Biological silicon removal from coal seam gas water

Abstract

High silica concentrations can cause silica-related fouling in reverse osmosis and heat exchanger water treatment processes. Conventional silica reduction processes to mitigate fouling produce a waste, which must be treated before disposal. This research investigates a waste free biological alternative to conventional silica reduction processes. The biological process utilises the growth of diatoms to convert dissolved silica into amorphous silica, which is the main constituent in the diatom frustule (shell). When removed from the water, the diatom provides a waste free product which contains lipids, protein and carbohydrates which can be used for biofuels and animal feedstock. The research involved growing diatoms in Coal Seam Gas (CSG) water then removing the diatoms using sieves, thus removing the original dissolved silica as amorphous silica. The research found the growth of both native and introduced diatoms resulted in a reduction of dissolved silica, provided the necessary nutrients were present and biologically available. The diatom growth rate varied from 0.8 to 5.5 g/m2.d (specific growth rate: 0.6 to 2.5 g/g.d). Diatom growth led to the removal of dissolved silicon (Si) at a rate of 120-800 mg Si/m2.d (or 1.2-8 kg Si/ha.d). The net biomass silicon content was 3 wt%. The lower growth rate occurred when trialling a single diatom species in 150mm deep containers, while the higher growth rate was achieved with multiple species in 300mm deep containers. It is expected diatom growth rate per unit area will increase with deeper containers until light availability constrains growth. Given these diatom growth and silicon removal rates, 1.5 to 10 ha of ponds per ML of water processed per day would be sufficient to remove all of the silicon from a typical CSG water treatment facility (Si load is typically 12 kg Si/ML); less area would be required to reduce the silicon concentration to manageable levels. For the biological process to effectively reduce silicon in CSG water the dissolved silicon concentration must be 2 mg/L prior to micro filtration. To achieve 2 mg/L for a typical 10 ML/day RO plant, 10 mg/L or 100 kg/day of silicon must be removed. If the mid range (500 mg Si/m2.d) silicon reduction rate achieved in the lab can be achieved on site, a 20 ha pond would be required.