Understanding and mitigating the toxicity of cadmium to the anaerobic fermentation of waste activated sludge

Highlights

• Solubilization was enhanced by low Cd levels but inhibited by high Cd level.

• 0.1 mg/g VSS Cd benefited both the hydrolysis and acidification.

• 10 mg/g VSS Cd inhibited all the hydrolysis, acidification, and methanogenesis.

• High level Cd decreased the activity of enzyme and microbial diversity.

• An effective strategy to mitigate the adverse impact of high Cd levels was proposed.

Abstract

Cadmium (Cd) is present in significant levels in waste activated sludge, but its potential toxicities on anaerobic fermentation of sludge remain largely unknown. This work therefore aims to provide such support. Experimental results showed that the impact of Cd on short-chain fatty acids (SCFA) production from sludge anaerobic fermentation was dose-dependent. The presence of environmentally relevant level of Cd (e.g., 0.1 mg/g VSS) enhanced SCFA production by 10.6%, but 10 mg/g VSS of Cd caused 68.1% of inhibition. Mechanism exploration revealed that although all levels of Cd did not cause extra leakage of intracellular substrates, 0.1 mg/g VSS Cd increased the contents of both soluble and loosely-bound extracellular polymeric substances (EPS), thereby benefitting sludge solubilization. On the contrary, 10 mg/g VSS Cd decreased the levels of all EPS layers, which reduced the content of soluble substrates. It was also found that 0.1 mg/g VSS Cd benefited both the hydrolysis and acidogenesis but 10 mg/g VSS Cd inhibited all the hydrolysis, acidogenesis, and methanogenesis processes. Further investigations with microbial community and enzyme analysis showed that the pertinent presence of Cd enhanced the activities of protease, acetate kinase, and oxaloacetate transcarboxylase whereas 10 mg/g VSS Cd decreased the microbial diversity, the abundances of functional microbes, and the activities of key enzymes. Finally, one strategy that could effectively mitigate the adverse impact of high Cd levels on SCFA production was proposed and examined. This work provides insights into Cd-present sludge fermentation systems, and the findings obtained may guide engineers to manipulate sludge treatment systems in the future.

Introduction

Activated sludge process has been widely used in the treatment of municipal sewage. Although this process is effective to clean wastewater, large amounts of waste activated sludge (WAS) would be produced as the byproduct (Ni and Yu, 2008, Zhao et al., 2015d, Chen et al., 2016). In 2015, the yield of WAS in China reached 34 000,000 tons (at a water content of 80%) (Luo et al., 2016). The treatment and disposal of WAS has become one of major challenges faced by wastewater treatment plants (WWTPs) nowadays. WAS contains high levels of organic matter, such as protein and polysaccharide, thus reutilization of WAS as a plentiful inexpensive source is regarded as a preferable method for WAS management (Wang et al., 2013, Wang et al., 2015a, Zhao et al., 2015c, Xie et al., 2016, Zhao et al., 2016a).

Generally, WAS is treated by anaerobic digestion to produce biogas, methane (Wang et al., 2015b, Wang et al., 2016). Recently, anaerobic fermentation has attracted growing interests as the produced short-chain fatty acids (SCFA) is the value-added product that could be used as a raw substrate for biodegradable plastic (such as polyhydroxyalkanoates (PHA)) synthesis or a preferred carbon source for biological nutrient removal (Li et al., 2016, Wang et al., 2015a, Wang et al., 2017a, Zhao et al., 2015c, Zhao et al., 2016b). Many efforts were performed on this topic in the past years, but most of them focused on the promotion of SCFAs production by applying pretreatment methods, controlling the operating conditions of fermentation reactors, or optimizing the characteristics of fermented sludge (Carrere et al., 2010, Yan et al., 2010, Wang et al., 2013, Zhao et al., 2015c). For example, pretreatment of sludge with 1.54–1.80 mg/L free nitrous acid for 2 d resulted in a 1.5–3.7 fold increase in SCFA production (Zhao et al., 2015c, Li et al., 2016). When enhancing the level of sludge PHA from 25 to 178 mg/g VSS, SCFA production was enhanced by ∼2 times (Wang et al., 2015a).

As the byproduct of wastewater treatment, WAS not only collects various compounds but also concentrates a large number of pollutants, such as persistent organic pollutants and heavy metals (Smith, 2009; Subedi et al., 2014, Luo et al., 2016, Yi et al., 2017, Zhao et al., 2017a). Actually, as the application of wastewater diversion collection system is not widespread in developing countries, large proportions of industrial wastewater, e.g., about 35.0% in China, enters WWTPs, bringing in large amounts of heavy metals, such as Hg, Cd, and Pb (Xu et al., 2012, Chen et al., 2014, Luo et al., 2016). Among them, Cd is a widely used metal of commercial importance, which has a wide application field in industry, such as batteries, chemical stabilizers, pigments, and metal coatings (Olabarrieta et al., 2001). The wide production and utilization of Cd inevitably leads to its substantial release into WWTPs (Karvelas et al., 2003). It is reported that the Cd²⁺ concentration in municipal wastewaters in India and Turkey is greater than 0.1 mg/L, even greater than 0.5 mg/L in Greece (Karvelas et al., 2003, Üstün, 2009). According to our survey on 8 WWTPs in Central China, Cd²⁺ level in municipal wastewater reached at 0.54–1.39 mg/L due to widespread Cd-based factories. The Cd entered in WWTPs would be adsorbed by biomass and therefore most of them would be transformed from wastewater to WAS in the wastewater treatment process (Santos et al., 2004, Chen et al., 2014). It was documented that total concentrations of Cd in activated sludge was greater than 1 mg/g VSS (volatile suspended solids) in central-south region of China (Chen et al., 2008). Thus, the impact of Cd on the operation of WWTPs raises increasing concerns.

Cd²⁺ is highly soluble and can be transported into cells, and once inside the cell, it can destroy protein structure and function and even damage its cache of DNA, leading to teratogenicity (Nies, 1991). Some essential metals from metabolic sites may also be replaced by nonessential metals (e.g., Zn²⁺ with Cd²⁺), and the function of specific physiological cations was inhibited (Nies, 1991, Amor et al., 2001). Yu and Fang examined the effect of Cd on the anaerobic acidogenesis of a simulated dairy wastewater in a upflow anaerobic sludge blanket reactor, demonstrating that the acidogenesis process was enhanced by the Cd dosage less than 20 mg/L but inhibited by the dosage over 20 mg/L (Yu and Fang, 2001). However, Lin found that Cd level at <15 mg/L inhibited the production of acetic acid from anaerobic digestion of glucose (Lin, 1993). Although useful information was obtained, these investigations only focused on either anaerobic treatment of synthetic wastewater or anaerobic digestion of model substrate (e.g., glucose). To date, no information is available on the impact of Cd on anaerobic fermentation of real sludge.

Compared with the digestion of model substrate or synthetic wastewater, anaerobic fermentation of real sludge includes more complex reactions (e.g., sludge solubilization) and microbial community. These differences lead us to raise several questions: Does Cd at environmentally relevant level have significant impact on sludge anaerobic fermentation? If it does, how does Cd affect sludge anaerobic fermentation? How to mitigate the adverse impact of Cd on sludge anaerobic fermentation especially in the areas with high Cd pollution?

By answering these questions above, this work aims to deeply understand the impact behavior of Cd on anaerobic fermentation of sludge and develop a potential strategy to mitigate the adverse impact. Firstly, the effect of different dosages of Cd on SCFA production from sludge fermentation was investigated under acidic condition (pH 4) (Alvira et al., 2010). Then, details of how Cd affects the individual steps of sludge anaerobic fermentation (i.e., solubilization, hydrolysis, acidogenesis, and methanogenesis) were explored via a series of batch test, microbial community comparison, and key enzyme activity analysis. It is known that the toxicity of Cd is mainly ascribed to the released cadmium ion, Cd²⁺. Thus, two possible strategies that might mitigate Cd toxicity to sludge anaerobic fermentation (i.e., adding certain amount of chelant, EDTA and controlling fermentation under alkaline condition) were finally tested, because EDTA is reported to have capability of mitigating some heavy metals’ inhibition on enzymes while alkaline condition could precipitate the released Cd²⁺ as Cd(OH)₂ and CdS. To the best of our knowledge, this is the first study revealing the mechanisms of how Cd affects SCFAs production from anaerobic fermentation of WAS, and the findings obtained here may guide engineers to manipulate sludge fermentation systems in the areas with high Cd pollution.