A novel forward osmosis reactor assisted with microfiltration for deep thickening waste activated sludge: performance and implication

Highlights

• A novel FO reactor assisted with MF for sludge thickening was developed.

• A deep thickening of WAS was achieved from approximately 7 to 50 g/L.

• An excellent water quality of FO membrane permeate was obtained.

• The flux decline of FO membrane was mainly due to membrane fouling.

• MLSS concentration significantly influenced the fouling mechanism of FO membrane.

Abstract

Waste activated sludge (WAS) treatment has gained growing interests for its increasingly capacity and high process cost. Sludge thickening is generally the first process of the WAS treatment. However, traditional sludge thickening approach was restrained by large footprint, low thickening efficiency, and tendency of releasing phosphorus. Here, we reported a novel microfiltration (MF) membrane assisting forward osmosis (FO) process (MF-FO) for sludge thickening. The MF-FO reactor achieved a sludge thickening of the mixed liquor suspended solids (MLSS) concentration from approximately 7 to 50 g/L after 10-day operation. More importantly, the effluent quality after FO filtration was superior with total organic carbon (TOC), ammonia nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃⁻-N) and total phosphorus (TP) of 1.94 ± 0.46, 0.02 ± 0.07, 4.55 ± 1.59 and 0.24 ± 0.26 mg/L, respectively. Additionally, the integration of MF membrane successfully controlled the salinity of the MF-FO reactor in a low range of 1.6-3.1 mS/cm, which mitigated the flux decline of FO membrane and thus prolonged the operating time. In this case, the flux decline of FO membrane in the MF-FO reactor was mainly due to the membrane fouling. Furthermore, the fouling layer on the FO membrane surface was a gel layer mainly composed of biofoulants and organic foulants when the MLSS concentration was less than 30 g/L, while it turned to a cake layer when the MLSS concentration exceeded 30 g/L. Results reported here demonstrated that the MF-FO reactor is a promising WAS thickening technology for its excellent thickening performance and high effluent quality of FO membrane.

Introduction

With increase of municipal wastewater capacity and improvement of the wastewater treatment process, waste activated sludge (WAS), a by-product of wastewater treatment, is growing substantially in daily operation (Zhu et al., 2012). It is estimated that the cost of WAS treatment is equivalent to wastewater treatment (Pei et al., 2010; Yuan et al., 2011; Nguyen et al., 2013, 2015; Collard et al., 2017). Accordingly, the treatment and disposal of WAS has aroused growing interests. Sludge thickening is the widely adopted approach to decrease the water content of WAS for achieving the reduction of sludge volume. Currently, the general sludge thickening methods are applied for WAS including gravity thickening, dissolved air flotation (DAF) thickening and centrifugal thickening (Wang et al., 2008a). Taking the gravity thickening process for example, it has some drawbacks such as a large footprint, a low thickening efficiency, a tendency of releasing phosphorus during long sludge retention time (SRT), and emission of unpleasant odors (Wang et al., 2008a; Zhu et al., 2012; Lee et al., 2017). In addition, it should be pointed out that the sludge supernatant with high concentrations of organic matters, nitrogen and phosphorus from the traditional thickening technologies has to be further treated via returning to WWTPs or discharging after post-treatment, which not only enhances the cost of thickening process but also complicates the thickening process.

Conventional sludge thickening technologies are low in efficacy and high in energy consumption, thus dedicated studies have devoted to developing new sludge thickening processes. Among these novel processes, applying microfiltration (MF) membrane for sludge thickening is an interesting attempt (Wang et al., 2008a, 2008b, 2009; Wu et al., 2009; Kim et al., 2013), which utilizes the effective separation of water and solid via MF membrane to achieve thickening of WAS. It can effectively reduce the water-content of WAS to about 97%, which is same as the traditional thickening technologies (Wang et al., 2008a, 2008b, 2009). In addition, it possesses a less footprint, a higher solid recovery and a better supernatant water quality compared with the traditional thickening technologies (Kim et al., 2010, 2013). Therefore, the MF process has been considered as a potentially alternative method for WAS thickening.

Although the MF process has many advantages over the traditional thickening technologies, it also has some bottlenecks retarding its wide application including serious membrane fouling and membrane permeate needing further treatment. Compared to conventional membrane separation technology, forward osmosis (FO) remains a unique, attractive and emerging technology after it was first proposed decade ago. FO utilizes a draw solution (DS) with a high osmotic pressure to “draw” the water from a feed solution (FS) with a low osmotic pressure through a semi-permeable membrane (She et al., 2016; Wang et al., 2016a). Based on the fact that FO membrane has high rejection and superior water flux stability against fouling (Gu et al., 2013; Li et al., 2019), a novel sludge thickening technology using FO membrane instead of MF membrane has been proposed (Hau et al., 2014; Nguyen et al., 2015, 2016; Lee et al., 2017; Ng et al., 2019; Sun et al., 2019; Zhao et al., 2019). In comparison with the MF process, the FO process had a better effluent quality and a similar thickening efficiency. However, the operation of FO process in most studies does not last for long time (less than 24 hours), and there is lack of evaluating membrane fouling and effluent quality in these reports. Besides, the WAS is only concentrated to about 35 g/L owing to the decline of FO membrane flux. The flux decline of FO membrane is mainly attributed to salt accumulation induced by high MLSS and reverse salt transport (RST) (Zhu et al., 2012). Salt accumulation is a common phenomenon in FO membrane reactor. For instance, many literatures have reported the cause and damage of salt accumulation in osmotic membrane bioreactor (OMBR) for wastewater treatment (Qiu et al., 2013; Wang et al., 2014a, 2016a, 2017a; Yang et al., 2018).

Recently, some researchers proposed a novel method to control salt accumulation in the OMBR via combining MF or UF membrane for discharging soluble salt (Wang et al., 2014b; Holloway et al., 2015). Inspiring by the successfully using MF membrane for alleviating salt accumulation in the OMBR, we intend to integrate MF membrane with FO membrane (called MF-FO process). In the MF-FO process, salinity build-up can be effectively controlled by the MF membrane, and thus enlarging the operation time of FO membrane. In this case, a deep thickening of WAS might be achieved in the MF-FO process, which will not only enhance the economic performance of MF-FO process but also be beneficial to follow-up treatment via reducing sludge volume. Although the integration of MF membrane and FO membrane has been widely reported in the OMBR process for wastewater treatment, this is the first attempt on applying the hybrid MF plus FO process for thickening sludge. The prior reported experimental techniques and data in the literatures cannot be justified and directly used in the context of sludge management. The objectives of this study are to concentrate WAS to about 50 g/L via FO membrane with a help of MF membrane for controlling salt accumulation and to further investigate the effluent quality and fouling behavior of FO membrane at a high MLSS condition.