Käppalaverkets nuvarande och framtida rötningskapacitet
en studie i labskala
Wastewater treatment plantAnaerobic digestionBatch laboratory testsContinuous test with small scale reactorsBiogasMethane gasPrimary sludgeExcess sludgeWater works wasteRestaurant waste
Käppala wastewater treatment plant situated on the island of Lidingö northeast of Stockholm is running a project during 2004 and 2005 with the purpose to map out the capacity of anaerobic digestion in the digesters that treat primary and excess sludge. The purpose of this thesis work, which is part of that project, was to characterize the present anaerobic digestion process and to investigate its capacity to treat other organic wastes such as restaurant waste and waste from water works. To decide the potential of both methane and biogas production from different substrates batch laboratory tests were carried out. To imitate the anaerobic digestion process at Käppala continuous tests with small scale reactors were carried out. These reactors were later fed with restaurant waste.The batch laboratory tests showed that primary sludge had a potential biogas and methane production of 0,62 and 0,35 Ndm3/g VS respectively after 40 days of digestion. After 15 to 20 days of digestion (average retention time in the digester at Käppala wastewater treayment plant that treats primary sludge) the biogas production was between 0,55 and 0,60 Ndm3/g VS, which is within the range of the production at the digester that treats the primary sludge. This leads to the conclusion that the digestion of primary sludge is well functioning at the plant.Batch laboratory tests showed that excess sludge had a potential biogas and methane production of 0,31 and 0,16 Ndm3/g VS respectively after 40 days of digestion.Batch laboratory tests with restaurant waste showed a potential biogas and methane production of 0,81 and 0,38 Ndm3/g VS respectively after 47 days of digestion. This means that the total production of methane gas can increase with 7-8 per cent with an annual load of 3500 tonnes of restaurant waste (estimated amount of available restaurant waste). Tests to imitate transport of restaurant waste with influent wastewater showed that 35-60 per cent of the increase of gas production would be ?washed out? if the waste was tranported this way to the plant.Batch laboratory tests with waste from water works showed that no gas was produced from this substrate.The continuous tests with small scale reactors which were fed with restaurant waste resulted in an increase of biogas production with 12 per cent (corresponding to an annual load of 3500 tonnes of restaurant waste) compared to the present process.