In CANDU plants, the decontamination of liquid waste is performed by the LRWS (The Aqueous Liquid Radioactive Waste System). The process is based on filtration and ion exchange, using a cellulose-ion exchanger premix. The study has estimated the treatment capacity of the product used at Cernavoda NPP, depending on the composition of the waste. The data obtained were validated experimentally and a calculation algorithm was developed to establish the volume of real waste that can be treated efficiently in the decontamination system installed at Cernavoda NPP. Also, the addition of an advanced treatment step, using selective ion exchangers, to the decontamination system, was studied.