N. Daltrophe

Novel ion-exchange spacer for improving electrodialysis II. Coated spacer

Ion-conducting spacers were prepared by applying an ion-exchange coating to commercially available polypropylene netting. Homogeneous and heterogeneous types of coating were used. Homogeneous anion-exchange coating consisted of bromomethylated and aminated polysulfone, homogeneous cation-exchange coating of sulfonated polysulfone. All heterogeneous coatings consisted of ground ion-exchange resin, embedded in crosslinked poly(vinyl alcohol). All the coated spacers increased the rate of desalting of sodium chloride solutions, at concentrations of 20 mM or less. The effect increased with the ion-exchange capacity of the spacer per unit area. The spacers suppress polarization, leading to increased current efficiency and decreased cell resistance. As expected, largest decrease of cell resistance is obtained in dilute solutions, <3 mM. The clearest effect on efficiency was observed in ED with heterogeneous ion-exchange membranes, which are by themselves highly polarizing. Most experiments were carried out with anion-exchange spacers, minimizing the water splitting which takes place at the surface of the ion-exchange membrane. Introduction of an anion-exchange spacer near the heterogeneous anion-exchange membrane and a cation-exchange spacer near the heterogeneous cation-exchange membrane led to a dramatic increase in current efficiency.

Improved compact accelerated precipitation softening (CAPS)

CAPS is a softening process in which adjusting water pH to the range 8-10.5 reduces calcium and carbonate alkalinity by accelerated CaCO 3 nucleation and growth in 2 regions: (a) in a pre-prepared slurry made of calcite small particles and, (b) within a CaCO 3 layer (cake) formed on the top of the filter through which water is pumped out. Whilst the largest degree of precipitation occurs within the slurry, the cake process is a polishing step in which calcium concentration is reduced further. Within the dense cake structure, the removal of the smaller calcium concentrations is possible within short contact times due to enhanced mass transfer rates made possible by large solution velocities within narrow pores and much larger surface to volume ratio. CAPS was first suggested for water softening and later tested for the possibility of simultaneous silica removal. CAPS was also studied as a pretreatment for RO (reverse osmosis) with water taken from fish ponds. The capability of reducing SDI, organics and hardness to levels satisfactory for prolonged RO treatment was demonstrated. In a previous paper, water was mixed with CaCO 3 particles, the slurry was circulated through a microfiltration module and the clear and softened permeate was then RO treated with a recovery rate above 80%. CAPS may be used as a stand-alone water treatment process or in conjunction with pressure and electrical driven membrane processes (UF, NF, RO, ED) as an effective pretreatment routine for increasing recovery and decreasing fouling rates. In this work, a new concept for CAPS, which comprises in-tank mixing and filtration is presented. This makes the CAPS device more attractive due to compactness and the process more attractive technically and in terms of cost. The advantages of in-tank filtration were appreciated in the past and it has been a subject for intensive investigation. Laboratory CAPS units were run continuously (up to 250 h) and for shorter time periods in order to investigate tap water softening. The effect of the initial CaCO 3 slurry concentration; residence time or pumping rate; pH; backwash frequency, duration and mode (dry or wet) and slurry mixing rate was investigated and analyzed in terms of Saturation Index (SI) reduction, separated effects of the slurry and the cake on the softening action and filter cake load.

Low-polarisation electrodialysis membranes

Heterogeneous ion exchange membranes were coated with hydrophilic charged layers. The cation exchange membrane was coated with polyvinylalcohol, crosslinked by a reactive dye containing sulfonic groups. The anion exchange membrane was coated by ground anion exchange particles embedded in polyvinylalcohol crosslinked by hexamethoxymethylmelamin. The coating substantially decreased polarisation on both types of membranes. Suppression of water splitting increased the current efficiency at low salt concentrations. In the concentration range of industrial water production, output and energy consumption of the modified heterogeneous membranes is equal to that of highly permselective homogeneous ones. The improvement of the heterogeneous membranes by the coating is due to suppression of polarization. The polyethylene based membranes are heat sealable.

A novel eductor-based MBR for the treatment of domestic wastewater

A novel aeration device has been developed that combines the mechanism of a venturi aerator with the flow multiplier effect of an eductor used for pump driven mixing. The performance of this novel eductor was evaluated in a flat-sheet immersed MBR and compared with the same MBR equipped with a conventional diffuser for the treatment of domestic wastewater. The eductor showed a higher rate of oxygen transfer both in clean and wastewater compared to the diffuser. The α value with the eductor (0.91) was also found to be more than that of the diffuser (0.75). Higher recirculation rate through the eductor resulted in a higher mixing/turbulance inside the MBR tank and thus alleviated membrane fouling significantly compared to the diffuser. The performance of the MBR in terms of organics removal was also found to be higher with the eductor than the diffuser. The eductor could have significant potential as a combined aerator and mixer in the field of wastewater treatment by MBR.

Performance evaluation of a novel eductor-based MBR for the treatment of domestic wastewater

AbstractThe performance of a patented novel eductor-based MBR was studied in comparison with a conventional diffuser for the treatment of domestic wastewater. The eductor showed a high rate of oxygen transfer over the diffuser (KLa of 18.49 and 5.6/h, respectively, for the eductor and the diffuser in clean water). Higher recirculation rate through the eductor increased mixing inside the MBR tank and resulted insignificantly less membrane fouling compared to the diffuser (when the MBR was operated in continuous permeation mode; without any back-washing). The COD and ammonia removal efficiency of the MBR was also found to be higher with the eductor than the diffuser. The eductor was found to have great potential to be used as an aerator as well as a mixer if operated in a large scale of application in wastewater treatment by MBR.