# Pilot-Scale Fenton-like System for Wastewater Treatment Using Iron Mud Carbon Catalyst
## 1 Introduction  
With the rapid development of industrialization and urbanization, the content of refractory organic pollutants in wastewater has gradually increased, posing a serious threat to water ecosystems and human health [1]. In addition, some key regions have more stringent effluent standards for COD in wastewater. For example, the effluent from the Wuxi wastewater treatment plant in China should meet the \"Discharge standard of main water pollutants for municipal wastewater treatment plant & key industries of Taihu area (DB32/1072-2018)\" [2], with a COD effluent standard of 40 mg\(\cdot\)L\({}^{-1}\). However, the COD concentration of the effluent after A\({}^{2}\)O treatment was 60-80 mg\(\cdot\)L\({}^{-1}\), which did not meet the local effluent standards in Wuxi. Therefore, it is necessary to introduce deep treatment to reduce the COD value of the effluent. The Fenton reaction has been widely applied in the field of treating refractory organic wastewater with simple operation, rapid reaction, and thorough mineralization [3]. Specifically, Fenton oxidation can generate hydroxyl radicals(\({}^{\bullet}\)OH) by activating H\({}_{2}\)O\({}_{2}\) with Fe(II). However, the traditional Fenton process can produce large amounts of iron sludge and react under acidic conditions (pH = 2-4), which limits its comprehensive application in industrial wastewater treatment [4]. To overcome the limitations of the traditional Fenton process, researchers have proposed a heterogeneous Fenton-like system based on hydrogen peroxide. This system shows promising application prospects in the treatment of refractory organic wastewater with a wide pH applicability range, reusable catalysts, and no iron sludge generation [5]. At present, the heterogeneous Fenton-like catalysts include metal and non-metal catalysts [6; 7], metal carbon-based catalysts [8], transition metal-modified composite catalysts [9], nanomaterials, and magnetic catalysts [10], etc. Fenton iron mud, as a by-product of the Fenton process, is rich in iron and carbon elements and can be used as a raw material for carbon-based catalysts [11]. Specifically, Fenton iron mud carbon catalyst (FSC) preparation via pyrolysis can achieve \"reducing waste with waste\", which presents practical application potential. However, the efficiency of the direct use of FSC for the activation of H\({}_{2}\)O\({}_{2}\) for the degradation of organic pollutants is relatively low [12]. Previous studies have demonstrated that the catalyst prepared from Fenton iron sludge can activate the single oxidant (H\({}_{2}\)O\({}_{2}\)). It achieves a COD removal rate of 65% at an initial concentration of 200 mg/L [13]. However, the experiments were conducted under laboratory water conditions, and the catalytic effect of the catalyst under actual wastewater conditions remains unknown. Research showed that the combination of H\({}_{2}\)O\({}_{2}\) with other oxidants could synergistically degrade organic pollutants and enhance oxidation efficiency [14]. For instance, the combination of H\({}_{2}\)O\({}_{2}\) and sulfite can rapidly degrade polycyclic aromatic hydrocarbons with a reaction rate constant (_k_) of 10.20 \(\times\) 10\({}^{-3}\) M\({}^{-1}\cdot\)s\({}^{-1}\). However, the single oxidant has a relatively low response to polycyclic aromatic hydrocarbons, with the \(k\) value being only 0.26 \(\times\) 10\({}^{-3}\) M\({}^{-1}\)s\({}^{-1}\)[14]. In addition, the combination of H\({}_{2}\)O\({}_{2}\) and calcium peroxide (CaO\({}_{2}\)) can generate singlet oxygen (\({}^{1}\)O\({}_{2}\)) within a wide pH range (3-12). At the same time, calcium peroxide is reduced to Ca(OH)\({}_{2}\), and tetracycline (TC) is degraded efficiently. The degradation rate of TC increased from 4% to 91% [15], indicating that the combination of oxidants can produce a synergistic effect and effectively enhance the degradation efficiency of pollutants. Therefore, introducing a second oxidant into the FSC/H\({}_{2}\)O\({}_{2}\) system could be beneficial for enhancing the oxidation performance of the system.