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.

Municipal wastewater treatment plants (WWTP) are important infrastructural components in a society and also important for sustainability. In a WWTP the most common treatment configuration is mechanical, biological, and chemical treatment of the wastewater. The treatments reduce nutrients and organic matter before the water is transferred to the recipient. During wastewater treatment, sludge is produced and then stabilized in an anaerobic digestion (AD) process.  The Käppala Association operates the third largest WWTP in Sweden. In the future the Käppala plant is expecting an increased number of connected households and also stricter sludge hygienization regulations. This implies that current strategies for the WWTP have to be developed. One idea has been to use a thermophilic AD process instead of mesophilic AD.Thermophilic AD has previously been shown to have higher capacities, lower hydraulic retention times (HRT) and increased pathogenic destruction compared to their mesophilic counterpart. A common negative aspect for a thermophilic process has been process instability.  In this study a 5 m3 pilot plant rented from Research Institutes of Sweden (RISE) was used to evaluate maximal organic loading rate (OLR) for a stable thermophilic AD process using mixed sludge as substrate. Four HRT were chosen, and each HRT was maintained for 3 retention times. Laboratory analyzes of the raw and digested sludge and on-line monitoring were performed regularly to collect information about process stability and efficiency. The pilot plant was controlled through a surveillance system where operating parameters were introduced. The main objectives of this study were to investigate how an increase of OLR affected pH, alkalinity, and volatile fatty acid (VFA) content and also how the alkalinity and VFA affected the process stability. Gas composition, gas production, degree of digestion (DD) and foaming were also investigated throughout this study. Results showed that VFA initially increased, and alkalinity decreased when every OLR increase occurred. The VFA and alkalinity returned to lower values after the process was given time to recover during constant OLR. The OLR increase caused slight variations in process pH but not enough for process failure. Increased OLR did not seem to have a large impact on the DD since it seemed to remain between 40-50% throughout the entire study. Composition monitoring showed an increased hydrogen sulfide content in the gas as a consequence of increased OLR. An OLR increase also lead to a volumetric gas production (VGP) and volumetric methane production (VMP) increase while efficiency of the AD process was seen to decrease when evaluating specific gas production (SGP) and specific methane production (SMP). The conclusion of the study was that a stable thermophilic AD process using mixed sludge as substrate could be operated with an OLR of 6.55 ± 0.06 kg VS m-3d-1and a HRT of 7 days. Maximal OLR for a stable thermophilic AD process was never achieved due to the process remaining relatively stable the entire experimental period. The process recovered from signs of instability during all HRT-transition times indicating it can maintain all investigated OLR. / Kommunala avloppsreningsverk är en viktig infrastruktur i ett samhälle och även viktigt för hållbarheten. I ett avloppsreningsverk renas spillvatten genom mekaniska, biologiska och kemiska behandlingar processteg. Behandlingarna minskar näringsämnen och organiskt material innan vattnet rinner ut i recipienten. Vid rening av avloppsvatten erhålls slam. Slammet kan användas som substrat för en anaerobisk nedbrytningsprocess. Käppalaförbundet driver det tredje största reningsverket i Sverige. I framtiden förväntas antal anslutna hushåll till reningsverket att öka samt strängare slamhanteringskrav appliceras. Detta innebär att nuvarande slamstrategi måste utvecklas. En idé är att tillämpa en termofilrötnings process. En termofil rötningsprocess har visat sig ha högre kapacitet, lägre uppehållstider och ökad patogen reduktion i jämförelse med en mesofil rötning. En nackdel med termofil rötning är den försämrade processtabiliteten.  I denna studie har en 5 m3 pilotanläggning inhyrd från RISE använts för att utvärdera den maximala organiska belastningen för en stabil termofil rötningsprocess med blandslam som substrat. Fyra uppehållstider valdes och varje uppehållstid hölls tre gånger. Regelbundna analyser av rå och rötslam utfördes för att erhålla information om process stabilitet och effektivitet. Pilotanläggningen styrdes genom ett människa-maskingränssnitt där driftparametrar matades in. Huvudmålen för studien var att se hur en ökning av organisk belastning påverkade pH, alkaliniteten och halt av flyktiga fettsyror samt att undersöka alkalinitetens och halten av flyktiga fettsyrors påverkan på processtabiliteten. Påverkan av en ökad organisk belastning på gas sammansättning, gasproduktion, rötningsgrad har även undersökts. Resultaten visade att flyktiga fettsyror initialt ökade och alkaliniteten minskade vid en belastnings ökning. Flyktiga syror och alkalinitet återgick till mer stabila värden efter processen fick återhämta sig vid en konstant belastning. Belastningsökningen orsakade små variationer i pH värdet men inte tillräckligt för att orsaka processkollaps. En ökad belastning verkade inte ha någon större påverkan på rötningsgraden då den låg runt 40–50% hela rötningsperioden. Skumutvecklingen visade sig inte heller vara ett problem. Resultaten visade även att väte sulfid ökade i gasen när en belastningsökning gjordes. Den volymetriska gas och metan produktionen ökade vid en ökad OLR medan specifika gas och metan produktionen minskade. Slutsatsen var att en stabil termofil rötningsprocess av blandslam kan drivas med en organisk belastning på 6.55 ± 0.06 kg VS m-3d-1 och en uppehållstid på 7 dagar. Maximal belastning för stabil termofil rötningsprocess uppnåddes dock ej då processen förblev relativt stabil under alla undersökta uppehållstider. Processen återhämtade sig även efter tecken på instabilitet vilket indikerar att AD processen kan behålla stabilitet vid de undersökta belastningar.

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.