Recovery of ammonia from centrate water in urban waste water treatment plants via direct contact membrane distillation: Process performance in long-term pilot-scale operation

Our current industrial ammonia cycle is far from being sustainable - while intense energy is required to produce ammonia - almost half of it ends up in wastewater treatment plants (WWTPs) where is finally transformed and lost. This work shows the application of flat sheet membrane distillation (MD) commercial modules for the ammonia recovery from WWTPs. First, optimized operating conditions - in terms of ammonia flux, specific thermal energy (STEC) and chemical demand - were investigated in the laboratory with a 2.3 m2 MD module. Second, optimized conditions were demonstrated in a pilot installation on site operating a 14.5 m2 MD module continuously (24/7) for three months. Results showed that MD is a robust, low-maintenance technology that can be operated at low temperature and corresponding STEC (i.e. 38 degrees C and 13.6 kWhth per kg1 NH3, respectively) and low pH (i.e. 8.7) for the recovery of 90% of the water-bound ammonia as an ammonium sulphate (AS) solution. The AS product reached a concentration of 5 g l- 1 N-NH4, lower than conventional fertilizers, however given its high quality and volume reduction factor, it constitutes a potential fertilizer solution for local needs. The maximum AS permeate concentration was limited by the increasing water vapour flux due to a higher osmotic distillation effect and a minor ammonia flux decrease with increasing permeate concentration. Further research will focus on optimizing the module configuration to minimize water flux and overall system operation for increased heat recovery and open-loop operation.