Role of polyurethane-modified layered double hydroxides on SCFAs extraction from waste activated sludge fermentation liquid for elevating denitrification: Kinetics and mechanism

Highlights

• A waste-to-resource concept was established for eggshell waste and WAS treatment.

• PU-LDHs can be repeatedly used as the carrier of SCFAs with high adsorption capacity.

• PU-LDHs retained SFL showed good performance as electron donor in denitrification.

• Recycle test confirmed PU-LDHs can be reused multiple times for biodenitrification.

Abstract

Extraction of short-chain fatty acids (SCFAs) from fermentation liquid of waste activated sludge (WAS) is the key bottleneck hindering its application as electron donor in denitrification. This study explores the feasibility of polyether-type polyurethane (PU)-modified layered double hydroxides (LDHs, prepared using eggshell waste as calcium source) in SCFAs adsorbing from WAS fermentation liquid (SFL). The adsorption parameters were first optimized by adsorption tests using artificial fermentation liquid (AFL). Then, adsorption kinetics, thermodynamic and isotherms were explored to further understand the adsorption mechanism. It revealed that SCFAs absorption by PU-LDHs from SFL was an endothermic and spontaneous process with positive enthalphy (ΔH^◦) values and negative Gibbs free energy (ΔG^◦) values. In addition, the maximum adsorption capacity of 208.0 mg SCFAs/g PU-LDHs was obtained from the Langmuir isotherm. Noting that both soluble carbohydrates and soluble proteins were simultaneously extracted, with efficiencies of 30.9%, 6.2%, respectively, compared with 62.9% SCFAs. The reuse tests confirmed that the prepared PU-LDHs can be used at least three times with high adsorptive capacity. With PU-LDHs-loaded SFL as external carbon source in the biodenitrification process, a denitrification rate of 0.014 mg NO₃⁻-N/mg mixed liquid suspended solids (MLSS)·d was recorded. This study provided a sound basis for the preparation of cost-effective biodenitrification carbon source from SFL by a novel adsorbent.

Introduction

Denitrification process was a vital procedure to alleviate water eutrophication by converting nitrite and nitrate to nitrogen gas (N₂) in the wastewater treatment process (Michal et al., 2002). During the process, sufficient carbon sources were in the need as electron donor, especially for small molecules such as acetic acid, glucose and methanol. However, the available carbon sources in the influent of municipal wastewater treatment plants (WWTPs) was relatively low with chemical oxygen demand to nitrogen ratio (COD/N) of 3~4, limiting improved efficiency of nitrogen removal. Thus, large amounts of external carbon sources were added in the biological nutrient removal (BNR) process, increasing the economic burden of WWTPs (Park et al., 2009; Sun et al., 2017). Recently, exploration of internal carbon source from waste activated sludge (WAS) was disposed by anaerobic fermentation has attracted increasingly attention due to its abundant organics. Short-chain fatty acids (SCFAs) produced from WAS fermentation was proved to be alternative carbon sources for denitrification, reducing the environmental risk caused by sludge disposal as well (Wang et al., 2017). It revealed that the denitrification rate raised by approximately 55% when SCFAs were used as electron donor (Dong et al., 2010; Reyes-Avila et al., 2004). Fermentation liquid from alkaline pretreated WAS were believed to be efficient as carbon source in nitrogen removal due to high proportion of available carbon sources, acetate acid, for instance (Sun et al., 2017). However, the waste activated sludge fermentation liquid (SFL) contains not only SCFAs, but also large molecules bio-metabolites and refractory organics, e.g., proteins, lipids and humus, which increase the operation overhead of biological nutrients process and are detrimental to microbes in wastewater treatment (Wang et al., 2019). Therefore, seeking for an efficient and reusable adsorbent to extract SCFAs is the key bottleneck for the in situ application of SFL as carbon source for denitrification.

Layered double hydroxides (LDHs) have a great anion adsorption capacity due to their M(II)-M(III) cation-pair layered structure (Theiss et al., 2016; Chen et al., 2017). LDHs was wildly used in wastewater treatment industries to remove pollutants such as heavy metal ions, bromate, dyes, and phosphate (Li et al., 2014b; Peng et al., 2014; Yu et al., 2015; Shen et al., 2017; Kou et al., 2018). Wei et al. found that F⁻ could be removed by Ca-Al-CO₃, and results showed that the adsorption competitions of F⁻ with Cl⁻ and NO₃⁻ were not observed during the adsorption process (Wei et al., 2020). Ca/Al LDHs was also showed an excellent selenium removal performance. Its desorption and regeneration results showed that Ca/Al LDHs could be reused at least three times (Li et al., 2020). Recently, LDHs, as an efficient carrier of SCFAs with high adsorption capacity and slow-release property, have attracted more attentions. It was reported that Zn/Al intercalated with SCFAs performed better than SCFAs-Na as carbon source of denitrification (Li et al., 2017). Mg/Al, Ni/Al, and Ni/Fe LDHs were also believed with good properties of slow-release, when they were used as carbon sources with acetate intercalated (Jiang et al., 2018). However, powdery LDHs were always used as adsorbents, which mean large cost in material preparation due to limited recyclability. Noting that Ca/Al LDHs was relatively untapped as carrier of carbon sources, although several studies have explored its excellent performance to remove F⁻, Cl⁻, NO₃⁻ and selenium with high efficiency (Li et al., 2020). In addition, calcium could be easily obtained from wide natural and rich source. Eggshell waste, as the common raw material of food processing industry (its production is estimated to be 31.79 million tons in 2020), owing abundant calcium carbonate (94%–99.63%), can be regarded as a low-cost calcium source (Lunge et al., 2012). Thus, in this study, the pretreated eggshell waste was used to release calcium ions for Ca/Al LDHs preparation.

Generally, powdery LDHs were used as adsorbents, which mean large cost in material preparation due to limited recyclability. Therefore, it was a reasonable solution by loading the powdery LDHs onto other materials. Polyurethane (PU) is a suitable adsorbent for chemical matters uptake due to its excellent thermal resistance and mechanical properties, porous structure and high specific surface area (Saeed et al., 2013). It was reported that the polar and non-polar groups in PU's structure in favor of the retention of free anions with high polarizability. Although, PU had been utilized a lot in separation (Mohammadi et al., 2014; Saranya et al., 2017), there are few studies considering the possibility of the combination of PU and LDHs to solve the bottleneck of non-recyclability of powdery LDHs.

In this study, PU-modified LDHs was synthesized to extract SCFAs from SFL. Scanning electron microscopy (SEM) and fourier transformed infrared spectroscopy (FTIR) were used to characterize the prepared PU-LDHs. Adsorption parameters were optimized by a series of adsorption tests using artificial fermentation liquid (AFL), including contact time, mass ratios of LDHs to PU (LDHs/PU), pH value, temperature and initial concentrations of SCFAs. The adsorption thermodynamics and isotherms were also used to reveal the mechanism of the extraction process of SCFAs by PU-LDHs. Finally, application tests were performed, including SFL adsorption tests, PU-LDHs regeneration tests and denitrification tests with PU-LDHs-loaded SFL as carbon sources.