Ammonia tolerance of filamentous algae Oedogonium, Spirogyra, Tribonema and Cladophora, and its implications on wastewater treatment processes

Filamentous algae have high potential for wastewater treatment due to their direct uptake of nitrogen and ease of harvesting. Theoretically, filamentous algae are best applied upstream to directly remove total ammonia nitrogen (TAN = ammonia + ammonium) from wastewater, however their tolerance to toxic free NH3 remains unclear. This study investigated the ammonia tolerance of four filamentous algae species from the Oedogonium, Tribonema, Spirogyra and Cladophora genera. The algae were cultivated using buffers at pH values from 7.0 to 9.0 at 15 °C and 25 °C using either 60 mg-N/L nitrate or ammonium as the nitrogen source. Using nitrate, an increase in productivity as a function of pH was observed for all four species. Comparison of the results and the corresponding experimental conditions with previous studies suggested that the effect of algal productivity enhancement via CO2 addition lies heavily on carbon provision rather than direct pH control. In the presence of ammonium, Oedogonium, Tribonema and Spirogyra showed stable growth at low pH followed by a sharp decline in productivity above certain pH thresholds, while Cladophora showed a very low growth rate irrespective of pH. At 60 mg-N/L initial TAN, the critical pH range was found to be approximately 8.0–8.6, 7.5–8.3 and 7.5–8.0 for Tribonema, Oedogonium and Spirogyra respectively. The critical threshold calculated based on the initial amount of free NH3 was 1.5–3 mg-N/L for Tribonema and Oedogonium and approximately 1 mg-N/L for Spirogyra. Oedogonium was also cultivated under TAN at pH 7.5 and 15 °C over three weeks, showing stable growth and capability to utilise TAN under controlled conditions. Nevertheless, a 30 % nitrogen loss, presumably due to ammonia volatilisation, was also observed. Based on the results of this study, it was concluded that the use of filamentous algae for downstream wastewater treatment rather than direct TAN removal may be a more practical option.