SE543955C2 - Method for the production of biogas - Google Patents

Abstract

A method for producing biogas in an anaerobic digestion chamber from an un-treated organic substrate, wherein said un-treated organic substrate has a dry matter of content of in the range of 20 to 90% of total solids, wherein the method comprises the steps of, pre-treatment of the un-treated organic substrate, to form a slurry having a dry matter content of in the range of 8 to 19.9% of total solids, feeding said slurry to a digestion chamber; digesting said slurry in the digestion chamber to produce biogas and a digestate, wherein in said pre-treatment step a mixture of a dilution fluid and a liquid digestate from said digestion chamber is used to dilute the un-treated organic substrate.

Description

METHOD FOR THE PRODUCTION OF BIOGAS Technical field The present document relates a method for production of biogas byadding a liquid digestate to the process.

BackgroundWhen producing biogas, it is possible to use several different substrates, such as slaughterhouse waste, food waste collected athouseholds, and industrial waste. Sweden has a national target to collect foodwaste from households and treat this material biologically to utilize bothnutrients and energy as biogas. ln practice, this means that food waste has tobe anaerobically digested such that biogas is produced, and that thedigestate is utilized as a bio-fertilizer, to return nitrogen and phosphorus toagricultural lands. The biogas production process thus closes the biologicalcycle of these nutrients.

One problem associated with food or organic waste from mainlyhouseholds is that it may also contain different types of impurities, such asplastic, metal, glass and textiles. These impurities, including the plastic bagsor containers used to collect and contain the organic waste, must be removedbefore the waste can be digested. This is done through a pre-treatment of thewaste, usually performed in several steps, before a so called slurry isobtained, which forms the base material for the digestion process. The foodor organic waste in a container or bag usually has a dry matter content ofabout 30-35°/> (i.e. 65-70°/> is water). After processing of the organic waste,the slurry usually has a dry matter content of around 8-19.9°/> (i.e.approximately 80.1 -92 °/> is water), which means that different types offluids/liquids have been added to the waste. The addition of fluids is made toobtain a liquid slurry to achieve a slurry which can easily be pumped throughthe system since, if no liquids are added, the viscosity would be unreasonablyhigh, which makes pumping and heat-exchanging difficult for thepasteurization step and the subsequent digestion chamber. l\/loreover, without addition of liquid, the separation of impurities is hard without simultaneouslyremoving organic matter. Conventionally a di|ution fluid made up of water(mainly freshwater/tap-water) and different types of diluted liquids, such as forinstance washing water from the dairy industry, milk, or other such fluids, areused for this process.

However, mixing di|ution liquids into the food waste increases theamount of bio-fertilizer, without increasing the actual amount of nutrients(such as phosphorus and nitrogen). This means that a loss of concentrationof the nutrients occurs in this process. The bio-fertilizer is transported byclosed lorries to the farmers, and the di|ution thus leads to increased costs, interms of water and transportation, and of course puts a load on theenvironment. Further to this, the retention time in the digestion chamber (i.e.the time that the material is in the chamber) is reduced and this leads to adecreased time for the slurry to be digested. There is therefore a need toreduce the impact on the environment, to increase the retention time in thedigestion chamber, and increase the concentration of nutrients in the bio-fertilizer.

There are also other applications within pre-treatments such as mixingof substrates that requires a liquid such as water or other liquids. ln order todigest products such as stillage, fat, grass silage, fodder or other organics,di|ution with a liquid might be needed in order to achieve a pumpable slurry.The lower viscosity the di|ution liquid has, the less liquid is required. Additionof a lot of water is not favorable since it dilutes the bio-fertilizer and shortenthe hydraulic retention time in the digester. l\/loreover, in some cases grit and gravel (heavy inert particles) need tobe removed from a substrate such as processed food waste (slurry) or otherpumpable waste. The most common way to remove inert particles from thesubstrate is by the use of a hydro-cyclone that amplifies the gravidity of theheavy particles, which then can be separated from the organic waste. Themain disadvantages with this method is that the cyclone requires water towork. This is both costly as well as negative for the bio-fertilizer quality due todi|ution. ln addition it shortens the retention time of the substrate in theupcoming digester.

Summarylt is an object of the present disclosure, to provide an improved method for producing biogas, and bio-fertilizers as well as an additive for dilution ofsubstrate material used in the in the production of biogas.

The invention is defined by the appended independent claims.Embodiments are set forth in the appended dependent claims and in thefollowing description and drawings.

According to a first aspect, there is provided a method for producingbiogas in an anaerobic digestion chamber from an un-treated organicsubstrate, wherein said un-treated organic substrate has a dry matter ofcontent of in the range of 20 to 90 °/> of total solids, wherein the methodcomprises the steps of:pre-treatment of the un-treated organic substrate, to form a slurry havinga dry matter content of in the range of 8 to 19,9 °/-.~ of total solids,feeding said slurry to a digestion chamber;digesting said slurry in the digestion chamber to produce biogasand a digestate, wherein in said pre-treatment step a mixture of a dilution fluidand a liquid digestate from said digestion chamber is used to dilute the un-treated organic substrate.

By “un-treated organic substrate” is meant a material which is deliveredfrom a source, the organic material may comprise different types of waste,usually food waste from households, food waste from industries, food wastefrom stores, slaughterhouse waste or other types of industrially producedorganic waste materials, such as fodder and silage. The organic substratemay also be different types of crops used for production of biogas. By “dilutionfluid” is meant conventional dilution fluids used for biogas production, such asfreshwater, washing fluids from the dairy industry etc. By “liquid digestate” ismeant a material which is taken from the digestion chamber and brought backinto the pre-treatment step. By “biogas” is meant a mixture of gases producedby the digestion of the organic substrate in the absence of oxygen. Thebiogases thus include primarily methane and carbon dioxide, and they aremost often used for fuel or for conversion to heat or electricity. The digestate is the material which is left after the digestion process of the slurry containingthe organic waste has been carried out.

The digestate is also considered as a bio-fertilizer, which is brought backto agricultural land, depending on the type of the organic waste used in theprocess.

By using a mixture of conventional dilution fluids and a liquid digestate theamount of bio-fertilizer is reduced, with an increased concentration ofnutrients, such as nitrogen and phosphorus as additional effect. This reducesthe transportation cost and increases the value of the bio-fertilizer (pervolume unit). By replacing some of the conventional dilution fluids with liquiddigestate the water consumption in the biogas production process can bereduced, which also has a positive impact on the environment. ln addition tothis, the retention time of the digestate is increased, which increases thedegree of degradation of the slurry.

According to the first aspect, the liquid digestate may be either anunaerated or aerated liquid digestate.

According to the first aspect the liquid digestate may be anaerated digestate, wherein the method further comprises aerating said liquiddigestate in an aeration chamber prior to the introduction into the pre-treatment step where the digestate is aerated with any one of the gasesselected from air, oxygen and nitrogen with a flow in the range of 0.1 to 100m3 gas/ms liquid h.

According to the first aspect the liquid digestate is an aerated digestateand wherein the method comprises aerating said liquid digestate prior to theintroduction into the pre-treatment step during a time period of at least 1 hour.

Preferably the retention time in the aeration chamber is longer than 1hour, conventionally between 1 and 24 hours.

The viscosity of the aerated liquid digestate is decreased compared tountreated digestate during the aeration step as measured in cP.

By aerating the liquid digestate it has been found that the viscosity asdefined in [cP] has been greatly reduced, compared to unaerated liquiddigestate. This means that by using the aerated liquid digestate in the pre-treatment step, the viscosity of the organic waste, and the slurry formed, can be reduced, without decreasing the concentration of the nutrients in thedigested slurry as much as when using conventional dilution liquids only. lt has further been found that by aerating the liquid digestate to form the slurryundesired side effects in the pre-treatment step and also in subsequentstorage and pasteurization steps, such as the formation of methane,hydrogen su|fide or other poisonous or explosive gases, can be greatlyreduced or abolished. lt has also been found that using an aerated liquiddigestate gives no negative effects on the biogas production in total, i.e. theproduction is neither increased nor decreased. lt has been found that the decrease of total amount of bio-fertilizer maybe up to around 20-25°/>, which means that the nutrients, such as ammoniumnitrogen, is accumulated in the digestion chamber, resulting in an increase ofthe nutrients by around 20%.

According to one embodiment of the first aspect the liquid digestatereplaces at least 5 % of the dilution fluid compared to conventional methods,or at least 25 °/-.~ of dilution fluid, or at least 50 °/> of dilution fluid.

According to a second aspect there is provided an additive for use inthe method according to the first aspect, wherein said additive comprises anaerated liquid digestate from a digestion chamber.

The aerated liquid digestate has a viscosity as measured in cP whichis lower than an untreated liquid digestate.

According to a third aspect there is provided a method of cleaning anun-treated organic waste substrate prior to introduction into a biogasproduction facility, wherein the cleaning is performed in a hydro-cyclone, andwherein said hydro-cyclone a cleaning liquid is used, wherein said cleaningliquid is at least partially composed of a liquid digestate from a digestionchamber.

According to the third aspect said liquid digestate may beeither an unaerated or aerated liquid digestate. Preferably the liquid digestateis an aerated liquid digestate that has been treated in an aeration chamber.

Said aerated liquid digestate has a viscosity as measured in cPwhich is lower than an unaerated liquid digestate.

Brief Description of the DrawinosEmbodiments of the present solution will now be described, by way ofexample, with reference to the accompanying schematic drawings.

Fig. 1 is a schematic view of a biogas production process according to theinventive idea.

Fig. 2 is a graph showing the viscosity of aerated and unaerated digestate.Fig. 3 is a graph showing a result of a pre-hydrolysis trial.

Fig. 4 is a graph showing a result of the gas composition from trials.

Fig. 5 is a graph showing hydrogen sulfide content in the off-gases from thepre-hydrolysis step (storage of slurry).

Fig. 6 is a graph showing the specific gas production in the continuous trail.Fig. 7 is a graph showing the amount of ammonium nitrogen in the digestionchamber.

Fig. 8 is a graph showing pH and alkalinity.

Fig. 9 is a graph showing the principle for the hydro-cyclone with water and/oraerated digestate as a liquid additive.

Description of Embodiments Fig 1 illustrate an overview of the process. ln a conventional biogasproduction organic waste material, such as food waste, industrial waste etc. isbrought into a pre-treatment tank 1, here the waste material is pre-treated andthus mixed with dilution liquids, such as water (freshwater), or liquids fromdairy orjuice industry. ln a facility such as the one at the Linköping municipalbiogas plant about 45 000-50 000 metric tons of liquids are used annually.After the pre-treatment of the food waste, forming a slurry, the slurry usuallyhas a dry substance content of 12-19.9 °/>, i.e. a slurry material. The pre-treated waste is usually brought to a second step in the pre-treatment, whichis at least one pasteurization chamber 1 or 2 or pre-hydrolysis chamber(storage tank for processed organic waste, slurry), wherein the pasteurizationchamber 2 pathogenic microorganisms are reduced or killed. Here a certaindegree of pre-hydrolysis occurs. After the pasteurization step the waste slurrymaterial is brought to digestion chamber 3, where the waste slurry material is continuously digested under anaerobic conditions to form the gases calledbiogas, i.e. methane, carbon dioxide etc. The residue after digestion is calleda digestate, and usually has a dry content of 3-9 °/-.~. The digestate is used asa bio-fertilizer and is brought to the farmers in closed tanks, or to a treatmentfacility to increase the dry content. From a facility as the municipal biogasplant in Linköping, Sweden, approximately 80 000 - 120 000 metric tons ofbio-fertilizer is produced annually, this bio-fertilizer usually has an ammoniumnitrogen content of 3,000-4,000 mg/L. The farmer normally wants as highammonium nitrogen and phosphorus content as possible. Also, the largevolume of digestate to transport to the farmers is associated with a high cost.

According to one embodiment (not shown in the flow chart of thefigures) of the invention a portion of the liquid digestate is brought from adigestion chamber 3 to the pre-treatment chamber 1 to replace some of thedilution liquids conventionally used in this pre-treatment step. By pre-treatment step is meant the step in the biogas process where the substratematerial, i.e. undigested organic waste material of different origin is treatedbefore it is brought to the digestion chamber.

A digestate from a digester chamber normally has a total solid (TS)content of 3-9°/> depending of the ingoing substrate and the degree ofdegradation of the material inside the digester. l\/loreover, the ammoniumcontent varies depending of the substrate type, but if food waste is the mainsubstrate the ammonium content typically is in the range of 2,000 to 4,000mg/L (express as ammonium nitrogen).

According to an alternative embodiment of the inventive method aportion, or all, of the digestate is brought to an aeration chamber or tank 4. lnthe aeration chamber air is provided to flow through the digestate, therebyforming a liquid aerated digestate material. The liquid aerated digestatematerial has a lower dry content compared to the untreated digestate,normally in the range of 2-8 °/> based on total solids. ln Fig. 1 it is shown thatall of the liquid digestate removed from the digestion chamber is brought toand aerated in the reaction chamber, however it is sufficient that only theportion needed to for dilution in the pre-treatment chamber is aerated.

The aeration may be performed, but not limited to, aeration with air,pure oxygen, and mixture of oxygen/nitrogen. ln one embodiment the gasused in the reaction chamber comprises in the range of 1 to 100 °/-.~ 02.Normally 21% of 02 in air is used as the aeration gas. ln one embodiment theflow of air in the reaction chamber is in the range of 0.1 to 100 m3 gas/m3liquid and hour, more preferably 0.2 to 50 m3 gas/m3 liquid and hour and evenmore preferable 0.5 to 5 m3 gas/m3 liquid and hour.

According to one embodiment the retention time of the liquid digestatein the aeration chamber is in the range of 1 to 500 hours, more preferably 2 to100 hours and even more preferable 12 to 48 hours. ln one embodiment theretention time is at least 1 hours, or at least 2 hours.

According to this embodiment the viscosity of the aerated liquiddigestate material has been reduced by at least 1°/-.~, more preferably by 10%and even more preferable by 25% compared to the untreated, i.e. not aeratedliquid digestate material. This reduction of viscosity may be essential for theslurry produced in the pre-treatment step as it must have a certain viscosityfor it to be fed in the system.

According to the invention around 50 °/> of the dilution liquids can bereplaced by the liquid digestate material, both untreated and treated.Preferably an aerated liquid digestate is used. A replacement of around 50%of the dilution liquids leads to a reduction of the total amount of bio-fertilizerby around 20-25 °/-.~ (under the conditions as set out in the municipal biogasfacility in Linköping, Sweden). This results in an accumulation of ammoniumnitrogen in the digestate chamber as well as in the bio-fertilizer, which in turnleads to an increase of ammonium nitrogen with about 20-25 °/>. Thus, thewater consumption decreases as well as the need for transportation of bio-fertilizer from the site to farmers land, thus providing a substantial economicalsaving. These amounts and improvements are all related to the specificconditions of the plant, and to the specific conditions of the raw material forthe process.

Experimental data Trials have been performed in several different sets. The first trial setwas aimed at simulating a semi-continuously fed storage tank for food waste,food waste and liquid digestate and food waste and aerate liquid digestate -naturally a pre-hydrolysis (controlled or uncontrolled) occurred when storing the slurry. ln the tank a microbial culture was formed which could perform pre-hydrolysis of the slurry. The pre-hydrolysis reactors operated at a temperatureof 55 °C. ln the second trial set both a pre-hydrolysis and a succeedingcontinuous biogas process for 275 days, i.e. a digestion chamber, were si m ulated.10 ln both trial sets a mixture matrix according to Table 1 were used.Table 1 _Pmcessed Theoretic dryF d t slurry °f f°°d Liquid Aerated contents in:få :gääáe Waste and Tap water Milk untreated liquid Total sum the pre-slaughterlmuse' Digestate digestate hydrolysiswaste TS = 15% reactorSlurry 57% 33% 10% 100% 16.2%Slurry withafiäïfåd 57% 8% 10% 25% 100% 173%digestateSlurry withliquid 57% 8% 10% 25% 100% 17.2%digestateSlurry fromday 180 in 75% 25% 100% 11.0%trail 2Slurry withaerated.hquml 75% 25% 100% 12.3%digestatefrom day180 in trail 2 Before the trials were initiated the viscosity of the liquid (untreated) digestate and the aerated liquid digestate (air; 24 hours, 0.5 I Oz/l h;temperature 38 °C.) was analyzed. These data showed that the viscosity wasgreatly reduced in the aerated liquid digestate, compared to the untreatedliquid digestate. This surprisingly turned out to be an important effect of theaeration process, since the dilution liquid for the pre-treatment of the organic waste material, since the slurry that is formed needs a certain viscosity to be able to feed in the system. The results of the measurements are disclosed inFig. 2. According to one embodiment the liquid digestate which is provided inthe aeration chamber has a significantly lower viscosity than untreateddigestate. At shear rate 50 (sf) in this example, the viscosity drops from 41cP down to 14 cP, a decrease with 66% compared to untreated digestate.

Pre-hvdrolvsis reactor trials The trial showed that the concentration of ammonium nitrogenincreased when liquid digestate was reintroduced instead of using water(alone) as dilution liquid. This is due to the fact that the liquid digestatecontains approximately 3,000 mg ammonium/L and water 0 mg/L. Since theliquid digestate is to be continuously reintroduced into the digestion chamber,this will have effect on the concentration of the bio-fertilizer too.

The trial showed that a certain production of gases occurred, where thegas mainly comprised carbon dioxide, and some hydrogen gas, but only traceamount of methane. On the other hand, the introduction of untreated liquiddigestate lead to the occurrence of a very high concentration of hydrogensulfide, which could be problematic due to its corrosive, toxic and inhibitiveeffect on materials, people and the biogas process.

When using aerated liquid digestate the hydrogen sulfide concentrationor level was very low, and also significantly lower in the comparative trial withfood waste handled according to the conventional process (i.e. conventionaldilution fluids). There are thus several advantages by using aerated digestateas liquid to mix and dilute the organic waste with.

The trial results are shown in Figs 3 to 5.

Aeration of the liquid digestate leads to inhibition of the production ofmethane gases (which has been disclosed in the article Aeration of the liquid digestate results in a substantial decrease inviscosity of the liquid digestate.

Mixing an aerated liquid digestate with an organic waste such as foodwaste or food waste slurry does not lead to any side effects with regards to the production of methane gas in the pre-treatment step, hydrogen sulfide, orother toxic or explosive gases (such as hydrogen).

Mixing of both aerated and untreated liquid digestate increases thenitrogen concentration from approximately 200 mg/L to 1000 mg/L in a pre-hydrolysis process of food waste, which leads to decrease in the amount ofbio-fertilizer, which in turn leads to decrease in transportation costs, increaseof value of the bio-fertilizer (as calculate based om unit volume), reducedwater use, and increased energy content in the slurry for digestion.

Continuous trialsln the continuous trial (feed once every day) the purpose was to not only simulate the pre-hydrolysis, but also to feed the formed slurry to acontinuous biogas production chamber. The experiment elapsed for 275days. All reactors where inoculated at the start with digestate from thedigesters at Linköping biogas plant (Sweden). The reactors used werespecially adjusted for the purpose, all described in the patent by Nordell et al(Patent SE1150954 A1). The pre-hydrolysis reactors operated at atemperature of 60 °C. The digesters were operated at a temperature of 42 °C.As a model plant the municipal biogas production plant of Linköping, Swedenwas used. ln this plant a dominating amount of food waste is used as organicmaterial, but also some slaughterhouse waste and other minor industrialwaste fractions such as fat, stillage and alcohols. The retention time wasdetermined to 35 days in the digestion step and three (3) days in the pre-hydrolysis step (storage tank), i.e. approximately the same as in the pre-hydrolysis trial. The organic loading rate in the digesters were 4.0-4.5 kgVS/ms day during the experiment. The first 30 days were used as start-upperiod were both reactors received the same slurry (food waste, milk, water).After that the experiment started and the pre-hydrolysis connected to theexperiment reactor started to receive digestate in the amounts described inTable 1. After 180 days, the digestate re-circulation was replaced withaerated digestate, until day 275 when the experiment ended. When untreateddigestate were used, fresh digestate from the experiment reactor were usedto recirculate the same day. The aeration of the digestate from the experiment reactor took place once a week with an air flow of approximately 1 L Oz/L h, T= 38 C and exposure time 24 h. To facilitate the trial, the gaseous phase fromthe pre-hydrolysis was continuously aerated and no gasproduction/composition was measured. The trial was designed this way sincethe focus was to examine the effect on the bio-fertilizer and the stability of theprocess in the digestion chamber. The pre-hydrolysis reactors were fed dailywith a mixture of food waste, water, milk, digestate or aerated digestate (seeTable 1). And subsequently the slurry from those were fed into the digesters,one digester fed with slurry formed from food waste, water and milk; andanother reactor fed with slurry formed from food waste, digestate (or aerateddigestate), water and milk. A comparative trial between using an untreatedand aerated liquid digestate is provided in Figs 6 to 8.

The trials verified that reintroducing both untreated and aerated liquiddigestate results in an increase of the ammonium nitrogen content in the bio-fertilizer (and also the total nitrogen content), moreover, an up concentrationsof all types of inert material, metals and mineral will of course appear in thesame rate. Fig 7. shows the ammonium nitrogen content of the digestate fromthe two digesters fed with different slurries. The reactor fed with slurryproduced from the aerated digestate-mixture has a steady higher ammoniumnitrogen content than the control. l\/loreover, at steady-state condition (3retention times, or ~105 days from the start of recirculation of digestate, thusday 135) the increase in ammonium nitrogen is 800 mg/L. The rest of theexperiment showed a steady increased value of ammonium with an averageincrease of 850 mg/L in the experiment reactor compared to the controlreactor. This corresponds to an increase with about 28% compares to theammonium nitrogen in the control reactor (Fig. 7). There were no negativeeffects on the production of biogas, i.e. the production was neither increasednor decreased. l\/loreover, the replacement of water with aerated digestate (ordigestate) results in a higher pH, 8.0, compared to 7.8 in the control reactor(Fig 8). This is due to the increased amount of ammonium ions which forcethe pH up and potentially stress the biogas process. However, the volatilefatty acids concentration was low or below the detection limit during the whole trail in both reactors (data not shown). l\/loreover, the increased amount ofammonium and carbonates results in a higher buffer capacity, which isdesirable in a biogas process (Fig 8).

Use of the additive in hvdro-cvclones to replace water ln some cases grit, gravel, glass etc. and heavy inert particles need tobe removed from a substrate such as processed food waste (slurry) oranother pumpable waste or material. The most common way to perform thisis by use a hydro-cyclone that amplifies the gravidity of the heavy particles,which then can be separated from the organic waste.

Example At Linköping biogas plant (Linköping, Sweden) a cyclone is mounted inthe circulation of the storage tank for processed food waste, the slurry. Theslurry has a TS content of 14-16°/> in normal case. On an annual basis around90 000 tons of slurry will pass the process. The cyclones use around 10-15 Lwater/min to work, resulting in 5,000 m3water per year in consumption. Thus,the dilution of the slurry is at this plant 5-6°/> due to the extra water added inthis step, to separate heavy inert particles such as (but not limited to) grit andgravel from the organic slurry.

The high water consumption is both a cost and dilutes the bio-fertilizeras well as shortened the retention time of the substrate in the upcomingdigester. By instead using aerated digestate (and thus a liquid with lowviscosity), the water consumption may be cut with 1-100°/-.~ compared to thecommon used technique with water. l\/loreover, since the aerated digestate ismixed-in to a substrate flow where the grit should be separated, there is ofgreatest importance to avoid gas bubbles and gas formation. Since theaerated digestate lacks dissolved methane and has low content of carbondioxide this is a suitable liquid. Finally, the aerated digestate has a lowviscosity, which is a requirement to be able to use the digestate as a counterflow in the cyclone. By using aerated digestate in the cyclone, dilution can beavoided as the aerated digestate contains high amount of nitrogen andphosphorus etc. which is desirable in the bio-fertilizer. Even more positively, this cuts the amount of transports needed to transport the bio-fertilizer to thefarmers at the end of the process. See Figure 9.

Claims (11)

1. Method for producing biogas in an anaerobic digestion Chamber (3)from an un-treated organic substrate, wherein said un-treated organicsubstrate has a dry matter of content of in the range of 20 to 90 °/> of totalsolids, wherein the method comprises the steps of: pre-treatment of the un-treated organic substrate, to form a slurryhaving a dry matter content of in the range of 8 to 19.9 °/-.~ of total solids, feeding said slurry to a digestion chamber; digesting said slurry in the digestion chamber (3) to produce biogasand a digestate, characterized in that in said pre-treatment step a mixture of a dilution f|uid and a liquiddigestate from said digestion chamber (3) is used to dilute the un-treatedorganic substrate.

2. The method as claimed in claim 1, wherein the liquid digestate is eitheran unaerated or aerated liquid digestate.

3. The method as claimed in claim 2, wherein the liquid digestate is anaerated digestate, wherein the method further comprises aerating said liquiddigestate in an aeration chamber prior to the introduction into the pre-treatment step where the digestate is aerated with any one of the gasesselected from air, oxygen and nitrogen with a flow in the range of 0.1 to 100m3 gas/ms liquid h.

4. The method as claimed in claim 2 or 3, wherein the liquid digestate isan aerated digestate and wherein the method comprises aerating said liquiddigestate prior to the introduction into the pre-treatment step during a timeperiod of at least 1 hours.

5. The method as claimed in claim 3, wherein the viscosity of the aerated liquid digestate is decreased compared to untreated digestate during theaeration step as measured in cP.

6. The method as claimed in any one of the preceding claims, whereinthe liquid digestate replaces at least 5 °/> of the dilution fluid compared toconventional methods, or at least 25 °/> of dilution fluid, or at least 50 °/> ordilution fluid.

7. An additive for use in the method according to claims 1 to 7, whereinsaid additive comprises a liquid digestate from a digestion chamber.

8. The additive as claimed in claim 7, wherein an aerated liquid digestatehas a viscosity as measured cP which is lower than an untreated liquiddigestate.

9. A method of cleaning an un-treated organic waste substrate prior tointroduction into a biogas production facility, wherein the cleaning isperformed in a hydro-cyclone, and wherein in said hydro-cyclone a cleaningliquid is used, characterized in that said cleaning liquid is at least partially composed of aaerated liquid digestate from a digestion chamber.

10.The method as claimed in claim 9, wherein said liquid digestate iseither an unaerated or aerated liquid digestate.

11.The method as claimed in claim 10, wherein said aerated liquiddigestate has a viscosity as measured in cP which is lower than an unaeratedliquid digestate.