All experiments were carried out in

All experiments were carried out in a continuous and non- recirculation mode (Fig. 1), i.e. under non steady-state conditions, using a flat stainless steel high pressure cell, as described in Cancino Madariaga et al. (2011). The high pressure cell was developed and designed by the GKSS Research Centre (Geesthacht, Germany) and included some modifications done at the Laboratories of the Inpromem (Chile). To drive the flow, a high pressure Speck GmbH pump (Germany) working between 0 and 140 bar was included, and to control temperature a Chiller, RESUN, model C-1500 was used (Cancino Madariaga et al., 2011). In the non recirculation mode, the permeate flow is discharged of the system and the concentration increase through the time, i.e. the membrane is used to concentrate the ammonia. The ammonia concentration is determinated in the initial solution and in the permeate through the time to observe if the ammonia go through the membrane. The idea is to concentrate the ammonia, obtaining a free ammonia permeate.
Ammonia concentration was measured using a multiparame- ter photometer Hanna Instruments model HI-83000 (resolution of 0.01 mg/L and deviation of ±0.02), used with an special kit and the tungsten lamp at 420 nm based on Nessler method. All the analyze were made in triplicate and the results were processed statistically using the commercial software centurion Statgraphics Centurion XVI (StatPoint, USA, 2010).
Four commercial membranes were used: NF 90, NF 200 and NF 270 from Dow Liquid Separations, USA and DSHR98PP provided by Alfa Laval, Denmark. Membrane characteristics, membrane pre-treatment and working conditions were described in Cancino Madariaga et al. (2011). The main characteristics of the membranes are showed in Table 2.
The initial ammonia model solution was prepared using distilled water with a conductivity lower than 5