Extracellular polymeric substances (EPSs) are very valuable and mainly derived from the autolysis, secretion, and shedding of microbial cells in excess sludge of wastewater treatment plants (WWTPs) [[1], [2], [3]], including polysaccharides, proteins, lipids, humus, nucleic acids, and other polymeric substances, accounting for 10 %–40 % of the dry weight of sludge [[4], [5], [6]]. EPSs are widely used to produce adsorbents [[5], [6], [7], [8]], fireproof materials [9], soil amendments [[10], [11]], and biological flocculants [[12], [13]]; furthermore, EPSs are utilized for biological phosphorus removal [14] and cartilage tissue engineering [15]. On the other hand, proteins and polysaccharides, which are the main components of EPSs, are recyclable substances with high-added value [1,[16], [17]].

The moisture content of EPSs extracted from excess sludge reaches up to 99.9 % [[5], [6]], therefore, concentrating EPS has become an important research focus. EPSs as biopolymers can be concentrated by hot-air, freeze, and vacuum drying; however, various problems, such as the loss of their biological activity [[18], [19], [20]], large energy consumption, and the time-consuming nature of these processes [[21], [22]], have been reported. Membrane separation has become a powerful concentration technology for biopolymers because it does not affect bioactivity. Compared to nanofiltration and reverse osmosis with high energy consumption [[23], [24]], micro- and ultrafiltration have been used to concentrate EPS solutions; however, strong membrane fouling occurs and further reducing moisture content is difficult because of extremely high filtration resistance [[5], [6]].

Forward osmosis (FO) is a novel concentration technology for biopolymers [[24], [25], [26]], which is driven by the osmotic pressure difference, including various advantages such as low membrane fouling, easy cleaning, and low energy consumption [[27], [28], [29]]. Chen et al. reported that the concentration effect of FO is higher than that of nanofiltration (NF) and reverse osmosis (RO), with minimal membrane fouling and concentration polarization observed when dairy products were employed as feed solutions [24]. Cai et al. reported that polysaccharides of Dendrobium officinale concentrated by FO achieved almost 1.3 times at the same time compared with that in NF and RO [25]. Therefore, the FO process may become an alternative technology to concentrate the EPSs; however, the conventional FO process has low concentration efficiency. For example, Ding et al. reported that the enrichment factor was only 1.27 for both 0.1 and 0.3 g/L bovine serum albumin (BSA) solutions after 6 h FO process [26]; Mondal et al. reported that the concentration factors of BSA (0.2 g/L → 1.46 g/L) and DNA (0.5 g/L → 1.781 g/L) were only 7.3 and 3.6 times, respectively, using FO [30].

Low concentration efficiency arises from the significant rise in viscosity within the feed solution during the conventional FO process, particularly in cross-flow mode on both sides of the FO membrane. This viscosity increase makes it difficult to further improve the concentration ratio. Dead-end forward osmosis (DEFO) technology, which employs a dead-end mode on the feed side and cross-flow mode on the draw side within the FO process, can contribute to maximizing the concentration rate of the feed and minimizing the effect of high viscosity [[31], [32], [33]].

In a previous study, we proposed a novel DEFO technology that has been successfully applied to the concentration and dewatering of sludge [31], in which the effect of the viscosity and fluidity of the feed solution can be disregarded because of the dead-end mode in feed side. Ma et al. also reported that the DEFO process can be used to concentrate microalgae dewatering and reduce the moisture content of biomass [33].

In this study, we achieved a significant milestone by successfully using DEFO to concentrate various biopolymers for the first time. A novel evaluation model, including the DEFO coefficient and osmosis resistance, was proposed to characterize the DEFO properties of EPSs and model biopolymers [sodium alginate (SA), BSA, and a mixture of both]. Various factors affecting the process such as types of draw solutes, osmotic pressure difference, biopolymer concentration, and crossing velocity on the draw side, were investigated. The components of the EPSs and concentrated substances and the mechanism underlying the interaction between the reverse osmosis metal ions and EPSs were analyzed. The results of this study confirm the feasibility of the use of DEFO for biopolymer concentration and provide insights into the use of this technology.