Production of a bioflocculant using old polyester fibre as a fermentation feedstock and its use in treatment of polyester alkali-peeling wastewater

Highlights

• The process described overcomes the high bioflocculant production cost.

• Polyester fibre -hydrolysates were utilized to generate bioflocculant.

• Alkaline degradation was used to pretreat polyester fibre waste.

• The bioflocculant was applied for the treatment of polyester alkali-peeling wastewater.

Abstract

Old polyester fibre (OPF) was used for the first reported microbial flocculant (MBF) production by Diaphorobacter nitroreducens R9. Pretreatment of OPF was performed by different bases individually. The optimal pretreatment conditions were determined by total organic carbon (TOC) content of hydrolysate supernatants. Although all alkaline OPF-hydrolysate culture media were investigated for MBF production, KOH hydrolysate produced the highest yield. The influence of KOH OPF-hydrolysate culture condition factors were applied throughout the response surface methodology, and the optimal conditions with the maximum MBF yield (4.02 g/L) were 33.5 °C, pH 9.5, and hydrolysate TOC (4.416 g/L). Furthermore, the MBF was applied to treat polyester alkali-peeling wastewater (PAPW) for the first time, and the highest removal efficiency was achieved when 1.0 g/L MBF was added to PAPW. In general, this contribution reveals that KOH OPF-hydrolysate could be used as a potential inexpensive carbon source for production of MBF by sustainable processes that could reduce environmental pollution.

Introduction

Polyester fabric (PT) is the most widely used artificial fibre worldwide because of its perfect combination of chemical and mechanical properties [1]. In 2020, global PT production reached 54.8 million tons for manufacturing of textile materials [2]. Large quantities of polyester alkali-peeling wastewater (PAPW) are released in the process of generating pliable and elegant polyester fabrics. During the alkali-decomposition process, the superficial polyester is peeled off from the fabric and hydrolysed in NaOH solution, resulting in the emission of terephthalic acid (TPA), ethylene glycol (EG), bis-hydroxyethyl terephthalate (BHET), mono (2-hydroxyethyl) terephthalic acid (MHET), and their polymers as pollutants in wastewater [3]. The discharged wastewater during alkali-decrement process is characterized by high organic concentration (COD: 20,000–100,000 mg/L) and high pH due to polyester hydrolysis. Incomplete hydrolyzed polyester is also presented, resulting in a poor biodegradation efficiency during the conventional biological processes [3]. It is toxic and carcinogenic in nature because it has endocrine disrupting ability [4]. Unlike other textile wastewaters, the micro-particles in PAPW are not easily precipitated due to its low-density and strong alkalinity properties [4], so the major challenge is to establish an efficient method for treating PAPW. Various of methods have been developed for the removal suspended solids from wastewater, and the most commonly used have been precipitation, adsorption, filtration, ion-exchange, electroplating and coagulation [5]. There are some significant problems when these methods are applied to the removal of suspended solids from wastewater, such as high energy consumption and a high yield of toxic sludge [6]. The development environment friendly technologies for the removal of suspended solids is thus of both academic and practical interest.

Flocculation is an efficient technology to deposit suspended particles in effluent, and flocculants have been practically applied in numerous industrial fields, such as dye removal and microalgal harvest [7], [8]. However, suspended matter in PAPW is difficult to precipitate due to this wastewater’s high pH and low-density properties which restrict the use of traditional flocculants [9]. Microbial flocculant (MBF) is composed of exopolysaccharides and proteins produced by microorganisms [10], [11]. It not only adapts to pH variation [12] but also shows excellent efficiency in flocculating various particles, including bacteria and protozoa [13]. Moreover, compared with traditional artificial synthetic flocculants, MBF exhibits more favourable biodegradability properties, preventing secondary pollution [14]. Although a few MBFs have been commercially produced, high production cost is the major hinderance impeding the industrial production of MBF. In recent years, various efforts have been exerted towards reducing the cost of producing MBF, including seeking inexpensive alternative feedstocks. For example, highly concentrated industrial and agricultural wastewaters, such as palm oil mill effluent [15] and livestock wastewater [16], have been successfully utilized as low-cost sources of nutrition; and the hydrolysates of bran and corncob have been used as cost-effective feedstocks [17]. Current attention is mainly focused on utilizing abundant natural wastes for MBF, but very few efforts have addressed artificial synthetic material wastes.

Polyester fibre is one of the most important textile materials [18]. For decades, application of polyester fibre has dramatically improved because of its low price, processability, and bio-inertness [19]. However, its high durability and non-biodegradability are a mixed blessing, and an increasing number of people are greatly concerned that polyester fibre waste can persist in the environment for dozens of years due to its ability to resist degradation [20]. OPF mainly consists of terephthalic acid (TPA) and ethylene glycol (EG), which could potentially be alternative carbon resources for value-added product biosynthesis [21]. In a recent study, the novel bacterium Delftia sp. WL-3 was isolated, which could degrade PET film and utilize PET as major carbon source [22]. Diaphorobacter nitroreducens R9 has been reported as a flocculant-producing strain, and the alkaline condition can induce strain R9 to secrete MBF [23]. In this study, strain R9 was found to be able to simultaneously produce alkaline lipase and bioflocculant. Thus, old polyester fibre (OPF) was first used as an inexpensive alternative carbon source for producing MBF. Furthermore, the obtained MBF from strain R9 was named as MBF-9 and used for the first time to treat PAPW which shows a low-density and high pH property. This work could improve the comprehensive utilization of OPF and the recycling of PAPW.