Evaluation of direct membrane filtration and direct forward osmosis as concepts for compact and energy-positive municipal wastewater treatment

Abstract

Municipal wastewater treatment commonly involves mechanical, biological and chemical treatment steps to protect humans and the environment from adverse effects. Membrane technology has gained increasing attention as an alternative to conventional wastewater treatment due to increased urbanization. Among the available membrane technologies, microfiltration (MF) and forward osmosis (FO) have been selected for this study due to their specific characteristics, such as compactness and efficient removal of particles. In this study, two treatment concepts were evaluated with regard to their specific electricity, energy and area demands. Both concepts would fulfil the Swedish discharge demands for small- and medium-sized wastewater treatment plants at full scale: (1) direct MF and (2) direct FO with seawater as the draw solution. The framework of this study is based on a combination of data obtained from bench- and pilot-scale experiments applying direct MF and FO, respectively. Additionally, available complementary data from a Swedish full-scale wastewater treatment plant and the literature were used to evaluate the concepts in depth. The results of this study indicate that both concepts are net positive with respect to electricity and energy, as more biogas can be produced compared to that using conventional wastewater treatment. Furthermore, the specific area demand is significantly reduced. This study demonstrates that municipal wastewater could be treated in a more energy- and area-efficient manner with techniques that are already commercially available and with future membrane technology.

Research on Forward Osmosis Membrane Technology Still Needs Improvement in Water Recovery and Wastewater Treatment

“…Based on the above characteristics, it is also widely used in the fields of seawater desalination research and industrial wastewater treatment. P. Zhao [60] studied the performance comparison of anion and cation in the FO process of eight commonly utilized inorganic draw solutes, NaCl, NaNO 3 , KCl, KNO 3 However, studies have shown that the ammonium ion in ammonium bicarbonate has a small difference in hydration radius and water molecules, resulting in the interception performance of the draw solute is undesirable, the RSF is relative intolerable and contaminates the FS, which limits its applications.…”

“…Ge [64] employed 1.0 M oxalic acid complex with the formula of Na-3[Cr(C 2 O 4 ) (3) ] (Na-Cr-OA) as a draw solute, and recovered fresh water from raw water of high As(III) concentration with 1000 ppm at 60 • C, the relatively high water flux of 28 L/(m 2 ·h)(LMH) in FO mode and 74 LMH in pressure delay infiltration (PRO) mode are achieved. Recovery rates of 21.6% (FO mode) and 48.3% (PRO mode) were also completed within 2 h. P. Zhao [60] employed a molecular weight of 3,000,000 polyacrylamide as a draw solution to study the FO performance in different modes, and further applied in dye wastewater treatment.…”

“…High water flux and solutes rejection are essential for the most ideal FO membrane. The following characteristics are necessary for ideal TFC-FO membrane which is as shown in Figure 1: (1) The active layer should be dense and ultra-thin to have a high rejection rate for solute; (2) The support layer should be thin enough and high mechanical strength; (3) The membrane should have a high hydrophilicity to increase the water flux, anti-fouling property and CP alleviation; (4) The membrane should have a wide pH and temperature tolerance and oxidation resistance range.…”

“…The process requires no additional pressure, as opposed to the RO process.Compared with other membrane technologies, FO has prominence advantages such as high energy-efficiency, low fouling propensity, high salt rejection, and low brine discharge. First of all, as a spontaneous infiltration process that does not require additional driven pressure, FO has the potential for low energy consumption [3]. With the appropriate DS and recycling method, this point will become the most attractive advantage of FO.…”

“…Besides, to separate the draw solutes, energy-intensive methods-such as NF, RO, and membrane distillation (MD)-are often integrated with FO. The investment and energy consumption of the integrated system has become a new problem [3]. Furthermore, CP and RSD are two significant factors impeding FO performance [14].…”