| Abstract | Reuse of domestic wastewater for fish cultivation has a long tradition in Yen So commune, Hanoi city. There are two systems of wastewater-fed fish pond existing in the commune: fish-only culture and rice-fish culture pond. Three types of cultivated fish are common carp, silver carp and tilapia. The research was conducted at two wastewater-fed fish ponds which receive directly the untreated wastewater of Hanoi city through Kim Nguu river. Performance of the wastewater-fed fish ponds in removal of the contaminants such as SS, BOD, total nitrogen, total phosphorus, fecal coliform and helminth eggs were found relatively high at the rates of 37,8% for SS, 60.4% for BOD, 75.2% for total nitrogen, 46.9% for phosphorus, 98.3% for fecal coliform and 98.3% for helminth eggs (with retention time of 7 days). Performance of the fish pond in removal of the contaminants between dry and rainy season vary with different contaminants. Removals of nutrients (nitrogen and phosphorus) and fecal coliform in the dry season are higher than those in the rainy season, whereas removals of SS and BOD are lower. For helminth eggs, there is no different of removals between two seasons. The physicochemical quality of the effluents is good, meeting the Vietnamese Standard of Wastewater Discharge. Nevertheless, the microbiological quality is not so good: the effluents contain total coliforms with concentration of 3.72 x 10⁵ CFU/100mL for the large pond and 4.81 x 10⁵ CFU/100mL for the small pond while the discharged standard is 5 x 10³ CFU/100mL. In addition the effluents are not safe for reuse in irrigation because of the higher concentrations of fecal coliform and helminth egg in the water in comparison to the recommended values of WHO's Guidelines for safe use of wastewater, excreta and greywater. With an area of 17 ha of the pond, it could remove the contaminants at a rate of 0.7% for SS, 2.4% for BOD, 1.3% for nitrogen, 0.5% for phosphorus and 0.2% for total coliform of the total loading of the contaminants in Kim Nguu river's wastewater. Heavy metals including copper (Cu), lead (Pb), zinc (Zn), cadmium (Cd), mercury (Hg) and arsenic (As) have been found in the influents at the concentrations of 3.80 ug/L, 4.31 ug/L, 25.9 ug/L, 0.84 ug/L, 0.48 ug/L and 2.85 ug/L respectively. The reduction of the heavy metal contents in the effluent suggests that a certain amount of the heavy metals is retained in the ponds and accumulation in fish tissues. The contents of Cd, As and Pb have been found in muscle, liver and skin of three types of fish raised in the fish ponds which mostly were low compared to EU threshold value or the Codex Threshold value. Fecal coliform has been found in pond water at concentration of 1.19 x 10⁵ CFU/100mL for large pond and 3.12 x 10⁵ CFU/100mL for small pond which may result in contamination of this pathogenic micro-organism in fish meat. The content of fecal coliform were found in skin, gut and flesh of three types of fish raised in the fish ponds but most concentrations in the flesh were lower than Vietnamese standards for microbiological safety of fish. It is recommended that the increase of wastewater loading should be considered to improve the performance of the ponds. The effluents should not be reused directly for irrigation because of the risks of pathogen contamination in the irrigated products. The fish should not be eaten in raw fish and some protective measures should be considered with respect to food safety for human health such as depuration. Further studies should focus on the improvement of the effectiveness of the system in term of wastewater treatment and safety in food production. |