Some of anaerobic, mesophilic sulfate-reducing bacteria that produce H₂S and cause microbial metal corrosion can degrade crude oil in anaerobic conditions. In this study, a mesophilic sulfate-reducing bacterial strain D107G3 isolated from Bach Ho gas-oil field in Vung Tau, Vietnam that is able to utilize crude oil in the anaerobic condition is reported. The strain D107G3 was classified as a Gram-negative bacterium by using Gram staining method. Basing on scanning microscopy observation, the cell of a strain D107G3 had a curved rod shape. The 16S rRNA gene sequence analysis showed that the strain D107G3 was identified as Desulfovibrio vulgaris with 99.7% identity. The suitable conditions for its growth that was determined via estimating its H₂S production was the modified Postgate B medium containing 1% (v/v) crude oil, 1% NaCl (w/v), pH 7 and 300C incubation. In these conditions, the strain D107G3 can consume 11.4 % of crude oil total and oxidize heavy crude oil (≥ C45) for one month at anoxic condition. These obtained results not only contribute to the science but also continue to warn about the dangers of mesophilic sulfate reducing bacteria to the process of crude oil exploitation, use, and storage in Vung Tau, Vietnam. / Trong bài báo này, chủng vi khuẩn khử sunphat (KSF) ưa ấm D107G3 phân lập từ giếng khoan dầu khí mỏ Bạch Hổ, Vũng Tàu, Việt Nam có khả năng sử dụng dầu thô trong điều kiện kị khí được công bố. Chủng D107G3 được xác định là vi khuẩn Gram âm nhờ phương pháp nhuộm Gram. Quan sát trên kính hiển vi điện tử quét cho thấy tế bào chủng D107G3 có hình que cong. Kết quả phân tích trình tự gen 16S rRNA đã xác định được chủng D107G3 thuộc loài Desulfovibrio vulgaris với độ tương đồng 99.7%. Thông qua đánh giá lượng H₂S tạo thành đã khám phá được điều kiện thích hợp cho sinh trưởng của chủng D107G3: môi trường Postgate B cải tiến chứa 1% (v/v) dầu thô, 1 % NaCl (gL⁻¹), pH 7 và nuôi cấy ở 30°C. Trong điều kiện đó, chủng D107G3 đã sử dụng được 11.4 % hàm lượng dầu tổng số, thành phần dầu bị phân huỷ là các n-parafin có mạch C≥45 sau 1 tháng nuôi cấy kỵ khí. Các kết quả này đóng góp về mặt khoa học và tiếp tục cảnh báo mối nguy hại của KSF ưa ấm đến việc khai thác, sử dụng và bảo quản dầu mỏ ở Vũng Tàu, Việt Nam.

För att minska klimatpåverkan är energisnåla processer och användning av fossilfria bränslen viktigt. Vid stabilisering av avloppsslam vid reningsverk är en vanlig metod rötning som förutom att ta hand om slammet även producerar biogas, ett fossilfritt bränsle med låga växthusgasutsläpp. Processer som drivs inom det mesofila temperaturområdet har visat sig vara stabila och ger en jämn gasproduktion. Det mesofila området sträcker sig mellan 25–40°C men de flesta processer drivs mellan 35–40°C. Den här studien undersöker möjligheten att sänka temperaturen inom det mesofila området för att få en lägre energiförbrukning och en energisnålare process. Då det är viktigt att biogasproduktionen inte försämras av en sänkt temperatur har skillnad i utrötningsgrad, metanpotential och utrötningstid undersökts vid tre olika temperaturer (32, 34,5 samt 37,5°C) via satsvisa utrötningsförsök. Utöver påverkan på biogasproduktionen har en energibalansberäkning utförts för rötkamrarna vid Uppsala reningsverk. Detta ger ett mått på hur stora vinster i värmeenergi en sänkt temperatur kan leda till. En betydande kostnad vid reningsverk är avvattningen av slam och det är därför viktigt att den inte riskerar att försämras om temperaturen sänks. Ett filtreringsförsök som mäter CST (Capillary Suction Time) ger ett mått på slammets avvattningsegenskaper och har därför utförts vid tre olika temperaturer. Resultaten visade ingen försämring i biogasproduktion vid en sänkning till 34,5°C och en minskning i metanpotential med 11 % vid en sänkning till 32°C. Nedbrytningshastigheten försämrades inte vid en sänkt temperatur. Vinster i form av lägre värmeförbrukning uppgick till 14 % vid sänkning till 34,5°C och 27 % vid sänkning till 32°C. Avvattningsförsöket visade ingen försämrad avvattning vid lägre temperaturer. Den här studien visar att det finns en möjlighet att sänka temperaturen i rötkammaren vid reningsverket i Uppsala och på så sätt sänka energiförbrukningen. För att bekräfta resultaten bör även kontinuerliga försök utföras men denna studie visar att det är möjligt att få en lyckad nedbrytning även i lägre mesofila temperaturer. Resultatet öppnar upp för fortsatta undersökningar om temperaturförändringar inom det mesofila området och kan leda till en optimering av rötningsprocessen och möjlighet att få en effektiv och energisnål produktion av biogas. / Energy efficient processes and the use of fossil free fuels play an important role in order to reduce the impact of climate change. Anaerobic digestion is a common way for stabilizing sewage sludge at wastewater treatment plants (WWTP). One of the benefits with anaerobic digestion is that it also produces biogas, a fossil free fuel with low greenhouse gas emissions. An operational temperature within the mesophilic range has proven to give a stable process with an unfluctuating production of gas. The mesophilic temperature range between 25-40°C but most processes are operated between 35-40°C. This study investigates the opportunity to lower the temperature within the mesophilic range in order to reduce energy consumption. It is important to maintain the production of biogas with a lower temperature. Therefore, the reduction in VS-content (VS-volatile solids), methane yield and time for degradation was determined by a BMP-experiment (BMP-Biochemical Methane Potential) in three different temperatures (32, 34.5 and 37.5°C). In order to quantify the reduction in heat consumption with lower operational temperatures the change in heat balance for a full-scale WWTP in Uppsala was calculated. A major part of the operational cost is dewatering of sludge and it is therefore important that it does not deteriorate with a lower temperature. The effect on the dewaterability at different temperatures was examined by a filterability test measuring CST (capillary suction time). The results from the study showed no significant difference in methane yield between 37.5°C and 34.5°C. The methane yield at 32°C was 11 % lower compared to 37.5°C but the degradation kinetic was not affected by a temperature change. The reduction in heat consumption was 14 % when the temperature was reduced to 34.5°C and 27 % when it was reduced to 32°C. The filterability test did not show a deterioration with lower temperatures. The study showed that it is possible to reduce the operational temperature for anaerobic digestion at the WWTP in Uppsala in order to reduce the energy consumption. To confirm these results a continuously experiment should be done, but this study shows that it is possible to get a successful degradation in a lower mesophilic temperature. This leads the way for further investigations within the mesophilic range and could lead to optimizing anaerobic digestion and the opportunity to get an energy efficient production of biogas.

Neste trabalho avaliou-se a produção de compostos de interesse biotecnológico potenciais vetores energéticos a partir de papel em lisímetros (20L), usando-se consórcio microbiano enriquecido do fluido de rúmen. Para tanto, foi realizado um delineamento composto central (DCC) para verificar a influência de três variáveis independentes na conversão de papel sulfite a hidrogênio e outros compostos orgânicos em lisímetro de bancada. As variáveis testadas foram massa de papel (X1: 500g, 750g e 1000g), teor de umidade papel (X2: 50%, 65% e 80%), e temperatura (X3:35°C, 45°C e 55°C). As respostas avaliadas no DCC foram produção de hidrogênio (Y1; mmol), ácido acético (Y2; mg/L), etanol (Y3; mg/L) e metanol (Y4; mg/L). Para o monitoramento dos lisímetros em relação à hidrólise e fermentação do papel, foram analisados biogás (H2, N2, CO2 e CH4) e a concentração de compostos no percolado, como açúcares totais, demanda química de oxigênio (DQO), ácidos orgânicos voláteis (AOVs) e álcoois. Além disso, monitorou-se o pH, alcalinidade e sólidos totais. Sequenciamento massivo do gene RNAr 16S via Plataforma Illumina foi usado para identificação dos micro-organismos do fluido de rúmen in natura, do consórcio enriquecido, e daqueles dos lisímetros R2, R5 e R9. Produção de hidrogênio só foi observada nos lisímetros R1 (25 mmol), R2 (35 mmol) e R5 (3 mmol), sendo os três com umidade inicial de 80%. Em R1 e R2, observou-se elevadas concentrações de ácido acético, de 21.500 e 17.000 mg/L, respectivamente, provavelmente devido à ocorrência de homoacetogênese. Sob temperatura termofílica, especialmente em R5, observou-se consumo de hidrogênio, e produção de etanol (2.300 mg/L) e metanol (5.600 mg/L). Na condição de 80% de umidade (R1, R2, R5, R6), verificou-se maiores percentuais de remoção de papel e atividade fermentativa mais acentuada, ao passo que abaixo de 80%, o desenvolvimento microbiano foi desfavorecido, independente da temperatura. Verificou-se consumo muito reduzido de papel e baixas concentrações de AOVs e álcoois para R3, R4, R7 e R8, todos com 50% umidade. Em R9 e R10, operados a 45 °C e 65% de umidade, também verificou-se produção atenuada de AOVs e álcoois, com ausência de hidrogênio. Por meio do DCC, observou-se efeito estatisticamente significativo da umidade do papel na produção de hidrogênio, ácido acético e etanol. Em relação à temperatura, verificou-se efeito positivo estatisticamente significativo na produção de hidrogênio e ácido acético. Por fim, para a massa de papel não se verificou nenhum efeito sobre as respostas analisadas. Os gêneros de bactérias mais abundantes foram: Prevotella no fluido de rúmen in natura (F.N.) Dysgonomonas no fluido de rúmen enriquecido e em R2 (35°C), Thermicanus em R5 (55°C) e Phaeospirillum em R9 (45ºC). A umidade foi o parâmetro mais determinante para promover a hidrólise e fermentação do papel; a temperatura foi a principal variável de influência na estrutura das comunidades microbianas dos lisímetros, confirmada pelas diferentes rotas metabólicas observadas sob temperatura mesofílica e termofílica; e os rendimentos de produção dos compostos não foram influenciados pela massa de papel adicionada aos lisímetros. / This study evaluated the production of compounds of biotechnological interest and potential energy vectors from office paper in lysimeters (20L), using a microbial consortium purified from rumen fluid. A central composite design (CCD) was performed to verify the influence of three independent variables on paper conversion to hydrogen and other organic compounds in bench lysimeter. The tested variables were: mass of paper (x1: 500g, 750g and 1000g), moisture content (x2: 50%, 65% and 80%), and incubation temperature (x3: 35°C, 45°C and 55°C). The dependent variables of CCD were production of hydrogen (Y1: mmol), acetic acid (Y2: mg/L), ethanol (Y3:mg/L) and metanol (Y4:mg/L). For monitoring the lysimeters in relation to paper hydrolysis and fermentation, analyses of biogas (H2, N2, CO2 and CH4) and the organic compounds' concentrations in the leachate, such as, total sugars, chemical oxygem demand, volatile organic acids (VOA's) and alcohols were conducted during operation. Alcalinity, pH and total solids content of the leachate were also monitored. Massive sequencing of rRNA 16S (Illumina) was carried out for identification of the microorganisms of the in natura rumen fluid, the purified consortium, and those collected from lysimeters R2, R5 and R9. Hydrogen production was detected only in lysimeters R1 (25 mmol), R2 (35 mmol) and R5 (3 mmol), all of them operated with 80% of moisture content. In R1 and R2, high concentrations of acetic acid, of 21.500 and 17.000, respectively, were due to the likely occurrence of homoacetogenesis. Under thermophilic temperature, especially R5, the hydrogen production was detected in low quantity, and the highlight was the production of ethanol and methanol, with concentrations around 2.300 and 5.600 mg/L, respectively. At 80% moisture condition (R1, R2, R5, R6), high percentages of paper removal and sharp fermentative activity were observed. However, at lower moisture conditions, the microbial growth was unfavored, independent of the temperature. Low paper consumption and reduced concentrations of OVA's and alcohols were detected in R3, R4, R7 and R8, all of them operated with 50% of moisture content. In R9 and R10, operated at 45°C and 65% of moisture, there was also attenuated production of VOA\'s and alcohols, with absence of hydrogen. According to CCD statistical analysis, paper moisture content had positive effect statistically significative on hydrogen, acetic acid and etanol production. The temperature had positive effect on hydrogen and acetic acid production. And the mass of paper dit not have effect statistically significative for any dependent variables. The most abundant bacterial genus was: Prevotella in the in natura rumen fluid (F.N.) Dysgonomonas in the purified consortium and R2 (35 oC), Thermicanus in R5 (55°C) and Phaeospirillum in R9 (45° C). In conclusion, moisture content was the main parameter to promote paper hydrolysis and fermentation; temperature was the principal variable that influenced the structure of microbial community, confirmed by the different metabolic route observed under mesophilic and thermophilic conditions; and the production yields of the compounds were not influenced by the mass of paper added to the lysimeters.

Neste trabalho avaliou-se a produção de compostos de interesse biotecnológico potenciais vetores energéticos a partir de papel em lisímetros (20L), usando-se consórcio microbiano enriquecido do fluido de rúmen. Para tanto, foi realizado um delineamento composto central (DCC) para verificar a influência de três variáveis independentes na conversão de papel sulfite a hidrogênio e outros compostos orgânicos em lisímetro de bancada. As variáveis testadas foram massa de papel (X1: 500g, 750g e 1000g), teor de umidade papel (X2: 50%, 65% e 80%), e temperatura (X3:35°C, 45°C e 55°C). As respostas avaliadas no DCC foram produção de hidrogênio (Y1; mmol), ácido acético (Y2; mg/L), etanol (Y3; mg/L) e metanol (Y4; mg/L). Para o monitoramento dos lisímetros em relação à hidrólise e fermentação do papel, foram analisados biogás (H2, N2, CO2 e CH4) e a concentração de compostos no percolado, como açúcares totais, demanda química de oxigênio (DQO), ácidos orgânicos voláteis (AOVs) e álcoois. Além disso, monitorou-se o pH, alcalinidade e sólidos totais. Sequenciamento massivo do gene RNAr 16S via Plataforma Illumina foi usado para identificação dos micro-organismos do fluido de rúmen in natura, do consórcio enriquecido, e daqueles dos lisímetros R2, R5 e R9. Produção de hidrogênio só foi observada nos lisímetros R1 (25 mmol), R2 (35 mmol) e R5 (3 mmol), sendo os três com umidade inicial de 80%. Em R1 e R2, observou-se elevadas concentrações de ácido acético, de 21.500 e 17.000 mg/L, respectivamente, provavelmente devido à ocorrência de homoacetogênese. Sob temperatura termofílica, especialmente em R5, observou-se consumo de hidrogênio, e produção de etanol (2.300 mg/L) e metanol (5.600 mg/L). Na condição de 80% de umidade (R1, R2, R5, R6), verificou-se maiores percentuais de remoção de papel e atividade fermentativa mais acentuada, ao passo que abaixo de 80%, o desenvolvimento microbiano foi desfavorecido, independente da temperatura. Verificou-se consumo muito reduzido de papel e baixas concentrações de AOVs e álcoois para R3, R4, R7 e R8, todos com 50% umidade. Em R9 e R10, operados a 45 °C e 65% de umidade, também verificou-se produção atenuada de AOVs e álcoois, com ausência de hidrogênio. Por meio do DCC, observou-se efeito estatisticamente significativo da umidade do papel na produção de hidrogênio, ácido acético e etanol. Em relação à temperatura, verificou-se efeito positivo estatisticamente significativo na produção de hidrogênio e ácido acético. Por fim, para a massa de papel não se verificou nenhum efeito sobre as respostas analisadas. Os gêneros de bactérias mais abundantes foram: Prevotella no fluido de rúmen in natura (F.N.) Dysgonomonas no fluido de rúmen enriquecido e em R2 (35°C), Thermicanus em R5 (55°C) e Phaeospirillum em R9 (45ºC). A umidade foi o parâmetro mais determinante para promover a hidrólise e fermentação do papel; a temperatura foi a principal variável de influência na estrutura das comunidades microbianas dos lisímetros, confirmada pelas diferentes rotas metabólicas observadas sob temperatura mesofílica e termofílica; e os rendimentos de produção dos compostos não foram influenciados pela massa de papel adicionada aos lisímetros. / This study evaluated the production of compounds of biotechnological interest and potential energy vectors from office paper in lysimeters (20L), using a microbial consortium purified from rumen fluid. A central composite design (CCD) was performed to verify the influence of three independent variables on paper conversion to hydrogen and other organic compounds in bench lysimeter. The tested variables were: mass of paper (x1: 500g, 750g and 1000g), moisture content (x2: 50%, 65% and 80%), and incubation temperature (x3: 35°C, 45°C and 55°C). The dependent variables of CCD were production of hydrogen (Y1: mmol), acetic acid (Y2: mg/L), ethanol (Y3:mg/L) and metanol (Y4:mg/L). For monitoring the lysimeters in relation to paper hydrolysis and fermentation, analyses of biogas (H2, N2, CO2 and CH4) and the organic compounds' concentrations in the leachate, such as, total sugars, chemical oxygem demand, volatile organic acids (VOA's) and alcohols were conducted during operation. Alcalinity, pH and total solids content of the leachate were also monitored. Massive sequencing of rRNA 16S (Illumina) was carried out for identification of the microorganisms of the in natura rumen fluid, the purified consortium, and those collected from lysimeters R2, R5 and R9. Hydrogen production was detected only in lysimeters R1 (25 mmol), R2 (35 mmol) and R5 (3 mmol), all of them operated with 80% of moisture content. In R1 and R2, high concentrations of acetic acid, of 21.500 and 17.000, respectively, were due to the likely occurrence of homoacetogenesis. Under thermophilic temperature, especially R5, the hydrogen production was detected in low quantity, and the highlight was the production of ethanol and methanol, with concentrations around 2.300 and 5.600 mg/L, respectively. At 80% moisture condition (R1, R2, R5, R6), high percentages of paper removal and sharp fermentative activity were observed. However, at lower moisture conditions, the microbial growth was unfavored, independent of the temperature. Low paper consumption and reduced concentrations of OVA's and alcohols were detected in R3, R4, R7 and R8, all of them operated with 50% of moisture content. In R9 and R10, operated at 45°C and 65% of moisture, there was also attenuated production of VOA\'s and alcohols, with absence of hydrogen. According to CCD statistical analysis, paper moisture content had positive effect statistically significative on hydrogen, acetic acid and etanol production. The temperature had positive effect on hydrogen and acetic acid production. And the mass of paper dit not have effect statistically significative for any dependent variables. The most abundant bacterial genus was: Prevotella in the in natura rumen fluid (F.N.) Dysgonomonas in the purified consortium and R2 (35 oC), Thermicanus in R5 (55°C) and Phaeospirillum in R9 (45° C). In conclusion, moisture content was the main parameter to promote paper hydrolysis and fermentation; temperature was the principal variable that influenced the structure of microbial community, confirmed by the different metabolic route observed under mesophilic and thermophilic conditions; and the production yields of the compounds were not influenced by the mass of paper added to the lysimeters.

Käppala wastewater treatment plant (WWTP) has, during a few years, observed an increase in organic loading rate (OLR) in the mesophilic anaerobic digester R100, due to an increased load to the WWTP. The digestion of primary sludge at Käppala WWTP is today high loaded, with a high organic loading rate (OLR) and low hydraulic retention time (HRT). This study aims to evaluate the effect of the maximum OLR and the minimum HRT for the anaerobic digestion of sewage sludge and to investigate further actions that can be taken into consideration in case of process problems in the digestion. The study consists of (a) a practical laboratory experiment of 6 pilot-scale reactors to investigate how the process stability is affected when the OLR increases and the HRT decreases. (b) A mass balance calculation based on the energy potential in the feeding sludge and the digested sludge. (c) A study of the filterability of the digested sludge. (d) The construction of a forecasting model in Excel, to predict when digester R100 will reach its maximum OLR and minimum HRT. The result of the study shows that the maximum OLR for Käppala conditions is 4.9 g VS dm-3 d-1, meaning that R100 will reach its maximum organic load around the year 2031. An OLR of 4.5-4.9 and an HRT of 12 days is optimal for R100, according to the present study. Keeping the anaerobic digestion process in balance is vital when it comes to the outcome of energy in the anaerobic digestion process. Pushing the process to produce more gas can become counterproductive since a high OLR can lead to process imbalance, which in turn leads to low biogas production. Imbalance in the digestion process can occur fast; therefore, the margin for overload in the anaerobic digestion process must be significant. The methane concentration in the converted biogas and the pH level in the reactor are the best stability parameters for the conditions at Käppala. Ammonia is the less efficient stability parameter since it did not predict or detect any instability during the experimental process. Furthermore, the OLR and HRT have a significant impact on the needed quantity for dewatering polymer. The higher digestion of organic material in the sludge, the bigger the need for the polymer to take care of the rest material.

Käppala wastewater treatment plant (WWTP) has, during a few years, observed an increase in organic loading rate (OLR) in the mesophilic anaerobic digester R100, due to an increased load to the WWTP. The digestion of primary sludge at Käppala WWTP is today high loaded, with a high organic loading rate (OLR) and low hydraulic retention time (HRT). This study aims to evaluate the effect of the maximum OLR and the minimum HRT for the anaerobic digestion of sewage sludge and to investigate further actions that can be taken into consideration in case of process problems in the digestion. The study consists of (a) a practical laboratory experiment of 6 pilot-scale reactors to investigate how the process stability is affected when the OLR increases and the HRT decreases. (b) A mass balance calculation based on the energy potential in the feeding sludge and the digested sludge. (c) A study of the filterability of the digested sludge. (d) The construction of a forecasting model in Excel, to predict when digester R100 will reach its maximum OLR and minimum HRT. The result of the study shows that the maximum OLR for Käppala conditions is 4.9 g VS dm-3 d-1, meaning that R100 will reach its maximum organic load around the year 2031. An OLR of 4.5-4.9 and an HRT of 12 days is optimal for R100, according to the present study. Keeping the anaerobic digestion process in balance is vital when it comes to the outcome of energy in the anaerobic digestion process. Pushing the process to produce more gas can become counterproductive since a high OLR can lead to process imbalance, which in turn leads to low biogas production. Imbalance in the digestion process can occur fast; therefore, the margin for overload in the anaerobic digestion process must be significant. The methane concentration in the converted biogas and the pH level in the reactor are the best stability parameters for the conditions at Käppala. Ammonia is the less efficient stability parameter since it did not predict or detect any instability during the experimental process. Furthermore, the OLR and HRT have a significant impact on the needed quantity for dewatering polymer. The higher digestion of organic material in the sludge, the bigger the need for the polymer to take care of the rest material.

Kommunala avloppsreningsverk renar avloppsvatten och avlägsnar näringsämnen innan vattnet släpps ut i den omgivande miljön. Under reningen avskiljs partiklar som bildar råslam. Genom att bryta ner råslammet anaerobt (syrefritt) via rötning kan användbara produkter, såsom biogas till bussar, erhållas. Rötning innebär att organiskt material bryts ner under kontrollerade former. Sveriges tredje största kommunala avloppsreningsverk är Käppalaverket i Stockholm. Framtidsprognosen för verket visar att belastningen kommer öka, samtidigt förväntas kraven på slamhantering bli strängare.  Syftet med denna studie var att utreda en metod för att öka Käppalaverkets slambehandlingskapacitet. Idag rötas slammet konventionellt med mesofil rötning. Ett förslag för att öka kapaciteten är att förtjocka och därefter recirkulera slammet in i samma rötningskammare. Möjliga fördelar med detta kan vara ökad biogasproduktion samt möjlighet till högre belastning i rötkamrarna.  Till experimentet nyttjades en pilotanläggning bestående av två rötkammrar på 7 dm3. Rötkamrarna användes för att studera konventionell mesofil rötning (i studien benämnd BR01), jämfört med mesofil rötning med recirkulering (i studien benämnd BR02). Råslammet och rötslammet analyserades löpande (på dag- och veckobasis) med avseende på parametrarna: biogasproduktion och gassammansättning, torrsubstanshalt, halt av flyktiga syror, pH, alkalinitet, organisk belastning, andel mineraliserat kväve, utrötningsgrad samt reduktion av makromolekyler. Uppehållstiden (HRT) för slammet i rötkamrarna var inledningsvis 14 dagar. HRT sänktes stegvis  under studien (från 14 till 11 dagar och därefter till 9 dagar) genom att inflödet av råslam ökades. Därmed pressades BR01 och BR02 i syfte att utröna vid vilken belastning rötningsprocessen förändrades från stabil till instabil. Resultaten visade att under 11-dagars perioden var processen i BR01 stabilare än i BR02. Förhöjda halter av flyktiga syror uppmättes i BR02 under denna period. Under 9-dagars perioden med högre belastning observerades att processen i BR01 var instabil. Processen i BR02 var stabilare under samma period och ansågs därför prestera bättre under högre belastning. Statistiska t-tester visade att flertalet reaktorparametrar (i en och samma reaktor) genomgick säkerställda förändringar från experimentets start till slut. / Municipal wastewater treatment plants (WWTPs) purifies wastewater from nutrients before releasing it to the recipient. Particles that are separated from the water creates a sewage sludge. Anaerobic (without oxygen) digestion is mainly used as a stabilization method for the sludge. Digestion of the sludge produces energy-rich biogas and a nutrient-rich digestate which can be used as a fertilizer. The third largest municipal WWTP in Sweden is Käppala WWTP, situated in Stockholm. In the future, the amount of incoming wastewater is expected to increase. Simultaneously, regulations regarding sludge management are expected to be stricter. The main purpose of this study was to investigate a method to increase the sludge load to the digesters at Käppala WWTP. Today the sludge is digested conventionally with mesophilic anaerobic digestion. One suggestion is to recirculate thickened sludge into the same digestion reactor. This method has been shown to give an increased biogas production as well as improved digester performance.   During the experiment a pilot-digester consisting of two anaerobic reactors (each with a volume of 7 dm3) were used. These reactors were used to study conventional mesophilic digestion (BR01 in this study) and mesophilic digestion with sludge-recirculation (BR02 in this study). Several parameters of the sludge (raw and digested) were analysed on daily and weekly basis: biogas production and gas composition, dry matter content, volatile fatty acids, pH, alkalinity, organic loading, mineralised nitrogen, degree of digestion and reduction of macromolecules. The hydraulic retention time (HRT) of the sludge was initially 14 days. HRT was lowered stepwise (from 14 to 11 and finally down to 9 days) by increasing the inflow of raw sludge. This way the two reactors were pushed to see at what point the digestion process became unstable. The results showed that during the 11-days HRT period BR01 was more stable compared to BR02. Heightened VFA-content was measured in BR02 during this period. During the 9-days HRT period the process in BR01 was deemed unstable. The process in BR02 was more stable during the same period and was considered to have a better performance during high sludge inflow. Statistical t-tests showed that several digestion-parameters (compared for the same reactor) underwent statistically significant changes from the start to the end of the experiment.