Sorption of 75 active pharmaceutical ingredients (APIs) to three different types of sludge (primary sludge, secondary sludge with short and long sludge age respectively) were investigated. To obtain the sorption isotherms batch studies with the APIs mixture were performed in four nominal concentrations to water containing 1 g of sludge. The range of APIs concentrations was between ng L-1 to mu g L-1 which are found in the wastewater effluents. Isotherms were obtained for approximately 45 of the APIs, providing distribution coefficients for linear (K-d), Freundlich (K-f) and Langmuir (K-L) isotherms. K-d, K-f and K-L ranging between 7.1 x 10(4) and 3.8 x 10(7), 1.1 x 10(-2) and 6.1 x 10(4) and 9.2 x 10(-3) and 1.1 L kg(-1), respectively. The obtained coefficients were applied to estimate the fraction of APIs in the water phase (see Abstract Graphic). For 37 of the 75 APIs, the predicted presence in the liquid phase was estimated to >80%. 24 APIs were estimated to be present in the liquid phase between 20 and 80%, and 14 APIs were found to have <20% presence in the liquid phase, i.e. high affinity towards sludge. Furthermore, the effect of pH at values 6, 7 and 8 was evaluated using one way ANOVA-test. A significant difference in K(d)s due to pH changes were found for 6 of the APIs (variation 10-20%). (C) 2011 Elsevier Ltd. All rights reserved.