Effect of reverse sodium flux and pH on ammoniacal nitrogen transport through biomimetic membranes

Forward osmosis can be used to treat wastewater using seawater as the draw solution. This has been done for both water purification and nutrient concentration. However, the loss of ammoniacal nitrogen to the draw solution may be a key issue, reducing nutrient recovery and preventing the discharge of untreated seawater draw solution – a cost-saving strategy for the industrialisation of forward osmosis for wastewater treatment. In this study, forward ammoniacal nitrogen flux was studied using digester centrate from a wastewater treatment plant as the feed solution. The draw solution contained various NaCl concentrations in order to determine the effect of reverse sodium flux on forward ammoniacal nitrogen flux. The forward ammoniacal nitrogen flux was measured to be 1.5 × 10 ^–6 –8.0 × 10 ^–5 mol m ⁻² s ⁻¹ , and increased with pH and sodium concentration in the draw solution. The forward ammonium flux increased with draw solution reverse salt flux below pH = 9, whereas it was unaffected by this flux above pH = 9. Therefore, the reverse flux of sodium ions facilitates the forward transport of ammonium ions at low pH. The transport of the positively charged ammonium was lower than that of the neutral ammonia due to its higher hydrodynamic radius.