Abstract
Recovery of nutrients and energy from municipal wastewater has attracted much attention in recent years; however, its efficiency is significantly limited by the low-strength properties of municipal wastewater. Herein, we report a pilot-scale forward osmosis (FO) system using a spiral-wound membrane module to concentrate real municipal wastewater. Under active layer facing feed solution mode, the critical concentration factor (CCF) of this FO system was determined to be 8 with 0.5 M NaCl as draw solution. During long-term operation at a concentration factor of 5, (99.8 ± 0.6)% of chemical oxygen demand and (99.7 ± 0.5)% of total phosphorus rejection rates could be achieved at a flux of 6 L/(m2 h) on average. In comparison, only (48.1 ± 10.5)% and (67.8 ± 7.3)% rejection of ammonium and total nitrogen were observed. Cake enhanced concentration polarization is a major contributor to the decrease of water fluxes. The fouling also led to the occurrence of a cake reduced concentration polarization effect, improving ammonium rejection rate with the increase of operation time in each cycle. This work demonstrates the applicability of using FO process for wastewater concentrating and also limitations in ammonium recovery that need further improvement in future.
Highlights
Forward osmosis (FO) is a membrane separation process with a semi-permeable membrane placed between a feed solution (FS) of a low osmotic pressure and a draw solution (DS) of high osmotic pressure, and is driven by the osmotic pressure difference across the membrane[13]
The above-mentioned studies lay the groundwork for understanding the behaviours of FO systems for concentrating wastewater; it is still insufficient to establish a general rule for these systems since most of the studies use lab-scale FO systems under batch-filtration mode and the experimental duration lasts for several hours to several days[18,19,20,21]
cellulose triacetate (CTA) membrane used in this study were determined to be 0.70 ± 0.07 L/(m2 h bar) and 0.53 ± 0.03 L/(m2 h), respectively, which are similar to previous publications[22,23]
Summary
CTA membrane used in this study were determined to be 0.70 ± 0.07 L/(m2 h bar) and 0.53 ± 0.03 L/(m2 h), respectively, which are similar to previous publications[22,23]. The measured water and solute fluxes increase with the increase of osmotic pressure; the water fluxes of CTA membranes deviate from the theoretical flux using the linear curve (Jv = A(π draw − π feed)) based on the classical solution-diffusion theory[13] but can be well modeled by Eq (1), indicating that ICP24 can significantly impact the water fluxes. The Km value can be used in the fouling-incorporated water flux model (Eq (3)) for evaluating the performance of this pilot-scale FO membrane system for concentrating real wastewater. The CCF of this pilot-scale FO system for concentrating municipal wastewater was determined to be 8, indicating that this FO system should be operated with CF less than 8, i.e., a sub-critical CF, for achieving a cost-effective performance. Step-wise diluting of the concentrated wastewater did not restore the water fluxes back to those in
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