Pressure-driven membrane operations and membrane distillation technology integration for water purification

In the last years, boron and arsenic are gaining wide attention in the water treatment community due to their adverse effects both on human/animal health and on agriculture. As a consequence, nowadays, the number of water treatment plants equipped with boron and arsenic removal facilities is growing. Usually it is difficult to remove them from water due to their size and charge. Therefore, in order to obtain substantial arsenic and boron reduction in the treated water, the most part of the desalination/purification plants have to use chemicals agents and Reverse Osmosis (RO) systems with several pass-stages. In this paper the potentialities of Membrane Distillation (MD) as a new and innovative technique for contaminants removal from water were investigated. The purposes of the present work are as follows: 1) to examine the efficiency of different pressure driven membrane processes for boron and arsenic removal from water; 2) to test experimentally the performance of Membrane Distillation for water purification; 3) to analyse and propose an innovative integrated system for the contaminants removal from water. In the proposed flow sheet, pressure-driven membrane operations have been synergically joined with membrane distillation technology for reaching the Process Intensification goals thanks to the possibility of overcoming the limits of the single units and, thus, to improve the performance of the overall operation. The designed system is constituted by a Reverse Osmosis (RO) step followed by a Membrane Distillation (MD) one. In this process in order to achieve, in the produced fresh water, a boron and arsenic concentration equal or less the maximum recommended values, only 36% of the RO permeate has to be sent to the MD module.

In this work a variety of membrane processes have been evaluated for their ability to reject both boron and arsenic. The successful application of membrane technology to the As removal from drinking water will depend upon matching the proper membrane to the characteristics of the feed-water. If arsenic in the feed-water is primarily speciated as As(III), only RO membranes or tight NF membranes appear to be able to sustain high rates of arsenic. Pre-oxidation of As(III) to As(V) followed by Table 11 Product characteristics, energy consumption and desalted water cost for three different flow sheets Flow sheet RO Feed flow rate (m /h) Brine flow rate (m3/h) Brine concentration (g/L) Fresh water flow rate (m3/h) Fresh water concentration (g/L) As concentration in fresh water (g/L) B concentration in fresh water (g/L) Elect. energy consumption (kW h/h) GV (kg/h) Primary energy (PE) (Mcal/h) Quantity of energy required per m3 of fresh water produced (kW h/m3) Quantity of energy required per m3 of fresh water produced (kW h/m3)a Unit cost ($/m3) Unit cost a ($/m3) a b RO with pre-oxidation step RO-MD 1048E+03 57.6 0.338 1.400E–05 4.500E–04 / / 5.24 1048E+03 57.6 0.338 1.020E–05 4.500E–04 / / 5.24 1048E+03 54.4 0.226 9.333E–06 3.000E–04 28.4/5.76 b 2.69 2.69 25.6/2.96 b 0.614 0.398 0.616 0.399 0.967/0.797 b 0.729/0.559 b If Pelton turbine is used as energy recovery device. If thermal energy is available in the plant or the stream is already at the operating temperature of the MD unit.