During Phase II pilot testing, the performance of the SRO unit, operated at either 50% or 68% secondary RO water recovery (i.e., 91% and 95% overall water recovery, respectively), was evaluated in terms of specific permeate flux, normalized salt passage, and normalized differential pressure drop. In the beginning of Phase II pilot testing, the SRO unit was operated at 50% water recovery by targeting an average feed and permeate flow rates of 11.6L/min and 5.8L/min, respectively (Fig. 6, region (i)). At this water recovery level, the SRO unit operated at consistent specific permeate flux and normalized salt passage (Fig. 6a and b, region (i)) of 1.2±0.1×10−8 m/s-kPa and 2.3±0.4%, respectively (i.e. the range reflected two times the standard deviation). In addition, the pressure drop for the retentate stream, across the SRO stage modules (Fig. 6c, region (i)), remained consistent at 224±11kPa, indicating that there was no significant occurrence of SRO feed spacers blinding caused bycolloidalparticledeposition(e.g.[33])orSROretentatechannel compression. The consistent SRO unit productivity and salt passage (to the permeate side) indicated sufficient removal of scale precursors during the demineralization stage (i.e., in the SCR) to enable total RO system water recovery of 91% without any indication of membrane scaling or fouling over a continuous operation of 830h.
After complete membrane element replacements in the SRO unit (during the SRO unit shut down period of 830–1100h), SRO desalting was carried out at 68% water recovery (resulting in95%overallproductwaterrecovery)bytargetingaveragefeed and permeate flow rates at 8.1L/min and 5.6L/min, respectively (Fig. 6, region (ii)). During this time period, the specific permeate flux declined from an initial value of ∼1.4m/s-kPa by about 44% over a period of ∼200h and stabilized at ∼0.8m/s-kPa for the subsequent ∼200h (Fig. 6a and b, region (iii)). This specific flux decline clearly indicated loss of membrane permeability due to membrane scaling/fouling, most probably due to poor SCR performance during this period (Fig. 5, region (ii)). In fact, SiO2 removal in the SCR during this period was very low, varying between 0% and 33% removal. Nevertheless, the specific permeate flux decline was not progressive, and both the normalized salt passage and normalized differential pressure remained relatively stable at 1.7±0.2% and 195±14kPa, respectively. After the above ∼400h of operation at 68% water recovery (1100–1510h), the SRO unit was shut down and flushed with service water (at 1510–1590h) without any membrane element replacement.
In subsequent SRO operation at 68% recovery, both feed and permeate flow rates were reduced to 7.7L/min and 5.3L/min, respectively (Fig. 6, regions (iii) and (iv)). The specific permeate flux and differential pressure drop were stable at 0.84±0.05×10−8 m/s-kPa and 182±15m/s-kPa, respectively, with a relatively consistent normalized salt passage of 2.9±0.7% over a total of 600h of SRO operation. The lower specific permeate flux and higher normalized salt passage (compared to those in the previous time period of 1100–1510h) signified the impact of operating with the previously fouled/scaled SRO membranes. Notwithstanding, no additional progressive membrane scaling/fouling occurred during the SRO operation at 1590–2350h as can be inferred from the consistent SRO unit productivity and salt passage (Fig. 6).
In order to identify the mineral salts that scaled the membranes in the SRO unit at the high overall product water recovery of95%,membraneautopsieswereconducted(attheendofPhase II test period) on the first, fourth and sixth elements of the secondary RO unit. SEM imaging (Fig. 7a) of the lead element (first element) revealed a patchy, discontinuous inorganic layer consisting predominantly of silicon as revealed by EDS analysis. This foulant layer appeared relatively uniformly across the membrane sample. Micrograph from the fourth element (Fig. 7b) suggested early stages of CaSO4 scaling as evidenced by the crystalline structure and the strong SO42+ and Ca2+ peaks obtained in the EDS scan. EDS analysis on a membrane sample from the sixth element (Fig. 7c) also indicated the presence of Al3+ and silicon, suggesting the possible presence of an aluminum silicate material. Aluminum silicate materials have been previously shown to be problematic in other CRW desalting tests [14,33]. It is noted, however, that BaSO4 scaling was not detected despite of its high oversaturation in both the PRO and SRO concentrate streams, possibly due to the wide metastable solubility zone of BaSO4 [3,4] and the effective action of the added antiscalants. The membrane autopsy results suggested that membrane fouling and scaling occurred most probably due tosomeperiodsoflowcalciumandsilicaremovalintheSCRand thus some periods of high CaSO4 and SiO2 saturation indices in the SRO reject stream (Section 4.2; Fig. 5c and e). Clearly,
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