Abstract Peat (Mekong River Delta, Vietnam) was naturally dried, and then mixed with coal slag in the ratio of 70:30 by weight to use as adsorbent to treat wastewater from Pangasius catfish ponds in batches. The adsorbent was used in a specific designed system to treat Pangasius pond wastewater in a continuous flow model at a speed of 0.3 m 3 .h −1 . The treatment system has operated for 9 consecutive hours in each experiment batch. The results show that the system can treat a large amount of wastewater (64.8 m 3 ) with the efficiency (%) of BOD5 of 95.78%, COD of 95.3%, total phosphorous of 73% and total nitrogen of 89.79%. After treatment, the quality of wastewater has met the standards to be discharged into the environment.