Optimisation of a forward osmosis and membrane distillation hybrid system for the treatment of source-separated urine

Abstract

The high concentration of nitrogen, phosphorous and potassium in human urine makes it a suitable raw material for fertiliser production. However, urine is often diluted with a significant amount of flushing water which increases the costs for the downstream nutrients recovery process. Re-using the water and the nutrients in the urine is paramount for enhancing the sustainability of our waste management system. In this work, a combination of forward osmosis (FO) and membrane distillation (MD) was used to extract distilled water from human urine. FO was chosen as MD pre-treatment to increase the overall nitrogen rejection and to prevent wetting of the MD membrane. The goal of this investigation was to tune the FO and MD operating parameters to reduce the nitrogen transport to the MD permeate. Urine pH, draw solution (DS) salt concentration and operating pressure were varied as a means to enhance the FO performances. On the other hand, feed temperature, nitrogen concentration and membrane characteristics were investigated to optimise the MD process. With 2.5 M NaCl as DS commercial FO membranes achieved a water flux between 31.5-28.7 L.m-2 .h-1 and a minimum nitrogen flux of 1.4 g.L-1. An additional 33% reduction in the nitrogen transport was observed by applying minimal hydraulic pressure on the DS. However, this was also found to significantly reduce the net transmembrane water flux. Acidification of the feed was also beneficial for both FO and MD nitrogen rejection. Finally, we demonstrated that, by tuning the MD membrane porosity and thickness, higher MD permeate quality could be achieved. To conclude, the hybrid FO-MD process is expected to be an effective solution for the production of clean water and concentrated fertiliser from human urine. This double barrier separation process could be suitable for both water reclamation in space application and resource recovery in urban application.

Membrane desalination and water re-use for agriculture: State of the art and future outlook

“…Widespread studies on MD hybrid processes in various applications have been reported for its high contribution to treat complex impaired water, high rejection towards different organic and inorganic components, improve water recovery, recover valuable nutrients, alleviate fouling/scaling, reduce brine disposal, low energy if natural power source used, and cost-effective [113]. It has been utilized in many fields such as seawater desalination, wastewater treatment, agriculture, oily wastewater, landfill leachate, and pharmaceutical industry [113,114]. Because the commercial membrane provided high quality and stable permeate, thereby improving the efficiency of the processes and preventing the drawbacks that hampered its realization in largescale operations [114].…”

Forward Osmosis as Concentration Process: Review of Opportunities and Challenges

“…In that context, the improved rejection of nitrogen (50−80%) in hydrolysed urine was observed in comparison with the low rejection of neutral organic nitrogen (urea-N) in fresh urine [ 168 ]. A pH adjustment of around 7 also favoured the higher rejection of ammonium (around 80%) [ 172 ] by avoiding the stripping of ammonia occurring at higher natural pH of urine (above 8) and limiting potential membrane scaling [ 173 ]. The concept has been integrated within a FO-MD hybrid for the simultaneous production of fresh water and urine concentration [ 173 , 174 , 175 ].…”

“…A pH adjustment of around 7 also favoured the higher rejection of ammonium (around 80%) [ 172 ] by avoiding the stripping of ammonia occurring at higher natural pH of urine (above 8) and limiting potential membrane scaling [ 173 ]. The concept has been integrated within a FO-MD hybrid for the simultaneous production of fresh water and urine concentration [ 173 , 174 , 175 ]. Given the moderate rejection of urea by the FO membrane, Volpin et al combined FO concentration processes with fertigation using a Mg based fertiliser as a draw solution.…”

Low-Temperature Direct Contact Membrane Distillation for the Treatment of Aqueous Solutions Containing Urea

“…The integration of FO with DCMD was also investigated by Volpin et al [ 6 ] who optimized the FO and DCMD operating parameters to reduce the nitrogen content in the produced distillate. In particular, for the DCMD unit, the effect of the feed temperature (from 40 to 60 °C) and membrane properties were analyzed, while keeping the cross-flow velocities (8.5 cm/s) and the temperature at the distillate side (20 °C) both constant.…”