Investigation of Bromate Formation during Ozone-Biofiltration Treatment Processes for Potable Reuse Application
Substantial population growth in addition to climate change has caused water scarcity in many regions. Consequently, as an essential component of water resource management, potable reuse of municipal wastewater has been taken into consideration. Potable reuse projects mainly employ multiple purification steps to provide effective inactivation of pathogens as well as organic and inorganic contaminants. Though conventional potable reuse treatment trains employ reverse osmosis (RO) as the main purification barrier, concentrate disposal, challenging requirements for influent water quality such as removing suspended solids and organic matter, and high capital and operation costs, limits the use of RO. However, as a non-membrane based alternative technology to RO, ozonation followed by biological activated carbon (BAC) or ozone/BAC is an effective treatment technology for eliminating organic matter from a variety of wastewater effluent streams through oxidation, biological and physical filtration. Additionally, ozone is a powerful oxidant used in water treatment to degrade contaminants of emerging concern (CECs) into less harmful moieties. Ozonation as a vital process in ozone /BAC system produces a high-quality effluent while reducing brine production and disposal and eliminates the potential health risks associated with waterborne pathogens and so, it is being widely adopted worldwide for water reuse applications. However, ozonation may produce disinfection by-products (DBPs) harmful to human health. Potentially carcinogenic bromate is a DBP formed during the ozonation of bromide-containing waters. In bromide containing water, especially above 50 μg/L, bromate formation becomes a major concern. To understand bromate formation, a comprehensive pilot-scale study of ozone/BAC was conducted at South Truckee Meadows Water Reclamation Facility (STMWRF) and Reno Stead Water Reclamation Facility (RSWRF). At STMWRF, five sampling events and a bromide spike experiment was conducted. On the other hand, extensive sampling at a weekly basis was carried out for five months at RSWRF. Water samples from locations in the ozone/BAC were collected and analyzed for bromate. The results show that bromate remains nondetected in ozone influent, increases in ozone effluent and decreases after the biofiltration process. As bromate formation is strongly influenced by water quality and operational variables, water quality parameters (i.e. bromide, ozone dose, pH, ammonia, TOC, alkalinity, conductivity and temperature) were measured during the experiments. For both of the plants, results showed bromate formation increased through increases in bromide, ozone dose and pH; and decreased by decreases in total organic carbon (TOC) and increases in temperatures. An empirical correlation was also established to predict bromate formation at various operational conditions using bromide, ozone dose, TOC, temperature, pH and conductivity. The predicted bromate concentrations achieved from the correlation did not under-predict the observed or experimental bromate concentrations (in few cases the model under predicted values). This study provides a clear understanding on the water quality parameters positively or negatively influencing bromate formation in ozone/BAC.
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