## 1 Introduction  
Heavy metals are inorganic elements naturally found throughout the earth's crust [1]. Their concentration above permissible limits is considered pollution. \"Heavy metals\" refers to a group of elements with a density greater than 4 g cm\({}^{-3}\), including metals and metalloids [2]. Industrial discharges, agricultural runoff, storm water, mining activity, and direct inclusion of sewage/ wastewater contribute to the heavy metal pollution load in fresh water, leading to various health and environmental problems. Among the commonly reported heavy metals, copper (Cu) is used widely in electroplating, batteries, pesticides, galvanized pipes, and alloys [3; 4; 5; 6; 7]. Regular consumption of copper-contaminated drinking water may cause stomach upsets, abdominal cramp and diarrhea. Nickel (Ni) is another metal found widely in water and wastewater. The electroplating industry, rechargeable batteries, and galvanized pipes are its main sources. High levels of nickel contamination cause serious lung and kidney problems as well as skin dermatitis and pulmonary fibrosis. In drinking water, the maximum allowable limit for nickel is 0.1 ppm [8]. Zinc (Zn) is used in many types of industry, such as metal production, galvanization, food preservation, agri-food and biological engineering, pharmaceuticals, electronics, mining and metallurgy, with major contributions coming from electroplating and mining effluents [9; 10]. Zinc is not considered highly toxic but its presence in drinking water if exceeding 15 mg/L is reported to cause nausea, vomiting and diarrhea [11]. These heavy metals are ingested directly either by drinking contaminated water or indirectly through the food chain, and subsequently affect human health [12; 13; 14]. Drinking of contaminated water has been reported to lead to around 70-80% of the total diseases in developing countries [15; 16], where the impact of increased pollution is particularly problematic because the population at large does not have sufficient resources to effectively treat the contaminated water or access to safe drinking water systems at their homes. According to a WHO (2017) estimate, around 844 million people do not have access to a basic drinking water source and 230 million people spend more than 30 min/day in collecting water from an improved water source, which may include piped water, boreholes, protected wells and springs, rainwater and packaged/delivered water [17]. According to the United Nations, an estimated 80% of all industrial and municipal wastewater in the developing world is released into freshwater bodies without any prior treatment [18]. Heavy metal removal can be achieved through different physical, chemical and biological methods such as fungal remediation [19], microbial remediation [12; 20], phytoremediation [21; 22], adsorption [23; 24], flotation, coagulation-flocculation [25], chemical precipitation or ion exchange [26]; selection between these may be based on the nature and quantum of the pollution load and merits/demerits of decontamination processes along with other factors. It is noted that removal of heavy metals from water/wastewater is still an evolving research area, and there is wide scope for case-specific evaluation, optimization and integration of new and/or available technologies. In this regard, it has been noted that removal of heavy metals from aqueous solutions, especially metal-laden water or wastewater displaying high and heterogeneous concentrations is one of the major challenges. For this, nanofiltration (NF) and electrocoagulation (EC) processes have been reportedly more reliable than bioremediation in terms of the shorter time taken in providing near complete removal, ease of setup, and predictability. The primary emphasis of the present study is to explore the efficiency of removal of Cu, Ni, and Zn by NF and EC processes from their synthetic aqueous solutions in low to high concentrations.