials useful for carrying out this process may be obtained primarily from industrial or municipal waste sources. Although it is the aim of this invention to make use of various sulphur oxide-containing wastes, as well as organic-containing wastes, it will be understood that the utilization of non-waste sources of the required materials is within the intended scope of the invention.

Industrial or municipal wastestreams which contain sulphur oxides may be used as a sulphur oxide source for practice of this invention. Previosuly, anaerobic treatment (i.e., for methane production) of high concentration sulphur oxide-containing wastes was not possible because the hydrogen sulphide produced during fermentation was toxic to the microorganisms. The process of this invention reduces that limitation since it makes possible the anaerobic treatment of such wastestreams for resource recovery and pollutant removal, and provides industry with a cost-effective anaerobic treatment technology.

Among the major industrial sources of sulphur oxide-containing waste is the phosphate fertilizer industry, which generates approximately 5 tons of gypsum (CaSO4) per ton of fertilizer. This gypsum currently is disposed of in landfills.

A second major source of sulphur oxide-containing waste is power plant combustion and other processes which burn sulphur-containing fuel. The combustion of sulphur-containing fuel produces more than 75% of sulphur dioxide emissions in the United States. The major method of desulphurization of this flue gas involves scrubbing with lime and/or limestone slurry. The scrubber effluent contains a calcium sulphate-calcium sulphite mixture which is useful in the process of this invention. Similarly, the effluent from a sodium carbonate scrubber system contains a sodium sulphate-sodium sulphite mixture which also is useful in this process. In some cases, magnesium, rather than calcium, oxide or carbonate may be used. The sulphur oxide-containing material useful in the new method can thus be a variety of soluble or insoluble sulphates or sulphites.

Other useful industrial sulphur oxides are generated by the pulp and paper industry as wastewater containing sulphite and sulphate residues together with degradable organic material. In addition, a variety of processes in the chemical industry generate waste water with high concentrations of organic material and sulphur oxides. Parts of the food industry generate wastestreams with high organics and sulphur oxides concentrations, and certain pharmaceutical processes generate sulphur oxide-containing waste.

Also, municipal waste at some locations may have a high sulphur oxide concentration.

As the organic material, a variety of municipal, industrial and agricultural organic waste sources may be used. For example, municipal waste or sludge is a useful source of organic material, as is synthetic wastewater composed of a variety of industrial organic chemicals. Manure may also be used, as may wastes from distillery, cotton kiering, citrus, wine, yeast, molasses and meat packing processes.

The primary limitation on wastestreams useful in this process is that wastes containing soluble contaminants which are toxic to the microorganisms should not be used. Non-toxic non-degradable soluble contaminants flow unchanged through the reductionloxidation process. In cases where the sulphur oxide-containing and organic materials are other than liquid, they may be fed into the reactor in the form of a slurry. Although the solids concentration of the slurry may vary, it is preferred that it be in the range of about 5% to about 10%.

A mixed culture of microorganisms is used to inoculate the reactor. There will be numerous types of microorganisms present in the inoculum; those which are suitable for this process (i.e. those adaptable to the environment within the reactor) will thrive and multiply to form the reactor biomass. Among suitable microorganisms are those which degrade larger organic molecules to smaller organic acids, and those which both oxidize the acids to carbon dioxide and water and reduce the sulphur oxides to hydrogen sulphide. The various microorganisms which actively further the reactions of the new method are referred to herein as a "mixed culture of microorganisms." An appropriate mixed culture of microorganisms may be obtained from several sources.The conversion of sulphur oxides to sulphide by anaerobic microorganisms occurs spontaneously and is noticed quite commonly in municipal sewers and other anaerobic waste treatment reactors. A starter culture may be obtained from any of these sources, or from a mixture of sources.

Once the fermentation process is underway, there will be more than adequate generation of the desired mixed culture of microorganisms to sustain the reaction, provided that the minimum solids retention times are maintained. In fact, excess biological growth should be periodically removed from the reactor.

The process of this invention is carried out in an anaerobic reactor with suspended growth biomass.

Provision is made for intake of organic materials and sulphur oxide-containing materials. The hydrogen sulphide reaction product is preferably purged from the system by gas sweeping, which may conveniently use nitrogen and/or carbon dioxide or, in some cases, methane.

In a preferred embodiment of the invention, a continuous upflow expanded bed reactor is used. The size of the reactor will vary on a site-by-site basis, depending on the amount of sulphur oxides and/or organic wastes produced at the site. The size of the unit is primarily determined by the required solids retention time and the rate of conversion of the organic material. The solids retention time, i.e. the time the biomass is retained in the system, is the prime design parameter determining the ability of the sys tem to degrade specific organic material and sulphur oxides, and to withstand process upsets.

Solids retention time (SRT) can be determined by calculating the increase in biological solids in the reactor as a function of the amount of organic material oxidized. For example, 10 pounds (4.5 kg.) of microorganisms may generate 0.050-0.15 pounds (0.22 to 0.67 kg.) of new bacteria each day. If 0.1 pounds (0.045 kg.) are generated: SRT 10 10 pounds (4.5 kg.) of bacteria SRT 0.1 pound (0.045 kg.) of bacteria/day = 100 days The SRT in this example is 100 days. Typical SRT's for anaerobic systems may be 30-130 days.

Because of the generally high conversion rate in this process, the requirement for lengthy hydraulic retention is low. Short hydraulic retention results in a high flow-through rate, and a high flow-through rate allows use of a relatively small volume reactor. Thus, the conversion is carried out more efficiently with a greater concentration of microorganisms per unit volume of reactor space. The volume per reactor can vary from 100 to 100,000 cubic feet (2.8 to 2830 m3) or more.

The reactor size is also determined by the sulphur oxide load. The design loading of the reactor (weight of sulphur oxide per unit volume of reactor per day) will vary according to the type of organic material used, but as a rule will be greater than with conventional aerobic or anaerobic processes.

The reactor shape may be similar to conventional expanded bed reactors. Typically, the reactor has a conical lower portion and a cylindrical upper portion. A sludge-liquid separator, or clarifier, is located in the upper portion of the reactor to control the quantity of solids flowing out of the reactor into the gas separation chamber.

The lower portion of the reactor is provided with intake means for introducing the required material into the reactor. Conveniently, the materials may be pumped into the reactor from a source such as a holding tank. The upper portion is provided with means for conducting the reaction effluent from the reactor to a gas separation chamber or compartment. The reactor is provided with means for introducing small amounts of gas into the lower portions of the reactor. In addition, means for removing accumulated hydrogen sulphide preferably by gas sweeping is provided external to the reactor. This hydrogen sulphide removal is described in more detail below. A convenient means for recycling at least a portion of the hydrogen sulphide-poor effluent from the gas separation chamber back into the reactor is also provided.

It may be desired to employ a fixed film reactor in the wastewater treatment process of this invention.

Any convenient growth media conducive to growth of fine fixed film bacterial slime may be used, plastic or ceramic, for example. The growth media may be oriented (channeled) or may be randomly packed.

The configuration should be such as will provide channels or voids for the flow of the wastestream.

The reactor is connected to a continuous source of sulphur oxide-containing material and a continuous source of organic material. These materials may be combined before introduction into the reactor, or there may be separate inlets for each material. In the preferred upflow reactor, these materials enter through the bottom of the reactor, but it is also possible to locate the inlet(s) on the side of the reactor.

In the preferred embodiment an expanded bed reactor is used. The upward flow velocity in the reactor is sufficient to maintain the biomass in an expanded bed without tending to flush significant amounts of the biomass from the reactor. "Expanded bed" as used herein includes within its scope particles maintained in a fluidized state as well as particles subjected to lesser relative movement. The upflow velocity preferably is about 0.5-1.0 gallons per minute per square foot (0.34 to 0.68 litres per second per square metre), but may be from about 0.2 to about 5.0 gallons per minute per square foot (0.14 to 3.4 litres per second per square metre), as desired in any particular case.

The reactor contains a sludge bed made up of a mixed culture of microorganisms, as described above, and a medium on which the microorganisms are grown. Calcium carbonate or other conventional bacteria growth medium may be used. When the sulphur oxide-containing influent contains gypsum, the presence of Ca2 ions greatly improves conditions for sludge bed formation and retention. Since insoluble calcium carbonate is generated as a by-product of the reduction reaction when calcium is present, operation of the invention provides self-regeneration of the growth medium. It has been found that stable sludge beds can be formed one to two months after inoculation with the mixed culture.

A reactor employing the method of this invention has an inherently higher resistance to environmental upsets when overloaded than methane reactors. In a methane reactor, the organic wastes are broken down in two steps. The first step converts the organic material to acids ("acid fermentation") and the second step converts the acids to methane ("methane fermentation"). In a well balanced reactor, these two steps occur simultaneously. However, when these two steps become unbalanced the reactor is subject to pH upsets. The microorganisms responsible for methane fermentation are very acid sensitive and cannot tolerate changes in pH.

The method of this invention also utilizes a two-step process. However, this method is capable of maintaining a stable reactor pH because it generates approximately four times the alkalinity generated by methane fermentation. The result is that reaction systems of this invention are virtually immune to pH upset.

It has been found that the pH in the reactor consistently remains in the 7.0-8.0 range, regardless of the pH of the influent. Influent pH can be varied from at least about 2.5 to at least about 6.5 without altering reactor pH. This facilitates the use of a wide variety of wastestreams without the added step of pH adjustment. Moreover, the system can tolerate a higher allowable load.

The two-step process of this invention is similar to methane fermentation in that "acid fermentation" is the first step. That is, microorganisms degrade larger organic molecules to smaller organic acids. However, in the second step, other microorganisms oxidize the organic acids to water and carbon dioxide and also reduce the sulphur oxides to hydrogen sulphide.

The overall reaction taking place in the reactor can be represented by the following (unbalanced) equations: CaSO4 + organics < CaCO2 + CO2 + H2O + H2S or Na2SO4 + organics < NaHCO3 + CO2 + H2O + H2S or MgSO4 + organics < MgCO3 + CO2 + H2O + H2S The ratio of carbon (in the organic material) to sulphur (in the sulphur oxide-containing material) should generally be in the range of from about 0.75 to 1 to about 3 to 1 for optimal performance. The conversion- rate of, e.g., calcium sulphate or sodium sulphate will be from about 0.5 to about 2 pounds per cubic foot (8 to 32 kg/m3) of reactor space per day.

The products of the reaction vary according to the wastestream. For example, insoluble calcium carbonate is produced in applications using gypsum as the sulphur oxide-containing material. Soluble sodium carbonate is formed where the wastestream is from a sodium carbonate flue gas scrubber system.

Magnesium carbonate is produced when this process is used with wastes from scrubber systems using magnesium carbonate.

In the preferred embodiment of the new process, accumulated hydrogen sulphide is removed from the reactor contents and from the effluent by a staged stripping procedure. Removal of hydrogen sulphide is critical since a buildup of hydrogen sulphide in the reactor quickly poisons the microorganisms. Hydrogen sulphide stripping is most conveniently accomplished by sweeping with a suitable inert gas, such as nitrogen, but other means may be employed, if desired. The presence of a gas phase in the effluent facilitates the liberation of hydrogen sulphide gas out of solution. Any gas inert with respect to the reactants and reaction products may be used. Nitrogen gas works well for this purpose, as does carbon dioxide or a combination of the two. If the reactor is operated near or in conjunction with a methanogenic digester, methane may be used for the gas sweeping step.

A two-stage stripping is preferred. In this mode of operation, the hydrogen sulphide concentration in the reactor is kept somewhat below the level toxic to the microorganisms by sparging the reactor with small quantities of gas. The gas-to-liquid ratio in the reactor is preferably about 2:1 to about 10:1 ("liquid" in the context of this ratio refers to the effluent flow from the reactor). With this relatively low gas flow rate, the hydraulic flow patterns of the reactor and the stratification of the expanded bed are not disrupted to any significant extent.

The hydrogen sulphide-reduced effluent from the reactor preferably is conducted into a separate chamber, compartment or other convenient location for the second stage of the stripping procedure and for gas separation. The chamber is swept with large quantities of selected gas to remove all or most of the hydrogen sulphide from the effluent. In this external stripping chamber, the gas-to-liquid ratio can range from about 5:1 to about 100:1, preferably about 5:1 to about 20:1, depending on the amount of hydrogen sulphide to be stripped and on the desired effluent quality. It is possible to use high ratios - i.e. high gas flow rates - in the stripping chamber since violent mixing of the gas and liquid external to the reactor will not adversely affect the hydraulics pattern or retention of biomass in the reactor itself.

The use of staged stripping is advantageous from an energy efficiency standpoint as it allows the use of two small gas-to-liquid ratios for the removal of hydrogen sulphide. Moreover, the total volume of gas needed for the stripping operation is conserved by reducing the flow rates.

Alternatively, the hydrogen sulphide may be removed either by sweeping only the reactor itself or by sweeping only in a chamber or zone external to the reactor. However, for large reactor volumes, the use of exclusively internal gas sweeping is inefficient from energy and technical standpoints. In order to remove sufficient hydrogen sulphide, large quantities of gas must be pumped through the reactor relatively forcefully, reducing the economy of the recovery process. Moreover, internal gas sweeping of this intensity would interfere with the hydraulic flow patterns of the reactor and the stratification of the expanded bed. Removing the hydrogen sulphide by sweeping only in an external chamber avoids the problems encountered by exclusive internal sweeping, but, as discussed above, it not as desirable for energy efficiency reasons as staged stripping.

After the effluent is stripped of hydrogen sulphide by gas sweeping or other suitable means, a portion of the hydrogen sulphide-poor effluent is recycled into the reactor for further reaction and for maintenance of the expanded bed. By continuously removing the hydrogen sulphide-contaminated reactor ma terials and recycling them after stripping out the hydrogen sulphide, the reactor is maintained in a healthy, actively digesting condition. It is possible to dilute the wastestream with water or other liquid, rather than recycling hydrogen sulphide-poor effluent. However, this approach is far less economical as it adds quantities of water to the system which ultimately would need to be removed and disposed of.

Regardless of the location of the hydrogen sulphide stripping, the hydrogen sulphide is conducted away from the reactor for further processing into a commercially useful product. This processing is conducted according to conventional technology. For example, hydrogen sulphide may be converted into elemental sulphur by the commercially available Claus process wherein the hydrogen sulphide is catalytically oxidized to sulphur plus water. Alternatively, commercial grade sulphuric acid may be generated by conversion of sulphur over a suitable catalyst system. Still another option is to scrub the hydrogen sulphide from the gas stream with sodium hydroxide, thereby producing sodium bisulphide, which is useful in the organic chemicals industry. The inert gas may be recycled for further sweeping of the reactor and/ or the external stripping chamber.

The liquid overflow from the gas separation chamber, may be disposed of as a non-polluting discharge, possibly after polishing treatment. The volume of overflow is equal to the volume of waste fed to the reactor. The overflow volume is thus controlled by control of reactor influent, and with a concentrated feed a greater proportion of reactor effluent is recycled. Conversely, a more dilute feed results in a higher volume of overflow.

Although feed concentration may be varied, there are practical limits which should be borne in mind.

The concentration should not be so high as to interfere with the expanded nature of the reactor bed. A maximum slurry feed concentration is about 10%. Conversely, an extremely dilute feed does not utilize the full conversion capabilities of the reactor. Thus, a concentration of 0.05-0.1% sludge would not utilize the full benefits of this invention. If a fixed film reactor is used, the feed should be comprised only or substantially only of soluble wastes or compounds.

Provision is also made for removal of solids from the system, either continuously or periodically. For example, when this process is used at a power plant using a calcium-based gas desulphurization system (see Application B, below), a continuous purge of solids from the reactor effluent may be desired. In this way, calcium carbonate is continuously reclaimed for re-use in the power plant's flue gas scrubbers.

In other applications, periodic purging may be desired, with the timing determined by the solids content in the reactor. That is, when the solids concentration in the reactor reaches a preset limit, some sludge should be withdrawn to reduce the solids concentration. This preset limit is determined by the condition of the reactor. The solids content is kept as high as possible without developing mechanical problems in the reactor. Thus, evidence of settling or solids plugging, or the inability to expand the bed, indicates that the solids concentration limit has been reached. A typical solids concentration limit may be 20-50 grams/liter.

The solids are typically de-watered and sold, re-used or otherwise disposed of. The rate of solids generation depends on the type of organic material used and the amount of calcium ions present in the wastestream. The volume of solids requiring disposal is less than the volume of solid waste used to feed the reactor. This is particularly true when there is a use for the alkali by-product, e.g. in neutralization, scrubbing, etc.

The benefits of this invention are multiple. Resource recovery is a primary benefit to industry. Hydrogen sulphide, elemental sulphur or sulphuric acid reclaimed by the process can be re-used. In addition, in some applications calcium carbonate is formed which provides a growth medium for the microorganisms, and which can also be removed and converted into pebble lime for re-use in the scrubbing of industrial off gases or neutralization of acid wastestreams. Moreover, the volume of disposable industrial and/or municipal wastes is significantly reduced by utilization of the process, thereby greatly decreasing landfill demands.

The following Examples and Applications illustrate the invention. The Examples (including the comparative Example) describe laboratory scale experiments. The comparative example is not a demonstration of the invention claimed herein, but is included to illustrate the improvement shown by the claimed treatment process. The experimental example summarizes an actual laboratory demonstration of one embodiment of this invention, using gas sweeping only in an external chamber for removal of accumulated hydrogen sulphide. The mathematical example is based on the results of the experimental example with mathematical calculations to determine the effect of staged stripping; this example has not actually been run. The Applications, while not actually run on plant-scale, are included to illustrate certain embodiments and industrial applications of the invention.

Example I (comparative) This Example utilized a continuous flow reactor with no gas sweeping. The reactor was plastic, conically shaped and oriented with the wider portion on top. The reactor volume was 1.3 liters. The hydraulic residence time of the feed was approximately one day. The sulphur oxides and organic materials were pumped into the reactor bottom. Effluent from the top of the reactor was conducted into a gas separation box and then was partially recycled back into the reactor. The recycle rate was such that a 5:1 to 10:1 ratio existed between the makeup flow rate and the recycle rate.

The immediate source of the bacterial inoculum used in Example I was another digester in the inven tors' laboratory; the original source was sludge obtained from a municipal treatment plant. The system was started with a mixture of piggery waste and gypsum. The stock food used was a solution containing a wide range of organics and nutrients.Its composition is as follows: Ingredient Quantity Formaldehyde (37% aqueous) 10 ml Methanol 25 ml Sodium Acetate 110 gm Benzoic Acid 45 gm Propionic Acid 55 ml Butanol 35 ml Ethanol 20 ml Monoethanolamine 35 ml Butyl Acetate 45 ml Diethylene Glycol 60 ml Ammonium Carbonate 160 gm Sodium Phosphate (.1H2O) 20 gm Potassium Phosphate (anhydrous) 20 gm Phenol 6 gm Sugar 50 gm Ferrous Sulphate (.7H2O) 2 gm Microelements* 40 ml *The "microelements" are added as a solution made up of: 5 mg/l NiSO4; 5 mgn MnCl2. 4 H2O; 1000 mg/l FeSO4. 7 H2O; 100 mg/l ZnSO4. 7 H2O; 50 mg/l CoCI2.

6 H2O; 5 mg/l CuSO4. 5 H2O: 100 mg/l H3BO3; 50mg/l Na2MoO4. 2 H2O The listed ingredients are combined and diluted to 18 liters to form a stock food solution.

The reactor was run for 43 days. Table I shows data collected and generated over period including Days 14 through 42. The reactor demonstrated a pH stability of 6.3 to 6.84, even with an influent pH range of 2.5 to 6.0.

TABLE I Results of experiments without gas sweeping Gas Volume Sulphide mg S in Gas pH Production mg/l So4 mg/l SOC+ of Feed in Gas Produced/ % SO4 Day Influent Effluent ml/day influent Effluent Influent Effluent l/day mg/day mg S Removal Removal 14* 6.0 7.76 50 - 325 - - - - - 15 6.0 7.62 130 - - - - 0.70 0 0 17 6.0 7.35 30 - - - - 0.86 - - 20 6.0 7.7 150 - - 680 90 0.85 - - 21** 6.0 7.4 100 - 176 745 210 0.65 24 - 25 6.0 7.84 220 - 388 370 120 0.88 56 - 27 6.0 7.7 120 - - 200 130 0.8 - - 28 6.0 7.84 0 1070 750 - - 0.95 - - 30 34*** 4.0 7.63 20 - - 800 95 0.9 - - 36 3.5 7.38 125 1480 580 390 75 1.5 21 0.14 62 37 2.5 7.72 160 1220 550 - - 1.5 49 0.15 66 39 4.2 7.3 120 760 550 400 90 1.25 28 0.32 41 41 4.2 7.8 170 760 530 415 85 - - - 90 42 4.2 7.8 50 - - - - - - - - * Feed: 65 ml piggery waste + 12.5 g CaSO4, in 1.5 1 water ** Feed: 150 ml piggery waste + 12.0 g CaSO4, in 1.5 1 water.

Feed: 10 ml piggery waste + 0.9g acetic acid, 40 ml stock food, 3.9 g CaSO4, in 1.5 1 water.

t SOC = Soluble Organic Carbon Cyclic drops in gas generation were noted. It is believed that the cause was cyclic increases of hydrogen sulphide concentrations above the inhibitory levels. When hydrogen sulphide concentration was low, i.a, below toxic levels, more gas could be produced. The increased gas production increased the hydrogen sulphide concentration which, in turn, resulted in a drop in gas production.

Example 11 The reactor from Example I was used, with the addition of a gas sweeping system consisting of a gas separation box external to the reactor. Nitrogen gas was used to sweep the effluent in the gas separation box. The resulting nitrogen-hydrogen sulphide mixture was conducted into a hydrogen sulphide trap consisting of a vessel containing 200 ml of a solution of zinc acetate (2-5 g/l) in H2O and with sufficient sodium hydroxide added to bring the pH to 10-11. In this way, the amount of hydrogen sulphide could be measured by titrating the zinc acetate. The hydrogen sulphide-poor effluent then was partially recycled from the gas sweeping compartment back into the reactor, with overflow being conducted into an effluent storage tank.

The reactor feed comprised 25 ml piggery waste plus 1.3 g acetic acid and 160 ml stock food (see Example I for stock food recipe). The influent sulphate content (CaSO4) varied as shown in column 5 of Table II. Approximately a 10% slurry concentration was used. The reactor was run with external nitrogen sweeping for 14 days. Table II shows the data collected and generated for Example II.

During the first eight days, hydrogen sulphide gas generation increased dramatically. On Days 9-10 an accidental spill of approximately 150 ml of reactor contents occurred. After replacement with 150 ml of the content of a dormant sulphur oxides digester, the system immediately resumed full performance.

It was found that the reactor pH was stable at 7.38 to 8.10, even with an influent pH of 2.0 to 3.4. TABLE II Results of experiments without gas sweeping Gas Volume Sulphide mg S in Gas pH Production mg/l So4 mg/l SOC+ of Feed in Gas Produced/ % SO4 Day Influent Effluent ml/day influent Effluent Influent Effluent l/day mg/day mg S Removal Removal 1 - 7.42 **** - - - - 1.5 186 - 2 3 7.38 **** - - - - 1.5 94 - 3 3 7.99 **** 1360 960 780 70 1.8 208 0.88 30 4* 3 - **** 1430 560 - - 0 282 - 5 2.5 7.82 **** 1580 570 780 44 1.1 477 1.30 64 6 2.5 7.63 **** 1790 600 - - 1.8 443 0.63 67 7 2.0 8.10 **** 1990 370 970 22 1.8 576 0.60 81 8** 3.4 7.78 **** 1600 335 - 26 1.0 275 0.72 78 9 3.4 - **** - - - - 0 100 - 10*** 2.5 7.52 **** 1670 73 - - 1.4 500 0.68 96 11 2.5 7.8 **** 1100 75 - - 0.83 368 1.31 93 12 2.5 7.50 **** 1800 96 - - 1.1 416 0.67 95 13 2.5 7.55 **** 1740 84 - - 0.6 - - 95 * Influent pump off.

** Accidental spill of portion of reactor contents.

Supplemented biomass with material from other reactor.

**** No true reading because of gas sweeping.

SOC = Soluble Organic Carbon During the last 4 days of operation of the reactor in Example II, the average conversion rate was about 1000 mg SO4/liter-reactor/day. Over 90% of this was converted to hydrogen sulphide. The corresponding soluble organic carbon (SOC) removal was about 720 mg SOC/liter-reactor/day and resulted in over 95% SOC removal.

Example 111 (Mathematical example) The economics of single stage stripping in a chamber external to the reactor versus staged stripping as described above can be calculated in the following manner. The wastestream is assumed to comprise 10,000 mg/l COD (chemical oxygen demand, calculated by determining the quantity of oxygen required to oxidize all the organics in the sample by chemical means) and 4,500 mg/l SO4. It is assumed that a 90% reduction in sulphate will occur by the treatment process of this invention, corresponding to 4,050 mg/l as SO, or 1,350 mgil as S. This will form 1,434 mgil hydrogen sulphide.

Total COD removal is assumed to be about 80% by this process, or about 8,000 mg/l. The amount of COD removed by the sulphate-reducing microorganisms will be about 4,000 mg/l. Therefore, about 4,000 mg/l COD will be converted to methane by methanogenic microorganisms at a rate of about 0.32 litres methane per gram COD removed. The total methane production will be 4.0 grams COD x 0.32 litres CH4 = 1.3 litres methane per litre of waste. The methane produced in the reactor will aid in the stripping of hydrogen sulphide, reducing the volume of gas needed to be introduced for in-reactor stripping.

One Stage External Stripping - A final hydrogen sulphide concentration of about 20 mgil in the external stripping chamber effluent liquid will be selected. The hydrogen sulphide concentration in the stripping gas leaving the chamber is also about 20 mgil due to an approximately 1:1 liquid-gas equilibrium.

Therefore, to strip out about 1,400 mg/l hydrogen sulphide will require about 70 litres of gas for stripping the external chamber.

Staged Stripping - A hydrogen sulphide concentration of 100 mg/l will be maintained in the reactor to prevent toxicity. The gas leaving the reactor will thus contain about 100 mg/l hydrogen sulphide. Therefore, to remove 1,300 mg/l hydrogen sulphide will require 13 litres of gas. In the external stripping chamber the hydrogen sulphide concentration will be reduced from 100 mgll to 20 mg/l. Since the gas hydrogen sulphide concentration therefore will be 20 mg/l, this will require 4 litres of gas for the external stripping stage. The total amount of gas required for staged stripping in this example will be 13 (in-reactor) plus 4 (external) or 17 litres of gas per litre of effluent.

Application A The process of this invention may be adapted for use at a fertilizer plant, which normally generates about 5 tons of waste gypsum for every ton of fertilizer produced. Currently, these facilities dispose of the gypsum in landfills. In addition, elemental sulphur used to produce the fertilizer must be purchased.

Since the cost of gypsum disposal typically is nominal, the primary industrial incentive for adopting this process is for sulphur recovery. However, there are also environmental benefits from decreased dumping of gypsum.

The reclamation facility includes the basic anaerobic expanded bed reactor with external gas separation chamber and recycle capabilities. The hydrogen sulphide gas generated by the process is converted to elemental sulphur by conventional methods as described above.

A reactor of this invention should be able to convert from about 0.5 to about 2 pounds of gypsum per cubic foot (8 to 32 kg/m3) of reactor space per day. Preferably the conversion rate is about one to two pounds per cubic foot (16 to 32 kg/m3) per day. The gypsum is pumped into the reactor in the form of a slurry, preferably containing about 5% to 10% solids.

A variety of suitable organic materials may be conveniently used. Depending on the location of the facility, undigested municipal sludges, manure or industrial organic wastes may be available locally at low cost or perhaps for the cost of transportation. The selection of materials will be site-specific and other available organics-containing waste sources may be used. A combination of sources also is contemplated. About 20% of the organic sludge is likely to be non-digestible and will require disposal, possibly as a soil conditioner with fertilizer value.

The energy liberated by the conversion of hydrogen sulphide to elemental sulphur is available for use in generating steam in the plant. The theoretical yield is about three parts by weight of steam per part by weight of sulphur produced. A portion of this may be utilized in the gas separation system of the sulphur conversion system, leaving about one part by weight of steam per part by weight of sulphur available for other plant energy needs.

Application B A second major industrial application for this invention is at sites where combustion of sulphur containing fuel takes place, power plants, for example. Flue gas desulphurisation processes generally involve lime and/or limestone slurry scrubbing, including alkaline-fly-ash scrubbing. These processes use a slurry of lime or limestone in water to absorb sulphur dioxide from flue gas in a gas/liquid scrubber. The slurry generally contains from 5% to 15% solids. Other scrubber systems utilize sodium or magnesium carbonate.

The effluent from a lime or limestone scrubber system contains a calcium sulphate-calcium sulphite mixture which is suitable as the sulphur oxide-containing material used in this invention. The effluent from a sodium carbonate scrubber system contains a sodium sulphate-sodium sulphite mixture which also is useful in this process. Alternatively, some systems will yield magnesium oxides. Various organic materials, as described above, may be combined with the scrubber effluent to feed the reactor.

A major problem with these scrubber systems is the disposal of large volumes of sulphur-containing sludge. Approximately half of the operating and maintenance expenses associated with scrubber systems is attributable to sludge generation and disposal. The sludge volume is greatly reduced by the process of this invention, thereby reducing both the problem and the costs associated with its disposal. Other major cost factors in using calcium-based flue gas desulphurisation systems are the purchase of lime or limestone and the preparation of slurry Limestone, for example, must be crushed prior to slurry formation.

The process of this invention produces calcium carbonate which can be re-used in the scrubber system.

Alternatively, the calcium carbonate-residual organic sludge mixture can be removed to a kiln where the sludge is burned off and lime is recovered for re-use in the scrubbers.

Effluent gas from the reactor and/or gas sweeping chamber of this process will contain hydrogen sulphide. This gas may be directed into an additional reclamation system. For example, elemental sulphur may be obtained by processing the gas in a Claus plant. The Claus plant will not require its own air pollution unit since exhaust may be routed back to the flue gas scrubbers of the main plant. In addition, excess energy liberated in the Claus process (about three parts by weight of steam per part by weight of sulphur) may be partly utilized to heat the lime kiln described above. Alternatively, the hydrogen sulphide in the effluent gas may be converted to sulphuric acid.

Application C The pulp and paper industry can realize significant benefits from use of this process in terms of treating wastestreams and reclaiming resources. One of the lowest cost methods of pulp preparation utilizes lime sulphited with sulphur dioxide. This method generates wastestreams containing sulphur oxides and soluble organic material. The organic material makes these wastes attractive for anaerobic fermentation, but the presence of sulphur oxides has hindered efforts with conventional methane fermentation. The toxicity of the hydrogen sulphides which are produced during fermentation has required very careful supervision of the methane reactors. In addition, sulphate/sulphite sludges, such as calcium sulphate-calcium sulphite, remain for disposal.

The sulphur oxides reduction method of this invention offers advantages over the methandgenic fermentation. It achieves both sulphur oxide and carbon removal, it is inherently more stable, and it can accept low influent pH without a need for base addition. Moreover, the wastestream from this industry contains both sulphur oxides and organic material, which eliminates the need for a separate source of the latter. Where all the wastes fed to the reactor are soluble, the reactor may be either an expanded sludge bed or a fixed film reactor.

Application D In order to achieve more complete digestion of organic material, it is possible to combine the sulphur oxide reduction/organic material oxidation process of this invention with a methanogenic digester. The two systems can be arranged in series. The first reactor, constructed in accordance with this invention, is utilized primarily for reduction of sulphur oxides, with partial organic oxidation also taking place. Effluent from the first reactor is pumped into the second reactor, which is a conventional methanogenic digester.

The second reactor allows the methanogenic bacteria to convert the remaining organics to methane gas without becoming endangered by sulphide toxicity in the reactor.

Claims

1. A method for reduction of sulphur oxides which comprises fermenting a mixture of sulphur oxidecontaining material and organic material, with a mixed culture of microorganisms, in an anaerobic reactor and removing accumulated hydrogen sulphide from the fermented mixture by gas stripping.

2. A method according to claim 1 in which the said reactor is an expanded bed or fixed film reactor.

3. A method according to claim 1 or 2 in which the gas stripping is effected inside the said reactor, or in a zone external to the said reactor, or both internally and externally.

4. A method according to claim 1, 2 or 3, in which at least a part of the fermented mixture from which hydrogen sulphide has been removed is recycled to the said reactor.

5. A method according to any of claims 1 to 4 in which the hydrogen sulphide is removed from the fermented mixture by first sparging the reactor with a small quantity of gas, and then conducting reactor effluent to a gas-sweeping chamber or compartment where a gas stream is used to remove at least a major part of the remaining hydrogen sulphide.

6. A method according to any of claims 1 to 5 in which said gas comprises nitrogen, carbon dioxide or methane.

7. A method according to claim 5 or 6, in which the gas-to-liquid ratio in the said reactor is 2:1 to 10:1 and the gas-to-liquid ratio in the said chamber or compartment is 5:1 to 100:1.

8. A method according to claim 7 in which the gas-to-liquid ratio in the said chamber or compartment is 5:1 to 20:1.

9. A method according to any of claims 1 to 8 in which the said sulphur oxide-containing material comprises soluble or insoluble sulphates or sulphites.

10. A method according to claim 9 in which the said sulphur oxide-containing material comprises calcium sulphate or sodium sulphate.

11. A method according to claim 10 in which the said calcium sulphate or sodium sulphate is fermented at a rate from 0.5 t6 2 pounds per cubic foot (8 to 32 kg/m3) of reactor space per day.

12. A method according to claim 10 or 11 in which calcium sulphate is fed to the said reactor in the form of a slurry of gypsum.

13. A method according to any of claims 1 to 12 in which the said organic material comprises municipal waste, manure, industrial organic chemicals, agricultural organic waste or a combination of these.

14. A method according to any of claims 1 to 13 in which the fermentation generates soluble or insoluble carbonates.

15. A method according to claim 14 in which said carbonates comprise calcium carbonate, sodium carbonate or magnesium carbonate.

16. A method according to any of claims 1 to 8 in which the sulphur oxide-containing material used is effluent from flue gas scrubbers.

17. A method according to claim 16 in which the fermentation yields calcium carbonate which is recovered for re-use in a flue gas scrubbing process.

18. A method according to claim 16 in which the fermentation yields a sludge containing clacium carbonate and residual organic material which is heated sufficiently to burn off the said residual organic material, yielding lime for re-use in a flue gas scrubbing process.

19. A method according to any of claims 1 to 8 in which the sulphur oxide-containing material and organic material fed to the said reactor are provided by waste from pulp and/or paper manufacturing processes comprising soluble sulphur oxides and organic material.

20. A method according to any of claims 1 to 17 or 19 in which at least a part of the fermented mixture from which hydrogen sulphide has been removed is fed to a second anaerobic reactor containing methanogenic microorganisms, and methane generated by the said second reactor is recovered.

21. A method according to any of claims 1 to 20 in which the ratio of carbon in the organic material to sulphur in the said sulphur oxide-containing material is from 0.75:1 to 3:1.

22. A method according to any of claims 1 to 21 in which the hydrogen sulphide produced is converted into elemental sulphur, sulphuric acid or a sulphide salt.

23. A method according to claim 1 substantially as described in Example II or Ill or Application A, B, C or D.

24. Hydrogen sulphide, sulphur, sulphuric acid and sulphide salts when produced by the process of any of the preceding claims.