# Electrospark Method in Galvanic Wastewater Treatment for Heavy Metal Removal
## 2 Materials and Methods  
The essence of the electrospray method is to pass through wastewater short-term electrical pulses with electrodes immersed in the solution (Figure 2). Pulsed electric discharge in a liquid is accompanied by a sharp increase of pressure (up to 100-200 MPa), exposure to strong electromagnetic and acoustic fields, cavitation, which creates a powerful flow of liquid that converges and diverges, a sharp rise in temperature, and so on. In this case, the discharge gap is practically a low-temperature plasma, which transmits energy to the treated medium. Figure 1: A schematic diagram of the discharge–pulse treatment of the liquid. 1—step-up transformer; 2—diode bridge; 3—capacitor; 4—electrodes; 5—treated fabric; 6—cylindrical mesh; 7—reactor vessel; 8—discharge zone; 9—liquid. Electric discharge--in fact, an explosion in the aqueous phase--causes complex physicochemical processes that lead to the decomposition of organic impurities present in wastewater, improving the coagulation of colloidal and suspended solids, as well as the precipitation of suspensions and chemical compounds. The raw material for the treatment is real wastewater after various galvanic production operations, which is supplied to the treatment facilities of a machine-building enterprise. Galvanic wastewater is multicomponent composition containing ions Cr\({}^{6+}\), Ni\({}^{2+}\), Cu\({}^{2+}\), and Zn\({}^{2+}\). A mixture of metals such as aluminum and iron granules with diameters from 4 to 6 mm is selected as the material for metal loading (Figure 3). This choice is due to the following reasons:  
* the positive result of complex water purification from heavy metal ions during high-voltage electric discharges in reactors, granular metal loading, and the use of these materials [46];
* the traditional use of iron or aluminum electrolytic soluble anodes during the implementation of the electrocoagulation method for the treatment of galvanic wastewater [15];
* experience in stabilizing spatially distributed discharges in a layer of aluminum and iron granules using low-voltage (up to 1000 V) electrical equipment [49; 50]. The layer of metal-loading granules in the form of a rectangular parallelepiped is characterized by the following sizes: length (l)--the distance between the electrodes; width (b), which is equal to the width of the electrodes, and height (h), which varied depending on the scheme of implementation of the fluid flow. The distance between the electrodes is chosen in such a way as to ensure a stable breakdown along the shortest chain of contacts between the granules from one electrode to another at a given capacitance of the capacitor bank in order to optimize the phase composition of electro erosion particles [49; 51; 52]. Figure 3: The appearance of metal-loading granules: (**a**) iron granules; (**b**) aluminum granules. Figure 2: A typical type of single electrospray. The discharge current and voltage at the inter-electrode gap are recorded with an OWON XDS 3202E oscilloscope, using a divider and shunt of our own manufacture. There are many variants of circuit solutions of sources of electric discharge pulses based on semiconductor switches. To solve the problems of this work, the basic scheme was chosen without the use of energy efficiency, feedback control, etc. This scheme eliminates any effects on the cleaning results, which is very important. The electric circuit based on semiconductor switches (a push-pull charge/discharge) was chosen to implement low-voltage electric discharges between the metal loading granules (Figure 4). The tacts were formed by the signals of the rectangular pulse generators (PG1, PG2), which were connected by the output/input of synchronization, either with a preset frequency of the master oscillator (PG1) or external launching (S). Pulse isolation transformers (IT) were used to decouple the power and control circuits.