Pilot demonstration of nitrogen removal from municipal wastewater by vacuum membrane distillation

Nitrogen in municipal wastewater must be removed prior to discharge to the environment. However existing energy intensive aerobic processes used for nitrogen removal currently do not capture this important resource. Instead, nitrogen capture in the form of ammonia using a lower energy technology such as the membrane distillation; so far mostly tested in lab-scale, could be a significant improvement. This study reports a vacuum membrane distillation (VMD) pilot trial conducted at a municipal wastewater treatment plant to uniquely demonstrate ammonium removal and recovery to meet effluent discharge targets. The pilot plant was located in two streams on-site in the plant's anaerobic digester liquor handling facilities. The first trialed stream was on the existing phosphorus removal lime dosed effluent, and the second stream was effluent from an experimental struvite phosphorous recovery operation. In the post-lime scenario, VMD recovered N as ammonium sulphate solution and achieved up to 93% reduction. In the post struvite scenario, the VMD system achieved up to 85% reduction within the duration of the demonstration. With VMD driving force derived from volatility of the compound of interest, which is reliably calculated via Henry's law, the ammonia mass transfer coefficient across the membrane was 0.16 kg/m2/h/bar (±37% RSD) over the trial period, being lower than typical best case laboratory test results. Overall, the nitrogen removal and recovery process via VMD was proven technically viable on actual plant effluent leaving existing lime dosed and potential future struvite phosphorus removal systems. However, further studies are required to assess the economics of VMD for nitrogen removal compared with other nitrification-denitrification methods used in wastewater treatment.