3.1. Pre-Treatment Processes
The appropriate dose of Na2
O for an efficient softening process was investigated experimentally. Sodium carbonate promotes the removal of Ca2+
ions through reactive precipitation as CaCO3
. Therefore, the proper Na2
addition was determined with respect to the Ca2+
ion concentration in the FGD wastewaters to be treated. The obtained results (Figure 1
) prove that Ca2+
removal efficiency increased by growing the coagulant dosage until Na2
molar ratio between 2.27 and 3.18, then a plateau is reached. In particular, in this range the removal of Ca2+
achieved up to 90%, whilst the removal of Mg2+
, at the same coagulant dosage, was of about 30%. The pH of the FGD wastewaters increased from 9.2 to 10.6 in the range of investigated Na2
molar ratio due to the increased release of OH−
ions in the solution.
Ayoub et al. [20
] investigated the removal of inorganic compounds (including calcium and magnesium) in seawater using Na2
as alkalizing agents and obtained similar results. In particular, the removal efficiencies recorded were of 90 ± 2.5% for Ca2+
and 15 ± 4.55% for Mg2+
. A similar Ca2+
removal efficiency (higher than 90%) was observed also by Zheng et al. [21
] during the treatment of textile and dyeing wastewaters using NaOH. Based on these experimental results, an average Na2
molar ratio of 2.6 was chosen for performing the softening.
Before RO, softened FGD wastewaters were preliminary submitted to a UF process. In the selected operating conditions steady-state UF permeate fluxes of about 480 kg/m2
h were obtained. The composition of FGD wastewaters before and after the chemical and the UF treatments is reported in Table 2
. As expected, in the optimized Na2
molar ratio conditions a higher removal of Mg2+
was obtained whilst, the removal of other analyzed compounds was lower (in the range 11–27%). The UF process, allowed to remove more than 60% of TOC, while the content of inorganic compounds and TDS remained unchanged in agreement with the MWCO of the selected UF membrane.
3.2. RO of Pre-Treated FGD Wastewaters
shows the time evolution of permeate flux for both RO membranes in the treatment of the softened and ultrafiltered FGD wastewater, in the selected operating conditions. Despite different operating TMP values, RO membranes showed similar initial permeate fluxes values (of about 14 kg/m2
h). The permeate flux declined rapidly in the first 30 min; then the rate of flux decline became slower and a pseudo-steady state flux of 2.3 kg/m2
h and 1.9 kg/m2
h for the ESPA and SWC membranes respectively, was reached. This behavior has to be attributed to the increase of the osmotic pressure of feed solution due to the high salt rejection of the selected membranes, and to the concentration polarization and membrane fouling phenomena. This leads to a severe driving force decrease and, consequently, to a flux decline [22
]. In particular, the observed flux decline of selected membranes was in the range of 84–86%.
A similar permeate flux decline was observed by Vourch et al. [23
] in the treatment of dairy industry wastewaters for water reuse, with a RO spiral wound membrane (TFC HR SW 2540) in thin film composite, having a NaCl rejection of the same order (99.5%).
Dolar et al. [24
] obtained a permeate flux decline of about 72% during processing of pre-treated landfill leachate with a RO (XLE) membrane from Dow/Filmtec (Midland, MI, USA).
A street correlation between the concentration of feed solution and permeate flux decline was reported by Tang et al. [25
] in the processing of semiconductor wastewaters with a RO ESPA membrane. They obtained a reduction of permeate flux of 10% at a concentration of feed solution of 10 ppm of perfluorooctane sulphonate (PFOS) and of about 60% at 500 ppm of PFOS. Authors attributed the flux decline with the entrapment of PFOS molecules in the polyamide layer and their accumulation on the membrane surfaces.
shows the fouling index for the selected membranes based on their water permeability before and after the pre-treated FGD wastewaters filtration. According to the obtained values, the ESPA membrane showed a lower fouling index (FI, 28.8%) in comparison with the SWC membrane (FI, 35.3%). This behavior could be attributed to the different morphology (surface roughness) and contact angles of selected membranes (Table 1
). As reported in literature, membranes with smooth and hydrophilic surfaces (as the ESPA) presented less fouling tendency than those with rough and hydrophobic surfaces (as the SWC) [26
After cleaning with water, the water permeability of both membranes was lower than 10% when compared to the initial one. After chemical cleaning, a complete recovery of the initial water permeability was observed for the SWC-2540 membrane and of about 92% for the ESPA 2540 membrane. The low fouling index measured for both membranes and the almost complete recovery of RO membranes performance could be attributed to the pre-treatment processes (softening, precipitation and ultrafiltration) that limited inorganic scaling and the possible deposition of inorganic substances on membrane surface or inside the membrane pores [26
The selected membranes were also compared in terms of permeate quality and removal of salt compounds. The chemical composition of permeate and retentate fractions produced with both RO membranes, at a WRF 2, is reported in Table 4
The electrical conductivity rejection for ESPA-2540 and SWC-2540 membranes, were of about 85.7% and 93.2%, respectively (Figure 3
). The TDS rejection of the SWC-2540 membrane was relatively higher (93%) if compared with the ESPA-2540 membrane (87%). In addition, the SWC-2540 membrane showed a better performance in the rejection of ions. Mg2+
ions were completely rejected by the SWC-2540 membrane (R of 100%), while the rejection towards monovalent ions such as Na+
was lower (R of about 95.5%). The ESPA-2540 membrane showed rejections towards Ca2+
higher than 86.5%; on the other hand, the observed rejection towards Na+
was of 80%. Considering that divalent ions are larger than monovalent ions, the main mechanism of ion rejection by RO membranes is size exclusion [27
]. However, the charges of ions and membranes employed cannot be neglected since they also interact each other electrostatically. At the pH of FGD wastewaters (6.5) both membranes are negatively charged [18
]. Therefore, electrostatic attraction forces between the negatively charged membrane surface and specific ions can contribute to the retention mechanism.
In Table 5
and Table 6
the mass balance of the RO process for both investigated membranes is reported. The balance is referred to RO experimental runs in which starting from 18 L of pre-treated FGD wastewaters, 9 L of permeate and 9 L of retentate were obtained. It can be noted that for both RO membranes, the main ions are mainly concentrated in the retentate streams; on the other hand, the amount in the permeate is lower than 10%. The balance is complete, indicating that no interaction ions-membranes, or adsorption of ions on the membranes surface, occurs during the process.
According to the obtained results, the SWC-2540 membrane exhibited the highest removal efficiency for all measured parameters. Therefore, the quality of the produced permeate was higher if compared with the permeate produced by ESPA-2540 membrane. The SWC-2540 permeate presented an electrical conductivity lower than 2 mS/cm; it resulted completely depleted of bivalent ions such as Mg2+, with a low amount of total dissolved solids (TDS) (<1 g/L) and Ca2+.
3.3. Experiments in Total Recycle Mode
For the SWC-2540 membrane experiments were also performed according to the total recycle configuration in order to evaluate the effect of TMP on the permeate flux and the removal efficiency of salt compounds. Figure 4
shows the time evolution of the permeate flux at different TMP values (increased in the range 16–50 bar) and fixed values of temperature and feed flowrate (25 °C and 240 L/h, respectively). An increase of initial and steady-state permeate fluxes from 0.86 kg/m2
h to 16.8 kg/m2
h and from 0.61 kg/m2
h to 10.4 kg/m2
h, respectively, was observed by increasing the operating pressure in the range of investigated values.
Mohammadi et al. [28
] reported a linear increase of permeate flux with pressure in the treatment of seawater with a polyamide FT30 RO membrane. A linear trend between permeate flux and pressure was also observed by Mondal and Wickramasinghe [29
] in the treatment of oily wastewaters with a BW 30 membrane (FilmTec Corporation, Minneapolis, MN, USA) in aromatic polyamide with a NaCl rejection of 99.4%.
A different behavior was reported by Liu et al. [30
] in the treatment of textile effluents for water reuse with the same membrane (BW30). In this case, a decrease of permeate fluxes by increasing the operating pressure, was observed.
From Figure 4
it is also possible to note that working at high pressure values (higher than 20 bar) there is an increase of the permeate flux decline by increasing the operating time. As previously discussed, this behavior could be attributed to membrane fouling and concentration polarization phenomena. Particularly, working at high pressure, concentration polarisation is known to be the main factor that contributes to the increasing salt concentration at the membrane surface hence leading to particle deposition and an increasing of scaling [31
In Figure 5
the effect of TMP on the rejection towards main ions and other analyzed compounds, is showed. It can be noted that the rejection of Mg2+
was not affected by TMP: indeed, the rejection was of 100% independently by the applied pressure. Adversely, an increase of rejection by increasing the operating pressure was observed for Ca2+
. In particular, the rejection for Ca2+
increased from 32.3% and 82.2% at TMP of 16 bar to 78% and 98% at TMP of 50 bar, respectively. A similar trend was observed for TDS rejection. Similar results, in terms of TDS rejection, were observed by Nataraj et al. [32
] in the removal of dye and salts from simulated wastewaters with a TFC polyamide RO membrane in spiral-wound configuration.
The increase of the rejections by increasing the TMP could be attributed to the increase of solvent flux at higher pressure, resulting in the decreasing of salts concentration and ions in the permeate. On the other hand, during the RO treatment more components (ions and salts) are transported from the bulk solution towards the membrane surface as permeate flux increases, which enhance concentration polarization and consequently the faster flux decline (from the initial to the steady-state values) at the increasing of TMP.