Pilot test of simultaneous removal of silica, hardness, and turbidity from gray water using circulating pellet fluidized bed

Abstract

A pilot test was performed by applying a circulating pellet fluidized bed (CPFB) system during the simultaneous removal of silica, hardness, and turbidity from gray water generated from the coal gasification of a coal-to-liquids company in Shanxi Province, China. The effects of a magnesium agent (MgO) and coagulant (Al2(SO4)3) in the CPFB system on the simultaneous treatment of silica, hardness, and turbidity in gray water were investigated, and the removal mechanism was explored. The test results demonstrated that when the MgO and Al2(SO4)3 dosages were 500 mg/L and 660 mg/L, NaOH and polyacrylamide dosages were 200 mg/L and 4 mg/L, respectively, and the removal efficiencies of silica and total hardness exceeded 80% and 55% in the effluent, respectively, with the effluent turbidity remaining below 5 nephelometric turbidity units. The comprehensive cost of dosage agents was approximately 0.22 USD/m3. Physical characterizations of the sludge using scanning electron microscopy, energy-dispersive spectroscopy, and X-ray photoelectron spectroscopy revealed that silica was primarily removed through silicate precipitation and adsorption by compounds such as Mg(OH)2. The findings confirmed that the CPFB technology can reduce the silica, total hardness, and turbidity contents within the desired range, and thus, this study provides useful design parameters and an engineering basis for the development of subsequent gray water treatment processes.