Interactions between feed solutes and inorganic electrolytic draw solutes in forward osmosis

A comprehensive transport model for Forward Osmosis (FO) is presented, based on Maxwell-Stefan theory. In FO, the oppositely directed fluxes give rise to frictional interactions, while the salinity gradient also causes to thermodynamic non-ideal behaviour of organic feed solutes, in the form of salting out. When using electrolytic draw solutes, unequal ion permeance of the draw solute creates an electrostatic potential difference across the membrane, which is an additional driving force for transport of ionic feed solutes. A sensitivity analysis is presented, assessing the effect of frictional interactions, partitioning of feed and draw solutes and salting out on feed solute rejection. It is shown that feed solute rejection is determined primarily by friction with the membrane polymer and partitioning, and secondary by salting out. Frictional interaction between feed and draw solutes is not significant for active layer transport, for a wide range of parameter variation. It can however be significant for transport in the support layer, once feed solutes have permeated through the active layer. Electromigration can be as important as diffusively-driven transport, provided that the length over which the electrostatic potential is established is limited to about the thickness of the active layer. Finally, additional interactions between membranes, organic and inorganic solutes are discussed

Mechanistic modeling of mass transport phenomena in Forward Osmosis

Forward Osmosis (FO) is a wastewater treatment technology in which water is abstracted from a feed solution through a membrane into a draw solution having a high osmotic pressure. FO membranes however are not perfect barriers for solutes: during FO, there are solute fluxes between the feed and draw solutions in both directions. FO would be applied on heavily impaired feed solutions, such as raw or partially treated wastewater, sludges or specific industrial waste streams. Such streams commonly contain organic micropollutants (OMPs), which are to be rejected by the FO membrane. In this dissertation, mass transfer phenomena in FO are studied experimentally and mechanistic models of these phenomena are presented. Water and draw solute fluxes were studied in the abscence of feed solutes, and a model was developed which allowed membrane permeability and draw solute mass transfer resistance to be determined. Subsequently, it was studied whether the draw solute flux hinders the flux of OMPs in the feed, as both fluxes are directed oppositely. No evidence for steric hindrance was found, however, draw and feed solutes engage in charge interactions. It was also found that draw solutes change the OMP-membrane affinity, which for some combinations of feed solutes and membranes causes enrichment of those feed solutes rather than rejection. Finally, the fate of OMPs in FO-RO closed loop applications was studied. It was shown that if the OMP rejection by FO was lower than RO, OMPs would accumulate in the draw solution, even to levels exceeding those of the initial feed solution. This would impair the quality of the water produced by RO

Crown ether containing polyelectrolyte multilayer membranes for lithium recovery

Achieving solute selectivity has always been a goal of membrane development studies. The continuing growth of global consumption of scarce metals by different industries has put a strain on traditional sources of these species. Achieving cation selectivity in membranes, especially among monovalent cations, is a major step in introducing alternative sources for scarce metals such as lithium. Polyelectrolyte multilayer membranes (PEMMs) are a novel class of membranes, offering great potentials in monovalent/bivalent ion selectivity. On the other hand, crown ethers are a well-studied family of macrocyclic ligands capable of forming stable complexes with cations. In the current study, for the first time, we report on a PEMM nanofiltration membrane with crown ether moieties embedded in its structure for the goal of achieving monovalent salt selectivity. The crown ether 15-crown-5 was successfully incorporated in the polycation polyethylenimine (PEI), which was then used as the polycation in PEMM formation through layer by layer deposition. Both the synthesized polymer and the polyelectrolyte multilayer (PEM) were characterized and the performance of the resulting membrane was studied. It was determined that crown ether containing polymer forms more stable complexes with lithium than potassium. This was explained by the limitation put on 2:1 potassiumcrownether complexes by steric hindrance from polymer chain. The manufactured membranes showed Li/K selectivity for a period of around 90 min, after which the crown ethers became saturated and selectivity was lost. The modified membranes became non selective after this point, but possessed high salt rejection potential

Increased carboxylate production in high-rate activated A-sludge by forward osmosis thickening

Domestic wastewater represents a considerable feedstock for organics but the high dilution makes their recovery typically unsuccessful. Here we investigated three routes to 10-fold concentrate the organics using Forward Osmosis (FO) (Draw solution (DS) 2.2 M MgCl2): directly on domestic wastewater, A-sludge, or secondary sludge, with the end goal of increasing volatile fatty acid (VFA) yield from subsequent 9-day fermentation tests. Forward osmosis concentrated the total COD by a factor of 8.2 ± 1.2, 10.1 ± 2.4 and 4.8 ± 0.2 with respect to the raw streams of wastewater, secondary sludge and A-sludge. The soluble fraction of the COD was concentrated up to 3.5 times in the A-sludge and 2.1 times in the secondary sludge; the result of a combined effect of the chemical action of Mg2+ (diffused from the DS) on sludge disaggregation and cell lysis, and the physical action of recirculation and air-scouring of the A-sludge in the FO-unit. The FO-concentrated A-sludge produced 445 ± 22 mg COD-VFA g-1 CODfed, which was 4.4 times higher than for the untreated A-sludge. No VFA were produced from untreated secondary sludge, but after FO-concentration 71 ± 5 mg COD-VFA g-1 CODfed could be reached. Due to the low organics in wastewater even after FO-concentration (1.08 ± 0.08 g COD L-1), no notable VFA production occurred. The combination of A-stage technology and membrane technology for dewatering and COD concentration could be a key advancement to increase VFA production from domestic wastewater, whereby at least 45% of the COD can be recovered as valuable VFA.<br/

Increased carboxylate production in high-rate activated A-sludge by forward osmosis thickening

Domestic wastewater represents a considerable feedstock for organics but the high dilution makes their recovery typically unsuccessful. Here we investigated three routes to 10-fold concentrate the organics using Forward Osmosis (FO) (Draw solution (DS) 2.2 M MgCl2): directly on domestic wastewater, A-sludge, or secondary sludge, with the end goal of increasing volatile fatty acid (VFA) yield from subsequent 9-day fermentation tests. Forward osmosis concentrated the total COD by a factor of 8.2 ± 1.2, 10.1 ± 2.4 and 4.8 ± 0.2 with respect to the raw streams of wastewater, secondary sludge and A-sludge. The soluble fraction of the COD was concentrated up to 3.5 times in the A-sludge and 2.1 times in the secondary sludge; the result of a combined effect of the chemical action of Mg2+ (diffused from the DS) on sludge disaggregation and cell lysis, and the physical action of recirculation and air-scouring of the A-sludge in the FO-unit. The FO-concentrated A-sludge produced 445 ± 22 mg COD-VFA g-1 CODfed, which was 4.4 times higher than for the untreated A-sludge. No VFA were produced from untreated secondary sludge, but after FO-concentration 71 ± 5 mg COD-VFA g-1 CODfed could be reached. Due to the low organics in wastewater even after FO-concentration (1.08 ± 0.08 g COD L-1), no notable VFA production occurred. The combination of A-stage technology and membrane technology for dewatering and COD concentration could be a key advancement to increase VFA production from domestic wastewater, whereby at least 45% of the COD can be recovered as valuable VFA.<br/