3.2. Variation of the Weight of the Acidic Solution
The acidic solution decreased in weight in the three GPM systems (Figure 3
). Total weight losses of the acidic solution at the end of the experiment were 11 ± 2% for ZM (R2 = 0.89), 10 ± 4% for PM (R2 = 0.95), and 5 ± 1% for FZM (R2 = 0.99). Weight losses in all cases were related with an evaporation process as leaks were not observed. The rate of water weight loss (g·d−1
) for each type of membrane was: 16.1 ± 6.1 g·d−1
for PM, 17.8 ± 2.9 g·d−1
for ZM, and 7.5 ± 1.2 g·d−1
for FZM. The hydrophobic nature of the membrane prevents the penetration of the acidic solution into the membrane pores, creating a liquid/vapor interface at each pore entrance. If a vapor partial pressure difference across the membrane is established, vapor transport across the membrane takes place [34
Majd et al. [23
] also observed volume losses in acid traps due to evaporation, with values between 1 to 2 mL·d−1
in suspended systems, with an acid volume of 190 mL and a flow rate 3 times lower than that used in this experiment.
The rate of weight loss of the acid was not affected by membrane density, porosity, and permeability; however, it was affected by surface area. Weight loss was higher in the two membranes (ZM and PM) with the larger diameter and surface area, even though they had different porosity and density, and the weight loss was lower in the FZM membrane with smaller diameter and surface area. Therefore, the greater surface area resulted in higher vapor transport across the membranes and acid weight loss.
3.4. Effect of the Type of Membrane on Ammonia Capture
The total NH3
–N mass emitted by the synthetic solution was similar in the three membrane systems: 5381 ± 451 mg N for PM membrane, 5260 ± 514 mg N for ZM, and 4764 ± 606 mg N for FZM (Table 3
Corresponding percent N removals were 46 ± 4%, 45 ± 4%, and 41 ± 5%. Similarly, no differences were observed in the total mass of NH3
–N present in the synthetic solutions at the end of the experiment (Figure 4
The ammonia emission rate of the synthetic solution varied with time. There was a higher emission rate on the first day and a lower and almost constant emission rate in later days. For example, rates of emission the first day were 4138 ± 47 mg NH3
for PM, 3555 ± 433 mg NH3
for ZM, and 3342 ± 463 mg NH3
for FZM, and afterwards the rates of emission were 207 ± 83 for PM, 284 ± 158 for ZM, and 237 ± 24 mg NH3
for FZM. This emission behavior was also observed by Rothrock et al. [23
] who noted that in the first 7 days, the concentration of NH4
–N present in the synthetic source solution decreased faster, from 500 mg to 300 mg approximately. In contrast, from days 7 to 21, the concentration only decreased from 300 mg to 200 mg. The high recovery observed on the first day could be due to the high concentration of ammonium in the synthetic solutions. This generates a high concentration of ammonia in the gas phase. After the first day, a significant percentage of ammonium had been eliminated and, therefore, the driving force for transport decreased.
The masses of NH3
–N recovered in the acidic N trapping solution were 3628 ± 27 mg, 3407 ± 49 mg, and 2661 ± 307 mg for ZM, PM, and FZM, respectively. At similar emission and capture conditions, the NH3
–N mass recovered by FZM was lower due to a lower surface area compared to the membranes ZM and PM. The surface area was 4.2 times higher for PM and ZM compared to FZM. Surprisingly, the mass of NH3
–N recovered was not affected by large differences in material density (0.45 to 0.95 g/cm3
) between PM and ZM (Table 3
), or by differences in porosity (5.6 to 21.8%), air permeability (2 to 10 L·min−1
at 1 bar pressure), and wall thickness (0.8 to 1.2 mm). This was surprising because it is logical to think that higher NH3
capture should be obtained with higher membrane porosity and air permeability, and with smaller wall thickness [35
]. However, in the range tested in the experiment, these characteristics did not affect mass of NH3
recovered by the membranes.
In all membrane systems, NH3
–N accumulation in the acidic solution during the 7-day experimental period was linear (Figure 5
). Capture rates (mg NH3
) were calculated based on the slope of the linear regressions; they were higher with PM and ZM (487 ± 71 and 518 ± 4 mg NH3
, respectively) with larger diameter and surface area, compared to FZM (380 ± 44 mg NH3
) with smallest diameter and surface area. The acidic solution had a similar composition of 0.3 ± 0.1% of nitrogen and 0.4 ± 0.1% of sulphur.
recovery (%) for each type of membrane was calculated based on the relationship between the NH3
–N mass recovered (final content of NH3
–N in the trapping solution) and the NH3
–N mass removed (difference between the initial and final content of NH3
–N in synthetic solution). Percent recoveries were not different (p
≤ 0.05): PM = 63%, ZM membrane = 69%, and FZM = 57%. The percent recoveries were not quantitative (100%) probably because the rapid release of NH3
in the first day of the experiment exceeded the capacity of the membrane. Other authors such as Rothrock et al. [23
] obtained similar results than under conditions of an NH3
emission flush. They achieved recoveries of NH3
–N of 67.7%, 73.6%, and 76.2% with hydrated lime addition treatments of 0.4 w/v, 2 w/v, and 4 w/v to 300 g of poultry litter. Therefore, design of the membrane manifolds should consider possible situations of rapid release that may occur in filed situations such as disinfection of manure with alkali compounds.
On the other hand, when the NH3
–N capture is expressed on a surface area basis (N-flux, Table 3
), the results show additional insight on the best operating conditions for the membranes. The N-flux obtained in the membrane FZM with the smaller diameter (5.8 ± 0.7 mg N·cm−2
) was significantly higher—approximately 3 times higher—compared with the N-flux obtained with the larger diameter membranes (1.8 ± 0.0 and 1.7 ± 0.2 mg N·cm−2
with ZM and PM, respectively). Rothrock et al. [23
] also observed higher N-fluxes in membranes with a smaller diameter (1.37 g·m−2
N-flux for a membrane i.d. of 4.0 mm and acid flow 70–80 mL d−1
and 0.7 g·m−2
N-flux 0.70 for a membrane i.d. of 8.8 mm and same flow). Majd and Mukhtar [25
] observed an N-flux of 0.2 g·m−2
in a suspended membrane system. However, higher ammonia fluxes have been obtained when the membranes were directly submerged in the liquid (liquid–liquid) instead of being suspended in the air (air–liquid). For example, Daguerre et al. [36
] obtained N-fluxes of 7.1 to 8.9 g·m−2
placing the membrane manifold in liquid swine manure (4940 mg NH4
), and Fillingham et al. [37
] obtained N-fluxes up to 51.0 g·m−2
using synthetic wastewaters containing 6130 NH4
and NaOH to pH 8.5.
In this study (experiment 1), the same recirculation flow of the acidic solution (1.25 L h−1
) was used with the three membranes with outside diameters ranging from 3 to 8.6 mm (inner diameters 1 to 7). As a result, the smaller diameter resulted in a higher fluid velocity inside the membrane (2653.9 cm min−1
) compared to the higher diameter membranes (54.2 to 69.0 cm min−1
) (Table 3
) and a more frequent renovation of the acidic solution in the submerged membrane manifold.
The Reynolds number (Re) is used in fluid dynamics to describe the character of the flow (flow is laminar when Re < 2300 and viscous forces are dominant characterized by smooth fluid motion, and flow is turbulent when Re > 3000 and it is dominated by inertial forces and vortices). Although the fluid flow was laminar in all three cases (Re 64 to 415, Table 3
), the higher fluid velocity and Re in FZM resulted in a higher N-flux. Therefore, to optimize the effectiveness of the ePTFE membranes to capture gaseous ammonia, the fluid velocity should be an important design consideration because this study showed that the efficiency can be increased 3 times with changes in acidic solution velocity.
A second experiment was done to further evaluate the positive effect of fluid velocity on N-flux. The study used two recirculation rates (0.83 and 1.5 L h−1
), and two membrane types with different diameters (id 2.9 and 6.2 mm) but the same surface area (125.5 cm2
) (Table 4
). As a result, the fluid velocity inside the membranes gradually increased in the range of 49 to 315 cm min−1
and Re varied from 49 to 155. These modest differences in fluid velocity and Re (within laminar flow) significantly affected both the mass of NH3
–N recovered in the acidic N trapping solution, and the N-flux (recovery per surface area). Figure 6
shows the relationship between N flux vs Re obtained using combined data from all seven treatments in experiments 1 and 2. It confirms that velocity of the circulating acidic solution should be an important design consideration to optimize the effectiveness of GPM system to capture gaseous ammonia.