Impact of Conventional Water Treatment Coagulants on Mineral Scaling in RO Desalting of Brackish Water

Abstract

The potential impact of coagulants on mineral scaling in reverse osmosis (RO) feed treatment of brackish water was assessed experimentally, with respect to calcium sulfate and barium sulfate scaling potential, via a bulk crystallization and membrane scaling tests. The scale suppression effectiveness of six commercial antiscalants was first ranked based on measured observed bulk crystallization induction times. Bulk crystallization tests with three standard coagulants (ACH, FeCl 3 , and polyDADMAC), when used independently, demonstrated retardation of the observed crystallization induction time. However, along with an antiscalant dosing, antiscalant scale suppression was significantly reduced and scaling occurred at the same or greater severity relative to the additive-free feed solution. RO membrane scaling tests also indicated that, when the coagulants were present, antiscalant effectiveness was significantly reduced, consistent with the expectation based on bulk crystallization tests. Although the coagulants demonstrated a slight scale suppression quality in bulk crystallization tests at low dosages (<10 mg/L), membrane tests, even at a low dose of ∼3 mg/L, demonstrated scale formation that was at the same or greater level relative to the additive free solution. Optical imaging of the membrane surface demonstrated that, in essentially all cases, greater surface scaling was linked to a higher surface number density of mineral crystals. The percent flux decline increased with increased percent of scaled membrane area. Percent flux decline that was lower than the percent area scaled was encountered for scaling tests without additives and for the case of ACH along with added antiscalant, suggesting partial permeation through the gypsum rosettes. Scaling tests with ACH and with the antiscalant along with FeCl 3 or polyDADMAC revealed higher percent flux decline than the percent scaled membrane area. This behavior was linked to scaling by barium sulfate as indicated by SEM imaging and elemental surface analysis and possibly the complexation and/or adsorption of the coagulant onto the membrane surface. The results of the present study suggest that, when feed treatment includes the use of coagulants, excess antiscalant may be needed in order to offset the adverse impact of the coagulants on antiscalant effectiveness.

Coupled 3-D hydrodynamics and mass transfer analysis of mineral scaling-induced flux decline in a laboratory plate-and-frame reverse osmosis membrane module

“…The evolution of mineral scaling along the membrane surface was determined in an optically transparent plate-and-frame RO (PFRO) cell, known as the EXSOD system, developed by Uchymiak et al [16] and is described elsewhere [23,34]. Briefly, the EXSOD RO cell has an active membrane area of 8.24 cm × 3.16 cm and a channel height of 2.54 mm (Fig.…”

“…The mineral scale coverage of the membrane surface was determined via analysis of real-time high resolution digital images of the membrane surface, recorded every 15 min, with near dark-field illumination, using the EXSOD image analysis software [34]. …”

A method for evaluating antiscalant retardation of crystal nucleation and growth on RO membranes

“…All inorganic chemicals were reagent grade obtained from Fisher Scientific (Pittsburgh, PA). The feed solution composition detailed in Table 1 was selected as a model solution to mimic the composition of RO concentrate from desalting of Colorado River water at ∼85% recovery [23], to illustrate the present methodology of assessing antiscalant effectiveness and mineral scale development. Bicarbonate was excluded from the model solution in order to focus on gypsum scaling [4].…”

“…Bicarbonate was excluded from the model solution in order to focus on gypsum scaling [4]. It should be noted that while the saturation index for barite in the feed solution was high (Table 1), previous work has shown that barite would not precipitate on the membrane surface for the present range of experimental conditions [23].…”

“…Addition of antiscalant chemicals (typically consisting of formulations of polyacrylic acid, carboxylic acid, or phosphonates; [23,24]) to the RO feed is a popular method of reducing the adverse impact of membrane mineral scaling [3,11,[24][25][26][27][28][29][30]. Experimental determination of antiscalant effectiveness have been generally limited to evaluations of bulk crystallization [24,28,30], quantifying scaling behavior based on flux decline measurements [13,31,32] or determining the extent of surface area coverage by mineral scale [4,26,33].…”