Electrocoagulation As an Alternative Drinking Water Treatment to Decrease Calcium and Manganese from Groundwater: Ecuadorian Case Study

Abstract

Only 37.2% of the rural population of the municipality of San Cristobal in the canton of Paute-Azuay received drinking water according to the population census carried out in 2010. Paute has 2794 inhabitants and this number is expected to grow to 3406 by 2041. Water analysis results published by the local authorities of Paute reported a high concentration of Ca2+ hardness, e.g. 442±32 mg/L of CaCO3 and a concentration of manganese of 0.97 mg/L in the groundwater catchment source. These results far exceed the concentrations suggested by the World Health Organization (WHO) for drinking water as hardness and Mn can affect the water distribution systems and the people health. Since levels under 100 mg/L of calcium hardness induce corrosion of the steel pipes of the plumbing systems, a concentration of Ca2+ ranging from 100 to 200 mg/L is recommended. On the other hand, levels of Ca2+ higher than 200 mg/L may cause scaling in the drinking water distribution systems. It is regulated that the level of Mn must be <0.4 mg/L due to the fact that this element can affect the nervous system.The aim of this work is to test the electrocoagulation process as an alternative water treatment to evaluate the removal of Ca2+ and Mn in water to design the reactor, considering technical, environmental, and socioeconomic criteria. The electrocoagulation process was performed under galvanostatic conditions in a single compartment electrochemical cell using a two-electrode setting (carbon steel) and filled with 150 mL of water coming from the sedimentation stage of the treatment plant. The experiments were performed under different current densities, pH, operating and sedimentation times using a batch process. The concentration of Ca2+ was measured by EDTA titrimetric method before and after the electrocoagulation process. In order to enhance the removal of Ca2+,Ca(OH)2 was added. Similarly, unstable parameters such as total dissolved solids, electrical conductivity, and pH were measured to assess their influence before and after the electrocoagulation process. In addition, chemical oxygen demand (COD) was measured to evaluate if additional chemicals are generated during the process. To estimate the amount of sludge generated, sedimentable solids were measured using the Imhoff cone. These results were used to design the electrocoagulation reactor. The experimental results showed that higher efficiencies are achieved in basic water (pH~8), using an applied current density of 9.56 mA/cm2 and operating and sedimentation time of 60 and 20 min respectively. Under these conditions, the efficiency of Ca2+ removal was 59.3%. It was observed that the sedimentation process did not strongly influence the efficiency of the process. The addition of 100 μL of Ca(OH)2 at 20% increases the Ca2 removal efficiency to 85% and the Mn removal efficiency was 97% under the same conditions.The electrical energy demand was calculated to be 1.13 kWh/m3, this energy consumption is associated with the dimensions for a tank with a flow rate of 11 m3/h. The dosage of Ca(OH)2 was calculated to ebe 0.13 Kg/m3 which is not linked to the reactor dimensions. No alteration in the COD concentration was registered and the generation of sludge was 2.2 times lower than the amount generated using a conventional flocculation process. Once the optimum conditions were obtained, the dimensions of the reactor were established, the diameter of the electrocoagulation reactor is 3.36 m which is 9 times of the electrode diameter, while the height of the reactor is 2 times the height of the electrode, e.g. 1.25 m. The electrode separation distance must not be longer than 0.35 times the diameter of the reactor. Electrocoagulation is a more sustainable process since it produces the coagulant in-situ with low production of sludge mostly biodegradable. In this study the removal of Ca2+ and Mn was optimized obtaining efficiencies >85% where the addition of Ca(OH)2 enhances the electrocoagulation process. The low energy consumption in the electrocoagulation reduces the operational costs of this treatment alternative.