When four electrodes were used, the higher amount of metal ions released to the water destabilized the solids in the fluid and, thereby, increased the removal of contaminants with lower energy
requirement, as shown also in Table 2. Under these experimental conditions, the smallest ECEI values were achieved as low as 20.1 kWh Kg_1 cm_2 to generate COD removal as high as 76.8%. The addition of electrolyte for experiments using four electrodes did not generated the expected improvement in the EC performance.
Increase in the COD removal was certainly observed from 76.8 to 88.7% by the addition of 0.1 M of NaCl; this result, however, was achieved through an important increase of the ECEI value, from 20.1 up to 89.4 kWh Kg_1 cm_2. These results agreed with previously reported (Merzouk et al., 2011) where no significant improvement of the EC removal of a dye was observed by increasing electrolyte concentration (i.e. NaCl) in the EC reactor.
However, the overall COD removal obtained is fairly comparable with the results obtained previously (Izquierdo et al., 2010) for similar concentration of NaCl in the treatment of soluble oils by electrocoagulation using aluminum electrodes. They found an important increase in COD abatement when electrolyte concentration
increased from 0 to c.a. 400 mgL_1 (up to c.a. 85e90% in COD abatement), further increases in the electrolyte concentration did not generate any significant improvement in the removal of contaminants. However, it can be seen that in these cases, by adding a certain concentration of electrolyte, may not be favorable to increase
the conductivity, which may occur due to the associative character of the particles and this will create a resistance and does not favor the transport of loads. This explains why it is better to work four electrodes unsalted although pollutant removal is slightly lower.