Recovery of polyelectrolyte from polyelectrolyte-enhanced ultrafiltration by precipitation process

Abstract

The recovery of polyelectrolyte from polyelectrolyte-enhanced ultrafiltration(PEUF) for chromate removal was studied as equilibrium experiments in a laboratory scale and batch and continuous operations in a pilot scale crystallizer. PEUF is a membrane separation process, which can be used in the removal of chromate anion from wastewater. In the process, a water-soluble cationic polyelectrolyte is added to bind to bhromate. The bound chromate-polymer is ultrafiltered from the solution, resulting in the purified water (permeate), which contains very low chromate concentration passing through the membrane. For an economical operation, the retentate solution not passing through the membrane can be treated to separate the polyelectrolyte and chromate ions to permit reuse of the polyelectrolyte and/or concentrate the pollutant ions for disposal. In the regeneration step, barium chloride can be added to the retentate to precipitate chromate anion as compact barium chromate solid waste. The solution containing the concentrated polymer can be directly recycled after the solid barium chromate is separated from the solution. Gravity settling is much less expensive than filtration or centrifugation, so this study investigates the abilities to recover polyelectrolyte and to separate barium bhromate solid from the solution in a crystallizer/settler. The effects of the residence time or feed flow rate, height of the column, polymer to chromate ratio, and other parameters were studied. Compositions of the batch and continuous crystallization effluent streams are compared to that estimated at equilibrium. The highest percentage of polymer recovery is obtained from the equilibrium precipitation followed by the batch crystallizer operation and the continuous crystallizer operation, respectively.