Disinfection reduces microbial contamination in drinking water. Unfortunately chlorine, the most commonly used disinfectant, reacts with organic matter to create hazardous by-products like trihalomethanes (THM) and haloacetic acids (HAA).
The D/DBP Rule, which became official in December 1998 and calls for full compliance by December 2001, seeks to retain chlorine disinfection while reducing by-product risk. It sets standards for removing natural organic matter (NOM) in source water before disinfection occurs, primarily through enhanced coagulation. NOM is measured as total organic carbon (TOC).
Enhanced Coagulation Under Steps 1 and 2
Step 1 sets required percent TOC removal as a function of raw water TOC and alkalinity. Reduction is higher at higher TOC and lower alkalinities (Table A).
Enhanced Coagulation Under Steps 1 and 2
If a utility cannot meet Stage 1 TOC criteria, it can comply under Stage 2. A utility does not need to perform enhanced coagulation if:
Enhanced Coagulation Under Steps 1 and 2
1. Raw water TOC is < 2.0 ppm.
Enhanced Coagulation Under Steps 1 and 2
2. Treated water TOC is < 2.0 ppm.
Enhanced Coagulation Under Steps 1 and 2
3. Raw water SUVA is #2.0 L/mg-m.
Enhanced Coagulation Under Steps 1 and 2
4. Treated water SUVA is #2.0 L/mg-m.
Enhanced Coagulation Under Steps 1 and 2
5. The running average raw water TOC is < 4 ppm, alkalinity is > 60 ppm (as CaCO3), and TTHM and HAA5 are less than 50 percent of the MCL (maximum contaminant level).
Enhanced Coagulation Under Steps 1 and 2
6. The running annual averages of the TTHM and HAA5 are less than 50 percent of the MCL in the treated water when chlorine is the primary disinfectant.
Enhanced Coagulation Under Steps 1 and 2
SUVA, or specific UV absorbance, is a good indicator of the humic content (long-chain organics) and how well coagulation can remove it from raw water. Waters with high initial SUVA typically have a significant drop in SUVA (humic content) after coagulation. The opposite is true of waters with low initial SUVA. A low SUVA suggests lower molecular weight organic compounds that are not easily removed by coagulation. This is why the D/DBP Rule exempts raw waters with SUVA values below 2.0 l/mg-m.
Enhanced Coagulation Under Steps 1 and 2
SUVA is measured by UV spectrophotometry at 254 nm, the wavelength at which carbon-carbon bonds absorb. It relates to dissolved organic carbon (DOC), a component of TOC, through the general formula: SUVA(DOC) (L/mg-m) = UV254 x 100/DOC (PPM). (Multiply UV254 by 100 for 1-cm.-long cell paths and by 20 5-cm.-long cell path lengths).
Point of Diminishing Returns
If a utility cannot meet Step 1 or 2, it can define a removal level based on a point of diminishing return (PODR) and negotiate an alternative percent removal with the state. The rule defines a target pH for four raw-water alkalinity ranges (Table B). The primary coagulant is added in jar tests at specific increments until the pH is lowered to the target pH. This is the maximum required coagulant dose under this portion of the rule.
Point of Diminishing Returns
Another series of jar tests is needed to pinpoint the PODR, which is defined as the point at which an additional 10 ppm of dry alum fails to remove 0.3 ppm of TOC. Plants that use alum will utilize the predetermined 10 ppm dry alum increments to meet the testing requirements.
Point of Diminishing Returns
Those using other coagulants must calculate the equivalent coagulant dose needed for incremental addition. Dosages are based on the addition of 0.91 ppm of aluminum metal for aluminum-based coagulants and the addition of 1.955 ppm of iron metal for iron-based coagulants. Required increments are listed in Table C for some commonly used coagulants.
