Introduction The number of wastewater treatment plants (WWTP) is increasing constantly due to restrictions on water poli- cies. Nowadays, wastewater treatment plants are recog- nized as point sources of heavy metal pollution into river systems. One of the major problems of the WWTP is sewage sludge utilization. This type of valuable wastes can very often contain various contaminations that exclude them as a compost material. The presence of heavy metals in composts causes significant environmen- tal hazard, and is one of most difficult contamination problem to solve. Several research studies investigating quality of sewage sludge and composts show significant differences of metals content [1-4]. Their concentration depends on type of wastes, seasonality and other meteorological con- ditions. From other hand, the toxicity of metal is related to its dominant form in environment. Operational speciation is applied to characterize the toxicity [5-10]. Commonly used selective sequential extraction (SSE) based on Tessier method or BCR procedure gives the information on chem- ical speciation of heavy metals and the toxicity of sewage sludge. However, physical speciation is also important from the point of view of sewage purification. The aim of present study was characterization of phys- ical speciation of selected heavy metals in sewage sludge. Split-flow-thin fractionation (SPLITT) system has been applied to separate different fractions of colloids derived from deactivated sewage sludge. SPLITT technique was developed by Giddings in early 80’s [11] and become one of the members of field-flow fractionation (FFF) family. Separation in SPLITT is based on same principles as other FFF methods [12] but it has some unique properties. The fractionation channel consists two inlets and two outlets (Fig. 1). Separation is based on combined action of the non-uniform flow velocity profile of a liquid through the channel and transverse gravitation- al field applied perpendicularly to this carrier. SPLITT has an ability to separate relatively large quantities of sample (milligrams or even grams) in rea- sonable amount of time. Fractionation can be performed Polish J. of Environ. Stud. Vol. 19, No. 2 (2010), 437-441 Letter to Editor Characterization of Natural Organic Colloids Derived from Sewage Sludge Tomasz Kowalkowski* Department of Environmental Chemistry and Bioanalitycs, Faculty of Chemistry, Nicholaus Copernicus University, Gagarina 7, 87-100 Toruń, Poland Received: 28 October 2009 Accepted: 25 January 2010 Abstract Fractionation of sewage sludge colloids has been achieved using SPLITT technique. It reveals that majority of organic carbon as well as heavy metals is carried by particles smaller than 1 μm. Optimization of volumetric flowrates and application of full-feed depletion mode of SPLITT fractionation allows to concen- trate smallest colloids. Additionally, HPLC and 13 C NMR analyses has been performed to characterize the organic colloids. 13 C NMR spectrum confirm that smaller fraction contained elevated content of aliphatic, aro- matic and carboxylic groups characteristic for humic compounds. HPLC analysis separates two fractions of substances and compare obtained chromatograms with separation of humic standard. Keywords: heavy metal speciation, sewage sludge, SPLITT fractionation *e-mail: pinez@chem.uni.torun.pl