Factors influencing photobiological treatment process to remove reactive silica from brackish groundwater reverse osmosis concentrate

Recently it was demonstrated that silica assimilating brackish water diatoms can remove the reactive silica and inorganic scalants from the reverse osmosis (RO) concentrate. Photobiologically treated RO concentrate can be further treated using a secondary RO for improved permeate recovery and minimal brine generation. Here, the factors influencing reactive silica uptake from a brackish groundwater RO concentrate containing 93 mg/L of reactive silica and 338 mg/L of calcium, by a brackish water diatom Pseudostaurosira trainorii PEWL001 are discussed. Several semi-batch experiments revealed that P. trainorii PEWL001 could remove > 85% of reactive silica and 50% of calcium within 28 h. Externally supplemented orthophosphate and nitrate-N were found to be the preferred sources of nutrients, whereas ammonia-N inhibited the growth of the diatom. Vigorous mixing negatively affected the reactive silica uptake by the diatom, possibly by causing mechanical damage to the cells. Carbon dioxide addition reduced the pH of brackish groundwater RO concentrate from 8.21 to 6.8, did not enhance the reactive silica uptake but drastically decreased the efficiency of calcium removal. Reactive silica uptake rate was slower under the direct sunlight than under the LED light. The results of this study have important implications in advancing the photobiological treatment process.