Assessing the removal of organic micro-pollutants from anaerobic membrane bioreactor effluent by fertilizer-drawn forward osmosis

Abstract

In this study, the behavior of organic micro-pollutants (OMPs) transport including membrane fouling was assessed in fertilizer-drawn forward osmosis (FDFO) during treatment of the anaerobic membrane bioreactor (AnMBR) effluent. The flux decline was negligible when the FO membrane was oriented with active layer facing feed solution (AL-FS) while severe flux decline was observed with active layer facing draw solution (AL-DS) with di-ammonium phosphate (DAP) fertilizer as DS due to struvite scaling inside the membrane support layer. DAP DS however exhibited the lowest OMPs forward flux or higher OMPs rejection rate compared to other two fertilizers (i.e., mono-ammonium phosphate (MAP) and KCl). MAP and KCl fertilizer DS had higher water fluxes that induced higher external concentration polarization (ECP) and enhanced OMPs flux through the FO membrane. Under the AL-DS mode of membrane orientation, OMPs transport was further increased with MAP and KCl as DS due to enhanced concentrative internal concentration polarization while with DAP the internal scaling enhanced mass transfer resistance thereby lowering OMPs flux. Physical or hydraulic cleaning could successfully recover water flux for FO membranes operated under the AL-FS mode but only partial flux recovery was observed for membranes operated under AL-DS mode because of internal scaling and fouling in the support layer. Osmotic backwashing could however significantly improve the cleaning efficiency.

Understanding the organic micropollutants transport mechanisms in the fertilizer-drawn forward osmosis process

“…The OMPs concentrations were obtained based on methods used in our earlier study (Kim et al, 2017c). After the experiments, 100 mL samples were collected, to which 10 µL isotopes were added (Cambridge Isotope Laboratories, Inc., USA).…”

“…The recovery ratio (i.e., the ratio of the peak areas before and after extraction) was considered to evaluate the loss of OMPs during the extraction and evaporation processes and to calculate the final concentration of OMPs by multiplying the measured concentrations by the obtained recovery ratio. Then, the OMPs forward flux (toward DS) was derived based on the mass balance for OMPs species (Kim et al, 2012;Kim et al, 2017c) using Eq. (3),…”

“…where Js refers to RSF (mol m -2 h -1 ) and ΔmDS is the change in the mass of the draw solutes in the FS during operation (mol). The detailed experimental procedures are described in our previous studies (Kim et al, 2017c;Kim et al, 2017d).…”

“…Also, biofouling on the osmotic membrane surface is considerably influenced by the properties of the fertilizers used as the DS (Li et al, 2017a). Despite these drawbacks, FDFO was found to be feasible for wastewater treatment, and it has exhibited high OMPs removal during the treatment of AnMBR effluent (Kim et al, 2017c). When commercial hydroponic fertilizer solutions were evaluated for use as the DS, the fertilizer solution diluted by a pilot-scale FDFO system was found to be appropriate for a hydroponic application (Chekli et al, 2017a).…”

“…For evaluation, three different fertilizers, namely potassium chloride (KCl), monoammonium phosphate, i.e., NH4H2PO4 (MAP), and diammonium phosphate, i.e., (NH4)2HPO4 (DAP)) were employed as draw solutes in this study because they have different properties and performances (i.e., water flux, RSF, and pH change of the FS) as reported in the FDFO process (Kim et al, 2017c). To identify the effect of the physicochemical properties of the OMPs surfaces on their transport, the pore-hindrance transport model, which has been utilized to estimate rejections by size exclusion (Rodgers and Miller, 1993), was employed and compared to the experimental data.…”