Removal of Selenium from Industrial Waste Water by Ferrous-Ferric Salt

Abstract

Natural water bodies are continuously facing excessive selenium discharge from chemical industries, refineries and various treatment plants in US. Selenium is a naturally occurring element which is soluble in water in the form of inorganic oxyacids i.e. selenate (SeO4-2) and selenite (SeO3-2). Both selenate and selenite have tendency to accumulate in few parts of animal/human body. Selenium has essential roles in mammalian metabolism but it is considered as dietary supplement at <4ppb concentration. Selenium discharge at higher concentration into natural water bodies can cause adverse effects on aquatic and aquatic dependent wild life. Under Clean Water Act (1977) Section 304 (a), Environmental Protection Agency US regulates discharge of selenium and other toxic pollutants into natural water bodies for the protection of aquatic life. Current EPA Regulation for selenium discharge is 5 ppb on a daily average and 4 ppb on a monthly average and expected to get stricter in future. Best Available Technology (BAT) suggested by US EPA can remove only selenite from industrial waste water. Hence, it is difficult to meet EPA freshwater criteria using BAT when both selenate and selenite are present in industrial waste water. Currently, there is no such process available which can constantly meet EPA regulation related to selenium discharge. In this study, a new process comprising addition of ferrous and ferric salt has been developed for synthetic waste water prepared in laboratory. The process has been tested in batch as well as in continuous experiments. The newly developed process is able to remove both selenate and selenite from synthetic waste water to <5ppb total selenium; therefore, it has potential to meet forthcoming stringent EPA regulations.