AWWA EDC52028 ✓ Most Recent

The US Environmental Protection Agency (USEPA) has recently (December 1998) promulgated the Stage 1 Disinfectants/Disinfection Byproducts (D/DBP) Rule. When effective (in 2001 for large systems), this rule, will require utilities to comply with maximum contaminant levels (MCLs) for trihalomethanes (THMs), haloacetic acids (HAAs), bromate and chlorite. In order to reduce exposure to other DBPs, this rule will also require utilities to achieve specified removal of natural organic matter (NOM) as measured by total organic carbon (TOC). All surface water utilities with conventional treatment systems (including sedimentation) are required to implement this treatment technique termed enhanced coagulation. Enhanced coagulation can be achieved by increasing the coagulant dose, lowering the pH of coagulation or a combination of the two. Increasing the coagulant dose typically results in the formation of greater quantities of flocs which incorporate NOM and remove them through settling and filtration. On the other hand, lowering the pH of coagulation results in better charge neutralization which increases the rate of incorporation of NOM into the settleable and filterable flocs. Although both of these techniques may adequately remove NOM to satisfy the enhanced coagulation requirement, the latter method will generate smaller quantities of sludge and is preferable for some utilities. Utilities which practice enhanced coagulation by lowering the pH of coagulation, typically uses a mineral acid (e.g., sulfuric acid) for pH reduction. An alternative to the mineral acid for pH adjustment is the use of carbon dioxide. Carbon dioxide is commonly used in softening plants to perform recarbonation after the softening process. It has recently been used as a means for pH reduction for enhanced coagulation at the City of Chandler, Arizona. This paper presents various design and operational considerations for a carbon dioxide system for enhanced coagulation purposes. Includes tables, figures. See more