An integrated biological-electrocatalytic process for highly-efficient treatment of coking wastewater
Introduction
Coking wastewater has received widespread attentions for its potential to cause serious environmental pollutions. Coking wastewater contains various kinds of pollutants such as phenolic compounds, cyanide, poly nuclear aromatic hydrocarbons, most of which are toxic, mutative and carcinogenic (Wu et al., 2018). The biological treatment of anaerobic/anoic/oxic (A/A/O) process is considered to be the primary option for refractory wastewater treatment (Sheng et al., 2020). However, the performance of conventional A/A/O systems is not entirely satisfactory, especially at low temperatures (Zhang et al., 2017). In recent years, electrochemical technology, especially three-dimensional electrochemical technology, has been focused on because of its easy operation, small footprint and short hydraulic retention time (HRT). Ji et al., utilized a three-dimensional electrochemical reactor (3DER) to treat rhodamine B wastewater and the chemical oxygen demand (COD) removal efficiency is 60.13% after 30 min of electrolysis (Ji et al., 2018). Li et al., achieve a COD removal efficiency of 48.95% using a 3DER for the treatment of acid orange 7 dyeing wastewater (Li et al., 2017). However, the electrochemical process requires considerable energy input. To save the operation costs, 3DER is primarily considered for the pretreatment of refractory wastewater.
Although 3DER can degrade organics in coking wastewater, associated nitrogen removal is less satisfactory, with NH4+-N being especially difficult to remove. Therefore, the treatment processes of nitrification and denitrification are necessary. Biological aerated filters (BAFs), a classic biological treatment technology, is widely applied in wastewater treatment. In general, BAFs that filled with biologically attached fillers have a stable nitrification function without NO2–-N accumulation (Su et al., 2021). Zhang et al., obtain COD and NH4+-N removal efficiencies of 49.70% and 75.80%, respectively, by using combined ozonation and BAF to treat coking wastewater (Zhang et al., 2014). Therefore, BAFs process can realize nitrification treatment of toxic coking wastewater.
In general, a certain amount of pollutants including residual COD and nitrate (NO3–) still present in the wastewater after BAFs treatment. Therefore, a post-treatment process is still necessary. After treating by 3DERs and BAFs, the stubborn COD residuals and the generated NO3– are difficult to be removed by conventional biological technologies. Three-dimensional biofilm electrode reactors (3DBERs) might be a good choice for denitrification by taking the advantages of biofilm and electrochemical technologies (Feng et al., 2018). Hydrogen (H2) generated at cathode can be also utilized as electron donors for improved NO3– reduction (Tang et al., 2020). Furthermore, the stimulation of weak currents can improve the activity of microbial enzymes, thereby enhancing the biodegradation effect (Hao et al., 2013). Therefore, it is necessary to design and optimize post-treatment processes (e.g., 3DBER) for the advanced treatment of COD and nitrogen in coking wastewater.
This work aims to develop an integrated 3DERs/BAFs/3DBER process for the advanced treatment of coking wastewater. 3DERs are responsible for degrading COD and NH4+-N via electrochemical redox reactions. BAFs convert residual NH4+-N to NO3–-N by microbial nitrification. 3DBER reduces NO3–-N by bio-electrochemical denitrification. The degradation processes of coking wastewater in the integrated system are explored, and the function of each reactor in the integrated 3DERs/BAFs/3DBER system is identified. Moreover, the potentials of 3DBER to eliminate COD and various species of nitrogen (NH4+-N, NO3–-N, NO2–-N and TN) are evaluated. The results provide a solution to practical coking wastewater treatment via this integrated 3DERs/BAFs/3DBER system.
Section snippets
Wastewater source
Coking wastewater was obtained from an adjusting tank in Baosteel Co. (Shanghai, China). The influent was prepared by diluting raw wastewater and tap water at a ratio of 1:1. The influent was filtered through coarse filter paper to remove large particles. Chemical analysis of the resulting influent was carried out to determine its composition (Table 1).
Experimental procedure
The 3DERs/BAFs/3DBER system was composed of two 3DERs, two BAFs and a 3DBER (Fig. 1). Series connection of multi-stages wastewater treatment
Electrochemical degradation of coking wastewater in the 3DERs
The evolutions of COD and nitrogen in the 3DERs are shown in Fig. 2. The concentrations of TN, NH4+-N and NO3–-N in coking wastewater decreased obviously, and the removal efficiencies were 44.42%, 38.02% and 91.46%, respectively. NH4+-N was removed mainly by the electrochemical oxidation process (Deng et al., 2019). The 3DERs showed excellent removal efficiency for NO3–-N. Generally, the elimination of NO3–-N was the result of cathode reduction (Li et al., 2010). The concentration of NO2–-N in
Conclusions
The integrated 3DERs/BAFs/3DBER system demonstrates good performance in the treatment of coking wastewater. 74.72–83.27% of COD and 69.64–99.83% of TN are removed after being treated by the integrated system. 3DERs are mainly responsible for degrading COD and nitrogen by electrochemical redox reactions. BAFs have the capability of converting NH4+-N into NO3–-N and remove residual COD. 3DBER transforms NO3–-N to N2 by bio-electrochemical denitrification. Flowing through the integrated system,
CRediT authorship contribution statement
Zhen-Yu Wu: Investigation, Data curation, Writing - original draft. Wei-Ping Zhu: Investigation, Validation. Yang Liu: Investigation, Validation. Lu-Lu Zhou: Investigation, Validation. Peng-Xi Liu: Investigation, Validation. Juan Xu: Conceptualization, Data curation, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors wish to thank National Natural Science Foundation of China (51978266).
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