Particle Removal and Disinfection Byproducts at the Fort Thompson Microfiltration Plant

The drinking water industry is highly concerned with the removal of pathogens to prevent the outbreak of disease. The United States Environmental Protection Agency (USEPA) currently requires 4-log removal of viruses and 3-log removal of Giardia, and will likely require 2-log removal of Cryptosporidium in the future. Typically, the USEP A grants log removal credit to treatment facilities based on their treatment technique and effluent turbidity readings. The microfiltration (MF) system at the Fort Thompson Microfiltration Plant (FTMFP) currently receives 2.5-log removal credit for Giardia and 0-log removal credit for viruses from the Region 8 USEP A. It is not yet known what log removal credit will be given for Cryptosporidium. The remaining log removal must be achieved during disinfection in the clearwell. Previous studies have shown that MF systems can achieve over 3-log removal of Giardia and 2-log removal of Cryptosporidium. This suggested that the FTMFP may not be receiving adequate log removal credit for removal of Giardia, and may already meet the upcoming log removal requirement for Cryptosporidium. In addition to the level of microbial removal, the level of organics removal achieved at the FTMFP was also unknown. Organics react with chlorine during disinfection forming disinfection byproducts (DBPs). As a result, the FTMFP may have difficulty complying with the upcoming Stage 1 Disinfectants/Disinfection Byproducts Rule (Stage 1 D/DBP Rule), and Stage 2 Disinfectants/Disinfection Byproducts Rule (Stage 2 D/DBP Rule). The research objectives were to evaluate the amount of particle removal and organics removal occurring in the MF system. Furthermore, an alternative disinfection method was tested to determine if it would help reduce DBP formation. Particle counters were used to analyze the removal of particles similar in size to Giardia and Cryptosporidium. Log removals were calculated and compared with current and future requirements. In addition, raw and finished water samples at the FTMFP were analyzed for total organic carbon {TOC), dissolved organic carbon (DOC), and specific ultraviolet absorption (SUV A) to determine the organics removal occurring in the MF unit. Samples from the clearwell effluent and end-user in the distribution system were analyzed for trihalomethane (THM) concentrations to determine whether the FTMFP would be able to meet the Stage 1 and 2 D/DBP Rule. Finally, a simulated distribution system (SDS) test was performed to evaluate an alternative disinfection method using ammonia to create chloramines. Disinfection with chloramines typically helps reduce THM formation. The particle counts data indicated that the MF system was achieving an average of over 4-log removal of Giardia and Cryptosporidium. These results suggest that the system will probably meet the 2-log removal requirement for Cryptosporidium, and should meet the 3-log removal requirement for Giardia. Disinfection is still necessary to meet the 4-log removal requirement for viruses. Results from the TOC, DOC and SUV A analyses show that the MF system is achieving minimal organics removal. In addition, the MF effluent has a SUV A level near 2.0-U(mg-m), which suggests that organics removal by enhanced coagulation may be ineffective. The organics in the filtrate of the MF system react with chlorine during disinfection to form DBPs. THM concentrations observed during the testing period were below the Stage I D/DBP Rule requirement of 80-μg/l~ however, testing was not performed during summer months when levels typically peak. SDS test results demonstrated that combined disinfection with ammonia to create chloramines significantly reduces THM formation.