Abstract:
The present invention relates to a method for producing biogas by anaerobic digestion of organic matter. The method comprises: feeding organic matter suitable for biogas production to a first tank reactor  41 , and in the first tank reactor, contacting the organic matter with biogas producing microorganisms for digestion under anaerobic conditions; and digesting the organic matter in the first tank reactor  41  while producing biogas. The method further comprises: providing digested sludge from an anaerobic digestion process in a second tank reactor  31; 46 , which differs from the first tank reactor  41 , said digested sludge containing a desired composition of nutriments; and feeding said nutriments into said first tank reactor  41 . The invention also relates to a biogas producing system.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a biogas producing system provided with means to reuse nutriments harvested from an earlier biogas producing system in a subsequent system to improve microorganism efficiency. 
       BACKGROUND 
       [0002]    It is well known in the prior art that sludge from a process in a biogas producing system is dewatered and partially reused in the same process, as described in the published European patent application EP 1914205. Any nutriments, such as cobalt, selenium, tungsten, nickel, etc. present in the sludge may also be reused in the same process to avoid the need to introduce elementary substances as nutriments to the microorganisms in the biogas producing system. 
         [0003]    A continuous supply of microorganisms needed to produce biogas may also be introduced into the tank reactor of the biogas producing system by mixing a part of the digested sludge with organic matter at the inlet, as described in the published international application WO 2007/114787, paragraphs [0023]-[0025]. 
         [0004]    Pre-treatment of organic matter before introducing it into a biogas producing system is also well known in the prior art, as disclosed in the article with the title “Pre-treatment of wastewater sludge before anaerobic digestion-hygienisation, ultrasonic treatment and enzyme dosing” by Åsa Davidsson and Jes La Cour Jansen, published in VATTEN 62:335-340, Lund 2006. Pre-treatment, e.g. hygienisation by thermal treatment, will increase the methane potential in biological sludge since the pre-treated organic matter is more accessible to the microorganisms in the tank reactor of the biogas producing system. 
         [0005]    DE 102007005786 discloses a dryer in which the digested sludge from a biogas producing system is dewatered and pellets are created from the drying process. The pellets may thereafter be used as fertilizer. 
         [0006]    Treatment of digested sludge before it is reused as nutriments in an anaerobic biogas producing process has also been proposed. The treatment described includes: thermal gasification for 10 minutes (U.S. Pat. No. 4,289,625), hydrolyzing (U.S. Pat. No. 5,141,646), and oxidation (U.S. Pat. No. 5,492,624). 
       SUMMARY OF THE INVENTION 
       [0007]    An object with the present invention is to provide a method, and a system, for producing biogas that requires less nutriments in the shape of chemical compounds of elementary substances to feed the microorganisms in a biogas reactor when producing biogas compared to the prior art. 
         [0008]    The object is achieved by a method comprising feeding organic matter into a first tank reactor containing biogas producing microorganisms for digestion under anaerobic conditions in order to produce biogas. Digested sludge, having a desired composition of nutriments, from an anaerobic digestion process in a second tank reactor is provided and the nutriments are fed into the first tank reactor. 
         [0009]    An advantage with the present invention is that digestion processes based on organic material with low concentration of necessary nutriments, e.g. whole stillage from an ethanol production plant, slaughter waste, crops, sugar beats, silage, starch, dairy waste, etc., may be greatly improved without having to add any nutriments in the form of chemical compounds of elementary substances as in prior art. 
         [0010]    Another advantage and aspect of the present invention is that an increased amount of biogas may be produced in an anaerobic digestion process for producing biogas compared to prior art systems. 
         [0011]    In a preferred embodiment the digested sludge is treated by hygienisation to form a nutriment additive, and the nutriment additive is fed to the biogas producing microorganisms in the first tank reactor. 
         [0012]    An advantage with the preferred embodiment is that nutriments contained in the hygienised sludge from one biogas producing facility may be more easily transported to another remote located biogas producing facility compared to prior art systems. 
         [0013]    Further objects and advantages will be apparent for a skilled person from the detailed description and the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  shows a prior art system for producing biogas from organic matter with dewatering of the digested sludge. 
           [0015]      FIG. 2  shows a prior art system for producing biogas from organic matter with pre-treatment of organic matter. 
           [0016]      FIG. 3  shows a first embodiment of a system for producing biogas from organic matter according to the invention. 
           [0017]      FIG. 4  shows a second embodiment of a system for producing biogas from organic matter according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0018]      FIG. 1  shows a first prior art system  10  for producing biogas from organic matter introduced into a tank reactor  11  via a tank inlet  12 . The introduced organic matter is brought into contact with biogas producing microorganisms for digestion under anaerobic condition, and the organic matter is digested while producing biogas, as indicated by reference numeral  13 . 
         [0019]    Digested sludge is also produced as a result of the anaerobic digestion process in the tank reactor  11 , which is available at a tank outlet  14 . The digested sludge is dewatered in a dewatering device  15  producing dewatered sludge and reject water. 
         [0020]    The reject water may be added to the organic matter introduced via the tank inlet  12 , as indicated by the optional feeding pipe  16 , and the digested sludge is suitable as a fertilizer but may also be reused in the anaerobic digestion process in the tank reactor as indicated by the optional feedback pipe  17 . It is also possible to further treat the digested sludge by drying it in a dryer  18  to form the fertilizer into the shape of pellets. 
         [0021]    The biogas producing microorganisms within the tank reactor continuously needs to be provided with nutriments, i.e. chemical compounds of elementary substances, to function properly, as indicated by reference numeral  19 . 
         [0022]      FIG. 2  shows a second prior art system  20  for producing biogas from organic matter introduced into a tank reactor  21  via a tank inlet  22 . The introduced organic matter is brought into contact with biogas producing microorganisms for digestion under anaerobic condition, and the organic matter is digested while producing biogas, as indicated by reference numeral  23 . Digested sludge is also produced as a result of the anaerobic digestion process in the tank reactor  21 , which is available at a tank outlet  24 . 
         [0023]    The organic matter originates from a waste water treatment plant  25  and the waste water is subjected to a pre-treatment process (e.g. ultrasound treatment, hygienisation, enzyme dosing, etc.) in a pre-treatment unit  26  before it is introduced into the tank reactor  21  via the tank inlet  22 . Nutriments, i.e. chemical compounds of elementary substances, also need to be added in order to provide a suitable environment for the biogas producing microorganisms, as indicated by reference numeral  29 . 
         [0024]    The prior art systems described in connection with  FIGS. 1 and 2  disclose treatment of the organic matter before digestion and treatment of the digested sludge after digestion. The purpose of the prior art system is to increase the biogas potential of the organic matter as such, but both systems require adding nutriments in the shape of chemical compounds of elementary substances to the biogas producing microorganisms to obtain a suitable biogas producing environment within the tank reactors  11  and  21 . 
         [0025]    Nutriments in the shape of chemical compounds of elementary substances, such as cobalt (Co), selenium (Se), tungsten (W) and nickel (Ni), are expensive and restrictions to use these substances are currently discussed within the European Union. 
         [0026]    One fundamental purpose of the present invention is to harvest the nutriments available in digested sludge from an anaerobic process; expose the digested sludge to a hygienisation process to kill off pathogens and optionally to reduce the volume of the digested sludge in a dewatering process; and to reuse the harvested nutriments in the same, or different, anaerobic process to reduce the need to add nutriments in the shape of chemical compounds of elementary substances (i.e. elementary substances, or elementary substances in its ionic form). 
         [0027]    The environment within the tank reactor primarily depends on the type of organic matter that is introduced into the tank reactor, and in order to create a suitable biogas producing environment nutriments need to be added for the microorganisms to function properly. Examples of elementary substances used in chemical compounds as nutriments are: nitrogen (N), phosphorus (P), calcium (Ca), sulphur (S), iron (Fe), cobalt (Co), selenium (Se), tungsten (W) and nickel (Ni). When certain types of organic matter are used, such as sugar beats, it is also essential to introduce nutriments that have an alkaline increasing effect, i.e. increases the pH in the tank reactor. 
         [0028]      FIG. 3  shows a first embodiment of a system  30  for producing biogas from organic matter. The system comprises a first tank reactor  41  and a second tank reactor  31 , which in this embodiment are arranged at the same location “A” (as indicated by dash dotted lines). The organic matter is introduced into the second tank reactor  31  via a tank inlet  32 . The introduced organic matter is brought into contact with biogas producing microorganisms for digestion under anaerobic condition, and the organic matter is digested while producing biogas, as indicated by reference numeral  33 . Digested sludge is also produced as a result of the anaerobic digestion process in the second tank reactor  31 , which is available at a tank outlet  34 . 
         [0029]    The organic matter introduced into the second tank reactor  31  originates in this embodiment from a waste water treatment plant  35 , and is introduced into the second tank reactor without any pre-treatment. It is naturally possible to include a pre-treatment device as described in connection with  FIG. 2  to further enhance the biogas potential in the organic matter. 
         [0030]    A treatment unit  36  receives digested sludge from the tank outlet  34 , and transforms the digested sludge into a nutriment additive. The treatment unit  36  includes a hygienisation device  37  and optionally a dewatering device  38  (as indicated by dashed lines). In addition to hygienisation of the digested sludge, the hygienisation device  37  may also reduce the amount of water in the digested sludge and stabilise the digested sludge depending on the chosen hygienisation process (as described below). 
         [0031]    Organic matter introduced into the first tank reactor  41  via a feeder attached to a tank inlet  42 . The introduced organic matter is brought into contact with biogas producing microorganisms for digestion under anaerobic condition, and the organic matter is digested while producing biogas, as indicated by reference numeral  43 . Digested sludge is also produced as a result of the anaerobic digestion process in the first tank reactor  41 , which is available at a tank outlet  44 . 
         [0032]    A treatment unit  54  receives digested sludge from the tank outlet  44 , and transforms the digested sludge into a nutriment additive. The treatment unit  54  includes in this embodiment a dewatering device  48  producing reject water that may be reused in the process, a hygienisation device  49  and a furnace  51 . The hygienisation device  49  is in this embodiment realised by a dryer or a chemical dispensing unit configured to add lime/slaked lime, since the dewatering process is performed prior to the hygienisation. The nutriment additive from the hygienisation may be introduced (dashed line) into the first tank reactor  41  via a first nutriment inlet  50 , and/or available as pellets for other applications, such as fertilizer. 
         [0033]    A part of the dried sludge (or nutriment additive) from the hygienisation device  49  is forwarded to the furnace  51  to produce a nutriment rich ash introduced via a second nutriment inlet  52 . It should be noted that the nutriment rich ash also has a pH increasing effect on the organic matter within the first tank reactor, which may be essential if the organic matter introduced at the tank inlet  42  has a low pH. Preferably, a combination of nutriment rich ash and nutriment additive is introduced into the first tank reactor  41 , e.g. 70% nutriment rich ash and 30% nutriment additive, to obtain a suitable pH environment and reuse of nutriments present in the digested sludge. 
         [0034]    The organic matter introduced into the tank inlet  42  has, in this embodiment, a low concentration of nutriments and low level of protein, such as silage, sugar beats, starch, cocking oil, vegetable based oil, fibres from pulp industry, effluent from cellulose based ethanol production (i.e. cellulose and hemicellulose), etc. Nutriments, as defined above, are needed in order to create a suitable environment for the biogas producing microorganisms is provided through a separate nutriment inlet  45 . Nutriments are in this embodiment provided as untreated digested sludge from the outlet  34  of the second tank reactor  31 . 
       Hygienisation Processes 
       [0035]    A first example of a hygienisation process is drying the digested sludge in a dryer. rying at a temperature of 70 degrees Celsius for one hour will be sufficient to hygienisate the digested sludge as well as reduce the amount of water and stabilise the digested sludge. 
         [0036]    A second example of a hygienisation process is a chemical process controlled by a chemical dispensing unit configured to add hydrogen peroxide H 2 O 2  into the digested sludge. The hydrogen peroxide will hygienisate the digested sludge during a chemical oxidation reaction. The oxidation reaction will also improve the possibility to reduce the amount of water in the hygienisated digested sludge during a subsequent dewatering procedure, see below, which stabilises the digested sludge, and reduces the amount of water. 
         [0037]    A third example of a hygienisation process is another chemical process controlled by the chemical dispensing unit configured to add lime CaO and/or slaked lime Ca(OH) 2  into the digested sludge. The lime/slaked lime will hygienisate the digested sludge by generating a high pH level that kill any pathogens within the digested sludge. A dewatering process will stabilise the digested sludge, as mentioned below. 
         [0038]    In order to further reduce the amount of water in the nutriment additive, i.e. the treated digested sludge, a dewatering device  38  is preferably implemented in the treatment unit  36 . If the hygienisation device  37  is implemented as a dryer (as described above), the digested sludge should be dewatered in the dewatering device  38  prior to hygienisation to achieve the best result. On the other hand if the hygienisation device  37  is a chemical dispensing unit configured to add hydrogen peroxide, the dewatering process should be performed after hygienisation, since the addition of hydrogen peroxide will help the dewatering device to produce cleaner reject water due to the binding of substances in the digested sludge. 
         [0039]    However, if the hygienisation device  37  is a chemical dispensing unit configured to add lime/slaked lime, the dewatering process may be performed prior to or after the hygienisation. 
         [0040]    Organic matter, such as grass, may be mixed with a percentage of digested sludge before hygienisation when the hygienisation process is performed by drying, and pellets may be formed containing a mixture of organic matter and necessary nutriments in a suitable ratio, e.g. 90% grass and 10% digested sludge. The pellets may thereafter be fed to a subsequent biogas producing process without having to add additional organic matter or any nutriments in the form of elementary substances. 
         [0041]      FIG. 4  shows a second embodiment of a system  40  for producing biogas from organic matter comprising a first tank reactor  41  arranged at a first location “A” and a second tank reactor  46  arranged at a second location “B”. The first tank reactor  41  has been described in connection with  FIG. 3 , but the nutriments provided to the nutriment inlet  45  are transported from the second location “B” Nutriments are in this embodiment provided in the shape of pellets, such as the nutriment additives obtainable from the treatment device  36  in  FIG. 3 , or dried sludge from a previous anaerobic process such as the biogas producing system including the second tank reactor  46 , similar to the system described in connection with  FIG. 1 , also comprising a treatment unit  46 ″ including a dewatering device  47 ′ and a hygienisation device  47 ″. Optionally, cobalt may be added to the pellets as indicated by  53 , and the pellets are transported to the separate nutriment inlet  45  of the first tank reactor  41 . In a preferred embodiment, the pellets (with or without the added cobalt) is further treated in a furnace  55  before transported to the nutriment inlet  45 . 
         [0042]    Normally the distance between the first tank reactor  41  and the second tank reactor is great and the cost for transporting the nutriment additive/nutriment rich ash depends on the weight and volume. The treatment unit  46 ″ will reduce the weight by 96-98%, i.e. 100 tons of digested sludge at 2-4% TS and 60% VS will be reduced to 2-4 tons of nutriment additive (dried sludge) at 70-90% TS and 65% VS. In order to further reduce the weight, the optional furnace  55  may be included in the treatment unit  46 ″, preferably at the site of the second tank reactor  46 ′, and the furnace  55  will further reduce the weight of the dried sludge by approximately 65%, i.e. 100 tons of digested sludge at 2-4% TS and 60% VS will be reduced to approximately 700-1400 kg of nutriment rich ash. 
         [0043]    The two embodiments described in connection with  FIGS. 3 and 4  illustrate the inventive concept of reusing the nutriments available in the digested sludge from an earlier biogas producing tank reactor in a subsequent biogas producing tank reactor. The transport of the digested sludge to the subsequent biogas producing tank reactor may be accomplished by pumping the digested sludge through a pipeline (if the distance between the tank reactors are not too great) or by surface transport (trucks, railroad, etc.) if the distance is large. When pumping the digested sludge through a pipeline, it is preferably that the digested sludge is untreated in order to be pumpable, but when using surface transport, the weight and volume are of more importance which require some kind of treatment as illustrated above. 
         [0044]    It should be noted that other types of transport between tank reactors situated at the same site may be implemented in any suitable way to facilitate transport of untreated, or treated, digested sludge.