As seen in Fig. 4, the mass balance was satisfactory for all heavy metals in the primary treatment with closures between )20% and +30%. These losses/gains might be attributed to analytical errors as well as to relative mismatching of influent and effluent samples. The mass
balance of heavy metals in the secondary treatment showed excellent closures between )7% and +1%, with only Ni exhibiting a loss of about 20%. Good closures for all heavy metals were found in the mass balance through the whole treatment process (±30%), with best values observed for Cr (10% loss).