Removal of arsenic and silica from geothermal fluid by electrocoagulation

Abstract

In New Zealand and internationally, two deleterious constituents in geothermal fluids are silica and arsenic. Silica because scaling is a significant limiting factor for effective geothermal energy production and arsenic due to its potential groundwater contamination. Electrocoagulation treatment offers a simple and cost-effective method of removing both constituents, compared to the traditional treatment methods of coagulation and flocculation. It is an electrochemical process that uses direct current to remove a wide range of contaminants. This method has the potential to allow efficient downstream utilization of heat in low enthalpy fluids through cascaded direct use applications and can provide a more cost-effective disposal option than reinjection. Laboratory and field experiments have shown that the process can remove both silica and/or arsenic rapidly from aged geothermal water, leaving a floc which readily settles on standing. In aged water, iron electrodes were most efficient at reducing arsenic levels from approximately 4mgL-1 to < 0.1mgL-1, while aluminium electrodes were the most effective method for reducing silica levels from approximately 600mgL-1 to < 100mgL-1. In a full-scale process, the silica and arsenic can be removed as two separate components, leaving an arsenic-free silica sludge to be disposed of in a landfill and a much smaller arsenic / iron fraction to be further treated or disposed of by reinjection.