WO2012079586A1

WO2012079586A1 - Methods for providing a scrubber liquid and bacteria substrate for use in biological desulphurisation of gases, particularly biogas

Info

Publication number: WO2012079586A1
Application number: PCT/DK2011/050485
Authority: WO
Inventors: Peter Thygesen
Original assignee: BIOGASCLEAN AS
Current assignee: BIOGASCLEAN AS
Priority date: 2010-12-17
Filing date: 2011-12-15
Publication date: 2012-06-21
Anticipated expiration: 2013-06-17
Also published as: BR112013015099A2; CA2821792A1

Abstract

The present invention relates to the use of waste water and/or raw water for the precipitation of sulphate originating from a gas desulphurisation process, as a substrate for sulphur-oxidizing bacteria and as a scrubber liquid as well as processes for desulphurising gas and providing a substrate for the sulphur-oxidizing bacteria.

Description

Method for providing a scrubber liquid and bacteria substrate for use in biological desulphurisation of gases, particularly biogas.

The present invention relates to the use of water, such as waste water, a liquid effluent from an anaerobic digestion and/or raw water for the precipitation of sulphate originating from a biogas desulphurisation process, the use as a substrate for sulphur-oxidizing bacteria and as a scrubber liquid as well as methods for desulphurising of gases, such as polluted air and particularly biogas, and providing a substrate for sul- phur-oxidizing bacteria.

Anaerobic digestion is a biological process used to degrade organic matter in waste water and organic waste. During the anaerobic degradation, biogas (typically comprising 50-70% methane, CH₄, and 30-50% carbon dioxide, C0₂, and 0-5% hydrogen sulphide, H₂S, equiva- lent to 0-50.000 ppm (V/V)) is produced as a commercially important by-product. The waste water and organic waste typically contains sulphur compounds, mainly bound as sulphate (S0₄) compounds. The sulphur compounds originate from two main sources; i.e. sulphur from the soil absorbed by plants during the growing season and sulphur used in various production processes e.g. in the food industry. During the anaerobic digestion, where no oxygen is present, sulphate is converted into hydrogen sulphide (H₂S) which is absorbed in the biogas produced. H₂S, contrary to sulphate, is not liquid and thus not bound in the liquid phase.

During combustion of the biogas, such as in engines and boilers, the H₂S will form sulphur dioxide (S0₂), which will react with water and form sulphuric acid (H₂S0₄). Thus, if the biogas is combusted without prior H₂S removal, severe corrosion will occur in the equipment. In addition, sulphur dioxide (S0₂) causing acid rain, will be exhausted from the chimney. Therefore, it is vital that the H₂S is reduced to acceptable limits before the biogas is used as a fuel in e.g. gas engines or boilers. Typically, gas engine manufacturers require max. 200-250 ppm H₂S in the clean gas if full warranty should be provided.

Thus, as hydrogen sulphide is highly unacceptable in the methane gas provisions must be present for removing the hydrogen sul- phide before the gas can be used as a commercial product.

Several means for removing hydrogen sulphide exist; both chemical and biological systems.

Chemical removal of hydrogen sulphide can for example be per- formed by scrubbing the gas with water and polyethylene glycol whereby carbon dioxide is removed together with the majority of the hydrogen sulphide. Hydrogen sulphide reacts readily with either iron oxide or iron chloride to form removable insoluble iron sulphide. Also, activated carbon filters may be used to remove the hydrogen sulphide.

In biological removal of H₂S from gases or polluted air, sulphur oxidizing bacteria of the Thiobacillus family are used. These bacteria convert, in the presence of oxygen, hydrogen sulphide to sulphate compounds which are typically removed using an aqueous medium. The bacteria require water and nutrients in the correct amount and quality for the process of oxidizing sulphur. This is for example described in DE 10 2009 014 221.

The aqueous medium used is typically soft water. If hard water containing calcium and magnesium were used directly in the process there would be precipitations of compounds formed by metal ions in the water and sulphur oxides formed by the bacteria in the equipment. This has a negative effect on the efficiency of the system because a high degree of sludge formation inside the equipment will require frequent cleaning thereof and result in many hours lost production. Therefore, water used in the process has preferably to be low in i.a. calcium (Ca²⁺) and magnesium (Mg²⁺) and other metals capable of forming heavy salts. Consequently, there is typically used soft demineralized water.

The source of nutrients is most often industrial fertilizers in the form of agricultural or horticultural fertilizers typically containing at least nitrogen, phosphorous and potassium (NPK) diluted in water. One of the main problems with this solution is that it can be difficult and expensive to obtain the right mixture of fertilizers, since fertilizers often are made for another purpose (e.g. farm use) and can contain chemicals and particles that are inhibitive for the bacteria and also cause sedimentation of sludge and scaling inside the equipment, which is a problem for a smooth operation and economical operation.

Alternatively, waste water inherently containing nutrients could be used in the process. However, waste water will besides organic matters, also known as the chemical oxygen demand (COD), typically also contain calcium and magnesium and give major problems with sludge formation inside the equipment if used directly in the process. Therefore, the bacteria substrate normally used in biological gas cleaning systems is demineralized water supplied with nutrients, often in the form of industrial NPK fertilizers. This will ensure the necessary supply of water and nutrients for the bacteria and at the same time the problems with sludge formation inside the equipment is avoided.

As described above the drawback of this solution is the limited access to acceptable fertilizers and the significant investment and/or operation costs for providing industrial fertilizers and demineralized water.

Though the methane gas produced is basically a by-product from the degradation of organic matter in waste water and/or organic waste typically imposed for environmental reasons, the opportunity of creating a profitable business of for example the biogas drives the desire for finding economical ways of treating the gas so it can be profitably exploited as a valuable renewable energy source.

In the present invention a more productive and economical process for biological removal of undesired sulphur compounds contained in gases, such as polluted air, biogas or other gases is provided which overcomes some or more of the problems mentioned above.

Summary of the invention

Thus, in a first aspect is provided a method for desulphurisation of a gas, comprising the steps of

a) providing the gas to a main process tank, the gas comprising at least H₂S and the main process tank comprising sulphur oxidizing bacteria;

b) supplying a clean scrubber liquid to the main process tank, providing in the presence of oxygen a desulphurized gas and a used scrubber liquid comprising S0₄ ^2"; wherein the clean scrubber liquid is provided by the steps of:

i) supplying at least a part of the used scrubber liquid to a sedimentation tank;

ii) contacting the used scrubber liquid in the sedimentation tank with water wherein the water is not demineralized or deionized;

iii) allowing a precipitation comprising oxidized sulphur compounds to form in the sedimentation tank providing a precipitation and the cleaned scrubber liquid.

By the present invention a system has been developed where waste water comprising the nutrients nitrogen, potassium and phosphorus, micro-nutrients and trace metals needed for the bacteria, as well as organic matter, also known as the chemical oxygen demand (COD), can be used in the desulphurisation process without causing problems with the formation of sludge and precipitation and/or scaling in the main process tank(s). This is achieved by the method of the invention where the problems with sludge and precipitation and/or scaling are avoided by the insertion of a loop where acidic sulphate containing water from the main process tank(s), i.e. the used scrubber liquid, is contacted with untreated water, such as waste and/or raw water. Hereby undesired com- pounds, possibly including a part of the sulphate formed, is removed in one or more sedimentation steps before being re-circulated to the main process tank(s) as a clean scrubber solution for scrubbing out sulphate formed in the main process tank(s) by the bacteria. In a preferred embodiment the gas is biogas or polluted air and more preferred biogas.

By this looping of the scrubber solution otherwise requiring frequent cleaning of the main process tank(s) is avoided. In addition, the requirement for soft, i.e. demineralized, water is avoided.

In a preferred embodiment the water is waste water whereby addition of nutrients in the form of industrial fertilizers is superfluous rendering the method even more economical.

In yet another embodiment the waste water is treated by anaerobic digestion and the gas is biogas, whereby both treated waste water and the biogas to be desulphurized is provided and both being usable in the method of the invention. The operating costs of the method of the invention is therefore typically 5-15 % of the prior art methods using chemicals and typically 25-50 % of the prior art methods using biological systems with soft water and industrial NPK fertilizer. In addition, the availability of the system regarding operation hours is above 98%.

The method of the invention has several advantages, first of all when the water used is waste water savings on operation costs for nutrients are achieved.

In addition, raw water comprising high levels of Ca²⁺ and Mg²⁺ can be used in the process instead of expensive demineralized or reverse osmosis treated water. Thus, saving both the cost of using soft water or, if combined with waste water, saving cost on both nutrients and soft water.

By using the waste water for supplying nutrients, the risk of adding unwanted chemicals or components such as dirt, soil and/or sand normally present in industrial fertilizers is avoided. Such industrial fertilizers are manufactured to be used in other applications, and consequently such "unwanted" chemicals do not need to be removed. Thus, availability of a suitable fertilizer for the purpose of supplying nutrients to the sulphur oxidizing bacteria is never an issue.

For large plants processing around 2,500 m³/h gas/air comprising 30,000 ppm hydrogen sulphide in the raw gas and 250 ppm in the desulphurized gas the total cost when using the method of the present invention is approximately 20,000 Euro/year while plants of the same size will cost approximately 80,000 Euro/year when using biological methods with soft water and NPK, while chemical removal is the most expensive solution amounting to approximately 360,000 Euro/year. Thus, a significant reduction in operation costs is achieved when using the method of the present invention.

The method of the invention is further economical in that a complete recycling of nutrients and scrubber liquid in the plant is provided in a simple manner. Only small amounts of liquid is drawn from the system in the form of a slurry comprising precipitated material, i.e. heavy salts and sludge and possibly a part of the sulphate formed. This slurry is taken from the sedimentation tank(s) where the precipitation allows to sediment.

Also, the pH in the process is stabilized due to the reaction of salts, organic matter and metals in the waste water. Biological prior art H₂S removal systems will frequently need neutralizing chemicals, such as sodium hydroxide (NaOH) in order to increase and/or stabilize pH in the system. The decreasing pH is due to the sulphate and acid formed in the reaction facilitated by the bacteria. By stabilizing the pH without use of external chemicals the cost of operation will further decrease and problems when recycling the waste water are avoided.

In a particular embodiment the water used may be both waste water and/or raw water. The contacting of ions in the raw water and the sulphate in the used scrubber solution results in the formation of a precipitation whereby the ions are substantially removed from the water. Therefore, the process of the invention also ensures that expensive de- mineralized or reverse osmosis treated water can be replaced by untreated water. This will dramatically reduce the cost of operation and consequently result in a more profitable production of biogas.

The sedimentation may take place in one or several steps such as 2, 3, 4, 5 or more. Thus, the sedimentation step may take place in one or several sedimentation tanks or in one compartmentalized sedimentation tank. The sedimentation tank may then be lined with a suitable material, preferably an acid proof material.

In a second aspect the scrubber liquid may be seen as a liquid substrate for sulphur-oxidizing bacteria, and hence the method comprises the steps of

a) leading an acid liquid comprising sulphate (S0₄ ^2~) to a sedimentation tank supplied with waste water and optionally raw water,

b) allowing the sulphate to react with chemical compounds and/or organic matter contained in said waste water and/or raw water such as to form a precipitation and a liquid substrate,

c) separating the liquid substrate from the precipitation.

In this aspect is provided a simple method for providing a substrate for the bacteria used to convert the H₂S to sulphate. In both aspects, it is contemplated that one or more serial connected sedimentation tanks may be present in order to optimize the precipitation. In this embodiment clean scrubber liquid from the first sedimentation tank may be led to a second sedimentation tank for additional precipitation and so on. Clean scrubber liquid may be taken from each of the sedimentation tanks and supplied to the main process tank(s) as required.

It is also contemplated that more than one main process tank is present in which simultaneous desulphurisation of the gas takes place. The used scrubber liquid taken from the main process tanks may be supplied to the same or different sedimentation tanks for precipitation.

In yet another aspect the present invention provides the use of the precipitation formed in the methods of the invention as a liquid fertilizer.

In still another embodiment is provided the use of waste water for precipitation of sulphate originating from a biogas desulphurisation process, as a substrate for sulphur-oxidizing bacteria and as a scrubber liquid.

From these aspect the multiple uses of the waste water accord- ing to the invention illustrates all the benefits of the invention where the supply of a waste product can be used as a multipurpose valuable product.

All detailed and specific embodiments described herein for each aspect equally apply to all aspect of the invention.

Figures

Figure 1 is a schematic illustration of an embodiment of the invention in which two sedimentation tanks and one main process tank is used.

Figure 2 is a schematic illustration of another embodiment of the invention where one sedimentation tank and one main process tank is used, the sedimentation tank being compartmentalized in two sections.

Detailed description of the invention

In the context of the present invention the water used in the precipitation step comprise any type of water - no provisions need to be present for deionizing or demineralizing the water. Preferably, the water is waste water and more preferred treated waste water. Treated waste water is in the context of the present invention waste water treated by anaerobic digestion whereby methane gas, biogas, is produced . Most preferred are embodiments where the water is treated waste water from the same source producing the biogas to be treated in the method. This will provide a complete process for recovering, often industrial, waste water and producing desulphurized biogas without extra supply of ex- pensive soft water. In this context soft water is deionized and/or demi- neralized water.

The method of the invention can be seen as a biological gas scrubber process. The main process tank or tanks house the reaction where H₂S is converted to sulphate by bacteria of the Thiobacillus family. The main process tank(s) can be designed to meet any gas flow and H₂S content in the raw gas. Typically, the H₂S is reduced to below 300 ppm, such as to 50-250 ppm but the method of the invention can reduce the H₂S content to lower values if required.

The process of the invention will now be described in details with reference to the plant schematically shown in figure 1 showing a main process tank (1) and two tanks, functioning as sedimentation tanks or tanks for supplying make up water (2', 2" etc.). Herein the terms sedimentation tank and make up water (MUW) tank may be used interchangeably referring to the same type of tanks. It is contemplated that there may be one or more main process tanks and that there may be one or more sedimentation tanks, such as 2, 3 4 or 5. As embodied in figure 2 it is also contemplated that the sedimentation tanks are compartmentalized thus capable of facilitating multi stage sedimentation in one tank. The communication between tanks and/or compartments in one tank can be in the form of pipe connections and by overflow from one tank to another or compartment to another. Communication by means of pipe connections is preferred when the system comprises more than one tank. While overflow from one compartment to another is preferred when one compartmentalized tank is used. The sedimentation tank(s) can be made of concrete, metal or fiber glass. In the sedimentation tank(s) regeneration of the scrubber liquid is performed, i.e. steps i)-iii) of the method of the first aspect as well as the providing of the liquid substrate for the bacteria according to the second aspect. In the sedimentation tank(s) chemical compounds of the water used in the process, such as calcium (Ca²⁺), magnesium (Mg²⁺) and COD, will be removed. These compounds will, if not removed, produce heavily dissolved salts with sulphate produced by the bacteria in the main process tank(s). This would require frequent cleaning of the main process tank(s) and thereby less efficient production of gas.

In the main process tank(s) H₂S contained in the gas is oxidized to S0₄ ^2" by the bacteria contained in the tank(s). The formed sulphate will be transported into the scrubber liquid (the mixture of water and nutrients) and leave the tank(s) diluted in the scrubber liquid used for the process, this solution is denoted the used scrubber liquid.

At least a part of the used scrubber liquid having a low pH typically pH 0.5 to 2.0 is pumped to the first sedimentation tank and mixed with waste water and/or raw water. The amount of waste water pumped to the sedimentation tank(s) is adjusted so that the correct amount of nutrients are present for the process taking place in the main process tank(s). The amount of effluent from the main process tank(s) pumped to the sedimentation tank(s) is adjusted to ensure that sufficient S0₄ ^2" is present in the mixture to ensure that a full reaction between compounds in the waste water that has to be removed before entering the main process tank(s) will occur, and to ensure that an acceptable pH of the solution is obtained. Such compounds that are undesired in the main process tank(s) are for example Ca²⁺ (> 100 mg/l), Mg²⁺ (> 100 mg/l) and organic matter (COD) (> 1000 mg/l).

The S0₄ ^2" (sulphate) reacts with Ca²⁺, Mg²⁺ and COD under the formation of CaS0₄, MgS0₄ and sludge; these compounds are allowed to sediment in the sedimentation tank(s). Consequently, in the first sedimentation tank the highest degree of sedimentation will occur while, the optional subsequent sedimentation tank(s) serve to further allow for sedimentation. The sediments are taken from the bottom section of the sedimentation tank(s) as a slurry which can advantageously be used as a liquid fertilizer.

The effluent from the main process tank(s), i.e. the used scrub- ber liquid, comprising the waste water, and optionally raw water, will after the sedimentation contain the diluted nutrients functioning as a substrate for the bacteria in the main process tank(s). This liquid is denoted the clean scrubber liquid. This clean scrubber liquid then enters or reenters the main process tank(s). Consequently, the main process tank(s) in which H₂S is being converted to sulphate and removed, only receives diluted nutrients with a very low content of heavy salts.

The amount of raw water and/or waste water added to the sedimentation tank(s) is adjusted to ensure both a sufficient supply of nutrients for the bacteria in the main process tank(s), but also to dilute and remove S0₄ ^2" produced by the bacteria in order to adjust pH of the liquid.

The supply of water for achieving the above will result in an increased liquid level in the system why drainage from the system may have to be done in order to maintain the liquid balance in the tank(s). This water balance is mainly adjusted by removing the slurry of heavy salts and sludge formed. The main point of drainage from the entire system (i.e. main process tank(s) and make up water tank(s)) is at the bottom of each of the sedimentation tank(s) where the sedimentation from the chemical reaction between S0₄ ^2", Ca²⁺, Mg²⁺ and other reactive compounds in the water and waste water ends up as a mixed sludge. Preferably the sludge is removed from the sedimentation tank(s) in the same frequency as it is produced.

The process parameters, including the liquid balance, addition of raw water, waste water, drainage, supply of air and possibly heating are preferably controlled by a programmable logic controller (PLC) system making the process fully automated. PLC systems are commercially available for example under the trade names TWD Compact, Micrologix and Simatec available from Schneider Electric, Allan Bradley and Siemens, respectively. The operation of such systems is within the skill of the art.

The operation of the entire cleaning process is to be carried out in a system comprising two main items; the at least one main process tank (MPT) and one or more sedimentation tanks. The operation may be automatic and may be controlled from a machine room comprising the PLC system. In addition circulation/drain pump(s), air blower(s), water supply, heating, etc. are present. The machine room may be housed in a standard shipping container, housing of plastic or glass fiber or mounted on a skid. The choice of these units is within the skill of the art.

The actual number and size of the sedimentation and main process tank(s)s respectively, depends on the sulphur load. The main process tank(s)s have internal pipe systems for gas distribution, supply and distribution of the scrubber liquid and sludge removal and are filled with packing media. The tank(s) are manufactured in PE (polyethylene), FRP (reinforced fiberglass) or concrete or mild steel with an acid-proof liner.

In operation, the clean scrubber liquid is pumped to the top of the main process tank(s), where spray nozzles distribute the clean scrubber liquid over the surface of the packing media containing sulphur oxidizing bacteria. The scrubber liquid flows through the packed bed and provides moisture and nutrients for the bacteria living on the surface of the packing media.

The gas, such as biogas, may be introduced, both from the upper and the lower part of the main process tank(s), and passes upwardly and/or downwardly, respectively, thereby bringing the gas into contact with the bacteria. The bacteria oxidize the H₂S to sulphate. Subsequently, the sulphate is absorbed in the scrubber liquid, said liquid flowing by gravity to the bottom part of the main process tank(s).

The used scrubber liquid leaving the bottom part of the main process tank(s) is re-circulated after being supplied with make-up water and drainage of used scrubber liquid at certain intervals as detailed above.

After passing the main process tank(s), for example desulphurized biogas is ready for combustion in engine(s) or boiler(s). The desulphurization will now be explained in more details. The biological oxidation of hydrogen sulphide to sulphate proceeds in two stages. In the first stage, hydrogen sulphide is partially oxidized to sulphur (2) and in the second step, sulphur is completely oxidized to sulphate (3).

H₂S -» H⁺ + HS^" (dissociation) (1)

2HS^" + 0₂ → 2S^U + 20H (2)

2S° + 30₂ + 20H → 2SO₄- + 2H⁺ (3)

The overall reaction can be described as follows:

2HS^" + 4 0₂ → 2 SO ₄- + 2 H⁺ (4)

This reaction both desulphurizes the biogas and causes the pH of the scrubber liquid to decrease.

The sulphate produced is discharged with the used scrubber liquid from the main process tank(s) containing up to the equivalence of 1- 30 % S0₄, typically 2-10%; the chemical composition of the effluent will depend on the raw water and/or waste water and/or treated digester ef- fluent added to the process.

The sulphur oxidizing bacteria are mainly belonging to the 777/- obacillus genus as the Thiobacillus thiooxidans (also known as Acidithi- obacillus thiooxidans), Thiobacillus acidophilus (also known as Acidiphi- lium) and Thiobacillus albertis (also known as Acidithiobacillus alberten- sis) among others. These bacteria are well known in the art and are readily available from several suppliers, such as LCG Standards. The sulphur oxidising bacteria have the following requirements.

The bacteria need a place to live and multiply, which in the present invention will be on the packed media inside the main process tank(s). The bacteria require basically sulphur (sulphide), oxygen, nutrients (NPK) and water to grow. In the present invention these requirements are met by supplying the gas or polluted air containing H₂S, an external supply of oxygen, preferably in the form of atmospheric air, and the water supplied according to the invention, e.g. raw water and/or waste water and/or treated effluent. When water is supplied as waste water the nutrients are contained therein, when the water supplied is raw water, liquid NPK fertilizer is supplied. The growth conditions of the bacteria also requires a temperature between 25-60°C.

The desulphurisation process in the main process tank(s) requires oxygen. The oxygen supply may be provided by the injection of atmospheric air. This may be achieved by injecting atmospheric air into the main process tank(s) or by adding air to the influent biogas.

The amount of air supplied depends on the H₂S level in the raw biogas.

When air is added the absolute volume of methane will remain unchanged, but the injection of air will dilute the relative methane content in the clean gas.

The air may be injected by a frequency regulated air blower, so that only air needed for the process is injected. The choice and operation of injectors and blowers are within the skill of the art.

The nutrients for the bacteria may be supplied in several ways. In particular embodiments of the invention the water is treated waste water such as industrial waste water or raw water and NPK fertilizer, preferably liquid.

The treated waste water is typically originating from industrial waste water treatment plants (WWTPs) connected to palm oil mills, cassava/tapioca production, ethanol production, breweries, food industries etc., but may also originate from other types of biogas plants, incl. municipal WWTP's and biogas plants treating animal manure. Such treatment is often anaerobic digestion of organic matter in the waste water and/or organic waste whereby treated waste water is produced as well as the biogas, for example to be treated according to the invention.

If the treated waste water is not supplied from an end pond but is supplied directly from anaerobic treatment, means for settlement is required to avoid particles.

In yet another preferred embodiment the water is raw water to which NPK fertilizer is added. Due to the sedimentation of heavy salts in the sedimentation tank(s) the need for soft water is omitted.

In still another embodiment the water used is a mixture of raw water and treated waste water. This will enable adjustment of the nutrients to be fed to the main process tank(s) without compromising the cost of the process.

The used scrubber liquid leaving the main process tank(s) is in the present invention mixed with the treated waste water and/or water, such as raw water in the sedimentation tank(s). Here the heavy salts and sludge formed by the contacting of sulphate containing used scrub- ber liquid and metal containing water and COD containing waste water respectively, cause the sedimentation of heavy salts and sludge in the form of a waste slurry. This waste slurry will be high in NPK and S. The slurry is continuously drawn from the sedimentations tank(s). Therefore, the slurry drawn from the sedimentation tank(s) in a preferred embodi- ment is used as a liquid fertilizer.

Traditionally, fertilizers are classified according to their content of N (nitrogen), P (phosphorus) and K (potassium). However, S (sulphur) is as important, and in most countries the fertilizer manufacturers must specify the content of N, P, K and S. For sugar cane production the recommended yearly application is 30-50 kg S per hectare. For cereals and oil seed rape the recommended yearly application per hectare is ap- prox. 20 and 40 kg S, respectively.

Thus, the slurry drawn from the process of the invention, a liquid fertilizer, may advantageously replace or supplement traditional ferti- lizers, the price for such fertilizers is approx. 0.5 EUR per kg. The process of the invention will produce approximately one kg S fertilizer for each kg H₂S removed.

The fertilizer produced according to the present invention is superior over traditional waste water from desulphurization processes us- ing soft water and NPK fertilizer because the pH is higher and consequently the plants will be less exposed to acid damages on the leaves.

The present invention will now be illustrated by way of the following non limiting examples. Examples

Calculation of operational costs for removing hydrogen sulphide and comparison with prior art.

The following calculations are based on a similar plant as out- lined in Figure 1 but with two main process tanks (1) and two sedimentation (MUW) tanks (2' and 2"). The inlet of water will be either raw water or waste water to the first sedimentation tank (2'). The water will be further supplied to the second sedimentation tank (2").

The gas to be cleaned is biogas and the flow rate to the main process tanks is set at 2,500m³/h and the biogas having an initial H₂S content of maximum 30,000ppm. After the desulphurisation process in the main process tanks, the content of H₂S is reduced to maximum 250 ppm. Thus, illustrating that the degree of purity obtained by the method of the present invention results in the required purity.

The desulphurisation of the present invention was calculated to be 113 kg H₂S per hour corresponding to a desulphurization rate of the biogas of approx. 2,700 kg H₂S per day.

The calculations are based of the data summarized in the table below.

Estimated costs of the present invention compared to conventional plants

Based on the data obtained from the calculation example above, i.e. re- moval of approximately 2,700 kg H₂S per day providing a biogas with maximum 250 ppm H₂S, the operation costs of the plant of the present invention will be given below.

The cost of the electrical units is estimated to 0.1 EUR/kWh. The cost of soft water is estimated to 0.5 EUR/m³, NPK nutrient to 0.4 EUR/kg, caustic soda to 0.35 EUR/kg, and the nutrient for partial rege- neration of the caustic soda in the chemical processes is estimated to 2.0 EUR/liter.

As the process of the present invention does not require soft water, NPK nutrients, caustic soda or special nutrients, the only cost will be electricity. Based on a plant as outlined above, the estimated demand for electricity would be 22 kWh, which would result in operational costs of approximately 20,000 EUR per year.

A prior art biological plant requires in addition to electricity soft water and NPK nutrients, why the operational costs of such a plant would amount to approximately 80,000 EUR per year. This is based on an electricity consumption of 22.0 kWh, 4.0 m³ soft water and 12 kg NPK nutrients per hour.

A state-of-the art chemical plant is estimated to use on average 158 kWh electricity, 2 m³ soft water, 52 kg caustic soda, and 3.3 liter special nutrients which on a yearly basis results in operational cost of approx. 360,000EUR. Thus, based on the presumptions above it is clear that the process of the present invention provides a significant saving without compromising the quality of the desulphurization process. Furthermore, the present invention significantly reduces the need for fre- quent cleaning of the main tanks which traditionally results in many lost production hours. Therefore, the present invention will in addition to substantial savings in operating costs also secure a very high availability which will maximize revenues from generation of power and/or heat from the biogas.

Claims

P A T E N T C L A I M S

1. A method for desulphurisation of gas, comprising the steps of a) providing a gas to a main process tank, the gas comprising at least H₂S and the main process tank comprising sulphur oxidizing bacte- ria;

b) supplying a clean scrubber liquid to the main process tank, providing in the presence of oxygen a desulphurized gas and a used scrubber liquid comprising S0₄ ^2";

wherein the clean scrubber liquid is provided by the steps of: i) supplying at least a portion of the used scrubber liquid to a sedimentation tank;

ii) contacting the used scrubber liquid in the sedimentation tank with water wherein the water is not demineralized or deionized; and

iii) allowing a precipitation comprising oxidized sulphur com- pounds to form in the tank providing a precipitation and the cleaned scrubber liquid.

2. The method according to claim 1 wherein the water is industrial waste water.

3. The method according to claim 1 or 2, wherein the gas is bio- gas.

4. The method according to claim 3, wherein the industrial waste water is processed in the plant wherein the biogas is produced.

5. The method according to any of the previous claims wherein the water is both industrial waste water and raw water.

6. The method according to any of the previous claims wherein the precipitation formed in step iii) is collected.

7. A method for preparing a liquid substrate for sulphur- oxidizing bacteria, comprising the steps of

a) leading an acid liquid comprising sulphate (S0₄ ^2~) to a sedi- mentation tank supplied with waste water and optionally raw water,

b) allowing the sulphate to react with compounds and/or organic matter contained in said waste water and/or raw water such as to form a precipitation and a liquid substrate,

c) separating the liquid substrate from the precipitation.

8. The method according to claim 7, wherein the sulphate supplied is contained in an effluent from a main process tank of a gas cleaning plant, such as a biogas cleaning plant.

9. The method according to claim 8, wherein, the sulphate ori- ginates from desulphurisation of biogas by sulphur-oxidizing bacteria in the main process tank.

10. The method according to any of the claims 7 to 9, wherein the liquid substrate is also used as a scrubber liquid.

11. The method according to any of the preceding claims where- in the sedimentation tank comprises 2, 3 or 4 sedimentation compartments in fluid communication.

12. The method according to any of the claims 1-10 comprising 2-5 sedimentation tanks in fluid communication.

13. Use of a precipitation formed in the method of any of the claims 1-12 as a liquid fertilizer.

14. Combined use of waste water and/or raw water for the precipitation of sulphate originating from a gas desulphurisation process according to any of the claims 1-12, preferably a biogas desulphurisation process, as a substrate for sulphur-oxidizing bacteria and as a scrubber liquid.

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