Journal of Membrane Science 301. High-recovery reverse osmosis desalination using intermediate chemical demineralization, страница 4

Accordingly,thepresentstudyisanextensionofRahardianto et al’s work [12] to the pilot scale to test the robustness of the PRO–ICD–SRO process for high recovery CRW desalting in a continuous single-pass treatment system. An important objective of this study was to evaluate the integration of an ICD process in the pilot scale to enhance overall product water recovery of CRW RO desalting up to 95%.

3. Experimental

3.1. Pilot studies

Pilot-scale studies were conducted over a 9-month period at the U.S. Bureau of Reclamation’s Water Quality Improvement Center in Yuma, Arizona. Pilot studies were designed to provide both a proof-of-concept demonstration of the two-stage RO process and design and operation parameters for subsequent demonstration-scale testing. Pilot studies focused on two portions of the two-stage RO process (Fig. 2), namely the ICD step in which a SCR was utilized prior to the SRO step. Optimizing the operation of the MF pretreatment and PRO was addressed in prior investigations [13,14]; both of these unit processes were run under a single, optimized operating condition for the duration of pilot testing.

3.2. Feed water source, pretreatment, and primary RO desalting

SourcewaterfromtheColoradoRiverwasprocessedthrough a 0.2m nominal pore size, hollow-fiber microfiltration (MF) unit (Model 12M10C, U.S. Filter/Memcor, Timonium, Maryland). A 2.5mg/L chloramine residual was maintained in the MF feed, using a 4:1 (w/w) Cl₂ to NH₃ ratio; no free chlorine was detected at any time in the PRO influent. The PRO, consisted of 2:1 array of polyamide membrane modules (arrays 1 and 2: 4040 ESPA 4, Hydranautics, Oceanside, CA, and array 3: 2540 BL, Saehan, Korea), operated at an average feed flow rate of 83L/min, an average feed pressure of 1041kPa (151psig), feed pH adjusted to 6.9, and an average of 83% water recovery. Chemical feeds to the PRO were sulfuric acid (93%) and antiscalants (3.0mg/L Flocon 260, FMC Corp., Philadelphia, PA).

3.3. Chemical precipitation

ConcentratefromthePRO(15L/min)wassenttotheSCRfor demineralization (Fig. 2). The SCR contained an inner recirculation and reaction zone (20min hydraulic residence time) and an outer clarification zone (100min hydraulic residence time). PRO concentrate and chemicals (30% NaOH) were introduced into the center draft tube where a mixer was used to re-circulate solids from the bottom of the reactor up through the center draft tube. The desired pH of the SCR effluent was set by adjusting the caustic feed rate. Sampling ports at the top and bottom of the draft tube enabled collection of samples to determine the solids content in the recirculation stream. An outflow with an automatic valve at the bottom of the contactor was used for removal of waste solids. Prior to the removal of residual solids from the SCR, water was flushed through the lower blow down valve to prevent solids bridging. Solids removal was carried out periodically approximately every 20–40min.

Maintaining steady-state SCR operation required control of: (1)recirculationflowratethroughtheinnerdrafttube,whichwas set by varying the speed of the recirculation mixer, and (2) reactor solids concentration, which was controlled by the frequency and duration of reactor solids removal. The recirculation and solids removal (blow down) rates were controlled based on the ratio of the solids percent volume content at the upper portion to that at the lower portion of the draft tube. The SCR was operated in an automatic solids-removal mode in order to maintain the target reactor solids content ratio of approximately 1/2 (v/v), which is the typical inlet/outlet volumetric solids content volume ratio in the operation of commercial scale SCRs. It is noted that, in the relatively small SCR used in the current study, the high solids volumes (∼10–15%) typically encountered in the draft tube inlet of large-scale commercial SCRs [15] were not reached. Due to its relatively small size and short residence time in the reaction zone, the present SCR was most effectively operated by maintaining a constant level of the sludge blanket in the bottom of the reactor resulting in relatively low solids content (2–5% by volume).