Abstract
The control of fouling and scaling on heat and mass transfer surfaces is of major importance in processes as superficial water treatments, since it also improves the efficiency of the whole process from an energy saving point of view. The aim of the paper is to present the experimental results obtained in the inhibition of the fouling and scaling by calcium ions on an ultrafiltration membrane surface, by using citric acid as an additive. The last is an environmentally friendly additive−a so-called “green additive”, which may represent a reliable alternative to phosphorous and nitrogen based compounds typically used as inhibitors, since it has the characteristics of being non-toxic, non-bio accumulating, and biodegradable. The experimental plant is made of a tangential flow system on a lab scale equipped with a flat sheet ultrafiltration polymeric membrane, whose cut-off is 650 nm. In the first series of experiments, the effect of water hardness and its fouling effect due to calcium ions on membrane permeability has been measured in the range of potable waters. Then, the scaling effect of high calcium concentration in solution (supersaturated conditions) has been quantified by measuring the increase in weight of the membrane, with and without the addition of citric acid as an additive; moreover, the retarding effect of citric acid has been evaluated through the measurement of the induction times for the nucleation of calcium sulfate dihydrate (used as model scalant for fouling). Experiments have been carried out at two different supersaturation ratios (S = 2.25–2.60), at room temperature, in the absence of any additive, and with a citric acid concentration varying in the range 0.01 to 0.50 g/L. Experimental results have shown that the addition of citric acid in solution delays the induction times for gypsum crystals nucleation; moreover, it mitigates the phenomenon of membrane fouling and reduces the pressure drops by allowing an acceptable permeate flow for a longer duration.
Highlights
IntroductionTogether with calcium carbonate scales, are the major causes of fouling on process equipment surfaces with severe drawbacks
Calcium sulfate scales, together with calcium carbonate scales, are the major causes of fouling on process equipment surfaces with severe drawbacks
Two experimental series were carried out: The first series was aimed at verifying the effect of calcium concentration on membrane fouling in superficial waters acceptable for human consumption
Summary
Together with calcium carbonate scales, are the major causes of fouling on process equipment surfaces with severe drawbacks. Calcium sulfate scale is one of the major cause of fouling in reverse osmosis (RO) membranes, resulting in a continuous decline in desalted water production, reducing the overall efficiency and increasing operation and maintenance. Membrane fouling may be a major obstacle to the implementation of surface water ultrafiltration, which causes higher costs of energy, operation, and maintenance [8]. The inorganic ions in water that exceed the equilibrium solubility product firstly reach the nucleation stage and go through homogeneous or heterogeneous crystal growth processes [10]. This occurs when an aqueous solution passes through a membrane [11]. Membrane crystallization is a mature technology that can replace the conventional crystallizers (e.g., in seawater distillation) because of its advantages due to its adaptability and the lower energy consumption [14]
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