Kraft mills are responsible for the massive discharge of highly polluted effluents, and new bleaching processes (i.e. Total Chlorine Free (TCF)) is presented as a feasible option to reduce this environmental impact. However, increased TCF pulp production is accompanied by an increase in chelate use. The most commonly used chelates, ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DPTA), are considered to be relatively persistent substances in water treatment plants, and consequently environmentally critical compounds. The purpose of this work is to investigate DPTA behaviour in an anaerobic system. An Anaerobic Filter (AF) was operated with three different DPTA load rates (LR DPTA = 0.07 - 0.28 gDPTA/L×d), and the operating strategy was to maintain the anaerobic system stable during the entire operation (alkalinity ratio below 0.3). The AF's maximum Chemical Oxygen Demand (COD) removal was 59%, whereas the Biological Oxygen Demand (BOD5) was around 95%. However, only 5% of DPTA removal was observed under anaerobic conditions during the first operating period. Scanning electronic microscopy indicates that the operating system reduced microorganism biodiversity