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10.8 CHAPTER TEN

TABLE 10.3 Baffling Classifications
Baffling condition Tlo/T Baffling description
Unbaffled 0. l No baffling, agitated basin, very low length-to-width
ratio, high inlet and outlet velocities
Poor 0.3 Single or multiple unbaffled inlets and outlets, no
baffles
Baffled inlet or outlet with some intrabasin baffles
Average 0.5
Superior 0.7 Perforated inlet baffle, serpentine or perforated
intrabasin baffles, outlet weir or perforated launders
Very high length-to-width ratio, perforated inlet,
Perfect (plug flow) 1.0
outlet, and intrabasin baffles

from the volume of water in the filter box; dividing by the flow rate; and multiplying by
the superior factor of 0.7. Experience has shown that the T10 for typical filters operating
at 4 gpm/ft 2 (10 m/h) is about 15 min.

Disinfectant Monitoring Requirements. The SWTR requires that the disinfectant resid-
ual of water entering the distribution system be continuously monitored by water systems
serving a population of more than 3,300. The residual cannot be less than 0.2 mg/L for
more than 4 h during periods when water is being served to the public. Anytime the re-
sidual falls below this level, the water system must notify the state. Systems serving a pop-
ulation of less than 3,300 may take grab samples on an ongoing basis in place of contin-
uous monitoring. Samples must be collected at least as frequently as given in Table 10.4.
The disinfectant residual must be measured at the same points in the distribution sys-
tem and at the same time as total coliforms are sampled. Disinfectant residuals must not
be undetectable in more than 5% of the samples each month for any 2 consecutive months
that water is served to the public.
Water systems must submit special reports to the state detailing the monitoring re-
quired by the SWTR.
SWTR Changes. The 1989 requirements of the Surface Water Treatment Rule have now
largely been superceded by the Interim Enhanced Surface Water Treatment Rule
(IESWTR) and the Long-Term l Enhanced Surface Water Treatment Rule (LT1ESWTR).

TABLE 10.4 Required Disinfectant
Residual Sampling Frequency for Small
Systems
Population served Samples per day*
< 500 1
501 to 1,000 2
1,001 to 2,500 3
2,501 to 3,300 4
*Samples must be taken at intervals prescribed
by the state.