Last update:
10/28/2004
Antiscalants
Crystallization of sparingly soluble salts such as calcium carbonate and calcium sulfate dihydrate is known to occur when certain surface and ground waters are used as the water source in membrane desalination processes. Most natural waters contain relatively high concentrations of calcium, sulfate and bicarbonate ions. In membrane desalination operations at high recovery ratios, the solubility limits of gypsum and calcite exceed saturation levels leading to crystallization on membrane surfaces. The surface blockage of the scale results in permeate flux decline, reducing the efficiency of the process and increasing operating costs. To avoid scaling difficulties, it is essential to restrict the fractional recovery of purified water below a threshold limit at which there is a risk of scale precipitation. In view of the economic benefit of high water recovery, the effective solubility limits of scaling salts, and hence the allowable water recovery, are usually extended by antiscalant treatment.
During the past two decades, new generations of antiscalants (AS) have emerged commercially, in which the active ingredients are mostly proprietary mixtures of various molecular weight polycarboxylates and polyacrylates. Optimal molecular weights have been reported in the range of 1,000- 3,500. Other polyelectrolytes including polyphosphonates and polyphosphates have also been applied successfully with certain types of feed waters.
It is often stated that antiscalants adsorb onto formed crystals or associate/complex with incipient nuclei (or crystals) and that these phenomena govern the inhibition of scale formation. The precise mechanism of scale inhibition is not clearly understood at this time. However, it is known that in supersaturated solutions of sparingly soluble salts, a significant delay in crystal nucleation and subsequent growth is observed in response to AS treatment. This delay is referred to as the “induction-time” of the system, which occurs at remarkably low “threshold dosages” in the order of 1-10ppm. The scale inhibition capability of antiscalants is related to chemical structure, molecular weight, active functional groups and solution pH –parameters.
An important factor in determining the success of surface and ground water desalination is the optimization of antiscalant treatment with respect to type and dosage. Prior to field testing or even laboratory studies on the performance of RO processes, it is important to identify the proper antiscalant to use and the dosage-induction time relationship for the expected level of supersaturation. Antiscalants can be ranked based on measurements of observed homogeneous crystallization induction time for various solution conditions of interest (e.g., composition, pH and temperature). Once candidate antiscalants are selected one can then proceed with experimental membrane performance analysis to establish optimal dosage requirement. Various methods of determining induction time for homogeneous crystallization of mineral salts such as conductivity, constant composition monitoring through pH control, light scattering through transmittance and absorbance studies have been proposed in literature. Induction time studies have also been conducted whereby a membrane element is fouled in a reverse osmosis system. Conductivity and constant composition methods detect the decline in dissolved ion concentrations. Light scattering techniques involve complex laser positioning designs, and turbidity monitoring in flow cells external to the main crystallization vessel or in grab samples from that vessel.
In our laboratory we are developing robust methods of evaluating dose effectiveness and
gypsum crystallization suppression capability of antiscalants based on homogeneous
crystallization studies coupled with membrane scaling studies. Batch crystallization studies involve continuous in-situ monitoring of precipitate formation via back-light scattering turbidity probe, with added monitoring of dissolved calcium ion concentration to provide a confirmation of the observed induction time. We have also developed a QCM surface crystallization cell in which the direct impact of antiscalants on surface crystallization can be quantified. Our studies also include evaluation of the impact of various organics (e.g., humic acids), other additives (e.g., Alum) and colloidal matter (e.g., silica) on the effectiveness of antiscalants.
Schematic illustration of the action of antiscalants.
A crystallization system for determining crystallization induction time.
Antiscalant impact on surface crystal morphology of gypsum scale
Reference
Wen-Yi Shih, K. Albrecht, J. Glater and Y. Cohen, “A Dual Probe Approach for Evaluation of Gypsum Crystallization in Response to Antiscalant Treatment,” Desalination, 169 (3), 213-221 (2004).
