This research investigated the effectiveness of water lettuce (WL; Pistia stratiotes L.) in improving the quality of wastewater from biogas systems. Two treatments were designed, one without WL and the other with WL. First, WL were raised in containers that had 15 L of wastewater with an initial ammonium concentration of about 15 mg/L at the still-water stage (days 0–7). Then, at the running-water stage (days 10–22), wastewater with a targeted NH4 + -N concentration of about 15 mg/L in a 5-L tank was gravitationally delivered continually into terraced Styrofoam containers designed as ponds 1, 2 and 3. Water samples were collected on days 0, 3, 7, 10, 13, 16, 19 and 22, and fresh weights of WL were measured on the same days of sampling the water. The results showed that at the still-water stage, WL contributed to the reduction of chemical oxygen demand (14.74 ± 4.14% and 8.69 ± 0.92%, respectively), total inorganic nitrogen (23.93 ± 2.35% and 12.80 ± 1.30%, respectively), ammonium (25.21 ± 5.44% and 1.12 ± 0.93%), nitrite (59.98 ± 3.22% and 22.37 ± 1.21%, respectively) and phosphate (71.84 ± 0.89% and 61.64 ± 1.65%, respectively) on days 0–3 more than on days 4–7 but did not help decrease nitrate concentrations on days 0–7. WL contributed to reducing organic matter less at the running-water stage than at the still-water stage. WL helped lower ammonium, nitrite and nitrate concentrations at the running-water stage more than at the still-water stage but did so more for ammonium and nitrate compared with nitrite at the running-water stage. No differences in pollutant concentration reductions between the two treatments (without and with WL) were found in ponds 1, 2 and 3. On days 10–22, no clear trend in increasing or decreasing pollutant concentrations emerged, except nitrite concentration, which lessened over time. No significant differences in the relative daily WL fresh biomass increase between the still-water and the running-water days were observed. The findings indicate that WL is an aquatic plant that can be used in treating wastewater from biogas systems, showing a high efficiency in lowering phosphorus concentrations and a potential for removing nitrite.