Nutrient removal by duckweed from anaerobically treated swine wastewater in lab-scale stabilization ponds in Vietnam.

In Vietnam, swine wastewater is generally treated using anaerobic processes. Nevertheless, the level of pollutants in effluent after anaerobic treatment remains very high, thereby necessitating further treatment. This research was conducted to assess the applicability of duckweed (Lemna minor) for purifying wastewater collected from a household swine wastewater treatment system in Hanoi, Vietnam. After the anaerobically treated wastewater was diluted 10-fold, it was fed continuously to lab-scale stabilization ponds with and without planted duckweed at a hydraulic retention time of 5 days under ambient conditions. The chemical oxygen demand (COD), total nitrogen (T-N), and total phosphorus (T-P) concentrations in the influent were, respectively, 260-290 mg/L, 24-28 mg/L, and 1.4-1.8 mg/L. The COD, T-N, T-P removals in the pond with duckweed (74%, 84%, and 84%) were much higher than in the pond without duckweed (71%, 55%, and 58%). The duckweed greatly enhanced the first-order removal rates by 1.4, 2.0, and 3.2 times, respectively, for COD, T-N, and T-P in the ponds. Although the primary purification mechanisms in the ponds were sedimentation and adsorption, the duckweed grown with the relative growth rate of 0.07-0.16 d showed nutrient uptake activity from the wastewater. Biofilms formed on the duckweed roots apparently promoted COD removal and degradation of organic nitrogen into ammonia. Stabilization ponds planted with duckweed are anticipated for use as co-beneficial systems for wastewater treatment and biomass production.