Nitrogen conversion efficiency in the integrated catfish farming system toward closed ecosystem in Mekong delta, Vietnam

In this study, the nitrogen conversion efficiency in the integrated catfish farming system was estimated based on the sludge and wastewater treatment processes. The nitrogen sources were recovered using the composting process (organic fertilizer). The wastewater treatment process was recovered using the aquatic plant uptake (water hyacinths). The sludge waste was mixed with cow dung and water hyacinth biomass at a ratio of 8:1:1. The compost improved the nutrient content of the poor soil, as well as providing additional nitrogen for the plant and limiting the loss of nitrogen sources in the environment. The nitrogen dynamics were estimated using the nutrient biomass flow equality method. The results show that nitrogen was successfully recovered through the composting process and via accumulation in the biomass of the water hyacinths. The nitrogen removal efficiency by using water hyacinth reached 60% after 25–30 days; nitrogen recovery efficiency was found to be 66.94%; and total nitrogen concentration in the soil increased by 53.5% after applying organic fertilizer. The system could be used to recover and reuse nitrogen within the integrated catfish farming circle, under the context of circular economy concept.