Aqueous silica removal from agricultural drainage water and reverse osmosis concentrate by brackish water diatoms in semi-batch photobioreactors

Abstract

Abstract A novel aqueous silica removal process using naturally occurring diatoms for water and wastewater treatment, in particular water reuse and desalination, was developed. Brackish agricultural drainage water containing 39 mg L−1 of total silica and 10 g L−1 of total dissolved solids was used as a source of silica-assimilating diatoms. The drainage water was statically incubated in 500-mL and 7.5-L photobioreactors at 27 ± 2 °C under continuous illumination using two 13-W compact fluorescent light bulbs as a light source. After 10 days, brown algal biomass became noticeable and silica removal started to occur. Silica removal accelerated as algal biomass accumulated. In the fourth semi-batch cycle, more than 95 % of molybdate reactive silica was removed within 28 h. Removal of nonreactive silica was also confirmed. Reverse osmosis concentrate samples from advanced water reclamation facilities with high silica concentration (>120 mg L−1) were also tested. More than 75 % of silica was removed within 6 days. Microscopic analysis revealed the presence of Pseudostaurosira, Nitzschia, and Halamphora species in the photobioreactors. To the best of our knowledge, this is the first report on the use of diatoms for aqueous silica removal in water treatment.