Biogasanläggningar, som behandlar fler än en typ av substrat, så kallade samrötningsanläggningar, är relativt unga anläggningar. De första byggdes under tidigt 90-tal och står idag för omkring 28% av Sveriges totala biogasproduktion. Felaktigt inkommande material och det matavfall som tas in som substrat ställer höga krav på anläggningarnas utrustning, med därtill kopplade problem. Syftet med detta examensarbete var att genom intervjuer genomföra en kartläggning av befintliga tekniska och processrelaterade problem för två biogasanläggningar med samrötning samt en förbehandlingsanläggning. För att undersöka alternativa tekniker och utrustningar till identifierade problem genomfördes litteraturstudier och kontakt togs med leverantörer av biogasutrustning. Slutligen genomfördes en kostnadskalkylering för en specifik anläggning för att få en uppfattning om vad ett byte av problematisk utrustning skulle innebära rent ekonomiskt. Resultatet av kartläggningen visade att anläggningarnas förbehandling stod för de flesta problemen. Ett gemensamt problem för samtliga anläggningar var svårigheten att separera oönskat material från matavfall. Plast och annat oönskat material avskiljs inte i tillfredställande grad utan följer med vidare i processen och orsakar driftproblem samt att allt för mycket rötbart material följer med utgående rejekt och går till förbränning istället för rötning. Provtagning och analys av substrat och rejekt visade att vid avskiljning med skruvpress återfanns mer än 40% av substratets potentiella teoretiska metanutbyte i rejektfraktionen. Separation med bioseparator samt separation med en kombination av separerings­bassäng, trappstegsgaller och silpress visade en mindre förlust av metan i det avskiljda rejektet. Övriga identifierade problem omfattas av slitage och låg servicevänlighet hos utrustningen och outnyttjad utrustningskapacitet samt problem på grund av att utrustning ursprungligen ej var avsedd för biogasanläggningar. Litteraturstudier och kontakter med leverantörer av biogasutrustning visade att det finns alternativa tekniker och utrustningar till de identifierade problemen. Uppvärmning av substrat innan separation samt skifte till papperspåsar som insamlingsmetod är två potentiella lösningar. Det finns ett flertal utrustningar på marknaden för avskiljning av oönskat material, som potentiellt skulle kunna ersätta ineffektiva separationsutrustningar vid de aktuella anläggningarna. Huruvida sådana utrustningar skulle ge en förbättrad avskiljning och därmed minskad förlust av rötbart material i rejekt är svårt att avgöra innan tester med aktuellt substrat under aktuella förutsättningar genomförts. Resultatet av kostnadskalkyleringen visade att de största årliga drift- och underhålls­kostnaderna för befintlig separationsutrustning var lön till personal samt kostnader för slitage och haverier. En nyinvestering visade sig vara mycket kapitalintensivt i form av den initiala investeringskostnaden. Däremot frigör ett eventuellt byte till ny separationsutrustning personaltimmar då ny utrustning potentiellt kan kräva mindre drift- och underhållstid. Frigjorda personaltimmar kan istället användas till förebyggande underhåll eller som en besparingsåtgärd. Bättre uppföljning av rejektmängder och oplanerade driftstopp vid anläggningarna är viktiga initiala steg för att få kunskap om vilka problem som är av störst betydelse för att därefter kunna förbättra processen. Befintliga separationstekniker på anläggningarna fungerar inte optimalt och en vidareutveckling alternativt ersättning av dessa bör undersökas ytterligare. / Biogas plants treating more than one type of substrate, so called co-digestion plants, are relatively new facilities. Mainly they were established in the early nineties and in Sweden these digesters produce approximately 28% of the total amount of biogas produced today. Faultily incoming material and the food waste used as substrate at the plants set high demands on the equipment used at the plants and many problems occur. The purpose of this thesis was to identify technical and process related problems at two co-digestion plants and one pretreatment plant by performing interviews with staff at the plants. Investigation of alternative techniques and equipment to abate the identified problems were performed by a literature study and contacts with distributors of biogas plant equipment. Finally, a cost-estimation was made at a specific plant in order to investigate the economic aspects of a replacement of old equipment. The result of the problem identification showed that the pretreatment units of the plants caused most of the problems. A common issue at the three plants was the problem to separate unwanted material from the food waste. Plastics and other unwanted material aren’t separated to a full extent and thereby cause problems downstream. Furthermore a large amount of food waste aimed for digestion, are separated together with the reject fraction and goes to incineration instead. Analysis of the incoming substrate and reject showed that by separation with a screw press more than 40% of the substrates theoretical methane yield were lost in the reject fraction. However, separation with a bio-separator or with a combination of sedimentation/flotation, a grid and a strain press showed a smaller loss of methane in the reject. Additional problems identified included extensive wear and tear at the equipment, unused equipment and problems caused by the fact that the equipment was not originally constructed for biogas plants. The literature study and contact with distributors of biogas plant equipment revealed alternative techniques and equipment to remediate the identified problems. The warm-up of substrate before separation and the replacement of plastic bags for paper bags for the collection of food waste are two potential solutions. There are several different equipments available, in order to separate unwanted material, which could potentially replace the ineffective equipment at the plants. Whether these would result in better separation and reduced loss of food waste via the reject is hard to tell before tests have been made at prevalent substrate and operation conditions. The result of the cost-estimation showed that the largest operating and maintenance costs for the old equipment were the labor cost and the cost for wear and tear. An investment in new equipment is very capital-intensive because of the large purchasing and installation cost.  On the other hand, a new equipment could potentially result in a release in working hours compared to a continuous operation of the old equipment. These extra hours could instead be used to maintain the new equipment and prevent break downs.  A better follow-up at the plants regarding the amount of reject and the number of occasions of unwanted stop is a first step towards gaining knowledge on the major problems in order to be able to improve the process. Current used separation techniques at the plants are not working as expected and an optimization or replacement of these should be evaluated.

<p>One of Sweden’s sixteen national environmental objectives strives to decrease the impact on the climate. By 2020, green house gas emissions should be 40 % less compared to the levels of 1990 and a minimum of 50 % of the energy consumption should come from renewable energy sources. Because of this there is a great need of increasing the production of renewable energy. This is where biogas comes in as a competitive alternative. However, there is a problem. Substrates for biogas production have become a scarcity due to the increasing demand for biogas that has been expressed lately.</p><p>   The objective is to co-digest fodder beet and cow manure in order to find the best methane yielding mixture(s), and based on these results be able suggest whether fodder beets are a suitable substrate to use in a greater extent. This might prove to be one possible way in dealing with today’s substrate shortage. The mesophilic co-digestion has been performed in lab scale, where seven glass bottles (800ml) have served as digesters. Several analyses of important parameters i.e. gas production and composition, carbon/nitrogen-ratio, total solids and pH have been performed during and after the digestion tests.</p><p>   The best results (25-28 % methane) were obtained when 5-15 % beets were added to the digester. This shows that fodder beets is best used as a supplement in a smaller extent.  Results also show that, when using fodder beets as substrate, two of the most important process parameters to control are pH and buffer capacity (alkalinity/volatile organic acid ratio). This is because the digester easily gets acidic.</p><p>   Based on these results it’s hard to suggest fodder beet usage in a greater extent when being mixed with only cow manure for biogas production. This is especially true for smaller biogas plants where the possibility of observation often is limited or deficient. However, because of the rapid initial degradation and gas production, fodder beets would make a good complement in digesters where more persistent substrates with slower gas production are being digested. Also, since fodder beets are rich in carbon and poor in nitrogen they would also make a good complement when one desire to increase the carbon/nitrogen-ratio in the digester</p>

One of Sweden’s sixteen national environmental objectives strives to decrease the impact on the climate. By 2020, green house gas emissions should be 40 % less compared to the levels of 1990 and a minimum of 50 % of the energy consumption should come from renewable energy sources. Because of this there is a great need of increasing the production of renewable energy. This is where biogas comes in as a competitive alternative. However, there is a problem. Substrates for biogas production have become a scarcity due to the increasing demand for biogas that has been expressed lately.    The objective is to co-digest fodder beet and cow manure in order to find the best methane yielding mixture(s), and based on these results be able suggest whether fodder beets are a suitable substrate to use in a greater extent. This might prove to be one possible way in dealing with today’s substrate shortage. The mesophilic co-digestion has been performed in lab scale, where seven glass bottles (800ml) have served as digesters. Several analyses of important parameters i.e. gas production and composition, carbon/nitrogen-ratio, total solids and pH have been performed during and after the digestion tests.    The best results (25-28 % methane) were obtained when 5-15 % beets were added to the digester. This shows that fodder beets is best used as a supplement in a smaller extent.  Results also show that, when using fodder beets as substrate, two of the most important process parameters to control are pH and buffer capacity (alkalinity/volatile organic acid ratio). This is because the digester easily gets acidic.    Based on these results it’s hard to suggest fodder beet usage in a greater extent when being mixed with only cow manure for biogas production. This is especially true for smaller biogas plants where the possibility of observation often is limited or deficient. However, because of the rapid initial degradation and gas production, fodder beets would make a good complement in digesters where more persistent substrates with slower gas production are being digested. Also, since fodder beets are rich in carbon and poor in nitrogen they would also make a good complement when one desire to increase the carbon/nitrogen-ratio in the digester