Wastewater treatment by forward osmosis (FO) based system for resource recovery

Abstract

Resource recovery from wastewater has attracted significant attention in the circular economy. Among different technologies, forward osmosis (FO) as an emerging membrane technology has the potential to transform municipal wastewater treatment plants into factories for water, nutrients and energy production by concentrating very diluted municipal wastewater for direct nutrients recovery and subsequent water and energy recovery. Although FO membrane has much lower fouling propensity than reverse osmosis (RO) due to much lower pressure applied to membrane, fouling is still one of the barriers to the FO application. The first technical chapter of this thesis investigated the concentration of synthetic and real municipal wastewater to 90% water recovery rate by two different configurations, i.e., hollow fiber and flat sheet thin film composite (TFC) FO membranes, and their associated membrane fouling and cleaning. Results show that the FO membrane had high rejection rates of COD, phosphate, Ca²⁺ and Mg²⁺ with concentration factors at around 8 when achieving a 90% water recovery rate, which facilitated downstream phosphate recovery by direct precipitation and energy production by anaerobic digestion (AD). The significantly increased Ca²⁺ and phosphate concentrations after FO filtration with 90% water recovery was found to be the main factor contributing to inorganic scaling, which was harder to be cleaned by physical cleaning compared with cellulose particles used to simulate suspended solids (SS) in synthetic wastewater. Both hollow fibers and flat-sheet configurations are found not to be suitable for treatment municipal wastewater without prior SS removal although flat sheet configuration was still able to achieve a 90% water recovery rate but with water flux of only 7.5 L/m²·hr at 90% water recovery rate while hollow fiber FO was completely clogged with operation failure. The use of a spacer in the flat sheet configuration did not alleviate membrane fouling during the membrane filtration process, but it improved the efficiency of the following physical cleaning by around 15%. This study highlighted the importance of the chemistry of feed solution (FS) and draw solution (DS) and FO membrane configuration on membrane fouling particularly at high water recovery rates and the necessity of pre-treatment of municipal wastewater. For wastewater treatment with FO, it has been widely reported that FO has very low ammonium rejection primarily due to the negative charge nature of membrane. This restricts the application of FO for wastewater treatment particularly with a purpose of ammonium and water recovery. The second technical chapter in this thesis thus aimed to enhance ammonium rejection in the FO process by selecting DSs with different physiochemical characteristics. Results show that under the same osmotic pressure divalent cation DS (i.e. ii Mg²⁺) with larger hydrated radius resulted in a higher ammonium rejection than monovalent cation DS (i.e. Na⁺ ) with lower hydrated radius. For the same cation based DS, DS with lower diffusion coefficient showed higher ammonium rejection. These results imply that impeding the exchange of cation in DS with ammonium in FS by increasing cation radius or reducing diffusion coefficient could minimize the ammonium permeation to DS. Non-ionic DS such as glucose, glycine and ethanol are able to minimize cation exchange between feed and draw solutions, leading to a 98.5-100% ammonium rejection rate. This further validate that even with negative charge of membrane, cation exchange between FS and DS is critical for ammonium permeation. The treatment of filtered municipal wastewater and sludge digestate with glucose and NaCl as DSs, respectively, proved that the non-ionic nature of glucose resulted in a high rejection rate not only for ammonium, and other cations such as Ca²⁺ and Mg²⁺ but also for anion such as PO₄³⁺. This is for the first time that a generic guideline could be proposed based on cation exchange mechanism for the selection of DS in wastewater treatment especially when ammonium and water recovery is a concern. To recover water from municipal wastewater, FO is usually needed to combine with other technologies. Membrane distillation (MD) offers a promising solution for simultaneous water recovery from DS and DS regeneration when waste heat is easily available. Although the integrated FO-MD system was studied in literature, temperature effects and heat balance in such integrated system are unclear. In the third technical chapter, the effect of FS and DS temperatures was examined. In addition, ammonium permeated from FS to DS was studied in the MD configuration for its potential contamination to the recovered water. It is found that higher FS and DS temperatures resulted in a higher water flux and a higher RSF from either NaCl or glucose as DS due to the increased diffusivity of molecules. However, the water flux increased at a higher rate with glucose DS than with NaCl DS by 10-14.8%, while the RSF increase rate with NaCl DS was two times higher than glucose DS. In addition, the use of NaCl DS at higher DS and FS temperatures such as 50 and 42 °C, respectively, resulted in more ammonium permeation from the FS to the DS, whereas ammonium was completely rejected with glucose DS even at high temperature. These results are very important because in the integrated FO–MD system at higher temperatures such as 50 °C, the advantage of high flux of NaCl as DS was not increased significantly while the disadvantages of high RSF of NaCl and lower ammonium rejection were greatly amplified. On the contrary, the disadvantage of low water flux of glucose as DS was overcome at 50 °C while 100% ammonium rejection was still maintained. This implies that temperature needs to be an important factor for the consideration in selecting DS in FO-MD system. Furthermore, a simple heat balance calculation suggests that if there was no heat recovery in the FO-MD system, waste heat from power plants is far less than the heat iii demand in such systems for municipal wastewater treatment. An internal heat exchange between different streams was proposed to maximize the efficiency of heat utilization, but unarguably it would increase the complexity of the FO-MD system. Further study is needed regarding heat efficiency. Overall, this project shows that the FO membrane fouling, the associated cleaning methods and ammonium rejection can be enhanced by understanding the feed and draw solutions chemistry. In addition, this project highlights the importance of the operation temperature when selecting the DS and the necessity of internal heat recovery in the FO-MD system.

More information

Identifiers

ORCID for Yongqiang Liu: orcid.org/0000-0001-9688-1786

Catalogue record

Export record