A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science, School of Environment, Resources and Development

Thesis (M.Sc.) - Asian Institute of Technology, 1999

A feasibility study was conducted from December 1998 to April 1999 to use wastewater from intensive hybrid catfish (Clarias macrocephalus x Clarias gariepnus) pond culture as fertilization for rice crop at the Center of Aquaculture Development and Training of Freshwater Aquaculture Department, Cantho University, Vietnam. In this experiment, hybrid catfish fingerlings with individual mean weight of 16.7 ± 4.0 g were stocked at density of 10 fish/m2 in a 400-m2 earthen pond. Hybrid catfish were fed with home-made feeds twice daily at feeding rates of 5 - 10% of body weight per day for 105 days. Water from the pond was pumped through a hose system to irrigate the rice fields. The quality of effluents from the pond was determined when supplying the rice fields. Rice plots in four treatments of rice cultivation with three replications each were conducted in twelve 50-m2 rice plots. Treatment 1 was irrigated with canal water without fertilizer input; Treatment 2 was irrigated with canal water and inorganic fertilizer inputs (nitrogen, phosphorus and potassium); Treatment 3 was irrigated with water from the catfish pond as the only fertilization input; Treatment 4 was irrigated with water from the catfish pond supplemented with phosphorus and potassium fertilizers. The nutrient contents in water from the hybrid catfish pond were higher than those of in the canal water. Total nitrogen and total phosphorus in pond water and were applied to the rice crop was 31.85% and 24.43% of total nutrient loading from feed to the pond water, respectively. The grain yields in Treatments 2, 3 and 4, ranging from 2.86 to 3.08 t/ha, were not significantly different (P>0.05). The highest filled grain yield was achieved in Treatment 4, which was not significantly different (P>0.05) from that in Treatment 3, but significantly higher (P<0.05) than that in treatments 1 and 2. There was no significant difference (P>0.05) in apparent nitrogen recovery from total nitrogen applied to Treatment 2 (17.32 ± 4.81 %), Treatment 3 (35.96 ± 22.21 %) and Treatment 4 (32.11 ± 20.63%). Apparent phosphorus recovery from total phosphorus applied in Treatment 3 (128.71 ± 73.97%) was significantly higher (P<0.05) than that in treatments 2 (25.35 ± 9.6%) and 4 (21.99 ± 8.97%). Nitrogen and phosphorus recovery was highest in Treatment 3 among all four treatments. Therefore, wastewater from the catfish pond can be reused for rice crop to reduce environmental impacts of intensive aquaculture effluents and lower fertilizer costs.