Effect of pretreatment and membrane orientation on fluxes for concentration of whey with high foulants by using NH3/CO2 in forward osmosis

•Lower initial but similar final water flux at forward osmosis with pretreatment.•Higher initial water flux as active layers of the membrane were in contact with whey.•Correlation of lower solute resistivity with high water flux.•Small molecular weight molecules rather than carbohydrates passed from whey to draw. Usage of forward osmosis membrane in FO mode, in which active and support layers of the membrane were in contact with the feed and the draw solutions respectively, provided higher initial water flux (12L/m2h) than the usage of membrane in PRO mode (6L/m2h) having opposite orientation but fluxes approached to each other after 4h during concentration of whey with NH3/CO2 as draw salt. High organic and inorganic foulants of whey was considered as reason for observed result in addition to lower solute resistivity. Initial water flux (8,5L/m2h) was lower when pre-treatment was applied before forward osmosis process but final flux (4L/m2h) was equal flux of non pre-treatment. Reduction of solute resistivity or absence of hydraulic pressure can be reasons for lower initial flux. Detection of organic carbon but absence of lactose in draw solution showed passage of molecules being different than lactose into draw solution.