DIKA, Driftstörningar i kommunala avloppsreningsverk - en studie av ...

7SummaryOperational problems have increased during the last 5-10 years in many sewagetreatment plants. It has mainly been problems with oxygen transfer, sludge settling andfoaming, leading to increased discharge. This project was started with the theory that theproblems were partly caused by increased use of readily biodegradable surfactants. Theirdegradation products can be expected to influence both oxygen transfer and themicrobial compositions of the biosludge.Great amounts of data have been collected from five treatment plants (in Borlänge,Gävle, Haparanda, Karlstad and Örebro). Samples were taken every second hour duringcertain sampling days when, in addition to the normal variables, the concentration ofsurfactants and fatty acids were analysed and oxygen transfer rate determined. All fiveplants also collected a large amount of operational data every week, including microbialand physical characterisation of the biosludge.Existing methods to quantify surfactants and fatty acids have been adapted to sewage.Analysis of surfactants, fatty acids and oxygen transfer rate (α-value) at several mediumsize Swedish sewage plants have confirmed the theory that surfactants, and also fattyacids, can decrease oxygen transfer rate. High concentration of surfactants and fattyacids can, though they are degraded in the aeration, increase the demand for airsignificantly. This means increased cost for aeration, and risk for sludge problems if theoxygen level can not be maintained.One way to reduce the problems with slow oxygen transfer can be decrease of sludgeload in the plants, leading to a faster and more complete degradation of surfactants andfatty acids. An increase of the sludge concentration in the Duvbacken plant in Gävleresulted in higher α-values, lower residual concentrations of surfactants and fatty acids,and less problems with oxygen levels and loss of sludge.Another way to decrease the problems caused by fatty acids can be to add the excessbiosludge to the primary settling, as in Örebro. A great part of the fatty acids can beadsorbed to the sludge, and thus not influence the aeration in the next step.Measuring of filtration resistance, floc strength and hydrofobicity (with CST- andcontact angel determination) of the sludge can give extra information about theseparation properties of the sludge. Data from these measurements and other analyseshave also resulted in a partly new surface chemical way to look at the oxygen transfer.Data collected every week from the five sewage plants were evaluated with multivariatemethods, in order to find different connections of interest for a more stable operation. Itwas for instance shown to be possible to calculate the concentration of totalphosphorous in the incoming water with relatively good accuracy (±15%), based upon