A.E. Simpson

The effect of pH on the nanofiltration of the carbonate system in solution

Abstract The rejection and flux performance of a commercially available charged ultrafiltration membrane (also called a nanofiltration membrane) has been described for the sodium carbonate system in solution. The influence of pH on the speciation of the feed solution and on permeate composition has been examined. In addition, the implications of increased feed concentration on ion exclusion has been investigated. Furthermore, the application of nanofiltration for the removal of potential scale forming (precipitation fouling) compounds from carbonate solutions has been determined. The enhancement of the rejection of scale forming compounds by the addition of a sequestrant has also been determined. The influence of anion species on the rejection of sodium by nanofiltration membranes has been determined over a range of pH values by comparing the rejections of sodium ions in carbonate/bicarbonate systems in solution with the rejections of sodium ions in sulphate and chloride solutions. Finally, summarised pilot plant results for the regeneration and reuse of sodium hydroxide from caustic effluents have been presented. Nanofiltration has been applied as a pretreatment softening stage for the separation of sodium bicarbonate/carbonate from scale forming and organic impurities in this commercially viable effluent treatment sequence.

The treatment and disposal of waste brine solutions

Abstract A serious limitation in the use of desalination plants producing concentrated solutions is the lack of adequate methods of waste brine disposal such that the brine does not re-enter and contaminate the raw water inlet system. Various options for the potential disposal of waste brines have been presented. Procedures which enable the separation, precipitation and chemical transformation of specific species have been discussed. In addition, the implementation of various waste brine disposal options has been illustrated by means of case studies for a range of industrial processes.

The treatment of industrial effluents containing sodium hydroxide to enable the reuse of chemicals and water

Abstract An economically viable treatment sequence has been developed and piloted at two textile factories for the recovery and reuse of water, chemicals and heat energy from sodium hydroxide effluent produced during the scouring of cotton fibre. The treatment sequence involves pretreatment of the scour effluent by neutralisation, using an acidic gas, cross-flow microfiltration and charged membrane ultrafiltration (also called nanofiltration). The sodium hydroxide is then recovered in an electrochemical membrane cell with the simultaneous evolution of acidic gas which is recycled within the treatment process. Two possible configurations of the treatment process, where the acidic gas is either chlorine or carbon dioxide, have been discussed. Pilot plant results have been presented for both systems. The carbon dioxide system was the preferred route and is discussed in detail. The pretreatment sequence neutralised the scour effluent, lowered its chemical oxygen demand by 86% and removed 65% of both the calcium and the organics and 50% of the magnesium. The sodium hydroxide (100 to 200 g/l) and depleted brine solution (total solids 500 mg/l) from the electrochemical membrane cell were of suitable quality for reuse in the factory process. The electrochemical membrane cell produced sodium hydroxide at 62% current efficiency at an electrical power consumption of 4 000 kWh/ton 100% NaOH. The effect of electrolyte, in particular, anolyte flow rate, temperature and concentration on the limiting current density and power consumption has been investigated. Some design data for a full scale treatment plant has been presented. The operation of an acceptable background concentration closed-loop recycle wash system in the scour process was found to reduce the required membrane area by 82%. Minimum dissolution of the precious metal oxide anode coating occurred and long anode life was predicted. Serious electromembrane fouling, with increased resistance, was not apparent.

The removal of sulphuric acid from natural and industrial waste waters

Abstract A process is described which enables the removal of sulphuric acid from effluents without the addition of chemicals to the effluent. The technique employs an anion selective membrane which separates the acidic effluent from a lime solution. In the case where no current is applied, facilitated transport occurs and the driving force for the demineralisation process is the difference in the chemical potential of the species on either side of the membrane. The equilibrium position may be shifted by the passage of a current through the membrane, in which instance the driving force becomes the applied electric potential. The sulphates are removed as calcium sulphate, which precipitates in the lime solution. The specific membrane area requirements are a function of the desired degree of acid reduction. The application of low current densities (500 A/m2) reduces the specific membrane area requirements for a particular duty by two orders of magnitude compared to facilitated transport. Results of laboratory investigations are given and indicate that the sulphuric acid component can be removed from waste waters without an undesirable build-up of additional dissolved solids.

The use of speciation and X-ray techniques for determining pretreatment steps for desalination

Abstract A major cost factor in the construction of desalination plants is the feed water pretreatment section. Pretreatment is necessary to prevent fouling of membranes (in electrodialysis or reverse osmosis) or heat exchange surfaces (evaporation). Various laboratory techniques are described which enable small-scale tests to be undertaken in order to determine acceptable pretreatment sequences and feasible water recoveries. A variety of effluents were subjected to evaporation or small scale reverse osmosis tests. Precipitates produced were identified by X-ray techniques. In some cases, the identification of the precipitate was, in itself, sufficient to indicate the cause of fouling, in another, identification enabled various pretreatment sequences to be modelled using chemical equilibrium computer programmes. A combination of conventional chemical analysis, X-ray techniques and computer modelling has proved successful in determining acceptable pretreatment sequences.

The treatment of industrial effluents with high salinity and organic contents

Abstract Particularly problematic industrial effluents are those which contain high concentrations of both inorganic salts and organics. Some advanced technologies with their limitations are described. A few examples of the application of some membrane processes are given to demonstrate their potential use in the treatment of effluents from industries such as maize starch milling, tanneries, both sulphite and soda pulp mills, and a cotton textile factory.