## 1 Introduction  
For the last few decades, electrochemical machining (ECM) has been used for machining the macro- and micro-components of tool steel, carbides, superalloys, and titanium alloys. These materials are heavily used in the automotive and aerospace industries, as well as electronics, optics, medical devices, and communications. ECM is used as a high-priority machining tool because of its specific advantages, such as minor tool wear, mechanical forceless machining, no heat-affected zones, high surface quality, low roughness, and stress-free surface products. ECM is the reverse process of electroplating and can machine any hard materials or complicated shapes [1]. In ECM, the workpiece and tool are connected with the anodic pole and the cathodic pole of the electrochemical circuit, respectively, and the electrolyte is pumped through the inter-electrode gap (IEG) between the tool and the workpiece. A high-current DC power supply is allowed to pass through this electrochemical circuit to dissolve the metal from the workpiece in the form of metal hydroxides (i.e., sludge). ECM is an efficient and low-cost machining method for Ni-based alloys [2]. Several works were carried out to study the machinability of Ni alloys by ECM [3; 4; 5]. Ni alloys are primarily used in aircrafts, power generation turbines, rocket engines, chemical processing plants, and nuclear power plants. Poor surface traits, a short insert life, high manufacturing costs, and low productivity are associated with the machining of nickel alloys. Monel 400 alloy is one of Ni alloys that exhibits excellent characteristics of corrosive resistance, strength, and toughness in challenging conditions [6]. When machining Ni-based alloys, ECM generates a large quantity of toxic sludge in the form of Ni and chromium hydroxides aside from gaseous byproducts, acids, sulphates, nitrates, oils, and metal ions. Environmentally sustainable machining of Ni-based alloys in ECM is needed while maintaining productivity and quality [7]. However, the discharge of nickel ions into the electrolyte slurry is significant. Therefore, it is necessary to use ECM with proper cutting conditions so that it produces sludge with fewer toxic byproducts. This hazardous emission may be retained in the electrolyte by repeated use of the electrolyte. However, such a process might decrease the machining performance. Thus, it is proposed to have optimized ECM process parameters to produce a better MRR and fewer hazardous material emissions. To the best of the authors' knowledge, investigating the amount of nickel present in the sludge during electrochemical machining of Monel 400 alloys has not been attempted before. Optimizing the process parameters is an essential task in the ECM process, as the optimum process parameters improve the performance and economics of machining. The selection of ECM process parameters is carried out based on the experience and expertise of the machinist or machining handbooks. The process parameters selected based on the operator's experience rarely assure high efficiency and quality machining. The machining handbooks can be a handy choice for a few applications only. In most cases, the selected optimum process parameters are far from the best, and this hampers the best utilization of ECM. Selecting the optimum values for the process parameters without proper optimization requires elaborate, costly, time-consuming, and tedious experimentation. Therefore, researchers have used different soft computing techniques to optimize the ECM process parameters in various operations [8]. [PERSON] and [PERSON] implemented a biogeography-based optimization (BBO) algorithm in ECM to determine the optimum machining conditions [9]. The BBO algorithm was inspired by the migration behavior of species among the habitats in nature. A genetic algorithm (GA) was integrated with a desirability function (DF) for simultaneous optimization of multiple objectives of the ECM process [10].