The Rock Creek Advanced Wastewater Treatment Facility (RCAWWTF) must balance low effluent ammonia limits, disinfection requirements, and potential low disinfection byproduct (DBP) limits.
The ammonia and disinfection limits are currently met through stable nitrification and use of sodium hypochlorite (SHC) disinfection, respectively. Effluent characterization identified the presence of two trihalomethane DBPs of potential regulatory interest when effluent ammonia concentrations were low: bromodichloromethane (BDCM) and chlorodibromomethane (CDBM). Initial estimates suggest that future discharge limits for CDBM and BDCM may be as low as 1.1 µg/L and 1.5 µg/L, respectively.
Preformed monochloramine (PFM) disinfection was identified as an operational strategy for reducing DBP production. This method utilizes monochloramines that are formed by mixing ammonia and sodium hypochlorite in carrier water before mixing with process water, greatly reducing the opportunity for DBP formation. Bench-scale testing followed by pilot testing was conducted to determine if utilizing a PFM disinfection approach could be an effective solution.
Preliminary bench-scale testing of insitu monochloramines (ISM) versus PFM indicated that PFM was a promising option to meet the disinfection and DBP formation goals; therefore, pilot-scale testing was pursued.
A flow-through pilot system was constructed at the RCAWWTF to receive tertiary effluent where a PFM solution could be added. Testing evaluated:
- Effects of varying PFM dose at a constant chlorine-to-ammonia ratio of 4:1, and
- Effects of varying chlorine-to-ammonia ratio.
Testing demonstrated the benefits of using PFM over free chlorine and ISM by meeting disinfection permit limits and significantly reducing DBP formation potential while still maintaining effluent ammonia concentrations that met permit limits. These results show the viability of implementing PFM disinfection as a solution to more stringent DBP limits. Other clean water utilities may benefit from modifying existing chlorine disinfection to PFM disinfection as a much more cost-effective alternative relative to converting to another disinfection technology.