Page 92 - Water and wastewater engineering

GENERAL WATER SUPPLY DESIGN CONSIDERATIONS 2-35

TABLE 2-16
Maximum contaminant level goals (MCLGs) and maximum contaminant levels (MCLs) for
disinfectant by-products (DBPs)
Contaminant MCLG, mg/L Stage 1 MCL, mg/L Stage 2 MCL, mg/L

Bromate Zero 0.010
Bromodichloromethane Zero
Bromoform Zero
Chloral hydrate 0.005
Chlorite 0.3 1.0
Chloroform 0.07
Dibromochloromethane 0.06
Dichloroacetic acid Zero
Monochloroacetic acid 0.03
Trichloroacetic acid 0.02
HAA5 0.060 0.060 a
TTHMs 0.080 0.080 a
a
Calculated differently in Stage 2.

The D-DBP rule was implemented in stages. Stage 1 of the rule was promulgated in November
1998. Stage 2 was promulgated in 2006.
When chlorine is added to water that contains TOC, the chlorine and TOC slowly react to form
THMs and HAA5. The concentrations of THM and HAA5 continuously increase until the reactions
go to completion. Compliance with the regulation is based on samples taken from the distribution
system. Although the number of samples may vary, generally it is about four samples collected
quarterly. In the Stage 1 rule, the sample points are averaged over four quarters of data. Thus, for the
case of four samples for four quarters, 16 data points are averaged to determine compliance. In the
Stage 2 rule, four samples (one from each quarter) from a single site are averaged. Each site must
be below the MCL. This is referred to as a locational running annual average (LRAA). Although
the MCLs in Stage 1 and 2 are the same, compliance is more difficult with the Stage 2 rule.

Surface Water Treatment Rule (SWTR). The Surface Water Treatment Rule (SWTR) and its
companion rules, the Interim Enhanced Surface Water Treatment Rule (IESWTR) and the Long-
Term Enhanced Surface Water Treatment Rules (LT1ESWTR and LT2ESWTR), set forth primary
drinking water regulations requiring treatment of surface water supplies or groundwater supplies
under the direct influence of surface water. The regulations require a specific treatment tech-
nique-filtration and/or disinfection in lieu of establishing maximum contaminant levels (MCLs)
for turbidity, Cryptosporidium, Giardia lamblia, viruses, Legionella, and heterotrophic bacteria,
as well as many other pathogenic organisms that are removed by these treatment techniques.
The regulations also establish a maximum contaminant level goal (MCLG) of zero for Giardia
lamblia, Cryptosporidium, viruses, and Legionella. No MCLG is established for heterotrophic
plate count or turbidity.

Turbidity Limits. Treatment by conventional or direct filtration must achieve a turbidity level
of less than 0.3 NTU in at least 95 percent of the samples taken each month. Those systems using
slow sand filtration must achieve a turbidity level of less than 5 NTU at all times and not more