4.1.1. Water Treatment Technology and Regulations

During this period, THMs were the only regulated category of DBPs. However, new USEPA regulations were anticipated—i.e., the DBP Rule [44,45]—prompting water agencies to initiate research on advanced water treatment technologies such as ozonation [46] or granular activated carbon (GAC) adsorption [47] and to expand testing for additional DBPs [48] in their finished waters. At the time, chlorine and chloramines were generally the preferred primary and secondary disinfectants, respectively, because of relatively low cost, control of THM formation (significantly reduced during post-chloramination) [49], and high effectiveness in controlling bacterial growth in water distribution systems. Among the water agencies participating in the MWQI program, most were employing these conventional water treatment technologies during the early years of the MWQI program; less than 10 percent of the combined treatment capacity of the agencies employed ozonation (Figure 2).

The DBP Rule, which was promulgated in 1998 [44], lowered the total THM standard from 100 μg/L to 80 μg/L and set limits on other DBPs, including a 60 μg/L limit on the sum of five species of haloacetic acids (HAAs) (another chlorination by-product) and a 10 μg/L limit on bromate, an ozone by-product formed from bromide. The Rule also requires removal of total organic carbon (TOC) via enhanced coagulation or softening. The best available technologies identified for meeting the DBP Rule requirements for THMs and HAAs include enhanced coagulation, enhanced softening, or GAC; ozonation process optimization was identified as the best available technology for meeting the bromate standard. Water agencies anticipated the need for extensive research, retrofitting and upgrading of treatment facilities to meet the DBP Rule. Moreover, the specter of a more stringent Rule magnified the urgency to understand the possible effectiveness of source control in addition to new treatment technologies.

It was clear that water agencies charged with protecting the public health through treating drinking water from the Delta would face serious problems in meeting anticipated regulations [50]. The water agencies recognized that the degree of success they would experience in complying with the new rule would depend, in part, on how well DBP precursors could be reduced in the raw water supply. By reducing the concentration of these precursors in the raw water supply, the formation of known and unknown DBPs (regulated and unregulated DBPs) can be lowered. Moreover, controlling DBPs involves balancing risk/risk tradeoff issues with disinfection requirements. The SWTR had removal requirements for *Giardia* and viruses [41]. An Enhanced SWTR, developed along with the DBP Rule [44,51,52], included removal requirements for *Cryptosporidium*, which can be inactivated with ozone but not with chlorine.
