Integrated membrane processes for augmenting water resources and silica recovery at geothermal power plants

Abstract

The interdependence of water and energy, or the “water-energy nexus”, exacerbates the stress on both fresh water and energy resources. Power plants require high volumes of water for cooling purposes. Treated impaired groundwater is one alternative source of cooling tower make-up water. These water sources often contain high concentrations of low-solubility minerals such as silica. Oversaturation of silica can cause polymerization, leading to colloidal deposits, which are very difficult to remove from surfaces. Water from a geothermal power plant located in northeastern Nevada was selected for this study. Currently, more than 37% of the make-up water in the plant is wasted as blowdown because of the presence of silica, despite chemical treatment with numerous antiscalants. This study explores the best operating conditions of three membrane treatment processes: nanofiltration (NF), ultrafiltration (UF), and membrane distillation (MD) to enhance water recovery and potentially recover colloidal silica for beneficial use. Dow’s NF90 membrane was selected for testing. A model to predict concentrations of silica on the membrane surface was experimentally validated and used to determine an optimal water recovery of 82% for the treated water. The NF concentrate was used as feed in the UF to concentrate colloidal silica. A sustainable UF operation was achieved, demonstrated through 90% water recovery and 0.4%/w colloidal silica in the concentrate—facilitated by chemically enhanced backwashing. UF was also investigated as pretreatment to NF, clarifying NF concentrate and returning the permeate into the NF feed; however, the operation was unsustainable. Lastly, MD was explored as a desalination process for water recovery from NF concentrate, and demonstrated that 95% water recovery can be achieved when treating water containing high concentrations of silica.