Aplicação da técnica de eletro-floculação no tratamento de efluentes de curtume

Abstract

In the present work has been studied the electro-flocculation efficiency on the reduction of organic and inorganic material concentrations such as Chemical Oxygen Demand (COD), turbidity, total solids and metal, mainly chromium, on an untreated tannery effluent using a pair of electrodes. For this purpose, a lab scale Electro-Flocculation (EF) reactor was constructed using an 8-L plastic cylindrical container, containing a valve in its lower part for sludge evacuation and a horizontal support in its upper part for fixing a pair of electrolytic cell in upright position into a 5-L available reactor volume. The electrodes plates, made of iron or aluminum, were connected to terminals of direct current power supply. All the physical-chemistry parameters were measured following the Standard Methods for water analysis, while metal concentrations were determined using the Synchrotron X-Ray Fluorescence technique, available in the Brazilian Synchrotron Light Laboratory. In order to determine optimal operating conditions, the effects of current intensity, electrodes distance, effluent initial pH, and EF time have been experimentally examined, according to a statistic planning and correlation analysis. For each electrode, a total of 11 EF experiments were carried out for maximum and minimum values of EF reactor operating parameters. Applying a 95% confidence level into the correlation analysis between EF parameters, the physical-chemistry parameters and metal concentrations were reduced in all of them when the effluent initial pH was increased, while the EF time and the applied voltage were reduced for small electrodes distances. Moreover, the EF treatment efficiencies tend towards higher values with an increasing on the current intensity. In order to determine the best reactor operating values, other EF experiments were performed varying the effluent initial pH from 6.5 to 9.0 and from 6.0 to 8.5 for iron and aluminum electrodes, respectively, fixing the distance between the electrodes in 4.0 cm, the current intensity in 5.5 A and the EF time in 30 minutes. Nevertheless, these EF experiments have shown that the reduction factor on parameter concentrations due to an increasing of effluent initial pH was not significant compared to prior experiments. Henceforth, the effluent pH value was maintained near to the original one for the next EF treatments, avoiding introducing other substances into the effluent. Keeping effluent pH value in 7.5, other EF experiments were performed changing the EF time from 15 to 120 min., fixing current intensities and electrode distances in 5.5 A and 4.0 cm, respectively. For these reactor conditions, the EF time setting up in 30 min. was enough to get a good efficiency according the reduction factor in the most of parameters examined, without increasing a lot the reactor electrical power, for both kinds of electrodes. The EF reactor purpose and examined has shown a high efficiency on turbidity and chromium concentration reduction factor (98%). Furthermore, this system can be also used to reduce other elementary concentrations such as calcium, potassium and zinc, for both kinds of electrodes (Fe/Al). In comparison with iron electrodes, aluminum one has reported a better reduction factor (80%) on COD values and a good reduction factor (50%) on iron concentration. On the other hand, the EF method for tanneries effluent treatment system was also compared with conventional method using coagulating and flocculating agents, showing a great similarity between them in the reduction factor. In the EF method is not necessary to introduce substances to start the coagulation and flocculation process in effluent treatment.